US2052183A - Television apparatus - Google Patents

Description

Aug.' 25, 1936.

H- M. LEWIS ET AL TELEVISION APPARATUS Filed Oct. 5, 1934 8 Sheets-Sheet 1 1 36 A9 1 20 I A?! INVENTORS H.411 ew/ls/t (awe/I2 ATTORNFYS I Aug. 25, 1936.

H. M. LEWIS ET A I- TELEVISION APPARATUS Filed on. 5, 1934 a Sheets-Sheet 2 W DW, M ATTORNEYS Aug. 25, 1936. H. M. LEWIS ET AL TELEVISION APPARATUS 8 Sheets-Sheet 3 Filed 001:. 5, 1934 fib il, I\ l\ l\ .57 I 105, -%/dc INVENTORS fiMlew/ a? ("am/ah,

(DM Q L MV-M ATTORNEY5 Aug. 25, 1936. LEWIS 51- AL 2,052,183

' TELEVISION APPARATUS Filed Oct. 5, 1934 8 Sheets-Sheet 5 e-Fe l 4 Nfi EM L k INVENTOR5 f/Mlem34/i Carve/h ATTORNEYS Aug. 25, 1936. H, LEWIS ET AL 2,052,183

TELEVISION APPARATUS Filed Oct. 5, 1934 8 Sheets-Sheet 6 dyad Sync/1 My; u

ATTORNEYS Aug. 25, 1936.

H. M. LEWIS ET AL TELEVISION APPARATUS Filed Oct. 5, 1934 8 Sheets-Sheet 8 Patented Aug. 25, 1936 UNITED STATES PATENT OFFICE TELEVISION APPARATUS Harold M. Cawein.

Douglaston, asset. N. Y., assignors to Hazel and Madison object of this invention is to produce current or I saw-tooth wave form through inductance to provide the field for magnetic control of scanning in a cathode ray television receiver. Another object of this invention is to provide circuits and apparatus for generating the saw-tooth and related wave forms in a precisely controllable and economical manner for use in television. Another object of the invention is to generate voltage and current wave forms related to the sawtooth wave forms which will serve to govern or control the generation of voltage or current of saw-tooth form.

These and other objects of the invention will be clear from the specification which follows particularly as it relates to the description the drawings in which:

Figs. la-ld inclusive, illustrate as applied to a cathode-ray tube, the manner in which voltages and currents of saw-tooth and related wave forms are employed to effect scanning in a television receiver.

Fig. 2 is a schematic diagram of a cathode-ray television receiver to illustrate the essential units required for receiving, scanning and controlling a television image.

Figs. Zia-3e inclusive, graphically depict a socalled derivative series of wave forms of which the saw-tooth form is one, to illustrate the voltage and current relationships relative to pure reactances, such as inductance and capacity shown diagrammatically in Figs. 3) and 30 respectively.

Figs. 4a, b and c, are graphs employed to explain the correct use oi the saw-tooth wave forms when used for scanning the lines in a television picture.

Figs. 50-5) inclusive, are graphs similar to those of Fig. 4, illustrative of the use of the sawtooth wave form for scanning at picture frequency.

55 Figs. fie-6g inclusive, are a series of fundamental circuits pertaining to the generation of voltages and currents of saw-tooth and related wave forms.

Figs. "la and b and Figs. 8a and b, are diagrams of the wave forms of current and voltage 5 resulting from operation of certain of the Figs. 6a to 69 circuits.

Figs. 9a, b and c, and Figs. 100, b and c, are graphs illustrating the use of combined sawtooth and impulse wave form for scanning in tele- 10 Vision.

Figs. 11a, b and c, are graphs illustrating wave forms suitable for causing current of saw-tooth wave form to flow in an impedance.

Figs. 12-20 inclusive, show various types of 15 oscillator circuits employing series resistance and capacity for generating saw-tooth and related wave forms.

Figs. 21a, b and c, illustrate circuits for causing current of saw-tooth wave form. to flow in go inductance.

Figs. 22 and 23 are circuit diagrams illustrating the association of television frequency amplifiers and synchronizing circuits with saw-tooth form generators. 25

The present application is one of a series of related copendi'ng applications to the same inventors, which in aggregate, describe a complete television transmitting and receiving system employing saw-tooth and related wave forms for line and picture scanning. In copending application of Harold M. Lewis, Serial No. 747,070, filed Oct. 5, 1934, there is described the transmitting portion of the system utilizing substantially sawtooth scanning for generating the image or vision as frequencies and also synchronizing impulses for controlling the scanning action or the receiving apparatus. The present application is directed more specifically to the generation and synchronized control in the receiving apparatus of the 40 saw-tooth line and picture scanning impulses.

It is contemplated that a television modulated carrier wave having such essential components as those described in the above mentioned copending application, or its equivalent, is to be received to provide the vision frequency signals and control impulses referred to in this application for reproducing the image at the receiver. The present application therefore, is restricted to just so much of the descriptions of the 00- pending application as is requisite to an understanding pf the novel aspects of this app ication.

In the several drawings, elements which perform the same function in each case, are similarly labeled; those of a fundamental nature being labeled by letters.

Other elements are labeled numerically.

,Referring to Fig. 1a, A represents the luminescent-screen end of a cathode-ray tube K, upon which the scanning traces are indicated as they would appear in their ideal form when no signal is being received and applied to modulate the grid of tube K to control the intensity of the electron beam striking the screen. This ideal trace is of saw-tooth wave form in that each line is a linear trace from left to right (indicated by heavy line m to represent a constant value of illumination of the screen) and a rapid (practically zero time) retrace from right to left (indicated by a light line n) since practically no electrons strike the screen in this brief retrace interval and the illumination is therefore weak. Similarly, the succession or rate at which the lines are traced is shown to be linear with time from top to bottom of the screen since the lines m and n are evenly spaced for the 20 lines shown and the retrace p, q from bottom to top of the picture is shown to be rapid in that it occurs in the time required for two lines to be traced; and hence these two lines appear oppositely sloped in the picture retrace. Thus, in the example illustrated the linefrequency m, n saw-tooth wave form has an almost infinite ratio between time of traceand retrace, and the picture frequency p, q,

saw-tooth wave form has a ratio of trace to re-' trace shown as 10 to 1. This manner of scanning the picture from left to right and from top to bottom is termed rectilinear scanning.

At the present time, for a television picture of reasonable quality, the number of pictures transmitted per second may be taken as 24, and the line frequency taken as 2880 per second. At least this is a suflicient definition of quality to serve in this application for purposes of describing the inventions.

The ratio of picture trace to retrace time is in practice about 40 to 1 (much better than shown in the illustration). Hence, with the line frequency of 2880 per second, there are 117 lines in each picture and three lost in the picture retrace. This ratio has a correspondence dimensionally to the ratio between the height of picture in a standard 35 mm. motion picture film and the Opaque space separatingadjacent picture frames (the film to space ratio being, in fact, about 30 to 1). Hence to scan motion picture film as described in the mentioned copending application Serial No. 747,070, only the time required for three or four lines can be lost in the retrace without losing a. part of the picture height. As for the line frequency, a reasonable trace-to-retrace ratio in practice is about 10 to 1 which is far from the ideal of zero time shown in Fig. 1.

To cause the cathode ray to scan the screen it may be deflected by magnetic or electro-static fields. Magnetic'deflection is illustrated in Fig. l.

. The coils Ll, L1 serve to provide the field for the picture frequency scanning while L2, L2 similarly serve to provide the field for line frequency scanning. Since the magnetic field changes proportionally to the current flowing in these coils, the current through L1, L1 must have a saw-tooth wave form recurrent at the picture frequency 1), qv rate, for example 24 per second; and the coils L2, L2 must carry current of saw-tooth wave form at the line frequency m, n rate, such as 2880 per second.

In order readily to understand the nature of the voltage required to cause such a current in the scanning coils, and to arrive at other voltage and current relationships which maybe desired, a series of wave forms related to the saw-tooth wave form has been plotted and is shown in Figs. 3a to 3e inclusive. This may be termed a derivative series, since any of the wave forms shown is the mathematical derivative of the wave form immediately below it in the series. The utility of this series as it stands relates to pure reactances, since fundamentally the voltage e1, across an inductance, Fig. 3 is the derivative of the current 11. through the inductance, and the current ic through a condenser, Fig. 3g, is the derivative of the voltage ec across the condenser. Hence using the notation at the left of the figure, if a current through a pure inductance has any of the wave forms shown, the voltage across the inductance must be of the wave form shown immediately above, and if a voltage across a pure condenser has any of the wave forms shown the current through the condenser is of the wave form shown immediately above in the series. Fig. 3c is the desired wave form for scanning and is shown as providing a ratio of 10 to 1 in the time of trace to retrace. Fig. 3a. will be referred to as a doubleimpulse; Fig. 3b as an impulse; Fig. 3c as a sawtooth, and Fig. 3d as a parabolic impulse wave form. The form Fig. 3e requires cubic equations to represent it mathematically.

In Fig. 1a, the current in coils L1, L1, and that in coils L2, L2 is labeled i3c to indicate its wave form as being that of Fig. 3c and the voltages applied in both cases are necessarily of impulse wave form and hence labeled eat to indicate the wave form as being that of Fig. 3b.

In Fig. 1b the cathode ray tube K employs deflecting plates for electrostatic scanning, wherein P1, P1 are the deflecting plates for the picture frequency and P2, P2 the deflecting plates for the line frequency scanning. Since the deflection of the electron stream is proportional to the voltage between the deflecting plates, the voltage across P1, P1 should be saw-tooth as indicated by the notation 83c and that across plates P2, P2 similarly should be saw-tooth in form as labeled 630.

Fig. lc illustrates magnetic deflection, coils L1, for the picture frequency and electrostatic deflection, plates P2, for the line frequency, and hence the voltage required to cause saw-tooth current in L1, L1 is of impulse wave form 63b as indicated, and the voltage across plates P2, P2 is of saw-tooth form 630' The saw-tooth wave is also important in mechanical systems of scanning, which do not use rotating elements but which, nevertheless, must accomplish rectilinear scanning. For example, if a light valve is to be employed to vary the light intensity in accordance with the picture detail and the light is then to be scanned to spread a pattern on a screen similar to the pattern in Fig. 10., then the arrangement of Fig. 1d is a solution requiring the use of saw-tooth wave forms. In this figure the cathode ray tube is the source of light as well as the light valve, having in its simplest form a cathode l, a control grid 2, an accelerating apertured anode or plate 3 and fluorescent screen A. The vision frequencies which represent the picture detail are, of course, applied as voltage variations between I and 2 to control the number of electrons which pass through 3 and hence produce light in proportion to their number on striking the fluorescent screen A. No special elements have been indicated here static of magnetic fields as best serves the construction of the particular tube employed. Lens serves to focus the spot of light from A on screen 8 via the mirror surfaces of oscillograph-type vibrator mirrors 6 and 1 which determine the path of the light. It will be clear from the description of Fig. 141 that vibrator 6 must act to deflect the light on' the screen according to a saw-tooth form at the line frequency and similarly vibrator I must cause the light to traverse the screen linearly from top to bottom and rapidly back to the top in saw-tooth form at the picture frequency. The angle through which the mirror surfaces turn and hence the motion of light on the screen is assumed to be directly proportional to the current throughthe vibrators and to the voltage applied; hence in each case, the actuat ing voltage required is, as indicated, of the form Fig. 30.

To better understand the specific circuits which follow, it is desirable first to consider broadly the functioning of the receiver and the cooperation of its parts, whereby the scanning operation at the receiver is synchronized with the transmitter and the picture correctly reproduced. Hence in Fig. 2 a diagram of the receiver is shown wherein unit blocks serve to indicate broadly the functions of the various receiver parts. The receiver illustrated is of the superheterodyne type in that theincoming carrier wave and sidebands are collected by antenna structure 9, amplified bythe radio frequency amplifier l0, and applied to modulator ll, to gether with heterodyne energy from oscillator l2, to produce an intermediate frequency carrier and sidebands which are amplified by the intermediate frequency amplifier I 3 and applied to detector H. The detector l4 develops the vision frequencies, which represent detail in the picture, and also line and picture frequency impulses, which are to serve for synchronizing the scanning at the receiver with that at the transmitter andto block out illumination of the screen during each line retrace and each picture retrace interval. In order that the picture appearing on the screen shall be a positive and the impulse modulation of the carrier shall function for block-out of retraces, the proper poling of the output from M as applied to vision frequency amplifier l5 and the proper poling of the output of i5 as applied to the cathode ray tube grid 23 must be observed, so that as the cathode ray traces its pattern on the screen 25, the trace lines will be light or dark in accord with the corresponding line being traced at the transmitter, and the retrace lines will be black.

To make the case general, the cathode ray tube is shown as having magnetic control coils L1, L 'for thepicture frequency and electrostatic control plates P2, Pa, for the line frequency as was the case in Fig 1c. The generator unit It! serves to generate and supply saw-tooth voltage (labeled 83c) of line frequency to the deflecting plates P2, P2 and the generator unit 2| here serves to cause saw-tooth current (in) to fiow in deflecting coils L1, L1 (1. e., it furnishes impulse voltage of form 63;; across these coils).

To control the frequency and timing (phase) of the outputs of units l8 and 2i, the line frequency impulses and picture frequency impulses developed by detector I! are also applied to filter units I6 and I9. Unit 19 is, for example, a lowpass filter suitable for passing the picture frequency impulse undistorted in wave form to unit 20 which is the picture impulse amplifier to pass the line impulse to amplifier II which in turn serves to apply the line impulses properly poled and adjusted in amplitude as a control or synchronizing voltage to l8.

The arrows in the lines connecting units IS. IT and I8, and similarly units I9, 20, 2!, indicate that these are one-way circuits and that no voltages from units l8 and 2i are to pass in the opposite direction so that these generators in themselves shall not affect each other nor shall they react back on the detector l4 and hence on the amplifier l5 and the cathode ray tube grid 23.

The wave form of the synchronizing impulses supplied by I4 to units l6 and I9, is clearly ex plained and illustrated in copending application Serial No. 747,070, and need not be gone into here. It is suflicient to say that for the line frequency, the synchronizing impulse is similar to that here shown in Fig. 8b so that this graph may be taken as the formof impulse from I4 applied to control generator l8. Thus, as shown in Fig. 4a, during the time interval t, u a line m is being traced on the screen, and during the interval u, t the retrace 12. occurs (the ratio of time intervals being shown as 10:1). Similarly in Fig. 8b the control impulse endures for an interval '0, w and is repeated after the trace interval 10, v. This ratio of intervals is also shown as 10:1. Hence, if the impulse peak at 0. occurs simultaneously with the end of the trace it in Fig. 4a, perfect synchronism is assured.

Remembering now that the impulse, Fig. 8b, is applied to the cathode ray tube grid 23 poled negative to cause the screen to darken simultaneously with its application to unit l6 for control, it will be evident that the screen will be dark during the retrace and bright during the trace as shown in Fig. 41). If, however, generator I8 is producing a saw-tooth voltage as in Fig. 4c in which the ratio of trace to retrace is, say '7 to 3 as shown, and if synchronism is secured in that the points u. of this voltage and v of the control impulse occur simultaneously, the result is that shown in Fig. 4c where the retrace is blocked out" as long as the synchronizing impulse interval v, w endures. Here, however, the transmitter's line trace starts at w which is prior to the cathode ray beam having reached the left edge of the picture so that in effect the left side of the picture is folded under". Other relations giving improper synchronism will be seen to be possible depending upon the relative phase of the synchronizing impulses and the relative ratios of trace to retrace time between the transmitter's scanning and the receivers scanning processes. Hence for synchronism it is important in order that no part of the transmitted picture he lost, that the trace to retrace ratio of units I8 and 2| be as large or larger than that of the transmitted picture, and that the scannings correspond in phase in order that each line of the cathode ray tube screen starts with the corresponding line of the transmitted picture and that the retraces correspond so that retraces will be blocked out as shown in Fig. 4b.

If generator [8 produces a voltage only approximating the required voltage e3c, then it is important that the trace 111. part of the cycle should be linear but the shape of retrace n is unimportant so long as its time interval is held sufliciently short. Non-linearity in the line traces would result in the traces (m in Fig. 4) being curved and the picture detail would appear crowded together in some places and too widely spaced in others. The illustration is more easily given when non-linearity exists in the picture trace, as shown in Figs. 5a to 5f inclusive. Thus in Fig. 5a the wave-form shown for the picture frequency is that of Fig. 3c and the proper uniform spacing of the picture lines which results are shown in the first corresponding pattern Fig. 5b. The second pattern Fig. 50 indicates how the retraces are absent when proper synchronization of the picture frequency is achieved and block-out occurs.

Fig. 5d indicates that the saw-tooth current from generator 2| is of exponential form in both trace and retrace and the subsequent crowding of the lines at the bottom of the picture is shown in pattern Fig. 56. With correct synchronization the retrace lines are blocked out as shown in pattern Fig. 5].

It will be clear then that rigid requirements of design are imposed on generator units I8 and 28 of Fig. 2, in that they must supply saw-tooth voltage orcurrent as the case requires having good linearity in the trace, adequate ratio of trace to retrace time, and proper response to synchronizing, control voltages. Furthermore, the units l6 and H for the line control and I9 and 20 for the picture control must fulfill two functions: (a) apply the synchronizing impulse undistorted and in proper amplitude and phase to units I8 and 2!, to effect synchronism; and (b) prevent reactions between these units and the rest of the system.

In the circuits which follow, fundamental circuits are first illustrated and specific circuits which perform the fundamental functions in a preferred manner are then given. It will appear that the units 3, I1, I9 and 20, Fig. 2, may be dispensed with, in part or in whole, where the designv of the units [8 and 2| is such as to render their assistance unnecessary.

In Figs. 6a to 69, a number of fundamental circuits are shown for producing current and voltage wave forms related to the saw-tooth derivative series. The circuits of Figs. 6a, b and c will be termed the R, C, type in that the desired wave form results from the charge and discharge of a condenser through resistance. The circuits of Figs. 6d, e, f, and y, are termed the L/R type in that the wave form results from the flow of current through an inductance as affected by resistance.

In the modification of Fig. 6a, a pendulum S periodically short circuits 2. capacity C through resistance 1 for a brief interval of time during each swing to the left, owing to closure of switch 9. S is assumed to be actuated bysome mechanism, as for example the usual clock escapement, so that the frequency of recurring short circuits is here determined by S. For the values which follow, the voltage across C relative to ground and the current through C are to a close approximation as shown in Figs. 7a and 7b, respectively.

E=300 volts, R=0.l megohm,

1:5000 ohms, 0:11.008 ,uf, Ratio of trace to retrace=b'=10: 1,

j=2250 cycles per second.

During each interval that the contacts of tially charged from source E through resistance R at a rate determined by the time constant of the circuit which depends on the product of. R and C. During the retrace, that is, while the contacts of switch g are closed by swinging of the pendulum to the left, the condenser 0 discharges through the resistance r at a rate depending upon the product of r and C. The effect of the path through R on the rate of discharge condenser C can be neglected since R is large compared with r. The current through C, Fig. 7b, is the mathematical derivative of the voltage, Fig. 7a. It will be noted that the exponential saw-tooth voltage of Fig. 7a approximates the ideal form, Fig. 3c, and that the exponential imgglse, Fig. 7b, approximates the ideal form, Fig.

The analogous case for an inductance is given in Fig. 6d, and the constants of the circuits, listed below, may be so chosen that the same curves of Fig. 7 represent the resulting wave forms; Fig. 7a in this case being the current through the inductance L and. Fig. 7b being the voltage across the inductance. In the circuit of Fig. 6b, the battery E supplies current through 1', L and R during the trace part .of the cycle. The pendulum is here labeled 0 to indicate that it is an opening device instead of the shorting devic of Fig. 4a. The contact to O is closed except during a brief interval during the end of the swing of O to the right. The circuit constants mentioned to satisfy Fig. 7, are

E=300 volts, r=5000 ohms, b'=10:1,

L=4h, R=100,000 ohms.

The current through L increases slowly and exponentially during the trace part of the cycle according to the time constant L/r. During the retracethe current falls rapidly and exponentially according to the time constant L/R. It will be noted that during the retrace the part of the circuit which includes E and r can be neglected (i. e., considered as of zero resistance) since R is large compared with r. The error in making this assumption is very small. The voltage across the coil is in this case the mathematical derivative of the current through the inductance and is as shown in Fig. 7b.

In the R, C case Fig. 6a, if circuit conditions permit values of R and C to be chosen so that operation occurs over only a small part'of. the exponential curve for the trace then the trace will be sufficiently linear to serve as a saw-tooth wave form for scanning.

Similarly, if L and r in Fig. 6b are chosen so that operation occurs over only a small part of the exponential trace (i. e., L/r is large) the resulting current wave form may be acceptable for scanning in television.

The exponentiality of retrace in both cases is unimportant but the time of the retrace interval is quite important. The frequency in each case is determined by the periodicity of the pendulum, and the ratio of trace to retrace isobvirously'determined by the ratio of the time that the contact to the pendulum is closed to the time it is open.

In Fig. 6b, the same elements of the R, C type switch W are open, the condenser C is exponencircuit are present with the exception that S in this case is a shorting device which acts to short circuit C through 1' when the voltage across C has reached a predetermined maximum value. Assuming that the device S when closed (conductive) has no resistance and has infinite resistance when open (non-conductive) the same wave forms of voltage and current as shown by Fig. 7 result here if all circuit constants are the same as in Fig. 6a and if the device S closes when the value of voltage across C reaches a maximum of volts, (1. e., when conversely the voltage between ground and point X has fallen to 150 volts) and opens when the voltage across C has fallen to 50 volts. A typical relaxation oscillater is this circuit in which S is a gaseous discharge tube such as a Thyratron.

practice the voperating'voltages of S may be controlled and when once they are fixed the fun- C(R log +r log Where:

I =frequency in cycles per second C =capacitance in farads V1=maximum in volts developed across R V2=minimum in volts developed across R E =maximum available battery voltage.

For a given adjustment of S the amplitude of saw-tooth voltage developed across C is independent of frequency.

In the analogous L/R type of circuit, Fig. 6e,

' the device 0 is substituted for the pendulum of O of Fig. 6d and is assumed to be a device normally closed or conductive and of zero resistance when closed, until the current through it reaches a predetermined maximum value at which instant it opens to become non-conductive.

.If the conditions are prescribed that O is conductive for all currents less than 30 milliamperes but non-conductive for currents exceeding this value, then with the remaining circuit constants as given for Fig. 6d, the wave forms of Fig. 7 again apply; Fig. 7a representing the current through L and Fig. 7b the voltage across L. In practice, if the adjustment of 0 remains unchanged, the frequency can be controlled primarily by changing r or L, or both. The equation for frequency is as given below, and again for a fixed condition of 0, the frequency can be varied without changing the amplitude of output current through L.

:12 (2) f L(r log l-R 10 I, g 1-1,

Where: v

f=frequency in cycles L=inductance in henries I1=maximum current that flows through L, in

amperes. Iz=minimum current that flows through L, in

amperes. I=maximum current that can flow through In part, many of the circuits which follow relate to apparatus and arrangements which serve as the device S in R,C type circuits and as the device 0 in the L/R type circuits. Furthermore,

synchronization is generally eflected in connection withv the controlling of S or 0 as the case may be, as will appear later. For the present, however, attention is directed to those parts of the circuits which control the trace of the cycle. Thus in Fig. 6c the device H replaces R. of the preceding R,C circuits, and in the simple form illustrated is a two-element vacuum tube having the cathode temperature adjusted (adjustment not shown) to give limited electron emission, so that current through the tube is the same throughout a wide range of voltage across it. The; tube is therefore a constant current device and the charging of C through H, during the trace of the saw-tooth voltage cycle is therefore linear with time as desired. For the circuit constants given below, the voltagoacross (whicl'ris also that across C alone plus a direct current component) is as shown in Fig. 8a and the current through C is as shown in Fig. 8b which is the mathematical derivative of Fig. 8a:

E=300 volts C=.008 [bf r=5000 ohms i=1.9 milliamperes j=2250 cycles b'=10: 1

Here the approach to the ideal form of Fig. 3c is all that can be desired since the trace is linear and the form of the retrace is immaterial providing its time interval issufliciently brief. The

frequency is:

Where:

i=constant current through H in amperes, and other symbols have same significance as in Formulas 1 and 2.

Since the current through H is a constant steady value of direct current, and by Kirchofi's law, the sum of the currents flowing to the point X must be zero, the wave form of current through S is identical with that through C with the addition of a direct current component. The voltage across a resistor inserted anywhere in this loop (as for example the voltage across r) will be .of an impulse wave form, Fig. 8b. This circuit is therefore a source of current or voltages of sawtooth or impulse wave form or of a combination of the two.

Since the condenser C passes no direct current it is immaterial as to whether its lower terminal is connected at point 1 as shown or whether this terminal is connected to ground G or to some intermediate point on the battery E. The action is entirely the same as to the current through C and the alternating voltage across it; a change in only the direct current voltage component across C results. Actually the curve, Fig. 8a, is the voltage between X and G and is obviously the voltage across C plus E or simply the voltage across H.

An advantage of returning C to the plus terminal of E results when the voltage source E has appreciable resistance since then E is located in the constant current branch of the circuit and no impulse current flows through it. Hence any reaction through the power supply source is avoided in practical circuit arrangements. When returned to point Git is more correct to say that C is rapidly charged when S operates and that C discharges at a constant current rate through E.

There is no reversal, however, of the saw-tooth voltage generated.

In the R, C type circuit just described a constant-current device serves to linearize the voltage trace. In an analogous manner a constant voltage device will serve to linearlze the current trace in the L/R type generator. Thus in Fig. (ie the trace is exponential due to the fact that as the current increases through L, it likewise increases through r and the voltage drop across 1' prevents the voltage across L remaining constant during the trace. If Fig. 3c is the current through L then the voltage across L as shown by Fig. 3b must be constant during the linear trace. Clearly if r were zero (and 1' includes any resistance present in the inductance L) then the voltage across L would remain constant during the current trace when is closed. Under such conditions Fig. 8a represents the current through L and Fig. 8b the voltage across L for the constants which follow:

E=200 volts L=4 henries R: 100,000 ohms I=2250 cycles E=constant voltage in series with L, and other symbols have same significance as in the preceding formulas.

To make 1 zero a negative resistance --1' may be introduced as is illustrated in Fig. (if.

The equivalent of introducing a negative resistance -r to maintain constant the voltage across L during the trace is shown in Fig. 69

' where a generator 83c of saw-tooth voltage properly poled and adjusted in amplitude is introduced in series with r and L. If the resultant current through L is of saw-tooth wave form then the voltage drop across 1' will be of saw-tooth wave form, and hence the insertion of generator e30 will compensate for the voltage drop across r to maintain the voltage across L constant during the trace. Fig. 8a represents the current through L and Fig. 8b the voltage across L for this figure. Also the voltage across 0 is of impulse form since the sum of the voltage drops across 1', L, R and 83c must add up to the constant direct current voltage E.

The L/R circuit may be developed as an amplifier of an R, C generators output to produce saw-tooth current through scanning inductances. Or it may be developed as a self-sustaining generator as will appear later. For various economic reasons in construction and operation of cathode ray tubes as television projectors, it appears that magnetic control of scanning may be favored. Hence the precise production and control of sawtooth current through an inductance is required.

It has been pointed out that the R, C generator may be employed to provide saw-tooth, impulse, or a combined saw-tooth impulse voltage. In Fig. 9a the ,two wave forms of Fig.0, both now understood to be voltages, are shown first separately and then combined to give a resultant saw-tooth plus impulse voltage wave form. Such a resultant voltage is, for example obtainable between points Y and Z of Fig. 60. If, for example, this voltage is applied across deflecting plates of the cathode ray tube for picture frequency scanning, (with a saw-tooth form of control simultaneously operating for the line scanning) the pattern on the screen will be as shown in Fig. 9b. If the transmitters retrace endures for a time interval only one-half as long as that of the impulse component of this scanning wave. the picture retrace block-out will appear as shown in Fig. 90. This blocks out that part of the picture retrace which lies across the field to be viewed and throws that part of the retrace which was not blocked out above the field of view. These lines showing above the field of view constitute, of course, the top lines of the picture which now must be sacrificed and may be obscured by an opaque mat (frame) which will present only the field of view to the observer. In effect, however, the retrace time has been shortened.

In practice, it is generally more dimcult to obtain a relatively short retrace time in the case of high frequencies such as the line frequency. Hence by employing the combined saw-tooth and impulse for the line frequency voltage (this illustration being for the case of electrostatic scanning) the folded under-part of the lines (as t, w in Fig. 4a) can be thrown to the left of the picture, and while this adds nothing of value to the left side of the image it does avoid the disturbing efiect of the folded-under lines giving a ghost picture. A proper proportioning of the relative amplitude of the two forms to be combined is important for correct results.

A perfect saw-tooth wave form having zero retrace time involves frequencies extending to infinity in the harmonic composition of the wave and it would be impossible to provide circuits for their use. It can, however, be shown that the plot of the summation of the first ten harmonics of the infinite Fourier series required to represent a saw-tooth wave having a 10:1 ratio of trace to retrace, Fig. 3c, is a curve very closely approximating the ideal. Similarly, it can be shown that the plot of the summation of the first ten harmonics of the infinite Fourier series required to represent a saw-tooth wave having zero retrace time does not, in comparison give a close approximation of its ideal.

Nevertheless, while recognizing this limitation, it is, at times, easier to generate a saw-tooth wave with a retrace interval which is too great and then improve this retrace time. The combination of an impulse and a saw-tooth having an exponential retrace (of form as in Fig. 8a) is given in Fig. 10. Here the resultant wave form is a perfect saw-tooth with zero retrace time. The form of impulse which is here used to combine with the Fig. 80. form is not that of Fig. 8b but can be derived, to a close approximation, from Fig. 812 by rectification.

An equally important need for the combined saw-tooth and impulse wave form is required when the inductance, through which saw-tooth current is to flow, has resistance in series with it which is nearly always the case. This is shown for the ideal case in Fig. 11a, where the load circuit is indicated as L and R. in series. The current is to be of the form of Fig. 3c and hence the voltage ea is indicated as of that form. The voltage e1. is accordingly of the form of Fig. 3b as illustrated, and the resultant voltage which must be applied across this load is the wave form 6 shown. The resultant voltage e is here obtained by arbitrarily adding the instantaneous values of these two forms. Since the resultant wave form of e depends on the relative values of L and R, a second showing 'of the resultant wave form labeled a is indicated in which the impulse voltage across L is taken as one-third of its value in the first case. In other words, the inductance L was taken as being smaller in value relative to R. the current remaining the same, in deriving the voltage wave form e.

The entirely similar case where the current trace is linear but the retrace is exponential as in Fig. 8, is shown in Fig. 11b and two cases of the resultant voltage wave form required are v shown as e and e" for this figure.

Where a very large value of R is employed in series with L, the'voltage applied to cause a sawtooth current to flow will necessarily reduce to simply the saw-tooth wave form, while in the other extreme where the series resistance is negligible the voltage wave form necessary to cause a saw-tooth current will reduce to simply impulse wave form.

Frequently as will be seen in circuits which follow, it is desirable to have capacity in series with the inductance to prevent any direct current flow through the scanning coils. Such a load circuit andits performance are illustrated in Fig. 11c. For a current of form Fig. 30 to flow in this circuit, the voltage e will be a resultant of Figs. 3b. 3c and 3d, each component being protperly adjusted in amplitude to fit the load circui s.

In the case of high (line) frequency scanning the capacity C, of Fig. 110, can generally be made sufficiently large so that its reactance is negligible. Hence a parabolic impulse voltage wave form Fig. 3d need not be included asa necessary component of e, and the required voltage wave form reduces to that shown as e, Fig. 11b.

The current through the scanning coils can, of course, be made very large in the type of load circuit of Fig. 110 by having L and C resonant at, or near, the fundamental frequency of the wave form. Such a design, however, materially attenuates the harmonic components and generally results in requiring that R be made large,

which again reduces the current amplitude as the saw-tooth current wave form is improved.

In the case of low (picture) frequency scanning L is generally so small that its reactance is negligible. Hence a voltage of impulse wave form, Fig. 3b, need not be included as a necessary component of e. It is, however, almost impossible to make C large enough in this case to pass the fundamental components of the very low frequency wave form, so that its reactance is usually appreciable for picture frequency scanning. When simply a saw-tooth voltage wave form is employed, it will be noted when viewing the scanning pattern on the fluorescent screen, under this condition, that the wave form appears approximately exponential. To counteract this eflect of exponentiality in the picture trace it is necessary to introduce a compensating wave form, Fig. 3d, of proper amplitude, in combination with the form, Fig. 30, for low frequency scanning.

Thus in Fig. lie the first cycle shown is of form Fig. 30, indicated as the wave form of voltage e applied. The current, and hence the voltage ea, Fig. 11c, is shown as having approximately exponential trace and retrace. The difference in voltage between e and ex is Co which is of the form Fig. 3d (parabolic impulse) as shown of 5 small amplitude in Fig. 11c.

Mathematically, it can be shown that this wave form is composed primarily of the low frequency fundamental and the lower harmonics; that is the amplitude of the harmonics decreases rapidly with frequency and hence the observation that discrimination against the low frequency components tends to make the saw-tooth wave form appear exponential is confirmed.

Referring to Fig. 12, the essential elements of Fig. 6c are embodied in a form involving no movable elements. Condenser C is charged by current through the constant current device H which is shown to be a vacuum tube of the pentode type. The well known typical load curves of such a tube show that for a given control grid potential and screen grid potential, the plate current is constant over a wide range of variation in plate voltage. The same, is true. (over a smaller voltage range of a tube of thetetrode, or screen grid, class and hence the tetrode would similarly serve as the constant current device. The potential from biasing source 26 applied to the control grid of H from variable tap f controls the value of current to C and hence the frequency as shown by Formula (3). The shorting tube S is here assumed to be of the grid controlled gaseous discharge type. S is normally non-conducting until the voltage across C, which is between cathode and plate of S, has increased to a maximum value determined by the characteristics of the tube S and the bias on its grid supplied from battery E through resistance 28. In this circuit there is a fixed voltage spread" between grid and plate of S, and as the voltage across C increases, the voltage between cathode and grid of S decreases until the critical value is reached when current can flow through S to discharge C,. The contact 21 is therefore a setting for amplitude of voltage developed across 0 as well as a control of frequency.

The discharge of C through S is rapid since such a gaseous discharge tube may have a very low, or even negative, resistance when conductive. For very rapid discharge of C a limiting factor in the use of gaseous tubes for S is the de-ionization time. In this circuit the discharge of C means a fall in the plate-to-cathode voltage of S whereby the grid again regains control. The saw-tooth output voltage developed across C may be taken off for use as shown between point x and ground, the capacity 3|) and resistor 3|, shown in the output circuit serving to eliminate the direct current voltage component of battery E. In case a saw-tooth plus an impulse voltage is desired a resistor can be included in the discharge circuit as shown at r, and the voltage between Z and ground will then be the out put voltage. If the contact 2Iislocated nearer to the positive end of E, a lower voltage across C will suflice to cause S to operate. Hence a voltage applied across resistor 28 will, if it be poled to makethe grid end of 28 positive, cause the discharge to C to occur earlier in time. Thus impulse voltages for synchronization, as for exam ple the impulse output voltage of unit I! or unit 2|) of Fig. 2, may be effectively applied to the grid of S to trip the shorting tube and control the generated frequency'in synchronism with the corresponding line or picture frequency of the transmitter. The poling of such a control impulse should be such that the impulse peaks are positive 'as applied to the grid of S, and

hence in the diagram the connection to the grid of 5 through condenser 29 has been labeled +synch. In practice I is adjusted to cause the including the resistor r in ,the circuit.

quency under which conditions, the controlling impulse voltage applied to 29 readily causes the frequency generated to pull in step" at the correct synchronous frequency.

When synchronism has been achieved, with the circuit of Fig. 12 and those which follow, it will be found that the phase of the generated wave form, as evidenced by the pattern on the cathode ray tube screen, in relation to the blockout of retraces, can be controlled to a considerable extent by the adjustment of the frequency control tap I while still maintaining synchronism. This in practice is generally an advantage and also reduces any requirement as to a critical setting of the amplitude of synchronizing impulse applied.

The circuit of Fig. 13 is the inverse of Fig. 12 in that the voltage generated between points 3 and G is oppositely poled. Since it is generally desirable to obtain output voltages relative to ground as shown the arrangement of the component units (which perform the same functions as in Fig. 12 and hence are similarly labeled) is somewhat different. The constant current device H is again shown to be a pentode through which C is charged, and the shorting tube S is of the grid controlled gaseous discharge type. Since the cathode of H is above ground potential and varies relative to ground G by the sawtooth voltage developed across C, the battery connection necessary to fix the potential of the screen grid relative to the cathode of H is made via resistor 32. This resistor 32, with capacity 33 which connects from screen grid to cathode, has a time constant R, C preferably lower than the lowest fundamental frequency at which the circuit is to oscillate whereby the screen remains fixed in its potential relative to its own cathode, regardless of the fact that the potential between cathode and the point on E, at which the tap is made, is varying. The voltage drop across resistor 34 serves as the bias for the control grid of H the position of contact 1 serving as a control of frequency.

The shorting tube S again has the voltage de veloped across C applied between its cathode and plate, the maximum value tg which this voltage will rise being determined bythe characteristics of S and the bias on its grid as determined by the position of contact 2? on battery 2E. The saw-tooth output voltage is developed across 3| via condenser 30 and the connection to 1;. An impulse voltage could be added by having this connection made to point Z, for example, thus Resistor 28 and condenser-29 permit the application of synchronizing control voltages between cathode and grid of S and again the poling of impulses for synchronous control should be, as indicated in the diagram, such that the peaks are positive as applied to this grid.

Fig. 14 is similar in form to the R, C type generator of Fig. 12, the elements being identical except that an important modification provides that S is here a thermionic vacuum tube and an additional vacuum tube Rv has "been added.

When the voltage across C has reached a pregenerator circuit to oscillate near the desired freis poled to make the grid of Rv negative whereby a similar impulse oppositely poled is developed across the plate resistor 38 of Rv to make the grid of S positive, thus lowering the plate-cathode resistance of S and causing a rapid discharge of C. The coupling of the plate of Rv to the grid of S may be established through a blocking condenser and a resistance from the grid of S to ground, or to some point on battery E which would be employed to give a definite voltage spread between grid and plate of S as in Fig. 12. The direct metallic connection of grid to plate as shown, is, however, the equivalent of such a return of the grid of S to a point on battery E,

and hence this more economical arrangement has I been illustrated. It will be clear that here the frequency is controllable by the setting of f as in Fig. 12a decrease of the negative bias voltage on the control grid of H giving a larger charging current and hence a higher frequency. This arrangement is in general preferable to that of Fig. 12 since the use of a gaseous discharge tube for S requires, as a rule, some length of time before the tube has heated up to. a permanent state of uniform operation. The adjustment of the amount of feedback due to the reversing tube Rv is not critical and can best be made by adjusting either resistors 35 or 38 or by tapping the grid of Rv to its biasing battery through resistor 37. In theory, the time constant of resistance- capacity combination 36, 31 should be lower than the fundamental of the lowest frequency to be generated. In practice, however, it is found that a high time constant may be employed here, without harming this operation. The combination of S with Rv as a feedback (reversing) tube is the equivalent of a gaseous discharge tube but with improved results. Furthermore, it offers a point where synchronizing impulses of opposite polarity may be employed. Thus the illustration shows that synchronizing impulses poled with peaks positive may be employed at the grid of S or the impulses oppositely poled may be appiled to the grid of Rv- Saw-tooth voltage of the form as in Fig. 8a (and hence labeled 88a) is available across C or (with a direct current component) between point X and ground. Impulse wave form voltage may be had as the voltage drop across 35 or, in opposite polarity, as the voltage drop across 38. The points a: and 2 form a source of combined saw-tooth and impulse voltage and, of course, a resistor inserted at any point in the loop circuit which includes S and C will furnish impulse voltages which may be combined with the saw-tooth voltage as desired.

The circuit of Fig. 15 is one possible inverse arrangement of Fig. 14. Here H may be a pentode arranged as the unit H of Fig. 13 or some other arrangement such as a diode as in Fig. 66 to serve as a constantcurrent device, or the unit may be merely a variable resistor as inFig. 6b. The device is is here, as in Fig. 14, a thermionic tube made to serve effectively as a shorting device by virtue of the action of the feedback or reversing tube Rv. The voltage drop across 35 is applied directly to the grid of Rv. The capacity and resistor 31 of Fig. 14 could, of course, be employed instead of the direct connection but it is more economical to use the arrangement shown which omits these elements. The oppositely poled impulse developed by By in resistor 38 is applied to the grid of S through capacity 39. Resistor 40 connects the grid of S to battery 26. The circuit of Fig. 15, therefore, serves as an im proved equivalent of Fig. 13 just as Fi 14 is an improved equivalent of Fig. 12. The points 1! on the circuit and ground G provide a source of combined saw-tooth and impulse voltage. A sawtooth voltage alone, or impulse voltage alone, can be obtained from the circuit and the circuit can be modified by one skilled in the art to supply such voltages as desired relative to ground without changing the described functioning of the circuit. The synchronizing points in the circuit are as labeled.

The circuit of Fig. 16 should be directly compared with those of Figs. 12 and 14. Here, as before, battery E charges C through the constant current device or resistor H, which is the control for the frequency generated. The shorting device S is, asin Fig. 14, a thermionic tube which is made tofunction' satisfactorily for this purpose by virtue of a feedback (reversing) transformer T, instead of by means of the reversing tube of Fig. 14. When the potential across C reaches a value where current starts to flow from plate to cathode of S. there is applied to the grid of S, due to transformer T, a voltage which makes the grid positive effectively to reduce the platecathode resistance of S. so that it serves effectively to short circuit or discharge C. The result is an impulse form of current in the pla e-cathode circuit of S. Referring to Fig. 3 it will be noted that for an impulse of type Fig. 3b through an inductance the voltage across the inductance is a double impulse of the form Fig. 3a, and inspection of the voltage at the grid of S (by means of a cathode ray oscillograph) shows that the wave form can be made qu te similar to that of Fig. 3a and that the poling is such that the first peak as applied to the grid of S is positive. Noting that this circuit is a vacuum tube arrangement having inductance in both plate and grid circuits with the coupling poled so as ordinarily to produce oscillations, there is a tendency for the circuit to oscillate as an ordinary vacuum tube oscillator at a frequency determined by the inductance and distributed capacities of the feedback circuit unless precaution is taken to prevent such action. Hence the use of either one or both of the two resistors 4i and 42, shown as shunting the windings of T, sufiices to prevent such spurious oscillations and to confine the frequency and wave form control to the units H and C. With the transformer T thus properly damped, the voltage across the windings will be found to be of impulse plus a. saw-tooth wave form and the current through the shorting tube of impulse wave form. The points for application of synchronizing voltage are as indicated. This circuit provides a definite voltage spread between plate and grid of S (as was the case in Figs. 12 and 14) and the amplitude to which the voltage across C can rise is readily adjusted by the setting of tap 21 on the battery E.

Fig. 1'7 is the inverse form of Fi 16 wherein S serves as a shorting device to discharge C when a predetermined v ltage maximum has been developed across C. e potential at wh ch S will act to short C depends upon the characteristics of tube S and the bias as determined by the setting of 21 on battery 26. The transformer T, damped by resistors 4| and 42, functions as the feedback to expedite the shorting of C by S as was described for Fi 16.

In the fundamental circuit of Fig. 6a a pendulum arrangement was shown; the period of the pendulum determining the fundamental frequency of the generated wave. This type of generatorjs embodied in the vacuum tube circuit of Fig. 18 in which E as before charges C through H. C is discharged by S which, in this case, is a vibrating contact electro-magnetically controlled at a frequency generated by the typical vacuum tube oscillator circuit of which V is the vacuum 5 tube, having a tuned grid circuit 43, 44 and plate circuit feedback winding 45. The alternating current voltage developed in the plate circuit is applied through condenser 46 to the windings 41 of unit S. Unit S might for example be similar to a telephone receiver whereby the vibrating element (the diaphragm) will move according to the frequency generated in response to the setting of condenser 43. The contact adjustment of S is assumed to be such that the contact is made 15 only for a brief interval during each cycle of vibration of the diaphragm. The functioning of" the circuit to generate a saw-tooth voltage across C and an impulse current in the loop circuit containing C and S will be understood from the de 20 scription of Fig. 6a. Synchronization is best effected at the grid of V. The adjustment of H determines amplitude of the generated saw-tooth voltage.

In Fig. 19, the control of frequency is likewise 25 consigned to a typical vacuum tube oscillator V identical to that of Fig. 18. The battery E again charges C through unit H which controls the amplitude of generated voltage.

Vacuum tube S acts as the short circuit device to discharge C. During the. trace part of the cycle, S is nonconducting. Once, however, during each cycle of the frequency generated by V, the voltage applied to the grid of S, via the capacity 46 and resistor 28, is suiflciently positive to cause S to short circuit C for the retrace part of the cycle. With this arrangement, it will be difficult to obtain a rapid retrace unless the voltage generated by V is of large amplitude, since V generates a voltage which is essentially sinusoidal. The positive voltage peak of this sine wave form will be less efllcient in actuating the grid of S than if tube V were arranged to generate an impulse wave form.

Generator V may be omitted and the synchronizing impulses from the transmitted s gnal applied instead to the grid of S in large amplitude and poled with the peaks positive to cause the circuit to generate the saw-tooth voltage across C in synchronism with the transmitters scanning. In effect, this arrangement is that of Fig. 12 wherein S is now considered to be a thermionic (non-gaseous) vacuum tube. The circuit of Fig. 12 using a non-gaseous tube will not generate oscillationssince a simple triode does not have the abrupt change in conductivity with voltage applied between cathode and plate to satisfy the requirements of oscillation. However, with the transmitters synchronizing impulses applied in suflicient amplitude at the point labeled 60 +synch. the circuit generates the saw-tooth wave form required. Such an arrangement, in which the scanning generator functions only under the stimuli of the transmitters synchronizing impulses, is entirely practical.

Fig. 20 is essentially that of Fig. 16 but the frequency determination has been transferred to tube S in that the grid circuit of S is tuned by the grid winding of T and condenser 43 and the circuit is undamped. This circuit is subject to the limitations of a slow retrace as was the case with preceding circuit Fig. 19 when V acted as a sine wave control generator.

Fig; 21a shows an amplifier arrangement in which the current through the scanning inductance List: will be of the same wave-form as that of the applied grid voltage 630. Here V is an amplifier, of the pentode type, having an energizing battery source 49 and bias" source 48 and a plate or load circuit which comprises resistive, capacitive, and inductive elements 50, Si, 52, 53. Leo and 54. The circuit having the elements enumerated comprises a band-pass filter in which the input arrangement is mid-shunt" and the output to resistor 54 is mid-series". The voltage applied between input elements of V is assumed .to be of saw-tooth wave form (as shown by the notation eat) and the design of the filter structure is such as to pass the fundamental and a sumcient number of the harmonics comprising the saw tooth form so that it is, in effect, a pure resistance load for'the band of frequencies under consideration. The voltage across the filter input resistor 50 will be an amplified replica of the grid voltage and hence the voltage across the filter output terminating resistor 54 is also of the original sawtooth'wave form. Since 54 is a pure resistance the current through it must also be of saw-tooth wave form and hence the current through filter element Lac in series with 54 is of the required saw-tooth wave form. Here Lac is assumed to function also as the scanning inductance to control the cathode ray beam so that current in the scanning coil is of the same wave form as that applied to the control grid of V. The saw-tooth voltage for the input of V can be had from any of the R, C type generators thus far described.

The efiiciency of a circuit arrangement such as that of Fig. 21a is rather low since to obtain large current in Lsc the energy dissipated by having this'current flow in resistor 54 may be prohibitive. Nevertheless, where 'the saw-tooth current requirement through L 0 is not large, the arrangement is satisfactory. The two circuits Figs. 21b and 210 which follow, are likewise dependent upon the requirement that the saw-tooth current for scanning be small so that resistance in series with the scanning coils serves in large'measure'to cause the current wave form to follow the applied voltage.

In Fig. 21b the amplifier tube V is of the pentode type, in which, because of its high platecathode'resistance, the output current will normally follow closely the wave form of grid voltage. The arrangement here shown, has been found particularly well suited for the line frequency 2880 cycles. The energizing potentials for the control grid, screen and plate of 'V are derived from a voltage divided GI, 55 and shunting the voltage source 49 which is closer to actual practice than the showing of simply a battery E. Elements 51, 58, 59, and Lao resemble closely in appearance the filter of 2la. In general it has been found suflicient simply to make the reactance of 51 high compared with that of Lsc and to make capacity 59 large so that its reactance is negligible. Resistors 58. and 60 are selected experimentally, 60 being made as low as practically possible without distorting the saw-tooth current wave form. If in employing the arrangement of Fig. 21b for the line frequency amplifier, the control to..the g rid is sii n ply a voltage of good saw-tooth wave form, the resulting saw-tooth current through the scanning coils Lsc will generally have a slower retrace than the controlling voltage wave. The inclusion ofan impulse wave form component will materiallyspeed up the current retrace for the reason discussed in connection with Fig. 9 (the illus- 'aotaies tration there being for picture frequency} though a part of the merit of adding an impulse component to the controlling saw-tooth wave form. to speed up' the retrace is due to the'efi'ec'ts discussed in connection with Fig. 11. The type 5 of effect set forth in Fig. 11 is of increasing importance when resistor 60 is small compared with L.

For lower frequencies (i. e., the picture frequency 24 per second) the form of an amplifier V (for'use with an R,C type saw-tooth voltage generator) such as is shown in Fig. 21c is satisfactory if the current requirements through Lso are not excessive. The triode arrangement is generally preferable for V as shown, and the reactance 51 of Fig. 21 is omitted since it can seldom be made sufliciently large to be effective at very low frequencies. The capacity 59, present primarily to eliminate the direct current component through Lsc, is made very large to 20 avoid a distortion of the saw-tooth wave form to an exponential form. Since a part of the output current of V will flow through 58 and to ground through the power supply system 59, 55, 62, a discrimination against low frequencies fre- 25= quently occurs which can be made relatively negligible by employing the series cathode bias resistor 63 to give a certain amount of uniform negativeregeneration for all the frequency components of the saw-tooth wave form. In prac- 30 tice resistor 63 is decidedly effective in this type of circuit to maintain linearity in the picture frequency trace. In effect the resistor 63 accomplishes the result which, in the theory relative to Fig. 110, would be effected by the inclusion of a component of parabolic impulse of form, Fig. 3d, in the grid control wave form;

Fig. 22 is intended to give a completed picture of that'part of the television receiver which includes (a) the final detector and vision frequency amplifier of the receiver proper: (b) the cathode ray tube; (0) the power supply, and (d) the scanning and synchronizing control wherein R,C type saw-tooth voltage generators are employed to eifct scanning by magnetic control of the cathode ray. Exclusive of power supply there are three circuit sections leading to the cathode ray tube. "The top section L is primarily the amplifier'for applying vision frequencies and block-out impulses to the control grid of the cathode ray tube, as well as to supply the synchronizing impulses to the other two lower sec tions M and N which are alike'in that they comprise units' for generating and synchronously controlling the scanning currents through th deflecting coils of the cathode'ray tube.

Vacuum tube 64 and interstagecoupling unit 65 are represented as the final intermediate frequency stage of the intermediate frequency amplifier of a superheterodyne-type of-receiver.

The received carrier wave is negatively modulated by the vision frequencies and the synchronizing impulse peaks are represented by increase of amplitude of -,the carrier wave, indi-' cated as the preferred form of carrier wave in 65' the mentioned copending application Serial No. 747,070. Vacuum tube 66 is the detector which is illustrated to be of the diode form havingras its load circuit the low-pass filter unit 61 across which the vision frequencies and synchronizing 70 aosaras tures being transmitted. The cathode ray tube 15 has been illustrated as of a type employing magnetic focus of the ray for which purpose coil 16 is employed to establish a non-varying magnetic field longitudinally along the axis of the tube. The intensity of this focus field is controllable by rheostat H which determines the amount of direct current which will flow in coil 16 from the power supply P in the lower left part of the diagram, since the terminals 16 there shown connect with corresponding terminals 19 of coil 16. A separate high voltage power sup ply of the voltage doubling type, comprising rectifiers 89, 8| and associated elements not labeled, develops the necessary direct current voltage across resistor 82, to serve as a source of potential between; the common ground 93 and the anode 64 of the cathode ray tube. A negative biasing voltage developed between ground and point 85 of source P is applied over connection 86 to the grid 13 of the cathode ray tube whereby the background" or average value of illumination on the screen can'be controlled by adjustment of potentiometer 91.

The circuit section M immediately below the amplifier section is the picture frequency generator and its control unit comprising tube 89 which is a synchronizing amplifier, 99 which is the constant current tube for the RC type generator, 99 the shorting tube, 9| the reversing tube and 92 the output amplifier. Synchronizing impulses as developed across theoutput of filter 61 are applied through condenser 93 and potentiometer 94 in adjusted amplitude to the control grid of tube 88, which in turn applies the control impulses properly poled with peaks positive, through capacity 96 and resistor 96, to the grid of tube 99. Tube 99 thus synchronously controlled by tube 88 andregenerated by reversing tube 9| serves effectively to short the charging condenser 91, exactly as described in connection with Fig. 14. The saw-tooth voltage thus synchronously generated is applied via resistor 98, capacity '99 and potentiometer I99 to the grid of amplifier 92 which is illustrated as a modern type of multi-grid tube connected to operate as a triode. The output circuit comprising elements I 9|, I92, I93 and scanning coils L1 of the cathode ray tube is like the arrangement of Fig. 21c. Resistor I94 in the cathode branch likewise serves the function described for corresponding resistor 63 of Fig. 210. Current through the picture scanning coils is of saw-tooth form so that the magnetic control of the cathode ray at picture frequency is saw-tooth. Potentiometer I95 of the power supply P is adjustable to set the bias on constant current tube, 99 to con; trol the picture frequency generated. Adjustment of potentiometer I99 determines the output current amplitude and hence the height of the picture projected on the screen. The setting of potentiometer 94 controls synchronization.

The third section N from the top of the diagram is the line frequency generator section consisting of the synchronizing amplifier I96, constant current tube I91, shorting tube I96, reversing tube I99 and amplifier tube II9. Theprincipal part of the circuit is entirely similar to the picture frequency section M already described. Potentiometer III in the power supply P controls frequency, potentiometer II2 controls synchronization, and potentiometer H3 controls the amplitude of saw-tooth current output and hence controls the width of the picture. The settings of potentiometers I99 and H3 set the ratio of width to height of the picture (the aspect ratio) to correspond to that of the picture being transmitted so that the subject depicted will be properly proportioned in heighth and width. Resistor Ill, included in the discharge circuit has voltage of impulse wave form developed thereacross. The wave form of voltage applied to tube II9 through potentiometer H3 is, therefore, of combined saw-tooth plus impulse wave form. For the line frequency the amplifier arrangement here shown is a pentode circuit the arrangement being similar to that of Fig. 21b. Bias for the grid of tube H9 is taken from a tap on the power supply over conductor H5. The plate or output circuit of tube II9 contains a filter circuit I I6 supplying the scanning coils La. Saw-tooth current will flow through coils L2. The retrace will be rapid due to the inclusion of the impulse component as explained in connection with Figs. 9 and 11.

The power supply circuit P needs no description. It will be noted, however, that capacity 'I I6 and inductance III serve effectively to prevent reactions at line frequency through the power supply. Similar by-passing for the picture frequency amplifier 92 is not illustrated since for the very low frequency fundamental of 24 cycles such an arrangement is not generally serviceable. The circuit comprising elements 98, 99, I99 and H9, II 9, H3 must be of high impedance and of good fidelity to effectively pass the generated voltages to the grids of amplifiers 92 and I I9 respectively without aflfecting the operational characteristics of the saw-tooth voltage generators.

Fig. 23 is an abbreviated showing of an amplifier section and the line and picture frequency generator sections of a receiver. Only one stage, tube I26, of vision frequency amplification is shown. Since the carrier wave is of the type having negative" modulation, the poling of the detector I2! is made as shown, the cathode being above ground potential to properly pole th the vision frequencies applied to the control grid of cathode ray tube 15 and the impulses as applied for synchronization. The point labeled +Synch." in the detector output is, therefore, a source of voltage relative to ground of synchronizing impulses with peaks poled positive. Similarly, the point labeled Synch. in the output of amplifier I26 is a source of impulse voltage with peaks poled negative relative to ground and hence of opposite poling.

The elements of the generator circuits for both line and picture frequency are labeled according to the notation employed in the several preceding diagrams, both circuits illustrated being of the R, C type similar to Fig. 14. The amplifiers indicated by V may be arranged for either saw-tooth current or saw-tooth voltage output according to whether the tube 15 is adapted to magnetic or electrostatic scanning. Instead of employing synchronizing tubes, the vacuum tubes used for S and Ry are each illustrated as having synchronizing grids. A positive voltage applied to' an extra control grid in the shorting tubes S will cause these tubes to become conductive earlier in the cycle. Similarly a negative voltage applied to an extra control grid of the reversing tubes R- will, in turn, result in a positive voltage appearing til on the grid of tubes S to trip" the circuit. Furthermore, there is no appreciable backvoltage from the generator circuit present on such an extra control grid to react back upon other circuits. Hence, no synchronizing amplifier stages are necessary for the function of preventing reactions due to a back E. M. F. from the line and picture frequency generators. The'dot-dash connection shown from the point +Synch. to the synchronizing grids of S will serve effectively for synchronization. Furthermore, synchronization can be alternatively achieved by connections from the point of the amplifier I26 marked Synch." to the synchronizing grids of tubes Rv as indicated.

In both Fig. 22 and Fig. 23 filter circuits may be employed between the amplifier section and the synchronizing grids to prevent particularly the line frequency from tripping the picture frequency as was illustrated in Fig. 2. If the picture impulse peaks are of greater amplitude than the line impulse peaks such precautions may be omitted, since the relative adjustment of 96 and 2 (Fig. 22) may be made such that the line impulse as applied to the grid of tube 88 will be insumcient to trip the picture frequency generator.

As has been pointed out in the descriptions of the several circuits, in which saw-toothed voltage a is developed across a condenser, the condenser may be considered as charged linearly with time for the trace and discharged rapidly during the retrace; or alternatively as being charged rapidly during the retrace and discharged linearly with time for the trace. However, for clearness in the claims the term charge has been used to designate voltage changes across the condenser during the trace part of the cycle, while the term discharge designates voltage changes across the condenser during the retrace.

We claim:

1. In an electric wave form generator: a source of. direct current potential, a condenser, a charging path connecting said condenser to said source, and a discharge path for said condenser, said charging path including the space path of a vacuum tube having at least a cathode, an anode, a control grid and ,a screen grid,.and means for supplying direct current operating potentials to said grids relative to the cathode to predetermine a c nstant current rate of charge, at least one of sa (1 means including a condenser directly conn cted between one of said grids and the cathode of said tube and a series resistance cooperating with said condenser to prevent alternating potentials from affecting said grid, said discharge path including the space path of a three-electrode dispath connecting said condenser to said source,

and a discharge path for said condenser, said charging path including a constant current device, said discharge path including the space path of a shorting vacuum tube, means connecting the cathode of said shorting tube between a terminal of said condenser and a terminal of said constant current device, a resistor joining the plate of said shorting tube to a positive potential point on said source, and means for feeding back energy from the output circuit of said shorting tube to its input circuit, comprising a vacuum tube having a 5 plate, a grid and a cathode, means coupling the plate of said shortingtube to the grid of said feedback tube, and a connection from the plate of said feed-back tube to the grid of said shorting tube, thereby to develop voltage of saw-tooth wave form across said condenser.

3. In an electric wave generator: a source of direct, current potential, a condenser, a charging path connecting said condenser to said source, and a discharge path for said condenser, said charging path including a constant current device and a resistor in series with said condenser, said discharge path including said resistor and the space path of a shorting vacuum tube, means for feeding back energy from the output circuit of said shorting tube to its input circuit, comprising a vacuum tube having a plate, a control grid, and a cathode, a connection from the junction of said condenser and said resistor to the control grid of said feed-back tube, means coupling the plate of said feed-back tube to the grid of said shorting tube, thereby to derive voltage of sawtooth plus impulse wave form across said condenser and said resistor in series.

4. In an electric wave generator: a source of direct current potential, a condenser, a charging path connecting said condenser to said source nd a discharge path for said condenser, said c arging path including a resistor variable over a range such that the time constant determined 35 by said resistanceand said condenser is large compared with the period of the generated wave,

said discharge path including the space path of a vacuum tube provided with a control grid, said condenser being common to the space path and grid circuit of said tube, means connecting the control grid of said tube to a point on said source for providing a substantia ly constant potentialdifference between grid andplate thereof; a

vacuum tube oscillator providing a source of aiternating voltage of predetermined frequency,

means for applying said alternating voltage to the grid of said first mentioned vacuum tube to effect periodic discharge of said condenser at said predetermined frequency, whereby voltage of saw-tooth wave form is developed across said condenser.

5. In a scanning current generator: a vacuum tube having input and output circuits, means for applying voltage of saw-tooth wave form to said input circuit, a load circuit included in said output circuit comprising resistance and a scanning inductance in series, said resistance being high relative to the reactance of said scanninginductance at the essential frequencies constituting said saw-tooth wave form thereby to cause the current in said load circuit to follow the wave form of voltage applied to said input circuit.

6. 'In a television scanning current generator, a vacuum tube having input and output circuits, means for supplying'voltage of saw-tooth wave form to said input circuit, a scanning inductance of high reactance at the essential frequencies constituting said saw-tooth wave, said scanning inductance being included in said output circuit, and said output circuit having an impedance in addition to that of said scanning inductance of such characteristic and magnitude that the current through said inductance is also of saw-tooth wave form.

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