US2309744A - Television system - Google Patents

Description

Feb. 2, 1943.

A. v. BEDFORD TELEVISION SYSTEM Filed Sept. '7, 1940 2 Sheets-Sheet 1 31mm or fllda I r Bedfo r2;

2. 1 3. v A. v. BEDFORD. 2,309,144

TELEVISION sysmm;

-' Filed Sept. 7,1940

2 Sheets-Shut 2 Fi q. 3.

Patented Feb. 2, 1943 Aida V. Bedford, Collingswood, N. 1.,

assl'gnor to Radio Corporation of America, a corporation of Delaware Application September 7, 1940, Serial No. 355,797

4 Claims.

This invention relates to television transmitters and more particularly to transmitters of the type employing cathode ray tubes.

Television signaling systems are adapted to provide images of' high definition over a relatively' wide band of signaling frequencies. It is well known that thermionic amplifying tubes, their associated circuits, and cathode ray transmitter tubes experience a falling off in response at high frequencies due to the fact that the in-- terelectrode and'inter'element capacity is of sufficient magnitude to provide relatively low impedance over the upper range of the required frequency band. One method of overcoming this decreased efficiency in the television system is to designthe output circuit of the cathode ray transmitter tube so that the response of the transmitter tube is substantially fiat over the useful band of frequencies. This has been ac complished by providing a low value load resistance for the transmitting tube so that the distributed capacity in the transmitter tube would have little effect on the response characteristic in the .useful band. Picture signal from the transmitter tube was then fed into I an amplifier designed to have a fiat frequency response whereby the signal appearing at the output of the amplifier was a true representation of the conditions of light and shade of the picture. The use of a low valued load. resistance in the output circuit of the transmitting tube reduces its output considerably, thus requiring a large amount of amplification which consequently introduces a large amount of extraneous noise and thereby deleteriously affecting the resultant transmitted image.

In order to obtain low noise to signal ratio from a high impedance wide band low output device, such -as a cathode ray transmitter tube, it is advantageous to operate the device into a load which has high shunt resistance compared to the shunt capacity reactance due to the stray and distributed capacity of the tube and circuit. At the highest, useful frequency the output resistor may well be one hundred or more times as high in value as the shunting capacitive reactance.

In the arrangement of my previous electrode capacity.

Patent No. 2,151,072, the loss in response at v high frequency resulting from a transmitter tube provided with a relatively high load resistance was equalized by an inductance connected in series with a in a subsequent amplifier stage.

In order that the effect-of low-valued plate resistor the interelectrode capacitance may be negligible, the reactance of the inductance at the highest useful, frequency must be small compared to that of the inter- The value of the load resistance will be only one hundredth of the reactance of the inductance, say 10 ohms, and

hence will be so small that at low frequency,

the signal on the control electrode of the falling amplifier stage will be so small that the low frequency interference due to unsteady power supply of the amplifier tubes may be objectionable.

This difficulty is greatly reduced by equalizing for the falling frequency characteristic in two or, more amplifier stages, thus allowing greater low frequency gain in each of the amplifier stages since the high frequency gain is limited by the interelectrode capacitance.

Two or more not be made to provide equalization, according to the correct law, to compensate for a single I input circuit. This is evident when it is considered that for all except the very low frequencies, the input signal to the amplifier from the cathode ray transmitter tube varies inversely as frequency. A single stage may be designed to provide gain which varies directly as frequency over a similar range, however, if two stages having similar frequency response characteristics were used the gain-would vary as the square of frequency which would not correctly compensate for the falling-off characteristic of the cathode-ray transmitter tube.

Two amplifier stages in which each provides gain varying in accordance with the square root of frequency would-be satisfactory but no simple circuit is now available to provide such a characteristic.

According to this invention, two or more equalizing circuits have different rising frequency characteristics and are connected in cascade so that one provides gain substantially proportional to frequency useful frequency band and the other or others provide substantially uniform gain for that first part of the useful gain proportional to the frequency for other parts of the useful frequency band. The response characteristic of the several circuits are also complementary at the transition points between diiferent parts of the frequency band.

The principalobject of this invention is to provide a. television system having a uniform response over a large range'of frequencies.

vvAnother object of this invention is to provide an amplifier which will compensate for identical stages in cascade canfor a first part of the frequency band but provides stray capacity of a transmitter tube and its associated circuits. 1

. A still further object of this invention is to provide a multi-stage wide-band amplifier whose frequency response is substantially flat over the lower portion of the band and increases proportional to frequency over the higher portion of the band.

Other and incidental objects of this invention will be apparent to those skilled in the art from the following specification consi ered in connection with the accompanying dra s in which t Figure 1 is a circuit of this invention,

Figure 2 is a graph illustrating the operation of this invention, and

Figures 3, 4, 5, 6, 7 and 8 are modifications of this invention. 1

Referring now in more detail to Fig. 1, the amplifier circuit comprises a cathode ray transmitter tube i having-an evacuated envelope 3 and electron gun therein for producing a beam of electrons, a second anode l and a mosaic I of electron-emissive elements. An optical image of the scene ii to be transmitted is projected upon the mosaic 9 by means of a suitable lens system i 3. -An electrical representation of the optical image is formed on the mosaic so that when the mosaic is scanned by the electron beam from gun 5 it produces picture signals representative of the optical image ii. The beam is caused to scan the mosaic 9 by means of deflecting coils i 5 or any-other suitable deflecting means.

A further and more complete description of the cathode ray transmitter tube may be found in an article by V. K. Zworykin appearing in the Proceedings of the Institute of Radio Engineers for January 1934, pages 16 to 32.

The train of picture signals is transmitted to diagram showing one form the control electrode ll of the amplifier tube i0 having a cathode and an anode 25; The grid leal: resistor R1 maintains the control electrode i! at a suitable bias potential. The distributed capacity of the picture tube is indicated by the broken line shown by capacity C1. The auxiliary electrode is supplied with positive potential through resistance 29 and bypass condenser ii. The anode 28 is supplied with positive potential through resistor 33 and the amplified signal is impressed upon the control electrode ill of the next amplifier tube 31 through coupling condenser 39. Suitable bias potential is applied to control electrode 30 through resistor 4i. Tube 31 contains cathode t3, auxiliary electrode and anode M. The positive potential for auxiliary electrode 45 is supplied through resistor er 5 I.

A frequency compensating circuit composed of resistances Ra, R: and inductance L1 is connected to the anode circuit of discharge device 31. The modified signal from anode 41 is impressed upon the control electrode 59 of tube 8i through coupling condenser 68. Capacity 0: represents the interelectrode capacity of the tubes 37, Si and their associated circuits. A second frequency circuit of the anode iii of tube 6|. The signal from the anode i3 is then amplified in tube 18 by transmitting it to control electrode 'ii through coupling condenser 79. The interelectrode capacity of tube 15 is shown by capacity C2.

The anode circuit of tube 3! and the anode 2i, an auxiliary electrode 20,

49 and bypass condenswhere fo=maximum frequency utilized (for example,

3,000,000 C. P. S.) f1=a lower transition frequency (for example,

30.000 C. P. S.) and fa=a much higher transition frequency (for example, 300,000 C. P. 8.).

Referring now to Fig. 2, T0, T3 and T4, respectively, show the response characteristic curve of the three anode circuits of the three discharge devices 3 (picture transmitter tube), 37 and ti.

If the falling oil in frequency response of the transmitter tube was to be compensated for in one stage of amplification, the anode circuit of the compensating stage would contain an inductance whose value is governed by the following equation:

Li=R2R1C1 (5) where Lr=the inductance in the anode circuit,

Ra=the series resistance in the anode circuit,

R1=theload resistance of the transmitter tube,

and

Ci=distributed capacity of the picture transmitter tube and associated circuit in order for the output voltage of the amplifier stage to be uniformly proportional to the current of the picture tube. If the fallingofi in frequency-response characteristic of the picture tube is to be compensated for in a single stage, the valve R: will necessarily be so low that the irregularities in the power supply voltage will be appreciable by comparison with the picture signal.

It is apparent that Equations 1 and 2 define L1 to have the same value as was given in Equation 5 for the one amplifier stage frequency re sponse characteristic equalizer. F1 defines the frequency at which the voltage across L1 has become equal to that. across R: and the voltage across Lr exceeds the voltage across R: for all higher frequencies. Hence F1 may be called the coming-in" transition frequency for Li. R: has

compensating circuit comprising the resistances R4, R5 and inductance leis connected to the.

little effect at this frequency because the reactance of L1 is relatively low with respect to Rs. If R: and C: were not present, this circuit would provide correct equalization. At F2 the goin out" transition frequency of L1, the value of Xm reaches R: so that L1 and Rs are equally responsible for the amplification of the stage comprising tube It.

The voltage across R2 is almost negligible at frequency F2. R; and R: are given such a value that C: will not appreciably load the circuitat the maximum frequency of the useful band. At the same transition frequency F2, the voltage across In in the plate circuit of discharge device M becomes equal to the voltage across R4. Above this frequency the voltage across L2 continues to rise and assumes the task ofcompensating for the continued fall of high frequency response of the picture transmitter tube i.

For the purpose of this illustration,

curve To tobe substantially fiat m L: by a resistor equal to Xmat Fo, making F atransition frequency. The third network would be added elsewhere as in anode circuit of tube 15. It would consist of a resistor in se es with an inductance having a reactance equa to the resistor at frequency F0. The maximum compensated frequency would then become equal to F0.

More equalizing stages could be added by either extending the band or by spacing the transition frequencies closer in the band. It must be observed that the coming-in transition frequency of each stage is the same as the, goingon transition frequency of the stage adjacent on the low frequency side in the frequency band.

It will be noticed that according to one form of this invention regardless of how many stages of'compensation there may be, the inductance shunting resistor will be omitted in the compensating stage having the highest transition frequency. It is, however, satisfactory'to use the in uctance-shunting resistor in each stage providing the going-out transition frequency of the stage, having the highest coming in transition frequency, is at the upper limit of the desired band.

Referring now to Fig. tion frequencies F1, F2 and F0 are not as much as several octaves apart in frequency, the resistor in series with the inductance becomes compar- 3, if the various transi' sistance circuit an inductance such as L3 in Fig. 6; therefore. the elements Ra, La and C: in efiect combine to produce a resistive load across the channel, thus resulting in a circuit, the equivalent of which is shown in Fig. 3.

Fig. 7 shows still another embodiment of this invention in which a compensated circuit comprising condensers'and resistances is used. Thecapacities and resistances are of such a value that the corresponding stages produce response curves similar to curves is and t4 shown in Fig. 2. One

compensating circuit comprises resistance R2 in series with resistance R6 and series circuit comprising resistanceRa and capacitor C4 connected in parallel with resistance R2. Another compensating stage including tube 6|, which has the highest coming-in transition frequency of all the compensating stages, is provided with a resistance R4 in series with resistance R1 in its anode circuit, resistance R4 having a condenser Cs shunted therewith.

A better understanding of the operation of Fig. 7 will'be had with reference to Fig. 2. F1 defines the frequency at which the admittance of C4 has become equal to that of R2, and the admittance of C4 is more than that of R2 for all higher frequencies. R: has little effect at this frequency I because the reactance of C4 is relatively high with respect to R3. Hence Fr may be called the coming-in" transition frequency for C4. At F2,

the going-out transition frequency of C4, the i value of X04, (the reactance of C4) reaches R3,

' so that the admittances of C4 and of R3 are qually able with the resistor shunting the inductance.

If this obtains, it is important whether the resistor R3 shunts the inductance L1 alone or shunts the series resistor R2 also, as shown in Fig. 3.

The coming-in" transition frequency of 'Li, forexample, then is effected by R: as well as by R2. Similarly, the goingeout transition frequency is affected by Rz also instead of by Rs alone, as

indicated by Equations 3 and 4. A slight revision- This circuit is commonly used for a coupling impedance between the stages for a uniform response video signal. amplifier.

It was shown in Fig. 3 that, according to one form of this invention, both the series resistance R2 and inductance L1 are shunted by a resistnegative feedback in ance R3. However, no account was taken of the unavoidable stray capacitance. Fig. 5 specifically takes into account the unavoidable stray capacity C2 of the circuit and tube electrodes. We know that, in order to provide a circuit which presents a resistive load to a definite range of frequencies and in which there is included a capacity such as C2, it is necessary to include in the shunt reresponsible for the amplification oi the stage comprising tube 31. At frequencies above F2, C4 has little effect because X04 is relatively lowv with respect to R3.

R2 and Re are given such a value that C3 will not appreciably load the circuit at the maximum frequency of the useful band. At the same transition frequency Fz, the admittance of C5 in the anode circuit of discharge device Bl becomes equal to the admittance of R4. Above this fre-v quency, the current through C5 continues to increase, thus assuming the task of compensating for the continued fall of highfrequency response of the picture'transmitter tube.

Referring now to Fig. 8, there is shown another embodiment of this invention in which the compensating circuit of each of the amplification stages is provided for in the cathode circuit of the discharge devices. Discharge device 31 has in its cathode circuit a series resistance R2 shunted by a series circuit comprising capacity C4 and resistance Ra. At least one of the stages,

that is, the stage compensating for that highest part of the useful frequency range, is provided with a cathode resistor R4 and a shunt condenser Cs.

By using resistance in the cathode circuit of a discharge the circiut' to control the response characteristics of a tube. By including in the cathode circuit a circuit whose impedance changes with frequency it is possible to change the frequency response of an amplifier. By referring again toFig. 2, the operation of the circuit shown inFig. 8 may be more readily understood. F1 defines the frequency at which the admittance of C4 has become equal to that of R2, and admittance of C4 is less than the admittance of R2 for all higher frequencies. Hence, F1 is called the coming-m transition frequency for the amplifying stage comprising discharge device 31 and its associated circuit. R: has little effect device, it is possible to introduce enough at this frequency because the reactance of C4 is relatively high with respect to Ra. At F2, the going-ou transition frequency of the stage comprising tube 31, the value of X04 reaches Ra. so that C4 and Rs are equally responsible for the amplification of the stage comprising tube 37. At frequencies above F2, C4 has little effect because x04 is relatively low with respect to Ra.

At the same transition frequency F2, the voltage across C5 in the cathode circuit of the discharge device il becomes equal to the voltage across R4. Above this frequency, the admittance of C5 continues to increase and assumes the task of. compensating for the continued fall of high frequency response of the picture transmitter I l V While several systems for carrying this invention into effect have been indicated and de-' scribed, it will be apparent to one skilled in the art that this invention is by no means to be limited by the particular organization shown and described but that many modifications thereof will be made without departing from the scope of this invention as set forth in the appended claims. 7

I claim as my invention:

i. In combination with a cathode ray pickup tube for generating picture signals occupying a 'wide frequency band, said tube having high in temal impedance and having output terminals that have unavoidable capacity therebetween, an

output resistor connected between said terminals, an amplifier including a first vacuum tube having input electrodes connected across said resistor, said resistor having such high resistance that the frequency response characteristic at said input electrodes falls with increasein frequency, said amplifier comprising a plurality of amplifier stages connected in cascade, one of said stages including a frequency correcting network which gives to said one stage a frequency response char-' acteristic which has a rising response with increase in frequency over a band of intermediate frequencies in said wide band and which has a substantially flat response over the rest of said wide band, and another of said stages includin a frequency correcting network which gives to said other stage a frequency response characteristic which has a rising response with increase in frequency over a band of frequencies at the high frequency end of said wide band and which has a substantially flat response over the rest of said wide band, the overall frequency response characteristic of said two stages with frequenc correcting networks being substantially complementary to the frequency response characteristic at said input electrodes.

2. In combination with a cathode ray pickup wide band, and another of said stages including 1 tube for generating picture signals occupying a wide frequency band, said tube having high internal impedance and having output terminals that have unavoidable capacity therebetween, an output resistor connected between said terminals, an amplifier including a first vacuum tube having input electrodes connected across said resistor, said resistor having such high resistance that the frequency response characteristic at said input electrodes falls with increase in frequency, said amplifier comprising a plurality of amplifier stages connected in cascade, one of said stages including a frequency correcting network which gives to said one stage a frequency response characteristic which has a rising response with increase in frequency over a band of intermediate frequencies in said wide band and which has a substantially flat response over the rest of said a frequency correcting network which gives to said other stage a frequency-response characteristic which has a rising response With Incrcase in frequency over a band of'frequencies toward the high frequency end of said wide band and which has a substantially flat response over the rest of said wide band, the rising frequency response characteristic of each of said stages with frequency correcting networks being substantially complementary to the frequency response characteristic at said input electrodes for corresponding bands of frequencies.

3; A frequency compensating television signal transmission circuit having a frequency response characteristic which rises in response to frequency throughout its entire pass band and comprising in combination a plurality of amplifier stages each having a frequency response characteristic which does not vary with frequency except wherein the response characteristic rises in response to frequency over a portion of said band other than that portion of said band in which each of the other of said stages have rising frequency characteristics, and wherein said portions make up the entire band.

4. A frequency response compensating television signal transmission circuit having a frequency response characteristic which rises substantially proportional to frequency over a rela tively wide frequency range comprising in combination a plurality of signal amplifying stages each of which has a frequency response characteristic which is substantially flat except wherein the response characteristic rises substantially proportional to frequency over a different portion of said frequency range and wherein said portions make up substantially the entire frequency range.

'ALDA V. BEDFORD.

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