US2724738A - Black and white signal limiter - Google Patents

Description

Nov. 22, 1955 c. H. BABBS BLACK AND WHITE SIGNAL LIMITER Filed June 26, 1950 Inventar Cycws H. BAs

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United States Patent BLACK AND WHITE SIGNAL LIMITER Cyrus Herbert Babbs, Cambridge, England, assignor to Pye Limited, Cambridge, England, a British company Application June 26, 1950, Serial No. 170,298 Claims priority, application Great Britain July 2, 1949 1` Claim. (Cl. 178-7.1)

The present invention relates to apparatus used in television picture waveform generation. In apparatus according to the invention the waveform is clipped at a predetermined level at the voltage corresponding to black level, which may not necessarily be picture black, thereby to give black level setting, followed by limiting of the picture amplitude corresponding to white. A feature of this invention consists in providing a variable setting to enable the maximum picture amplitude to be set `in relation to normal peak white, for instance at plus or minus 50%, of peak white.

In order that the invention may be more fully understood one embodiment will now be described by way of example with reference to the accompanying drawings. In the drawings:

Fig. 1 is a circuit diagram showing one arrangement for putting the invention into practice and Fig. 2 shows the voltage/time waveform at various parts of the circuit,

Fig. 2A shows the sawtooth video waveform on the anode V1 in the absence of an input signal to V2,

Fig. 2B shows the waveform at the anode of the blanking amplifier V2 in the absence of an input signal to V1,

Fig. 2C shows the combined waveform of Figures 2A and 2B when input signals are fed to V1 and V2 and with* out the clipping valve VSA conducting,

Fig. 2D is similar to Fig. 2C but with clipping valve VSA non-conducting and without squelch diode VSB,

Fig. 2E shows the output from the clipper valve VSA without the squelch diode VSB, and

Fig. 2F shows the waveform at the anodes of V1 and V2 with clipping diode VSA and squelch diode VSB in operation.

Referring to the drawings, the valve V1 of Fig. l is a pentode valve, operating as a D. C. amplifier having video signals applied to its control grid through the condenser C7; the polarity of the signals being such that black corresponds to the positive edge. The valve V1 also has its control grid connected to earth through the resistor R18, and its cathode connected to earth through the resistor R19. The D. C. insert A is connected across R18 and supplies a reference voltage to the video signal. The

. is developed across R11 and L1 without the presence f "the squelch diode VSB. This squelch diode `VSB is inwaveform at the anodeof V1 is as shown in Fig. 2A.

Sharing the common anode load of' V1 is a pentode valve V2 that has positive-going blanking pulses applied to its control grid through the condenser C8, the control grid of V2 also being connected to earth through the resistance R20. These blanking pulses are so applied that the valve V2 is cut-off during the picture periods and in between picture periods the Valve conducts and develops a large t The common anode load of V1 and V2 consists of R3 in parallel with R11, L1 fed via the diode VSA.. The D. C.

potential on the cathode of VSA is such that the negative excursions of anode potential of V1 and V2 cause the diode VSA to be biased to cut-off. The valves V1` and ,V2 being pentodes have a constant current characteristic` and their amplification is directly proportional to the anode load impedance. Therefore when diode VSA is cut off i. e. below the clipping level, the load circuit R11, L1 is isolated from RS, and the effective load impedance of `valves V1 and V2 is greatly increased, thereby causing a greaterV voltage drop across R3 and a large increase in amplication. This change of amplification due to the clipping action of valve VSA is shown in Fig. 2D. The

waveform shown in Fig. 2E is the output voltage which cludcd in the circuit to limit the negative excursion of the anodes of V1 and V2 and so prevent large pulses api pearing across VSA. Large pulses Would tendto be capacity coupled by the self capacity of VSA into theload circuit. According to a feature of this invention the squelch diode VSB is itself biased by R8, C2 to a Voltage which allows a small change of amplification to take place at the anode of V1 and V2, giving a sharp clipping action to VSA, see waveform Figure 2F.

The clipping potential of VSA is set by the cathode follower valve stabiliser V4 having its anode connected to the high tension positive line through R7 and its grid fed through the grid stopper R6 from the potential divider comprising resistors R4 and R5 connected across the stabilized high tension supply, the resistor R5 being decoupled by C1. The clipping potential of VSA is also controlled by the anode potential of V1 and V2 set by the signals fed into these valves and also by the manual control R2 in series with R1. Variation of R2 allows the user to set the black level clipping to any position from below picture black through to peak white, picture black being set as required by this lift control and the signal amplitude fed into V2.

According to another feature of this invention the peak white limiting is accomplished by the diode VSB, fed by the isolating cathode follower VSA which is D. C. coupled to the black level clipping diode load circuit R11, L1 via the grid stopper R13 through the cathode follower stage VSA the cathode of which isdirectly coupled to the cathode of the diode VSB. These cathodes are also coupled to the high tension negative line through resistors R14 and R15, the latter being decoupled by C5. The anode of VSA is connected to HT positive through R16. The biasing of VSB is set by the potentiometer R10 which is connected to the high tension positive line through resistor R9. Excursions of voltage on the cathode of VSB are such that picture white is positive going and if this voltage exceeds the biasing` potential of VSB the diode current will be cut off and no signal voltage greater than the biasing potential will appear across` the output load circuit R17, L2. The biasing potenitometer R10 can be set to give the required limiting potential.

Whilst a particular embodiment has been described it will be understood that various modifications may be made without departing from the scope of the invention.

I claim:

In a generator` for the generation of a television picture waveform, a circuit for setting the black level of the picture waveform and for limiting the picture amplitude corresponding to white, comprising a first electronic amplifier valve having an input electrode and an output electrode, a D. C. inserter connected to said input electrode, means for feeding the video signal to the inputelectrode of said first amplifier valve, a second electronic amplifier valve having an input electrode and an output electrode,

Patented Nov.` 22, 1955 means for feeding blanking pulses to the input electrode of said second amplitier valve during the between-picture periods such that the peaks of the blanking pulses have the lsame polarity as that portion of the video signals which rcorresponds to black, fed to said first amplifier valve,r'a`comn`10n load circuit connected to the output electrodesl of said rst and second amplifier valves across which the mixed Waveform of vdeo signals and blanking pulses appears, a tirst electronic' rectifier having an anode and a cathode, a direct connection between the anode of said rst rectifier and the output electrodes of said first and second amplier valves, a source of stabilised D. C. potential, means for applying a D. C. reference potential derived from said stabilised source to the cathode of said fir'strectiiier, means for manually varying the potential applied to the anode of said lirst rectifier, a second electronic rectifier having an anode and a cathode, for damping the mixed waveform when the potential ofthe waveform'falls belowlthe clipping level, a direct connection between the cathode of said second rectifier and the anode of the first rectifier, means for applying a D. C. reference potential derived from said stabilised source to the anode of the second rectifier, a self-biasing circuit connectedto the anode of said second rectifier, a cathodefollower stage having an input electrode and an output electrode, a D. C. connection between the input of the cathode follower stage and the cathode of the first rectier, a third electronic rectifier for limiting the amplitude of said waveform, having an anode and a cathode, a direct connection between the output of the cathode follower and the cathode of the third rectier, means for applying a D. C., reference potential derived from said stabilised source to the anode of said third rectifier, means for varying said D. C'. potential applied to the anode of the third rectifier, a load connected in series with the anode of said third rectifier and an output lead` connected to the junction of the anode of said thirdy rectifier and References Cited in the tile of this patent UNITED STATES PATENTS Dallos Jan. 4, 1944

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