US2299361A - Television system - Google Patents

Description

Oct. 20,1942. w, o soN 2,299,361

TELEVISION SYSTEM Filed ne 29, 19:59 4 Sheetls-Sheet 1 H0472. 04-7-1. C/AGU/T li 057. 4 asc/unme 1 (Ittorneg Oct. 20, 1942. w. A. TOLSON TELEVISION SYSTEM Filed Dec. 29, 1939 4 Sheets-Sheet 2 Oct. 20, 1942. w, TOLSON 2,299,361

' TELEVISION SYSTEM Filed Dec. 29, 1959 4 Sheets-Sheet 4 P bi Zhwentor attorney Patented Oct. 20,1942 I TELEVISION SYSTEM William A. Tolson .Westmont, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 29, 1939, Serial No. 311,653

11 Claims. (Cl. 178-75) My invention relates to cathode ray tube ap-- paratus and particularly to television receivers of the type in which horizontal and vertical defleeting circuits are employed for causing a cathode ray' to scan a fluorescent screen or the like. In television receivers the cathode ray is caused to scan the screen in synchronism with the scanning at the transmitter by transmitting horizon-- tal and vertical synchronizing pulses along with the picture signals. In the type of television system which is preferred at the present time, the synchronizing pulses are so transmitted that the scanning is interlaced by the so-called odd-line method. This method of interlacing is described in Ballard Patent 2,152,234, issued March 28, 1939, and assigned to the Radio Corporation of America.

It has been found that one of the major causes of improper interlacing is cross talkfrom the horizontal deflecting circuit into the vertical deflecting circuit. Specifically, if pulses from the horizontal deflecting circuit appear in the vertical deflecting circuit with suii'icient amplitude or energy content, they will directly aifect the operation of the oscillator ordinarily employed in the-vertical deflecting circuit. However, these Radio Corporation of'America, or in systems including the type or receiver described hereinafter in which'the vertical deflecting circuit is caused to synchronize on the back edge of a vertical synchronizing pulse.

It is an object of my invention to reduce the effect of cross-talk between horizontal and vertical deflecting circuits in cathode ray tube ap-' paratus on the vertical deflection of the cathode ray.

A further object of my invention is to provid an improved television receiver. 4

A. still further object of my invention is to pro- I vide in a television receiver means for and a method of improving the interlacing of a reproduced picture.

pulses from the horizontal deflecting circuit can affect the-vertical deflecting circuit oscillator in a this way only during its period of susceptibility. By periodof susceptibility is meant that time during which the oscillator is drawingplate current. This is the period during which any voltage applied to the oscillator input circuit will appear directlyin its output circuit.

Cross-talk between deflecting circuits may disturb the interlacing in another way in television receivers of the type wherein frequency separation of the horizontal and vertical synchronizing pulses is utilized. quency separation means generally includes an integration circuit through which the vertical synchronizing pulses are fed to the vertical defleeting circuit. Such an integration circuit may carry over into the period of susceptibility the efiect of some of the horizontal synchronizing pulses preceding this period. This undesirable effect usually is not serious in properly designed systems and it may be avoided entirely in certaintypes of systems, for example, in systems employing amplitude separation of horizontal and vertical synchronizing pulses, in systems utilizing different steepness of wave fronts of synchronizing pulses, suchas that decribed in application Serial No. 180,493, filed December 18, 1937, in the name of Aida V. Bedford and assigned to the In such receivers the fre-' In accordance with my invention I reduce the duration of the period of susceptibility oi the vertical deflecting circuit oscillator to a time which is so short that a horizontal synchronizing pulse or a pulse from the horizontal deflecting circuit will not appear during this period. Therefore, the first mentioned cause of poor interlacing due to cross-talk is eliminated.

This is advantageous even though some synchronizing pulses are fed into the vertical deflecting circuit through an integrating circuit. However, if, when practicing my invention, the use of an integrating circuit is avoided, there will be no disturbance ofthe interlacing even though no particular care has been taken to keep the horizontal synchronizing or deflecting circuit pulses out of the vertical deflecting circuit. I

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a circuit diagram of a television receiver embodying my invention,

Figures 2a, 2b, 2c, 2d and 2e are curves which are referred to in explaining the invention,

Figure 3 is a circuit diagram of another embodiment of my invention, and

Figures 4a, 4b, 4c, 4d and 4e are curves which are referred to in explaining the operation of the circuit of Fig. 3.

Referring to Fig. 1, the invention is plied to a 'television receiver of the type designed, in the example illustrated, to receive television signals transmitted in. accordance with the present R. M. A. standards in the United States. This signal includes horizontal synchronizing pulses which occur at the end'of each scanning line and at the rate of 13,230 per show'n apsecond, and framing pulses which occur at the end of each picture frame and at the rate of 60 per second, this being referred to as the field frequency. The framing pulses are of the same amplitude as the horizontal synchronizing pulses but they are of longer duration (and slotted) whereby the two groups, of synchronizing pulses may be separated from each other by means of frequency discriminating circuits. The received signal has the odd line relation between the horizontal and vertical synchronizing pulses as taught by the above mentioned Ballard patent in order to produce interlaced scanning. Each slotted vertical synchronizing pulse is preceded and followed by several pulses occurring at double the line frequency.

- The receiver may comprise the usual elements of a superheterodyne receiver, as indicated by the block l0, and the usual push-pull second.

detector Il including the diodes l2 and I3. The second detector supplies the picture signal and synchronizing pulses through a video amplifier I4 to a cathode ra tube l6 of conventional construction. In the example illustrated, horizontal deflecting coils l1 and vertical deflecting coils l8 are provided for causing the cathode ray to scan the fluorescent screen 19 at the end of the tube.

Signal is also fed from the output of the second detector II to a suitable separating circuit 2| which may comprise a grid leak biased tube 22 for passing the. synchronizing pulses to. the

' synchronizing pulses and of the pulses comprising the slotted vertical synchronizing signal (shown in Fig. 2a) produce a voltage of any substantial amplitude across the inductance coil 24, there is substantially no vertical synchronizing pulse applied to the horizontal deflecting circuit.

The horizontal deflecting circuit may be of conventional design comprising a suitable oscillator 3| such as a blocking oscillator of the type described in 'Tol'son and Duncan Patent 2,101,520,.

issued December 7, 1937, and assigned to the Ra dio Corporation of America.

The blocking oscillator 3| produces a sharp voltage pulse of large amplitude in response to the occurrence of each horizontal synchronizing pulse (but in response to alternate pulses only where they occur at double the line frequency); and this blocking oscillator pulse is impressed upon the grid of a discharge tube 32 to discharge a condenser 33 across which a saw-tooth voltage wave is produced. It will be understood that the condenser 33 is charged through the plate resistor 34 of the discharge tube at a comparatively slow rate in the usual manner to produce the desired saw-tooth. The usual peaking resistor 36 is connected. in series with the condenser 33 whereby a combined saw-tooth wave and impulse wave is impressed upon the grid of the output tube 31. As is well known in the art, such an impulse component is desirable for the production of a good saw-tooth current wave through deflecting coils where the output tube is of the low impedance type. p

In order to 'synchronizethe vertical deflecting circuit in this embodiment of'the invention, synchronizing pulses are also fed from the output circuit of the separating tube 22 through an amplifier 4| and through an integrating circuit 42 to a suitable oscillator 43 in the vertical deflecting circuit, this oscillator being of the abovementioned blocking oscillator type in theexample illustrated. The integrating circuit may comprise the plate resistor 44 of the tube 4| and a shunt condenser 46 whereby there appears across the condenser 46 a voltage wave of the character indicated in Fig. 217. As is well understood in the art, when the synchronizing voltage, as represented by the curve of Fig. 2b, reaches a certain predetermined amplitude it triggers ofi the oscillator whereby it goes through its cycle synchronizing pulse 12 as indicated in Figs. 2b

and 2c.

Voltage may be applied to the plate of the oscillator tube through the usual filter resistor 40,. there being the usual filter condenser 45.

The positive voltage pulse produced during the time t is impressed upon a discharge tube 41 to quickly discharge a condenser 48 which has been gradually charged in the usual manner through the plate resistor 49 of the discharge tube. Since this circuit, like the horizontal deflecting circuit, includes a low impedance output tube 5| for supplying saw-tooth current to the deflecting coils I8, there is included a peaking resistor 52 inseries with the saw-tooth condenser.

A condenser 53 is connected across the peaking resistor 52 in order to-increase the length of time that the output tube is held at plate current cut off. This has been found necessary or at least desirable because of the extremely short duration of the period t when practicing my invention.

By referring to the curve shown in Figs. 2a to 2e, it will be seen that the period t of the blocking oscillator, that is, its period of susceptibility, has been made so short that if the blocking oscillator is adjusted to trigger 01f at the time indicated by the dotted line .1: immediately following a horizontal pulse in the horizontal deflect The period t when given the duration above specified .is much .shorter than previously employed in vertical deflecting circuits. It will be understood that the oscillator 43 is self-oscillatory, that is, it oscillates at a certain free runused in the horizontal deflecting circuit and may have a natural period of the order of 150 kilo-- e e When practicing-my invention it is preferable to give the integration'circuit a shorter time constant than commonly employed whereby the front or leading edge of the vertical synchronizing pulse is comparatively steep as shown at v in Fig. 2b. In this way any variation in the time i: at which the oscillator is triggered off is held I to a minimum. It'will be noted that the second and third triangular portions of the integrated vertical synchronizing signal are inefiective inthe receiver and occur only because the'standard :elevision signal with long slotted vertical pulses is being received.

It will be understood that Figs. 2:! and 2e represent primarily the timing of those pulses or voltage kicks occurring in the horizontal defleeting circuit which feed into the vertical deflecting circuit through wiring capacity, stray magnetic fields, etc., to give the cross-talk difliculties above-described, and that no attempt has been made to indicate accurately either the amplitude or the wave shape. ,The pulses in Fig. 2d

are the evenline pulses corresponding to, the

synchronizing pulses shown in Fig. 211 while the pulses, in Fig. 2e are the odd line" pulses, the corresponding snychronizing pulses not being shown.

Referring to Fig. 3; there is shown an embodiment of my inventionin which the vertical defleeting circuit is synchronized on the back side of the firstportion of the slotted vertical synchronizing pulse, Thisembodiment comprises a television receiver which may comprise the usual radio receiver 56 which supplies the demodulated picture and synchronizing signal to a cathode ray tube 51 through 'a video amplifier 58.

It is assumed that the receiver is for the reception of the standard signal previously described. The horizontal and vertical synchronizing signals as they appear at successive onesixtieth second intervals are shown in Figs. 4a and 4b, respectively, the picture signal not being indicated.

, A portion of the amplified received signal is suplied to a grid leak biased separating tube 59 which supplies the synchronizing pulses only to an amplifier tube 6|. The amplified pulses are then passed through a condenser 62 and res'istors. 83 and 64 in series whereby, as a result .of condenser 62 charging rapidly through tube 6| but discharging slowly throughthe plate resistor 66, there appears across the resistor 64 the voltage waves shown in Figs. 4c and 4d. These voltage waves are, of course, produced by the synchronizing pulses shown in Figs. 4a. and 4b, respectively.

The voltage appearing across the resistor 64 is applied to the oscillator 61 which, in the example illustrated, is'of the blocking oscillator typeand a duplicate of the oscillator 43 of Fig. 1. The

voltage appearing on the grid of this oscillator, when it 'is'triggered oil by the voltage wave of the blocking oscillator 12 may be the same as Fig. 4c, is shown in Fig. 4e. In accordance'with my invention, the period of susceptibility t is made less than the time 1' where r is the time between the instant the vertical oscillator is triggered off by the back edge of a pulse and the instant the horizontal oscillator is triggered off by the front edge of the next succeeding pulse. Thus, in this embodiment of the nvention, as in the first one, the horizontal deflecting circuit pulses do not occur-during the period of susceptibility of the oscillator.

It should be understood that the invention applies where oscillatorsother than those of the blocking type are utilized. For example, a dynatron oscillator or multivibrator may be substituted for the blocking oscillator, although the blocking oscillator is preferred.

The remaining portion of the vertical deflecting circuit is indicated by the block H and may be the same as the corresponding portion of the circuit shown in Fig. 1. I

The horizontal deflecting circuit which includes that shown in Fig. 1. In the example illustrated, no attempt is made to prevent the vertical synchronizing pulses from being applied to the horizontal deflecting circuit.

As a refinement of the invention, the synchronizing pulses 'may bepassedthrough an inductance coil I3 to accentuate the low frequency components of the synchronizing signals by the same amount they are attenuated as a result of the network 62B3-64 being across the plate circuit of the amplifier 6|. The coil 63 is in series with a blocking condenser 14 and the resistors I 16 and 11.

' circuit be fiat "at the input circuit'of the oscillator 12, the circuit constants may be given the .following relation for a frequency of 16,000

cycles:

Xz=Xt=R at, a certain frequency such as 16,000 cycles per second, where XL is the reactance of coil 13, XI: is the reactance of condenser 62, and R is the resistance of resistors 16 and 11 in series and of 63 and 64 in series. The frequency of 16,000 per second is taken as the frequency at .which X1. and Xe should be equal because of the following considerations: It is assumed that the condenser 62 and associated discharge circuit should have the proper values to make the condenser 62 lose approximately 60 percent of its charge in-the time required for the cathode ray to traverse one scanning line. This time is 'less than 5 of a second (because about 20 percent of the time is used for blanking) and may be assumed to be about $5 of a second.

Merely by way of example, I have indicated the values of various circuit elements, these values being in ohms, megohms, microfarads, micro-microfarads and henrys.

I claim as my invention:

1. A television receiver for the-reception of picture signals, horizontal synchronizing pulses occuring at the-end of each scanning line, and vertical synchronizing pulses occurring at the end of each picture frame, said receiver comprising horizontal and vertical cathode ray deflect synchronizing pulses whereby cross-talk-from the horizontal deflecting circuit to the vertical defleeting circuit is minimized.

2. A television receiver for the reception of picture signals, horizontal synchronizing pulses occurring at the end of each scanning line, and vertical synchronizing pulses occurring at the end of each picture frame, said receiver compris-.

ing horizontal and vertical cathode ray deflecting circuits, said vertical deflecting circuit including only one oscillator, the said oscillator being adjusted to be self-oscillatory or free running in the absence of synchronizing pulses and having a period of susceptibility which is shorter than the time between successive horizontal synchronizing pulses whereby cross-talk from the horizontal deflecting circuit to'the vertical deflecting circuit is minimized. l

3. A television receiver for the reception of picture signals, horizontal synchronizing pulses occurring at the end of each scanning line, and vertical synchronizing pulses occurring at the end of each picture frame, said receiver comprising horizontal and vertical cathode ray deflecting circuits, each deflecting circuit including an oscillator, the oscillator in said vertical deflecting circuit being adjusted to be self-oscillatory or free running in the absence of synchronizing pulses and having a period of susceptibility which is shorter than the time between the instant the vertical deflecting oscillator is triggered ofl and the next succeeding time the horizontal deflecting oscillator is triggered off whereby cross-talk fromthe horizontal deflecting circuit free running in the absence of synchronizing pulses and which has a period of susceptibility which is shorter than the time between successive horizontal synchronizing pulses whereby cross-talk from the horizontal deflecting circuit to the vertical deflecting circuit is minimized.

5. A television receiver for the reception of a composite signal comprising picture signals, horizontal synchronizing pulses occurring at the end of each scanning line and vertical synchronizing pulses occurring at the end of each picture irame, said horizontal and vertical pulses occurring in odd-line relation to produce interlaced scanning,

I said receiver comprising horizontal and vertical cathode ray deflecting circuits, said vertical deflecting circuit including an oscillator which is -free running in the absence of synchronizing pulses and which has a period of susceptibility which is shorter than the duration of a-horizontal synchronizing pulse whereby cross-talk from the horizontal deflecting circuit to the vertical deflecting circuit is minimized.

6. A television receiver for the reception of a composite signal comprising picture signals, av

horizontal synchronizing pulse occurring at the end of each scanning line and a slotted vertical synchronizing pulse occurring at the end of each picture frame, said horizontal and vertical pulses occurring in odd-line relation to produce interlaced scanning, said receiver comprising :horizontal and vertical cathode ray deflecting cir- 7 5 cuits, said vertical deflecting circuit includingan oscillator which is free running in the absence of synchronizing pulses and which has a period of susceptibility which is shorter than the duration of a slot in said vertical synchronizing'pulse whereby cross-talk from the horizontal deflecting circuit to the vertical deflecting circuit is minimized.

7. A television receiver for the reception of a' composite signal comprising picture signals, horizontal synchronizing pulses occurring at the end of each scanning line and vertical synchronizing pulses occurring at the end of each picture frame, said horizontal and vertical pulses occurring in odd-line relation to produce interlaced scanning, said receiver comprising horizontal and vertical cathode ray deflecting circuits, each deflecting circuit including an oscillator which is free running in the absence of synchronizing pulses, the vertical deflecting oscillator having a period of susceptibility which is shorter than the time between the instant the vertical deflecting oscillator is triggered ofl and thenext succeeding time the horizontal deflecting oscillator is trigged ofi whereby cross-talk from the horizontal deflecting circuit to the vertical deflecting circuit is minimized.

8. A television receiver for the reception of a composite signal comprising picture signals, horizontal synchronizing pulses occurring at the end of each scanning line and vertical synchronizing pulses occurring at the end of each picture frame, said horizontal and vertical pulses occurring inodd-line relation to produce interlaced scanning,

said'receiver comprising. horizontal and vertical cathode ray deflecting circuits, said vertical defleeting circuit including a condenser across which a saw-tooth voltage is to appear, means for charging said condenser comparatively slowly, and means for discharging said condenser comparatively rapidly, said last means compris ing an oscillator including a vacuum tube, said oscillator being so adjusted that said tube draws 'plate current for a period which is shorter than the period between successive horizontal syncronizing pulses whereby cross-talk from the horizontal deflecting circuit to the vertical deflecting'circuit is minimized.

9. A television receiver for the reception of a composite signal comprising picture signals,

horizontal synchronizing pulses occurring at the end of each scanning line and vertical synchronizing pulses occurring at the end of each picture frame, said horizontal and vertical pulses occurring in odd-line relation to produce interlaced scanning, said vertical pulses being slotted and of greater duration than said horizontal pulses, said receiver comprising horizontal and vertical cathode ray deflecting circuits, said verticaldefleeting circuit including an oscillator which is free running in the, absence of synchronizing pulses and which has a period of susceptibility which is shorter than the time between successive horizontal. synchronizing pulses whereby cross-talk from the horizontal deflecting circuit to the vertical deflecting circuit is minimized, and means comprising an integrating circuit forimpressing the vertical synchronizing pulses on said oscillator substantially to the exclusion of said horizontal synchronizing pulses, the. charging and discharging time constants of said integrating circuit being such that the voltage thereacross rises to a comparatively large value before the occurrence of the first slot in said vertical pulse and drops to a comparatively low value during the occurrence of said first slot.

10. A television receiver for the reception of a composite signal comprising picture signals, horizontal synchronizing pulses occurring at the end. 5

of each scanning line and vertical synchronizing the absence of synchronizing pulses and which 15 has a period of susceptibility which is shorter than the time between successive horizontal synchronizing pulses whereby cross-talk from the horizontal defiecting circuit to the vertical defiecting circuit is minimized, and means for causing said oscillator to synchronize on the back edge of the first portion of the slotted vertical synchronizing pulse.

11. The method of producing an interlaced picture in a television receiver of the type including a horizontal deflecting circuit and including a vertical deflecting circuit having an oscillator therein which is free running in the absence of synchronizing pulses which method comprises receiving a signal having horizontal and vertical synchronizing pulses occurring in odd-line relation, and operating said oscillator with a period of susceptibility which is less than the period between successive horizontal synchronizing pulses whereby cros's-talk from the horizontal deflecting circuit to the vertical deflecting circuit is minimized.

WILLIAM A. TOLSON.

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