US2236066A - Television apparatus - Google Patents

Description

25, 1941K. w, J, POQH v 2,236,066 TELEVISION Auuwxmius r Filed May 3l. 1935 2 Shnets-Sheet l fnesg; l L .l/lfaldenar` Pochi Z5, 1941. w. J. POCH TELEVISION APPARATUS Filed May 51, 1935 -2 Sheets-Sheet 2 VOLT@ 190 10.00 10,000 FREQUENCY IN YCLE' PER SECDND INVE'N''OR Patented Mar. 25, i941 PATENT OFFICE ,$236,066 TELEVISION APPARATUS Waldemar J. Poch, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 31, 1935, Serial No. 24,207

4 Claims.

My invention relates to television apparatus, and particularly to a method of and means for obtaining proper synchronization between scanning at the transmitter and scanning at the receiver.

In British Patent No. 407,409 there is described a television system in which scanning at the receiver is maintained in synchronism with scanning at the transmitter by transmitting a horilo zontal synchronizing impulse of short duration at the end of each scanning line and a vertical impulse of the same amplitude and longer duran tion at the end of each picture frame. The synchronizing impulses are of greater amplitude than the picture signals and preferably are in the black direction.

In a television receiver for the reception of composite signals of the above-described character, the horizontal and vertical deiiecting cire 2O cuits which produce the deiiecting` currents or voltages for scanning at the receiver, and which lare controlled by the synchronizing impulses, usually each include an oscillator such as a relaxation oscillator or an oscillator of the dynatron or blocking oscillator type. Such deiiecting circuits are described in the article by Holmes, Carlson and Tolson which was published in the Proceedings of the Institute of Radi-o Engineers for November, 1934, pages 1266 to 1285. It has been found that the strong voltage impulses produced by the oscillator in one deflecting circuit tend to get into the other deflecting circuit and adversely affect the scanning.

Accordingly, it is an object of my invention to provide an improved television receiver of the above-mentioned type in which one deiiecting circuit does not produce a disturbance in the other deecting circuit.

It is a further object of my invention to provide an improved method of and means for separating picture signals from synchronizing signals in a television receiver.

In one embodiment of my invention, I obtain the desired isolation of horizontal and vertical deiiecting circuits by employing a vacuum tube in each deflecting circuit channel for separating picture signals from synchronizing impulses.

Instead of employing two separating tubes, a`

single screen grid tube may be utilized, the plate circuit being coupled to one deflecting circuit and the screen grid circuit being coupled to the other deecting circuit whereby at least one delecting circuit is substantially isolated from the other. ,55 Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawings, in which Figure l is a circuit diagram of a television receiver embodying my invention;

Fig. 2 is a circuit diagram of a picture signal and synchronizing impulse separating circuit which may be substituted for a portion of the circuit shown in Fig. 1;

Fig. 3 is a group of curves which are referred 10 to ,in explaining the operation of the circuit shown in Fig. 1;

Fig. 4 is a circuit diagram of another embodiment of my invention; and

Figs. 5 and 6 are curves which are referred to 15 in` explaining the invention.

Referring to Figure 1, there is illustrated a television receiver which is designed to receive picture and synchronizing impulses which have been transmitted by means of a modulated car- 20 rier wave in accordance with the teachings of the above-mentioned British patent. The receiving apparatus includes a radio receiver l which selects and demodulates the desired carrier wave whereby picture signals and synchro- 25 nizing impulses of the character described above appear in the output circuit thereof.

The picture signals and synchronizing impulses are amplified in a signal channel including resistance coupled ampliers 2 and 3 and are im- 30 pressed upon the control grid 4 of a cathode ray tube 5. The cathode ray tube 5 may be of well known construction comprising an evacuated envelope having therein a cathode E, the control grid 4, a rst anode 1, and a second anode 8. 35 Suitable deecting means, such as deiiecting coils 9 and I0 are provided for deflecting the cathode ray both horizontally and vertically to effect scanning of a fluorescent screen at the end of the tube 5. 40

'Ihe horizontal defiecting coils 9 are supplied with saw-tooth current having a comparatively high frequency from a horizontal deflecting circuit indica-ted at l I. The vertical derlecting coils l0 are supplied with saw-tooth current havingV a 45 comparatively low frequency from a vertical deflecting circuit indicated at I2. The deiiecting circuits Il and l2 may be any of the designs described in the above-mentioned article, such a circuit Aincluding a blocking oscillator which 50 can be locked instep with synchronizing impulses impressed thereon.

The number of ampliiier stages in the radio receiver l is so selected that the polarity of the synchronizing impulses impressed upon the con- 5s i trol grid i of the cathode ray tube 5 is negative whereby the cathode ray is blocked at the end of each scanning line, this being desirable in order to prevent a trace von the uorescent screen during the return line period.

The scanning of the iluorescent screen 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 deiiecting circuits II and I2. In accordance with my invention, the composite signal comprising picture signals and synchronizing impulses is impressed upon two separating tubes I3 and I4, the tube I3 supplying synchronizing impulses to the vertical deflecting circuit I2 and the tube I4 supplying synchronizing impulses to the horizontal deflecting circuit II.

In the particular receiver illustrated, the separating circuit is of the general type described and claimed in Patent 2,178,766, issued Nov. 7, 1939, in the name of William A. Tolson and assigned to the same assignee as this application. While a particular type of separating circuit is illustrated which is for separating synchronizing impulses having a negative polarity, other types of separating circuits may be employed, such as those which are designed for separating synchronizing impulses having a positive polarity.

The separating tube I3 which supplies the vertical deflecting circuit may be a vacuum tube of the three element type, having a cathode I6, a control grid I1 and a plate I8. The input circuit of the tube i3 includes a resistor I9 next to the grid I1 and a resistor 23 across which the picture and synchronizing signals are impressed. Preferably, a biasing battery 2| or other biasing means is provided for the-purpose of applying a positive bias to the grid I1 in order to obtain better separation of picture and synchronizing signals as will be explained later.

The composite signal, which includes the negative polarity synchronizing impulses, is supplied from the output of the amplier 3 through a conductor 22 and a condenser 23 to the input circuit of the separating tube I3. For reasons which will appear later, only the synchronizing impulses appear in the output circuit of the separating tube I3 and these impulses are sup- -plied through the coupling condenser 24 to the vertical deflecting circuit I2.

In order to separate the vertical synchronizing impulses from the horizontal synchronizing impulses, the plate impedance of the separating tube I3 is made low for the comparatively high frequency horizontal impulses. This low plate impedance is obtained by connecting a comparatively high capacity condenser 25 across the plate resistor 26. The capacity of the condenser 25 is suflicient to make the plate circuit of the tube I3 present a very low impedance to the higher frequency synchronizing impulses whereby substantially all of these impulses are eliminated and only the vertical synchronizing impulses are impressed upon the deflecting circuit I2.

The picture separating circuit for supplying synchronizing impulses to the horizontal deflecting circuit II may be of the same type as the separating circuit described above. In the circuit which is illustrated, it comprises the vacuum tube I4 which has a cathode 21, a control grid 23 and a plate 29. The input circuit of the tube includes a resistor 3| next to the grid 28, a resistor 32 across which the composite signal' isV vthe vertical synchronizing impulses.

impressed, and a biasing battery 33 for applying a positive bias to the grid.

The composite signal is supplied to the input circuit through a condenser 34. Just as in the case of the other separating circuit, only the synchronizing impulses appear in the plate circuit of the tube I4 and these impulses are supplied through a coupling condenser 35 to the horizontaldeflecting circuit II. Only the high frequency horizontal synchronizing impulses are supplied to this circuit, however, the plate circuit of the tube I4 having a low impedance to This low plate impedance is` obtained by supplying positive voltage to the plate 29 through an inductance coil 36 which presents a high impedance to the high frequency horizontal synchronizing impulses, and a comparatively low impedance to the lower frequency synchronizing impulses. Also, the coupling condenser 35 may be given a small capacity whereby it presents a high impedance to the vertical synchronizing impulses.

From the foregoing description it will be seen that the two deflecting circuits II and I2 are isolated from each other by means of the picture separating tubes I3 and I4 whereby impulses produced by an oscillator in one deecting circuit cannot be fed back into the other deflecting circuit.

In addition to the filtering action obtained in the plate circuits of the separating tubes I3 and I4 for separating vertical and horizontal synchronizing impulses, an additional ltering action can be obtained in the input circuits of these separating tubes by giving the condenser 23 a comparatively large capacity and the resistor I9 a comparatively large resistance and by giving the condenser 34 a comparatively small capacity and the resistor 3| a comparatively small resistance.

Thel action of the separating tubes I3 and I4 in separating synchronizing impulses from picture signals, especially when they have a positive bias on the grid, will be understood by referring to Fig. 3. In this figure, the picture signals are indicated at 31 and the horizontal synchronizing impulses at 38. The grid voltage-plate current characteristic of a' picture separating tube is indicated at 39. As explained in the above-mentioned Tolson patent, only negative impulses which are impressed upon the input circuit of the separating tube will appear in the plate circuit since the grid-cathode impedance of the tube becomes very low when positive impulses are impressed upon the grid. When the grid is negative, however, the grid-cathode impedance is high and the negative impulses appear in the plate circuit.

By referring to the curve representing picture signals in Fig. 3, it will be seen that the picture signals sometimes have a negative polarity as at 31a. It is in order toV prevent such negative polarity picture signals from being passed by the separating tube that a positive bias is applied to the control grid. The effect of such a bias is apparent from an inspection of Fig. 3, it being evident from this figure that the positive bias has shifted the A. C. axis of the composite signal to the right sufiicientlyv to prevent the negative polarity picture signal at 31a from driving the control grid negative.

In the embodiment of my invention shown in Fig. 2, a single tube 4I of the screen grid type is employed in place of the two separating tubes I3 and I4 shown in Fig. l. The particular tube illustrated is a. vacuum tube having a cathode 42, a control grid 43, a' screen grid 44, a suppressor grid` and an anode 46. While the separation of picture signals and synchronizing impulses may be obtained by employing an input circuit of the type shown in Fig. 1, it may be preferable to employ a different circuit of the type which separate positive polarity synchronizing impulses from the picture signals. In the circuit shown in the drawings, the input circuit of the tube' 4I includes a grid condenser 41 and a grid leak resistor 48, the grid condenser 41 to be connected to a conductor 40 in the output circuit of the ampliiier 2 shown in Figi* 1. The positive polarity synchronizing impulses cause a periodic ilow of grid current which causes a negative bias to be applied to the control grid 43. This bias is sufficient to bias the tube beyond cut-off whereby only the positive polarity synchronizing impulses are passed by the tube. So long as the amplitude of the synchronizing impulses does not change, the negative bias ap plied to the control grid 43 is substantially constant since the circuit including the grid condenser 41 and the grid resistor 48 has a time constant great enough to hold over the condenser charge between horizontal synchronizing impulses.

[In this embodiment of my invention, I take advantage of the fact that the synchronizing impulses Iappear in both the plate circuit and the screen grid circuit. The plate circuit is designed to have a low impedance to vertical synchronizing impulses by applying the volt-age .to the plate 46 through an inductance coil 49 whereby the horizontal synchronizing impulses are applied to the horizontal deiiectlng circuit III substantially to the exclusion of the vertical synchronizing impulses.

The screen gri-d circuit is designed to have a low impedance to the horizontal synchronizing impulses whereby only the venti-cal synchronizing impulses are impressed upon the vertical deflecting circuit I2. This low plate impedance is obtained, as in the circuit shown in Eig. 1, by connecting .a comparatively large capacity condens-er 50 between the screen grid 44 Vand ground.

It will be apparent .that impulses produced by an oscillator in the horizontal deflecting circuit II cannot get back into the vertical defiecting circuit I2. This is of particular importance in television systems employing interlaced scanning, since horizontal synchronizing impulses appearying in the vertical deflect/ing circuit may destroy the inter-lacing. llt is of less importance that the impulses produced by an oscillator in the vertical deflecting circuit I2 be prevented from getting into .the horizontal deecting circuit Ill, aithough this is desirable. However, the circuit shown in Fig. 2 may be of :particular value in television receivers which must be manufactured at low cost.

Referring to the embodiment'l of my invention shown in Fig. 4, the picture signals and synchronizing impulses are separated `from each other by means of any suitable separating circuit such as .a circuit including :a separating tube 5.2 which is biased beyond cut-off by means of grid leak biasing, as described in connection with Fig. 2. The vertical synchronizing impulses and the horizontal synchronizing impulses are separated from each other by means of ill-ters, the vertical synchronizing impulsesbeing supplied to the vertical delecting circuit or saw-tooth current generator I2 through a vacuum tube 61, and the horizontal synchronizing impulses being supplied tube sz.

tothe horizontal deilecting circuit or saw-tooth current generator III through a vacuum tube 69. Like parts in Figs. 1, 2 and 4 are indicated by the same reference numerals.

The vacuum tube 61, which may be of. the three-element type, has in its input circuit a resistor 53 of high resistance next to the grid and a resistor 54 having comparatively low resistance across which .the synchronizing impulses are impressed. These impulses are supplied from the tube 52 through a coupling condenser 51. The input circuit also preferably includes a biasing battery 10 ou' other biasing means for applying a positive biasing potential to the control grid. A filter condenser 55 is connected across the input electrodes of the vacuum tube 61, this condenser, in conjunction with the resistor 5'3, serving to substantially eliminate the horizontal synchronizring impulses whereby only the vertical synchronizing impulses .are impressed upon the vertical defiecting circuit I2.l

It will be apparent from a comparison of Figs. 1 and 4 that the input circuit 'of the vacuum rtube 61, in addition to serving as a llter, will function to eliminate yany positive polarity picture signals which may have passed through the separating In other words, only negative polarity signals in the output of tube 52 are passed by the tube 61 for .the reasons given in describing the acti-on of the separating tubes shown in Fig. 1.

The input circuit of the vacuum tube 6'9 includes a grid resistor 65 connected between the giri-d .and the cathode, the tube preferably being operated with .the grid at zero bias. The grid of the tube 69 is coupled to the output circuit of the separating tube 52 through a condenser 63 having such small capacity that it presents a high impedance to the lower frequency vertical synchronizing impulses. Therefore, because of the filter action of .the condenser 613 .and the grid resistor 55 only the horizontal synchronizing impulses appear in the output circuit of the tube 69 :and vare impresse-d upon the horizontal deflect ing circuit I I.

It will be seen that .the vacuum tubes B1 and 69 isolate the two deflecting circuits from each other. Thus, voltage impulses produced by an oscillator in one deilecting circuit cannot be fed back into the other deflect-ing circuit to have a detrimental effect upon the scanning.

The character of the iilter action obtained by the Icircuits shown in Fig. 4 will be better understood by referring to Figs. 5 and 6. Eig. 5 represents the composite signal previously referred to which consists of the picture signal 31, horizontal synchronizing impulses 38 land vertical synchronizing impulses 12. It may be noted that this composite .signal is the same as the one represented in Fig. 3, except that in Fig. 3 each honizont-al synchronizing impulse 38 is shown placed on top of a pedestal in accordance with the latest practice.

It will .be seenuby referring to the curves in Fig. 6 that the characteristic of the filter circuit preceding the vacuum tube B1 is that it passes the comparatively low frequency components of the synchronizing impulses and excludes the 'higher frequency components, while the characteristic of the filter circuit preceding the vacuum tube 69 is that it passe-s these higher frequency components which are excluded by the rst illter circuit and excludes the lower frequency components. Thus the filter circuits shown in Elie. 4 substantially isolate the vertical synchronizing impulses and the horizontal synchronizing impulses, since impuIseshaVing..the general character shownin--Fig5 vare Icharacterized by the fact that most of the energy in the vertical synchronizing impulses `appears in a frequencyband below :a cert-ain frequency (in general, the horizontal syn-chronizing frequency), whil-e most of,

the -energy inthe horizontal synchronizing impulses appears in a frequency ban-d above .this frequency.

In the drawings, the values of certain elements have been indicated in ohms, megohms, microfarads, and -henries The values which are given are satisfactory for a television system employing interlaced scanning in which the horizontal syn- :chronizing impulses occur at the rate of 10,290

pensecond andthe vertical synchronizing impulses occur at the rate of per second. It

should Abe understood, however, that these values are given merely by Way of example and that they may be varied Within rather wide limits.

I claim as my invention:

1. In a television receiver of the type including a' cathoderay tube having deflecting means for deflecting `the cathode ray vertically and deiiectling means for `derlecting the cathode ray horizontally, la vertical deec'ting circuit coupled to said first means, a horizontal deecting circuit coupled to said second means, a signal channel` for the reception of a composite signal comprising ;picture signals and vertical'and horizontal synchronizing signals, said horizontal impulses occurring at a higher frequency than said ver-tical impulses, an electric discharge tube having a' cathode, a control grid jand an anode, said elec- ;tric discharge tube having an output circuit ina' control grid and an anode, said second tube having an output circuit including said second anode, means including said second tube for reducing the amplitude of said picture signals relative to said impulses, means for impressing said .composite signal upon the grid of said second tube Wherebysaid synchronizing impulses of relatively greater amplitude than said picture signals appear insaid second output circuit, and means for coupling said second `output circuit to said horizontal rdeflecting circuit.

2. The invention according to claim 1 char-acterizedin that the anode to cathode circuit of :the rst electric discharge tube has a substantially lowerimpedance to said horizontal synchronizing impulses than to said vertical synchronizing impulses, and further characterized in that .the anode to cathode circuit of the second electric discharge hasl a substantially lower impedance to said vertical` synchronizing impulses than to said horizontal synchronizing impulses.

3. In a television receiver, a signal channel for lthe reception of a composite signal comprising picture signals, horizontal synchronizing impulses, and vertical synchronizing impulses, a cathode ray tube, horizont-al. and vertical deecting circuits for said tube, means including a vacuum ltube having a grid circuit and a plate circuit for reducing the amplitude of said picture signals relative to said synchronizing impulses, said grid circuit being coupled to said signal channel and said plate circuit being coupled to said horizontal de'ecting circuit, said plate circuit having a substantially lower impedance to said vertical synchronizing impulses. than to said horizontal synchronizing impulses, and means including a second vacuum tube having a grid circuit and a plate circuit for reducing the amplitude of said picture signals relative to said synchronizing impulses, said second grid circuit being coupled .to said signal channel .and said second plate circuit ybeing coupled yto said vertical dellecting circuit, said second plate circuit having a substantially lower impedance to said horizontal synchronizing impulses than to said vertical synchronizing impulses.

` 4. The invention according to claim 3 characterized in that the grid .circuit of said rst Vacuum tube includes means'for attenuating said vertical synchronizing impulses more than said horizontal synchronizing impulses and further characterized in that the grid circuit of said second vacuum tube includes means for attenuating said horizontal synchronizing impulses more than said vertical synchronizing impulses.

WALDEMAR J. POCH.

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