US2223812A - Television system - Google Patents

Description

Dec. 3, 1940. A. v. BEDFORD TELEVISION SYSTEM Filed July 30, 1938 5 Sheets-Sheet l Dec. 3, 1940. A. v. BEDFORD TELEVISION SYSTEM Filed July 50, 1938 5 Sheets-Sheet 2 :inventor Gttorneg Dec. 3, 1940. i A. v. BEDFORD 2,223,812

TELEVISION SYSTEM Filed July 30, 1938 5 Sheets-Sheet 5 l Dec.3, 1940. A V, BEDFORD Y 2,223,812

TELEVI S ION SYSTEM Filed July 50, 1938 5 Sheets-Sheet 4 Dec. 3, 1940. A, v BEDFORD 2,223,812

TELEVI S ION SYSTEM Filed July 30, 1938 5 Sheets-SheerI 5` i e sg e. e, S Summer 51 70/1 N99 ya Kvfd Gnome" scanning.

Patented Dec. 3, 1940 UNITED STATES TELEVISION SYSTEM Alda V. Bedford, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application .my at, 193s, sensi No. mausiv i e ciiims.A (c1. 17a- 7.2)

My invention relates to television systems and particularly to a method of and means for improving the synchronization of the scanning at the receiver with that at the transmitter where interlaced scanning is employed.

In my Patent No. 2,192,121, issued Feb. Z7, 194), entitled Television systems and the method of operation thereof, and assigned to the Radio Corporation of America, there is described a television system of the type in which picture signals and horizontal and vertical synchronizing impulses are transmitted in the same channel, the synchronizing impulses having rthe proper frequency relation to produce interlaced scanning, and wherein there 'are impulses occurring in the region of the verticalV synchronizing impulse at a multiple of the frequency of the horizontal synchronizing impulses, specifically at double this frequency for double interlacing. Such multiple frequency impulses are desirable, as explained in said patent, because succeeding vertical synchronizing impulses occury in different time relations to the horizontal synchronizing impulses in a system of this type employing odd-line interlaced By odd-line interlaced scanning is meant scanning where the vertical deflecting frequency goes into the horizontal deflecting frequency a whole number of times plus a fraction such as one-half, one-third, one-fourth, or the like. This diiference in time relationof the impulses tends to cause succeeding vertical synchronizing impulses as impressed upon the receivers vertical deiiecting circuit to be unlike, whereby the quality of the -interlacing at the receiver is impaired. However, by employing the multiple frequency impulses, the succeeding vertical synchronizing impulses are caused to be almost identical at the receiver defiecting circuit, whereby good interlacing is obtained.

In a system of the above-mentioned type, it has been found that the receiver will lock into synchrcnism at two diiferent points, one being a stable point on the vertical synchronizing impulse at which there is good interlacing, and the other being an unstable point in front ci' the vertical impulse where there is a rise in the voltage caused by the integration of the double frequency impulses, the latter point giving poor interlacing and a coarse line structure.

It is the principal object of my invention to provide a television system of the above-menticned type in which the receiver will lock into .synchronism only at a point where the synchronization is stable and gives satisfactory interlaclng.

In practicing a preferred embodiment4 of -my invention, I produce a synchronizing signal of the character described above, having double frequency impulses preceding and followingv `each vertical synchronizing impulse with the double frequency impulses having the same energy content as the horizontal synchronizing impulses occurring in a like time. Since all impulses have the same height, this means that I produce the double frequency impulses with ,a width or dural0 tion which is one-half that of the horizontal synchronizing impulses.

The invention will be better understood from the following description taken in connection with the accompanying drawings, in which 1b Figs la and 1b are curves showing the picture, blanking and synchronizing signals which are produced in accordance with .one embodiment of my invention at and in the` region of succeeding vertical synchronizing impulses; I f

Figs. 2a and 2b are curves similar to those in Figs. 1aJ and 1b illustrating the signals produced by another embodiment of my invention;

Fig. 3 is a diagramof a television transmitter which produces the signals shown in Figs. la and 25 1b; f

Fig. 4 is a View of the disc illustrated in Fig. 3 for producing synchronizing and blanking impulses;

Fig. 5 shows enlarged portions of the disc illustrated in Fig. 4; Y

Fig. 6 is a circuit diagram of a television receiver for use with the transmitter shown in Fig. 3; and

Fig. '7 is a group of curves which are referred to in explaining my invention.4

The invention will be described in detail by rst referring to Figures la and 1b, which illustrate the video signal which is produced and transmitted in accordance with one embodiment of my invention. It will beunderstood that this video signal modulates a carrier wave at the transmitter for transmission to a receiver in the usual way. v

Referring particularly to Fig. la, the video signal. may be described as a composite signal consisting of horizontal blanking impulses l which occur at the end of each scanning line, picture signals 2 which are generated during the horizontal scanning line period, horizontal synchronizing impulses 3 which are added to or set on top of the blanking impulses l, a blanking impulse d, of comparatively long duration as indicated by the legend, which occurs at the end of each picture frame or vertical deflection period,

a slotted vertical synchronizing impulse 6 which is added to or set on top of the vertical blanking impulse 4, and impulses 'I preceding and following the vertical synchronizing impulses which occur at double the frequency of the synchronizing impulses 3 and which have one-half the width or duration of the impulses 3.

Since the vertical synchronizing impulse 6 is slotted, it may be looked upon as comprising a plurality of impulses 6a. The front edge of each impulse Ba occurs in the same time relation as the front edges of the double frequency impulses 1. Thus the front edges of the impulses 6a function to hold the horizontal deflection of the television receiver in synchronism during the occurrence of the vertical synchronizing impulse just as in the case where horizontal synchronizing impulses are set in slots in the vertical synchronizing impulse, this latter arrangement being described in my above-mentioned copending application and being illustrated in Figs. 2a and 2b, which will be described later. The reason for this action is that in the horizontal synchronizing circuit of the receiver, it is the front or sharply rising edge of a horizontal synchronizing impulse which provides the synchronizing voltage.

In accordance with a preferred embodiment of my invention, each double frequency impulse 1 is made to have one-half the width of and, therefore, one-half the energy content of each horizontal synchronizing impulse 3. 'Ihe reason for this will be described in detail hereinafter.

It may be noted that alternate double frequency impulses 'I may be considered to be equalizing impulses in that they, in cooperation with impulses 'I occurring in the same time relation as impulses 3 whereby they may be considered as true horizontal synchronizing impulses, correct for dissymmetry of the horizontal synchronizing impulses with respect to the vertical synchronizing impulse for successive vertical synchronizing impulses, as described in my above-mentioned patent.V

The curve shown in Fig. 1b is a continuation of the curve shown in Fig. 1a and shows the video signal as it appears during the vertical deflection period immediately following the deflection period represented in Fig. 1a. In other words, for the specic case assumed for the purpose of illustration, where vertical synchronizing impulses occur at the rate of 60 per second, the vertical synchronizing impulse shown in Fig. 1b occurs one-sixtieth of a second later'than the vertical synchronizing impulse shown in Fig. 1a.. In Fig. 1b, impulses corresponding to those in Fig. la are indicated by the same reference numerals marked with a prime.

It will be noted that, while the vertical syn-- chronizing impulses of Figs. la and 1bl occur in the same time relation, the horizontal synchronizing impulses in Fig.I 1b fall halfway between the horizontal synchronizing impulses shown in Fig. la, as indicated by the broken line x, whereby interlaced scanning is produced as described in Patent 2,152,234, issued March 28, 1939, to Randall C. Ballard, and assigned to the Radio Corporation of America.

The signal illustrated in Figs. la and 1h may be produced by apparatus or circuits of various types. By Way of example, there is illustrated in Fig. 3 a transmitter of the type wherein the blanking signals and synchronizing and equalizing signals are generated by means of a rotatable disc ID having properly shaped openings therein and cooperating with lightsources I I and l2 and photoelectric cells I3 and I4.

Referring to Fig. 3, I have shown merely by way of example a transmitter in which the disc Ill rotated by a motor I6 is employed for producing synchronizing impulses of the desired shape. Near the periphery of the disc I0 there are openings which rotate between light source II and photocell I3 for producing the synchronizing impulses. The other row of openings is located between light source I2 and photocell I4 for producing blanking impulses. The shapes and spacings of the openings Will be described later in connection with Figs. 4 and 5.

To produce the horizontal synchronizing impulses, the source of light I I, a mask II having a slit therein, and a suitable optical system form a vertical slit of light on the row of synchronizing openings. As an opening rotates past this slit of light, an amount of light dependent upon the shape of the opening passes therethrough and impinges upon the photoelectric cell I3 to produce voltage impulses of the desired shape at the input of an amplifier I8.

Blanking impulses are produced in a similar manner by means of the light source I2, a slotted mask I9 and a suitable optical system which form a vertical slit of light on the row of blanking openings. Thus, an amount of light dependent upon the shape of a blanking opening reaches the photoelectric cell I4 whereby voltage impulses of the desired shape and duration are supplied to the input of an amplier 2 I.

The device for generating the picture signal is illustrated as a cathode ray transmitter tube 22 of Well-known design. It comprises a highly evacuated envelope haw'ng therein a cathode ZI, a grid 25, a rst anode 23 and a second anode 24 for producing an electron beam. The envelope also contains a mosaic 26 of photoelectric elements which are insulated from each other and from a metallic signal plate 21.

An optical image of the subject to be transmitted is projected upon the mosaic 26 whereby an electrostatic image is formed thereon. Thus, when the mosaic is scanned by the cathode ray, picture signals are produced which appear across an output resistor 28 connected between the signal plate 21 and the second anode 24. These pictures are then amplified by a picture ampliiier 29 and supplied to a radio transmitter 3l.

The cathode ray may be caused to scan the mosaic 23 by means of horizontal and vertical deiiecting coils 32 and 33, respectively, which are supplied with saw-tooth current from their respective deilecting circuits 34 and 36.

The horizontal deflection and the vertical deflection of the cathode ray of the transmitter tube are made to occur in the proper time relation by means of the synchronizing impulses supplied from ampliiier I8 over a conductor 31 to a separatingr or lter circuit 38 which separates the horizontal and vertical synchronizing impulses from each other.

The deecting circuits 34 and 36 may be of any suitable design, ea-ch deecting circuit including an oscillator such as a blocking oscillator. A deflecting circuit of this character is described in Toison Reissue Patent 20,338.

Blanking signals from the output of amplifier 2| are supplied over a conductor 4I to the grid 23 of the cathode ray transmitter tube for blocking the cathode ray during the horizontal return period and during the vertical return period.

Since the synchronizing and blanking signals are to be transmitted togetherl with the picture signals to a receiver, the blanking signals are supplied through an amplifier 42 to a suitable amplifier stage in the picture amplifier 29 for removing undesired transient signals from the picture signals and for producing pedestals upon which the synchronizing impulses are to be located.

The synchronizing impulses are supplied through an amplifier 43 to a later stage in the picture amplier 29 Where they are added to the picture and blanking signals.

Referring more in detail to the impulse-generating disc, the shapes of the synchronizing and blanking openings and their relative locations are shown in Fig. 4 and in the fragmentary views in Fig. 5. Referring to Figs. 4 and 5, the slotted vertical synchronizing impulses b and b" (Figs. la and 1b) are produced by the diametrically opposed gro-ups of openings Vi and V2. The horizontal synchronizing impulses are produced by the openings BI, and, as will appear later, by alternate double frequency openings h.

As shown in Fig. 5, in particular, the openings h have one-half the width of the openings H whereby they produce the half Width impulses 1 and 1 of Figs. 1a and 1b. The slit of light which is projected upon the row of synchronizing openings is indicated at 45.

The relation between the openings H producing horizontal synchronizing impulses and the groups of openings Vi and V2 producing vertical synchronizing impulses is such as to produce Y interlaced scanning in the manner described and claimed in the above-mentioned Ballard patent. As taught by Ballard, the odd-line double interlacing is obtained by having the number of horizontal synchronizing impulse openings and the number of vertical synchronizing impulse openings (i. e., groups V1 and V2) such that the former number divided by the latter number equals a whole number plus one-half.

For the specific transmitter being described, the disc is'rotated at 30 rotations per second, whereby 60 vertical synchronizing impulses per second are transmitted. To prod-nce a 44-1line picture, 441 horizontal synchronizing openings are required, these being the openings H, onehalf the openings' h and one-half the openings or slots in the groups oi' impulses V1 and V2. On the drawing, no attempt has been made to draw the disc openings in the proper scale to include all. 441 openings on a disc of the size shown.

Referring now to the openings for producing the blanking impulses, openings 45 and 41 are opposite the groups of openings V1 and V2, respectively, and the adjacent double frequency openings h for producing the blanking impulses 4 and 4' (Figs. la and 1h). Opposite the openings H, there are openings 48 for producing the b-lanlring impulses l and I. The slit of light which is projected upon the row of blanking openings is indicated at 49. y

Referring to Fig. 6, there is illustrated a television receiver which is designed to receive picture and synchronizing impulses which havel been transmitted from the above-described transmitter. The receiving apparatus includes a radio receiver 5I which selects and demodulates the desired carrier wave whereby picture signals and synchronizing impulses of the character described above appear in the output circuits thereof.

The picture signals and synchronizing impulses are amplified in resistance coupled amplifiers 52 and 53 and impressed upon the control grid 54 of a cathode ray tube 56. The cathode ray tube 56 may be of well-known construction comprising an evacuated envelope having therein a cathode 51, the control grid 54, a rst anode 59, and a second anode 6l. Suitable deflecting means, such as delecting coils 02 and B3, are provided for deflecting the cathode ray both horizontally and vertically to effect scanning of a fluorescent screen 64 at the end of the tube.

The horizontal deflecting coils 62 are supplied with saw-tooth current having a comparatively high frequency from a horizontal deflecting circuit indicated at E6. rlhe vertical deflecting coils t3` are supplied with saw-tooth current having a comparatively low frequency from a vertical deflecting circuit indicated at 61. The deflecting circuits 65 and 61 may be any of several wellknown designs, such circuits generally including an oscillator which can be locked in step with synchronizing impulses impressed thereon.

The number of amplier stages in the radio receiver is so selected that the polarity of the synchronizing impulses impressed upon the control grid 54' of the cathode ray tube is negative whereby the cathode ray is blocked at the end of each scanning line, this being desirable in order to prevent a trace on the uorescent screen during the return line period.

The scanning of the fluorescent screen 64 by the cathode ray is maintained in synchronism with the scanning at the transmitter by separating the synchronizing impulses from the picture signal and impressing these impulses upon the deflecting circuits 66 and 61. The combined picture signals and synchronizing impulses are taken from a point in the circuit, such as the output circuit of the amplifier 52, where the synchronizing impulses are of positive polarity and are impressed upon a separating tube 68.

The separating tube 58 may be of the threeelement type having a cathode E9, a control grid 1I` and a plate 12. The picture signals and synchronizing impulses are fed to the grid 1I- through a grid condenser 13. A grid leak resistor 14 is connected between the grid 1| and the cathode 69. A positive potential from any suitable source is applied to the plate 12 through a plate resistor 16.

For reasons which will be explained hereinafter, only the synchronizing impulses appear in the output circuit of the separating tube il and these impulses are fed through a coupling condenser -11 to two filter circuits 18 and 19. The filter circuit 18 includes a resistor 8l of fairly high resistance value connected in series with a condenser 82 having comparatively large capacity whereby, due to an integration action, substantially only the lower frequency vertical synchronizing impulses appear across the condenser 82 as illustrated in Fig. '7. A battery 83 or other suitable biasing source may be provided for supplying a bias voltage to the amplifier tube through a resistor 84. l'f preferred, the battery 83 and resistor 84 may be omitted and, instead, a grid leak resistor shunted across the condenser 32. Merely by way of example, it may be noted that the following values for lter 18 are satisfactory: Resistor 8i equals 2 megohms, resistor 84 equals 100,000 ohms, and condenser 82 equals m. m. f.

The lter circuit 19 consists of a small capacity condenser 86 connected in series with a resistor 31 of fairly low resistance value whereby only the comparatively high frequency horizontal synchronizing impulses pass through the condenser 86 to develop voltage impulses across the resistor 81 which are impressed upon the horizontal deflecting circuit B6.

Referring briefly to the operation of the separating tube E8, it will be apparent that, since the synchronizing impulses are of positive polarity when impressed upon the grid 1|, they will cause a ow of grid current which will charge the grid condenser 13, thereby producing a negative bias on the grid. It will be apparent that the grid 1| and the cathode 69 function as rectifier electrodes to keep condenser 13 charged. The time constant of the circuit including the grid condenser 13 and grid resistor 14 is such that the horizontal synchronizing impulses cause a substantially steady bias to be applied to the grid 1|. The circuit of the separating tube is so adjusted that the biasing voltage applied to the grid 1| is sufficient to bias the tube beyond the cut-01T point, except during the synchronizing impulses. Hence, only the synchronizing impulses are present in the plate circuit of tube 68.

The action of my improved synchronizing signal is shown in Fig. '1, where the synchronizing signal in the region of the even vertical impulse is indicated at 3, 3, 1, 6a and 1 (corresponding to Fig. la), and where the synchronizing signal in the region of the odd vertical impulse is indicated at 3', 3', 1, 6a and 1 (corresponding to Fig. 1b).

The dotted line curves A and B represent the voltage which would appear across the integrating condenser 82 (Fig. 6) for even and odd `vertical synchronizing impulses, respectively, if the double frequency impulses 1 were the same width as the preceding impulses. It will be apparent that under these conditions the voltage Lacross the integrating condenser 82 would rise as soon as the double frequency impulses were fed into it. Because of this, the vertical delecting circuit might lock into synchronism ahead of the steep voltage rise caused by the impulses 6a. This would cause poor interlacing and a coarse line structure, because A and B are different in the region of the rst few double frequency impulses, whereas they .approach each other closely, in the desired manner, in the region of the vertical synchronizing impulse.

The solid line curves C and D represent the voltage which appears across the integrating `condenser 82 when the double frequency impulses are one-half the width of the preceding impulses, as illustrated in Fig. 7 curves C and D being for even and odd vertical synchronizing impulses, respectively.

It will be seen that curves C and D do not rise to a higher voltage level in the region of the double frequency impulses than the voltage level in the region of the impulses 3 or impulses 3', this being due to the reduced energy content of the double frequency impulses. Consequently, the vertical deflecting circuit of the receiver will lock into synchronism only on the steeply rising part of the curves C and D produced by the impulses a, where the curves are nearly identical and the operator cannot carelessly so adjust the receiver that the interlacing is poor.

It should be understood that my invention is not limited to the use of double frequency preparatory impulses of one-half the width of the line frequency impulses. The important feature of my invention is to make the energy content of the preparatory signals small enough to prevent the undesired voltage rise shown by curves A and B in Fig..'1. For example, the double frequency preparatory signals may be reduced to less than one-half the width of the line frequency impulses. Or, they may have .a width slightly greater than one-half the width of line frequency impulses providing the voltage rise during the preparatory impulses is kept so low that the receivers vertical deecting circuit will not lock in on it. Also, if the preparatory signals are produced at four times the line frequency, the width of the preparatory signals should be approximately one-fourth the width of the line frequency impulses or less.

As previously indicated, the synchronizing impulses may be of the character indicated in Figs. 2a and 2b, if preferred, where the double frequency impulses are set in slots in the vertical synchronizing impulses as described in my abovementioned copending application.

The signals illustrated in Figs. 2a, and 2b may be produced by means of a rotatable disc having suitable openings therein or they may be produced by means of a system like that described in Patent 2,132,655, issued Oct. 11, 1938, in the name of John P. Smith, entitled Systems for producing electrical impulses, and assigned to the Radio Corporation of America. In either case, the double frequency impulses are narrowed to approximately one-half the width of the line frequency impulses or less for ously explained.

I claim as my invention:

1. In a television system, means for transmitthe reasons previting horizontal synchronizing impulses which occur at a relatively high frequency, means for transmitting framing signals which occur at a comparatively low frequency, and means for transmitting during the period immediately preceding each framing signal impulses which occur at a rate which is a whole number multiple of the frequency at which said horizontal synchronizing impulses occur and which have a width or during which is substantially less than the width or duration of each of said horizontal synchronizing impulses.

2. In a television system employing interlaced scanning, means for transmitting horizontal synchronizing impulses occurring at a relatively high frequency, means for transmitting framing signals occurring at a relativelylow frequency, said high and low frequencies having the relation that the low frequency goes into the high frequency a whole number of times plus one-half, means for transmitting additional impulses during the period immediately preceding each of said framing signals, which additional signals occur half way between the horizontal synchronizing impulses whereby, in effect, there is a group of double frequency impulses preceding each of said framing signals, said double frequency impulses having a width or duration which is not greater than substantially one-half the width or duration of the preceding horizontal synchronizing impulses.

3. In a television system of the type employing odd-line interlacing and wherein the horizontal and vertical synchronizing signals are separated from each other at the receiver by means of lter circuits including an integration circuit, means for transmitting horizontal synchronizing signals which occur at a comparatively high frequency, means for transmitting framing signals which occur at a comparatively low frequency and means for transmitting impulses which occur at a whole number multiple of said comparatively high frequency, which immediately precede said framing signals and which are sufficiently short in duration to make the energy content of said multiple frequency impulses occurring in a certain time period not substantially greater than the energy content of the said comparatively high frequency signals occurring in an equal time period.

4. The invention according to claim 3 wherein said comparatively low frequency goes into said comparatively high frequency a whole number of times plus one-half to produce double interlacing and wherein each of said multiple frequency impulses has a Width or duration which is not substantially greater than one-half the width or duration of each of the signals occurring at said comparatively high frequency.

5. In a television system of the type employing odd-line interlaced scanning, means for transmitting horizontal synchronizing impulses which occur at a comparatively high frequency, means for transmitting framing signals which occur at a comparatively low frequency, and means for transmitting during the periods immediately preceding and immediately following each framing signal impulses that occur at a rate which is a whole number multiple of said comparatively high frequency and which have awidth or duration that is substantially less than the width or duration of each of said horizontal synchronizing impulses.

6. The invention according to claim 5 wherein said comparatively low frequency goes into said comparatively high frequency a whole number of times plus one half, wherein said multiple frequency impulses occur at twice the rate at which said horizontal synchronizing impulses occur, and wherein each of said multiple frequency impulses has a width or duration which is not substantially greater than one-half the width of each of said horizontal synchronizing impulses. 20

ALDA V. BEDFORD.

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