US2219579A - Television apparatus - Google Patents

Description

Oa. i29, 1940.

'W. J. POCH TELEvIsioN APPARATUS Filed July 3l, 1935 2 Sheets-Sheet 1 Oct. 29, 1940. w J, POCH l 2,219,579 f TELEVISION APPARATUS Filed July 31, l935` '-2 Sheets-Sheet 2y RTEP VOL T065 0N PL@ TE Ac. ,4x/5 I c1. IPP/)v6 L VEL 0F .incasso/N6. 2065 I AI I I I I I I I I I I I I I I I I I I I ZWZVIaw/v e -y www Patented Oct. 29, 1940 PATENT OFFICE TELEVISION APPARATUS Waldemar J. Pooh, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware,

Application July 31,

io claims.

My invention relates to television apparatus,

and particularly to a method ofand means for Y obtaining proper synchronization between scanning at the transmitter and scanning at the reelvel.

In British Patent No. 407,499 there is described a television system in which scanning at the receiver is maintained in synchronism with scanning at the transmitter by transmitting a hori'- zontal synchronizing impulse of short duration at the end of each scanning line and a vertical impulse of the same amplitude and longer duration at the end of each picture frame. 'Ihe synchronizing impulses are of greater amplitude than the picture signals and preferably are in the black direction. An improved system for transmitting a composite signal of this character is described in application Serial No. 728,147, iiled May 29, 1934, patented Feb. 27, 1940, No. 2,192,121, in the name of A. V. Bedford, and assigned to the Radio Corporation of America.

In a television receiver for the reception of composite signals of the above-described'chanacter, the horizontal and vertical deilecting cir-A cuits which produce the deiiectlng currents or voltages for scanning at the receiver, and which are 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.v Such delecting circuits are described in the article by Holmes, Carlson and Tolson which 'was published in the Proceedings of the Institute of Radio Engineers for November, 1934, pages 1266 to 1285.

It has been found that the synchronizing oscillators in the deecting 'circuits operate best when the synchronizing impulses which are ap-A plied to them .are all of uniform amplitude. When the amplitude of these impulses is irregular, several objectionable results may be noticed. vUsuallylthe adjustment of the oscillator for causing it to lock in with the synchronizing impulses becomes more critical, the edges of the picture patternbecome irregular, and the picture has a. tendency to tear out at the point where the synchronizing impulses arel of minimum amplitude. The circuits which are generally preferred for separating the synchronizing impulses from the picture signals will not eliminate ir-` 1935, serial No. 93,956

of hum is introduced both at the transmitter and at the receiver. Irregularities in the height of the synchronizing impulses are sometimes caused by phase shift at low frequency. This phase shift may` occur at either the transmitter or the receiver or at both points.

It has also been found that, in the vertical deilecting circuit after the synchronizing lmpulses have been passed through the frequency separating circuit, the horizontal synchronizing impulses are still present at a low amplitude and appear super-imposed upon the vertical deiecting impulse when a picture is transmitted by the Y' above-mentioned Bedford system. Such irregularities in the vertical synchronizing impulse tend to produce objectionable results, especially in systems employing interlaced scanning. In such systems, irregularities on the vertical synchronizing impulses may cause the scanning lines to pair or they may completely destroy the interlacing.

. Accordingly, it is an object of my invention to provide an improved method of and means for applying synchronizing impulses of substantially uniform amplitude to the delecting circuits in a television receiver.

A further object of my invention is to provide an improved method of and means for impressing upon a vertical deecting circuit vertical syn- .chronizing impulses which are free from irregularities.

In one of the preferred embodiments of my invention, after the transmitted carrier wave has been received and demodulated, the composite signal comprising picture signals and synchronizing impulses is impressed upon a grid leak biased separating tube with the synchronizing impulses having a positive polarity. The synchronizing impulses are then impressed upon two frequency separating networks, one of which'permits the Vpassage of vertical synchronizing impulsesto a vacuum tube in the vertical deecting channel substantially to the exclusion of the horizontal synchronizing impulses, and the other of whichy permits the passage of the horizontal synchronizing impulses to a vacuum tube in the horizontal detlecting channel substantially to the exclusion of the vertical synchronizing impulses.

Preferably the above-mentioned vacuum tubes are so adjusted that the negative synchronizing impulses which are impressed upon their control grids drive them beyond cut-oil?. This adjustment may be accomplished by applying a voltage to the plates of the tubes which is much less than the rated voltage for the tubes.

In the above-described circuit the separating tube is so negatively biased that it clips the synchronizing impulses at a level near their base while the succeeding' amplifier tubes clip them at a level spaced away from the first clipping level whereby all synchronizing impulses are given the same amplitude.

Other objects, features, and advantages vof my invention will appear from the following description taken in connection with the accompanying drawings, in which Figure 1 is a circuit diagram of a television receiver embodying myinvention;

Figures 2, 3 and 4 are curves which are referred to in explaining my invention; and

Figures 5 and 6 are circuit diagrams of modications ol a portion of the circuit shown in Figure 1.

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 carrier wave in accordance with the teachings of the above-mentioned British patent or Bedford application. The receiving apparatus includes a radio receiver I which selects and demodulates the desired carrier wave whereby picture signals and synchronizing 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 re- `sistance coupled amplifiers 2 and 3 and are impressed upon the control grid 4 of a cathode ray tube 6. 'I'he cathode ray tube 6 may be of Well known construction comprising an evacuated envelope having therein a cathode 1, the control grid 4, a rst anode 8, anda second anode 9. Suitable defiecting means, such as defiecting coils `I I and I2 are provided for deflecting the cathode ray both horizontally and vertically to effect scanning of a fluorescent screen I3 at the end of the tube. v

The horizontal defiecting coils II are supplied with saw-tooth current having comparatively high frequency from a horizontal deflecting circuit I4. The vertical deflecting coils I2 are supplied with saw-tooth current having a compara-l tively low frequency from a vertical defiecting circuit I 6. The deflecting circuits may be any of the designs described in the above-mentioned article, such a circuit including, for example, a blocking oscillator which can be locked in step with synchronizing impulses impressed thereon. The number of amplifier stages in the radio receiver I is so selected that the polarity of the synchronizing impulses impressed upon the control grid 4 of the cathode ray tube 6 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 fluorescent screen during the return line period.

'I'he scanning of the fluorescent screen I3 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 defiecting circuits I4 and I6. The separation of the picture and synchronizing signals preferably is accomplished by means of a circuit comprising an electric discharge tube I1 having a cathode I8, a control grid I! and a plate 2I 'I'he control grid I9 is connected through a grid resistor 22 to the cathode I8, a negative bias being maintained on the grid I9 by means of the synchronizplate voltage preferably having a value considerably below the rated plate voltage for the tube. 'I'he separating circuit just described is the same as that described and claimed in application Serial No. 23,136, filed May 24, 1935, in the name of William A. Toison and assigned to the Radio Corporation of America. As previously mentioned, the separating tube I1 is so biased that it clips off the picture signals and the lower part of the synchronizing impulses.

While the separating circuit illustrated is the preferred one, other circuits may be employed such as a vacuum tube provided with a self-bias resistor in the cathode lead. Preferably, any separating circuit employed should be of the type in which the bias on the clipping tube varies automatically in response to changes in the amplitude of the synchronizing impulses whereby the synchronizing impulses occurring over a number of picture frames are leveled It may be noted that one cause of the variation in the average height of synchronizing impulses in one picture as compared with their average height in a different picture frame (which variation makes leveling desirable) is the change in the average illumination or background of successive picture frames.

The vertical synchronizing impulses and the horizontal. synchronizing impulses are separated from each other by means of filters, the vertical synchronizing impulses being supplied to the vertical defiecting circuit I6 through a vacuum tube 26, and the horizontal synchronizing impulses being supplied to the horizontal deflecting circuit I4 through a vacuum tube 21. k

The vacuum tube 2 6, which may be of the threeelement type, has in its input circuit a resistor 28 of high resistance next to the grid 25 and a resistor 29 having comparatively low resistance across which the synchronizing impulses are impressed. These impulses are supplied from the tube I1 through a coupling condenser 3|. 'Ihe input circuit also preferably includes a biasing battery 32 orother biasing means for applying a positive biasing potential to the control grid 25. A filter condenser 33 is connected across the input electrodes of the vacuum tube 26, this condenser, in conjunction with the resistor 28, serving to substantially eliminate the horizontal synchronizing impulses whereby only the vertical synchronizing impulses are impressed upon the vertical deflecting circuit I6.

The input circuit of the vacuum tube 26, in addition to serving as a filter, will function to eliminate any positive polarity picture signals which may have passed through the separating tube I1. The particular-input circuit for the tube is described and claimed in my copending application, Serial No. 24,207, filed May 31, 1935, and assigned to the Radio Corporation of America.

The input circuit of the vacuum tube 21 includes a grid resistor 34 connected between the grid 36 and the cathode 31,l the tube preferably being operated with the grid at zero bias in order to remove noise or picture signal which may pass through the separating tube I1. The grid 36 is coupled to the output circuit of the separa-ting tube I1 through a condenser 38 having such small capacity that it presents a high impedance to the lower frequency vertical synchronizing impulses. Therefore, because of the iilter action of the condenser 38 and the grid resistor 34, only the horizontal synchronizing impulses appear in the output circuit of the tube 21 and are impressed upon the horizontal deecting circuit I4.

While it is preferred to separate the vertical synchronizing impulses from the-horizontal synchronizing impulses at the particular point shown,

it should be understood that such separation may be done at other points in the circuit. For example,.a single clipping tube may follow the tube I1 and the separation of synchronizing impulses done in the plate circuit of this second clipping tube.

In accordance with my invention, a direct current voltage is applied to the plate of the tube 26 and to the plate of the tube 21 which is low compared with the rated plate voltage. The plate voltage for these tubes is made so low that the synchronizing impulses of minimum amplitude drive the tubes beyond the cut-off point.

Thus, the synchronizing impulses are clipped the secondV time with the result that they are impressed upon the deflecting circuits with uniform amplitude. y

The action of the above described circuit lwill be better understood by referring to curves and Figures 2', 3 and 4.

Referring to Figure 2, the plate current-grid voltage characteristic curves of the vacuum tube 2B for three different values of plate voltage are illustrated. The curve 4I represents the characteristic which is utilized in the circuit as Vdescribed. The curve 42 represents a verticall synchronizing impulse as it appears when impressed upon the grid 25 of the vacuum tube 26. It will be noted that the horizontal synchronizing impulses have not been entirely eliminated by the filter but that they appear as irregularities upon the vertical defiecting impulse. By applying a slight positive bias to the grid, as indicated, the irregularities appearing near the base of the impulse are clipped off since they drive the grid 25 more positive and cause the grid-cathode impedance of the tube 26 to become -very low. This action is described and claimed inlmy abovementioned copending application. The irregularities appearing near thetop of the impulse 42 are removed since the portion of the impulse .located on the left of the dotted line marked cut-olf drives the tube vbeyond cut-oil' as a result of the low plate voltage.

The impulse which appears in the output cirycuit of the tube 26 and which is impressed upon the oscillator in the vertical deiiecting circuit I6 is indicated at 43. It will be noted that this iinpulse is free from irregularities.

. Referring to Figures -3 and 4, these figures ssynabove-mentioned Bedford application. This line,

therefore,A represents black in the picture. 'I'he impulses appearingv in the region A are the double frequencyhorizontal synchronizing impulses and the slotted vertical synchronizing impulse generated by the Bedford transmitter. -The impulses appearing in the region B above the line 44 are C are the picture signals. In Figure 3 the com- Y posite signal is shown with a certain amount of 60 cycle hum in it as indicated by the envelope of the signal.

The composite signal is clipped at the level v indicated by the lower dotted line for removing the picture signal from the synchronizing impulse. This clipping is done. as previously described, by the separating tube ITI.l The synchronizing impulses which appear in the output circuit of the separating tube I1 after this iirst clipping operation are theimpulses above this dotted line and it will be noted that they are of varying amplitude. their polarity has been reversed by the tube I1. vIn order` to make them of uniform amplitude they arek again .clipped atthe level indicated bythe upper dotted line. -In the vertical deflecting channel this clipping is done by the vacuuml tube 26 as .previously described. In the horizontal deflecting channel the second clipping is done by the vacuum tube 21. It will be apparent from the foreggmg description that by the time the vertical synchronizing impulses and the horizontal synchronizing impulses reach the vertical deflecting circuit and the horizontal defiectin'g circuit,

the curve shown in Figure 3 except that the effect of a phase 'shift at the low frequencies has been indicated. It will be noted that by clipping a composite signal of this character inthe same way as described invconnection with Figure 3, synchronizing impulses of uniform amplitude are supplied to the deilecting circuit.,

If desired, either of thecircuits shown in Figures5 and 6 may be substituted for the vacuum tube 21 and its associated circuit shown in Figure l. Like parts' in the three figures are indicated by the same reference numerals. In the circuit shown in Figure 5, a vacuum tube 46 has a negative bias applied to its grid 41 Ato prevent flow of grid current while a low voltage is applied to its plate 48. This plate voltage is made so low that the synchronizing impulses of minimum :amplitude drive the tube beyond the cut-off point.

In Figure 6 a screen Agrid tube -5I is employed, the control grid 52 being biased negatively a small amount and the plate 53l having normal voltage applied thereto. `The screen grid 54, however, is supplied with ,a positive voltage which is small enough to permit synchronizing impulses of minimum amplitudeto drive the tube beyond cut-off,

It may be noted that in Figure 1 the vacuum tube 2,6 also may be of the screen grid type having normal voltage applied to its plate and having avnaage appned to its screen grid which is so much lower than the rated screen grid voltage that the lowest amplitude synchronizing impulses drive the tube beyond the cut-oil point.

'Ihe values of certain circuit elements have been Vindicated in ohms, megohms, and microfarads,

merely by way of example. Likewise, other types of vacuum tubesl than those indicated may be employed'. The tube voltages are not cri-tical and may be given values varying considerably from those indicated on the drawing. The specific circuit illustrated is designed forthe reception of a composite signal in which vertical synchroniz- It will be understood that` ing impulses occur at the rate of 60 per second u and in which horizontal synchronizing impulses occur at the rate of 10,290 per second. f

From the foregoing description it will be apparent that various modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim as my invention:

1. In a television receiver for thereception of a composite signal comprising picture signals and synchronizing signals, part of the picture signal voltage being of opposite polarity to the synchronizing signal voltage with respect to the alter-I nating current axis of said composite signal, means foi` so clipping said composite signal near the base of said synchronizing signals that the greater part of said picture signals are elimiknated, means for next clipping oif the tops of said synchronizing signals whereby synchronizing signals of substantially uniform amplitude are produced, a deflecting circuit for a cathode ray tube, and means for impressing said uniform amplitude synchronizing signals upon said deflecting circuit.

2. In a television receiver for the reception of a composite signal comprising picture signals, Vertical synchronizing' impulses and horizontal synchronizing impulses, part of the picturasignal voltage being of opposite polarity to the synchronizing impulse voltage with respect to the alternating current axis of said composite signal, means for so clipping said composite signal near the base of said synchronizing impulses that the greater part of said picture signals are eliminated, means for next separating said vertical synchronizing impulses and said horizontal synchronizing impulses, a vertical deiiecting circuit for a cathode ray tube, means for clipping oi the tops of said vertical synchronizing impulses after they have been separated and impressing themupon said vertical deflecting circuit, a horizontal deflecting circuit for said cathode ray tube, and meansy for clipping oiT the tops of said horizontal synchronizing impulses after they have been s'eparated and impressing them upon said horizontal deflecting circuit.

3. In a television receiver for the reception of a composite signal comprising picture signals, and synchronizing impulses, said impulses being of greateramplitude than said signals, means for clipping said impulses near their base, means for clipping oi the tops of said impulses, a deflecting circuit for a cathode ray tube, and means for impressing said twice clipped impulses upon said deflecting circuit.

4. In a television receiver for the reception of a composite signal comprising picture signals and synchronizing impulses, said impulses being of greater amplitude than said signals, means including an amplifier tube for clipping 'oil the picture signals from said composite signal whereby said impulses are clipped near their base, means including a second amplifier tube for clipping off the tops of said impulses, a deilec-ting circuit for a cathode ray tube, and means for impressing said twice clipped impulses upon said deilecting circuit.

5. In a television receiver for the reception of a composite signal comprising picture signals and both vertical and horizontal synchronizing impulses, said impulses being of greater amplitude than said signals, means for clipping oil? the picture signals from said composite signal whereby said impulses are clipped near their base, means for substantially isolating said clipped vertical synchronizing impulses and said clipped horizontal synchronizing impulses,A means for clipping off the tops of said isolated vertical synchronizing impulses, a vertical defiecting circuit for a cathode ray tube, means for impressing said twice clipped vertical synchronizing impulses upon said vertical deflecting circuit, means for clipping offthe tops of said isolated horizontal synchronizing impulses, a' horizontal deflecting circuit for said cathode ray tube, and means for impressing said twice clipped horizontal synchronizing impulses upon said horizontal deflecting circuit.

6. In a television receiver for the reception of a composite signal comprising picture signals and both vertical-and horizontal synchronizing impulses, said impulses being of greater amplitude than said signals and said horizontal synchroniz- .ing impulses occurring at a greater frequency than said vertical synchronizing impulses, means for separating at least the'greater part of said picture signals from said composite signal, means for next filtering said vertical synchronizing impulses from said separated impulses, means for so clipping oil the base and the top of each illtered vertical synchronizing impulse that any irregularities thereon due to imperfect filtering are removed to produce an impulse free from irregularities, a vertical deflecting circuit for a cathode ray tube, and means for impressing said irregularity free impulses upon said vertical deiiecting circuit.

7. In a television receiver, a circuit for clipping synchronizing impulses, said circuit comprising a vacuum tube having a cathode, a grid and an anode, said tube having an output circuit including said anode and a plate resistor, said tube having input circuit including a resistor section, and a second resistor section and a biasing means all connected in series between said grid and said cathode, the rst of said resistor sections being next to said grid, thev polarity of said biasing means being such as to bias said grid positively, means for applying a positive voltage through said plate resistor to said anode which is low in value compared with the rated anode voltage for said tube, and means for impressing said synchronizing impulses across said second resistor section with the synchronizing impulses in the negative direction and of such amplitude that they drive said tube beyond its cut-oil point.

8. In a television receiver for the reception of a composite signal comprising picture signals and synchronizing signals, said composite signal having an alternating current axis and said cynchronizing signals having a greater amplitude of picture signals of like polarity, means for so clipping said composite signal near the base of said synchronizing signals that the greater part of said picture signals are eliminated, said means comprising an electric discharge tube having an electron emissive electrode and a cold electrode and further comprising a condenser and a resistor for applying a potential across said electrodes which increases in value in response to an increase in amplitude of said synchronizing signals, means for next clipping off the tops of said synchronizing signals whereby synchronizing signals of substantially uniform amplitude are produced, a deflecting circuit for a cathode ray tube, and means for impressingv said uniform amplitude synchronizing signals upon said deflecting circuit.

9. The invention according to claim 8 characterized in that said means for clipping oil' the tops of said synchronizing signals comprises a vacuum tube having a cathode, a grid and an anode, said tube having an output circuit including said `anode and having an input circuit including a resistor section, a second resistor section and biasing means all connected in series between said grid and said cathode, the rst of said resistor sections beingnext to said grid, the polarity of said biasing means being such as to bias said grid positively, and means for applying a positive voltage to said anode which is low in value as compared with the rated anode voltage for said- 'ing impulses when it is impressed thereupon, a

vacuum tube following said separating circuit and having a cathode, a grid and an anode, said tube having an output circuit including said anode and a plate resistor, said tube having an input circuit including a ilrst "resistor section arid a second resistor section and a source of biasing potential connected in series with each other and connected between said grid and said cathode, the iirst of said resistor sections being next to said grid, the polarity of said biasing potential beingsuch as to bias said grid positively, a condenser connected between said grid and said cathode, means for applying a positive voltage through said plate resistor to said anode which is low in value compared with the rated anode voltage for said tube, and means for impressing said synchronizing impulses across said second resistor section with the synchronizing impulses in the'negative direction and of such amplitude that -they drive said tube

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