US2313915A - Synchronizing circuit for television receivers - Google Patents

Description

March 16, 1943. A. V. BEDFORD 2,313,915 'SYHCHRONIZING CIRCUIT FOR TELEVISION RECEIVERS I 3 Sheets-Shee t s I Filed July 31, 1940 Fi e. 4. I

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'I'O WET/(15L Zinuento'r v Clttbmeg Patented Mai-.16, 1943 I SYNCHRONIZING CIRCUIT FOR- TELEVI SION RECEIVERS Aida of Delaware V. Bedford, Collinmood, N, 1., assig'nor to Radio Corporation of America, a corporation Application July :1, 1940, Serial No. 348,878 4 Claims. ((2.178-75) This invention relates to television circuits, and more particularly to synchronizing circuits for television receivers.

In receiving television signals which include pulses for maintaining the operating action of the receiver in synchronism with the transmitter, the disturbances of static and man-made interference sometimes cause the receiver to fall out of synchronism with the transmitter. This trouble is experienced when the strength of the incoming signals drop to a level at which the eflect of the interference on the receiving apparatus is enough to disturb the frequency of the cathode ray deflection oscillator. I I

It has been discovered that by reducing the level of the synchronizing pulses applied to the deflection oscillators when receiving television signals of low strength, the oscillator will not be greatly disturbed by the interference. The principal reason for this improvement is probably tude of synchronizing signal should be applied to the oscillator since the percentage of inter-' ference is lower and, under these conditions, the adjustments of the oscillators are not critical.

According to this invention, an optimum level of synchronizing pulses is maintained over a large range of incoming signal strength by automaticaily controlling the gain of the synchronizing signal channels so that, under conditions of low incoming signal strength, the gain of the an automatic control of the gain in the synchronizing signalchannels.

Still another object of this invention is to provide a method and means for decreasing the amplitude of the synchronizing signals under conditions" of lowsignal strength and increasingtheir amplitude under conditions of high signal strength.

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of the followingspecification and an inspection of the accompanying drawings, in which Figure 1 is a circuit diagram showing one form ofthis invention,

, Figure 2 is a circuit diagram showing another form of this invention, 7

Figure 3 is a graphical illustration of the operation of the invention as shown in Fig; 2, and

Figure 4 is a circuit diagram showing a further form of this invention.

Referring to Fig. 1, a portion of a television receiver, for example, one such as described by E. W. Engstrom and R. S. Holmes in Electronics Magazine for April 1939, page 23, is shown having an intermediate 'frequencypicture signal input from a first detector impressed on the control electrode 1 of a tube 3 which is in the first stage of the intermediate frequency amplifier. The signals are then impressed on control electrode' 'I' and further amplified in tube 9, which is part of a second intermediate frequency stage and further amplified by the third and fourth stages of the intermediate frequency amplifier represented in block H.

The l s s e of video signal intermediate frequency amplification is followed by a second detector l3 whose coupling from the preceding amplifier stage is accomplished through transformer l5 having primary I1 and secondary l9.

A push-pull detector circuit is shown and the detector anodes 2i and 22 are connected to coupling transformer winding 19, whose center tap ture tube.

This rectified signal voltage is also applied to an anode 25 and a diode 21 operating as an automatic gain control'rectifier for obtaining a control voltage in accordance with the height of they synchronizing peaks in the video signal. The rectifier cathode 29 is connected to the control electrode 3| of an automatic gain control amplifier tube 33. The cathode 05 of tube a isv also maintained at a negative potential, for example, 50 volts, in order that the control electrode II will be negative with respect to its associated cathode ll.

The anode $1 is supplied with a potentialinore trolling the vertical deflection oscillator.

positive than its cathode through resistances 38, 39 and bypass condenser 4|. For purposes of illustration, this potential maybe 3 volts. The signals taken from the anode 31 are filtered through resistances 43 and 45 and-condensers 41 and .48 and, in turn, are impressed on control electrodes and I of tube 3 and 9 of the first and second stages, respectively, of the video signal intermediate frequency amplifier.

This automatic gain control circuit tends to maintain the second detector input at a substantially uniform level. Any of the well-known automatic gain control circuits may be substituted without departing from the spirit of this invention.

The output of the second detector i3 is also fed to an amplifier shown as block 49 which, in turn,

impresses a video signal on the control electrode 5| of the synchronizing signal separating tube 53 which has a cathode 55 and a anode 5T|. function of this tube 53 is to clip the synchroniz ing pulses from the composite video signal.

The synchronizing signal is made up of two trains of pulses, one train controlling the horizontal deflection oscillator and the other con- The horizontal synchronizing pulses are impressed upon the control electrode 59 of 'the horizontal synchronizing pulse amplifier tube 6| through i series condenser 63. The verticalsynchronizing pulses are integrated in the filter circuit comprising resistances G'I'and 69 and condensers 99 and ID. This integrated pulse signal is then passed to the control electrode II of the vertical synchronizing pulse amplifier tube 13. The signals from the amplifying tubes 6| and I3 are then passed on to the'horizontal synchronizing oscillator circuit and to the vertical synchronizing oscillator circuit, respectively.

A portion of the automatic gain control amplifier output voltage from anode 31 of tube 33 is also fed through the filter comprising resistance l5 and condenser 11 to the control electrode 19-of a synchronizing pulse automatic volume control signal amplifier tube 8|. The cathode 83 of tube 8| is maintained at a negative potential and, for the purpose of illustration, mi

nus 7 volts. The output signal of tube 8|,

through its anode 85, is filtered by condenser 81 i and impressed upon the controlelectrodes 59 and II of the tubes 9| and 13, respectively, causing a of the, automatic gain control on the synchronizing pulse amplifier tubes is to cause them to provide more gain and thus a higher pulse signal output to their respective synchronizing oscillators. Conversely, when the signal input level from the first detector of the television receiver is at a relatively low value, the input to the second detector I3 is substantially the same because of the automatic gain control action'of the amplifier tube 33 on the intermediate frequency amy plifie'r, but the'gain of the synchronizing pulse amplifier tubes is decreased accordingly.

The

In Fig. 2, similar reference characters designate corresponding parts throughout. In this modification, the horizontal synchronizing ,pulse component of the output from the synchronizing pulse separating tube 53 is fed to a grid 89 of the horizontal synchronizing pulse amplifier tube 9|. The vertical synchronizing pulse-component of the output of the synchronizing separating tube 53 is fed. to a grid 93 of the vertical synchronizing pulse amplifier tube 95. A portion of the voltage from the automatic gain control amplifier tube 33 is impressed upon the input circuits of tubes 9| and 95 through resistances 96 and 91 such that the gain of each of the tubes 9| and 95 is changed in accordance ,with the strength of, .the incoming signal such that it increases upon an increase in incoming signal strength and, conversely, decreases upon a decrease in incoming signal strength.

It will be noted that an auxiliary tube for in- .further explained with reference to the curve shown in Fig. 3. It will be noticed that the anode current decreases more rapidly as the grid No. 3 bias potential increases in a negative direction to a value of approximately minus 5 volts. Therefore, an increase in negative bias, resulting from an increased automatic volume control negative voltage caused by an incoming signal of high'amplitude, will cause greater amplification in the synchronizing pulse amplifier stages.

Tubes 9| and 95-may be of the type more commonly known as the GUI, provided with a cathode, an anode, and'four intermediate grids. The cathode 98 of tube 9| is maintained at a negative potential, and for the purpose of illustration, negative 6 volts, the first grid 99 is connected to a negative voltage source, for purposes of illustration minus 7% volts, the screen grid |9| is connected to a positive voltage source, and

the suppressor grid I03 is connected directly to its cathode 98.

The grid 99 operates to limit the number of electrons flowing from the cathodei98 to the anode I05, causing cathode 98-to operate as a cathode would operate in its saturated condition so that, regardless of the attraction caused I by a high amplitude signal on grid 89, the number of electrons allowed to pass through grid 99 and flow to the anode I is limited. The screen grid |9l isolates the grid 89 from the grid 99 so that any fluctuation in the potential on grid 89 will not prevent the gradual limiting action" 01' grid 99. It can thus be seen that, by oper-' vating tube 9| such that its grid 89 varies between 93, while grid I9! is kept at a fixed potential, for

example, zero volts. The cathode I99 of this tube is maintained at'a negative potential and, To!" 2,s1s,e1s

the purposes of illustration, minus 4 volts. The

response characteristic of tube 25 is similar to that described for tube 9|. However, in the case of tube 95, the'curvature in the response characteristic, or the decrease in amplification factor as the control potential bias goes in a positive direction upon decreased signal strength, is caused by the increased drop in anode supply potential through resistances Hi and, H3 at increased anode current.

Referring now to Fig. 4, wherein similar reterence characters designate corresponding parts throughout, a manual level control is used in place of the automatic level control. A potentiometer II! is provided with a variable tap Ill, which is connected to resistance is supplying a negative bias to manually control the amplification iactoroi tubes 8 and 9. This control potential is applied to control electrodes l and I oitubes 3 and l in the intermediate frequencyvideo signal amplifier. The potentiometer H1 is mechanically coupled to a potentiometer III whose variable tap [23 provides an adjustable greater amplification o! the separated synchronizing pulses under conditions of high amplitude-- incoming signal and a corresponding decrease in amplification of the separated synchronizing pulses under conditions or low amplitude incom ing signals.

While several systemsior carrying this inven tion into effect have been indicated and described, it will be apparent to one skilled in the art that this invention is by nomeans limited to the particular organization shown and described but that many modifications may be made without departing from the scope of this invention as set forth in the appended claims. Having now described my invention, I claim: 1. In a television systemincluding asynchronizing signal channel, the combination or means for applying signals to said channel and means for increasing the gain 01' said signals in said means to control the level of synchronizing pulses irom the separating tube 53 to pass to the pulse amplifier I25 and 13.-

The potentiometer H1 is. connected from ground to a negative voltage, and, for the purposes of illustration, minus 25 volts, and the potentiometer Iii is shown connected inthe anode voltage supply circuit of tube 3. Tube Ill is of conventional design and, for the W of this illustration, is shown as a discharge device having a cathode I21, 9. control electrode I29 and an anode Iii.

when receiving signals oi high amplitude, the

video signal intermediate frequency amplifier manual gain control I [1 will be adjusted such as to reduce the gain or the amplifier in order to provide a desired level or signal input to the second detector. The gang controlcauses a simultaneous change in the potentiometer Ill whichisconnectedtoincreasethegainorthe signalamplifier. Thisresultsina channel with an increase in applied signal strength and decreasing the gain otsaid signals in' said channel with a decreasein applied signal strength. 1

2. In a television system including a synchro- V nizing pulse channel, means for applying synchronizing pulses to said channel and means for increasing the gainin said channel with an increase in applied pulse strength and decreasing the gain in said channelwith a decrease in applied pulse strength.

3. A system in accordancewith claim 1 and wherein said means-tor controlling the gain of said signals in the synchronizing signal channel also maintains substantially constant, the level or said signals applied to said synchronizing sig-' 4. A system in accordance "with claim 2 and wherein said means for applying synchronizing pulses to the synchronizing pulse channel includes an ampliflenand wherein said means for controlling the gain in the pulse channel also controls the gain insaid amplifier;

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