US2303968A - Television system - Google Patents

Description

Dec. 1, 1942:

E. L c. WHITE TELEVISION SYSTEM Filed May 10, 1939 SYNcM 516N440 INVENTOR ERIC ZAWREIVCE CASH/V6 WHITE ATTORNEY which constitutes one of the whiter than Patented Dec. 1, 1942 iilTED STATES PATENT ICE TELEVISION SYSTEM Eric Lawrence Casling White, Hillingdon, Middlesex, England, assignor to Electric & Musical Industries Limited,

Hayes, Middlesex, England,

a company of Great Britain Application May 10, 1939, Serial No.

In Great Britain May 18, 1938 '4 Claims.

917, filed May 11, 1939, in which a method of obi taining a rapidly acting automatic gain control as required for example in aircraft television sys tems is described, although the method is applicable to television systems generally. In carrying out the method of United States application Ser. No. 141,917, filed May 11, 1937, the waveform transmitted is a carrier wave modulated by picture signals and includes sets of both whiter than white and blacker than black signal pulses which occur at predetermined intervals, and the receiver is provided with an automatic gain control system which is designed to be responsive to the difierence between the amplitude of said signal pulses, the receiver being also provided with a synchronizing system which is designed to heresponsive to one set of the signal pulses. The blacker than black pulses constitute line-synchronizing signals and each of such pulses forms a portion of a double pulse, the other portion of white pulses. The carrierwave is also modulated by frame synchronizing signals which modulate the carrier wave in the whiter than white sense, the

frame signals being interrupted by the line synchronizing signals in'the blacker than black sense so that automatic gain control potentials and line synchronizing signals can be obtained during the occurrence of the fram synchronizing signals.

According to the present invention, a modlfication of the invention set forth in the specification of United States application Ser. No. 141,917 filed May 11, 1937, consists in modulating the tted carrier wave with frame synchroniz- 111g Si nals in the blacker than black sense. In order to eliminate at the receiver undesirable effects due to the pulses which modulate the carrier wave in the whiter than white sense, such pulses are suppressed by mixing them with the line synchronizing signals which are separated, delayed and/or broadened for this purpose.

In a particular television transmitting and receiving system embodying the invention the waveform sent out by the transmitter is a carrier wave modulated by picture signalsin such a sense that an increase in picture brightness is represented by an increas in carrier amplitude, said waveform including blacker than black line and frame auxiliary signals and wherein the receiver is provided with an automatic gain control system which is designed to be responsive to the absolute amplitude of said whiter than white auxiliary signals.

In order that the invention may be more clearly understood and readily carried into effect a waveform modulated in accordance with the invention will now be more" fully described by way'of example with reference to the drawing filed with the specification in which Fig. 1 shows such a waveform, and

Fig. 2 shows a circuit arrangement for preparing the line synchronizing signals to efiect suppression of the gain controlling pulses.

If the waveform described in the specification of application Ser. No. 141,917 filed May 11, 1937, is changed so that the frame synchronising pulses modulate the carrier wave in the blacker than black sense without other modification, a waveform results having th disadvantage that the line synchronizing pulses are interrupted by the synchronizing signals and whiter than white 55 cause disturbance in the line frame synchronizing pulses and this is apt to scanning at a receiver. An improvement is effected if the same step is adopted and in addition the automatic gain control pulses which modulate the carrier wave in the whiter than White sense are made to precede instead of to follow the line synchronizing pulses which modulate the carrier wave in the, blacker than black sense. In this case the trailing edge of a gain control pulse will coincide with the leading edge of the line synchronizing pulse and since the automatic gain controlling pulses continue throughout the frame synchronizing pulses, the line scanning oscillator at the receiver will continue to be synchronized by the trailing edges of the automatic gain controlling pulses.

Since the automatic gain control pulses occur I before the line synchronizing pulses, they result in a bright vertical bar appearing on the extreme right-hand edge of the received picture and this can be easily masked off. However, this does represent a loss of time during which picture signals could otherwise be transmitted and the waveform shown in Fig. 1 of the accompanying drawing overcomes this disadvantage.

Referring to Fig. l, the waveform shown is for a line frequency of 10,000 per second, and it will be necessary that the automatic gain control pulses I, whichare of 10 micro-seconds duration, follow the line synchronizing pulses 2 as in the waveform described in the specification of application Ser. No. 141,917 filed May 11, 1937, the

duration of these line pulses being 5 micro-seconds. e frame synchronizing pulses 3 consist of 10 to broadened line synchronizing pulses but with the automatic gain control pulses continuing through them. The picture signals 4, the duration of each line of which is 85 micro-seconds, are prevented from coming below a level represented by the line 5 which is of the order of 30% of the peak carrier wave output, and it is also convenient but not necessary to limit the upward excursions of the picture signals to about 90% as represented by the dotted line 6. 00% modul tion of the carrier wave is represented by the line I. The picture signals are suppressed to the 30% level 5 during the period of the frame synchronizing pulses.

The effect oi the automatic gain control pulses of Fig. 1 on the received picture, which would be to brighten the end of the return stroke and the beginning of the working stroke, can be suppressed by separating the synchronizing pulses, delaying them by an amount not quite equal to their width, broadening them to a breadth slightly greater than the automatic gain control pulses and mixing them the picture signals.

A circuit arrangement for effecting the delaying and broadening of separated line synchronizing pulses is shown in Fig. 2. The separated negative line synchronizing pulses are applied to the terminals 8 between which and a diode valve D1 is a network giving an initial delay 151. The

delays introduced by the following networks are each equal to t2 and lessthan the width of the original pulse. These delay networks are connected to diode valves D2, D3 and D4, the anodes of which are connected together and with that of D1. The output from the diodes is passed to a valve V, the output from which consists of delayed and broadened positive pulses.

In operation, the grid of the valve V is maintained negative so long as incoming pulses are applied through the delay network to any of thediodes D1 to D4. It should be arranged that the amplitude of the incoming line synchronizing pulses is 'sufiicient to cause the valve V to be shut off so that the waveform of the output is regular.

Although four diodes are shown, any number n greater than one may be used. The width of the output pulse will be increased by (12-1) is. Thus, if the width of the line synchronizing pulse is t5 and that of the automatic gain control pulse is t it must be arranged that t is greater than n and t1+(n-l)ta is greater than t.+t The delayed and broadened pulse obtained from in negative sense with asoaoes the valve V will overlap the automatic gain control pulse on both sides and will therefore suppress it if mixed in the negative sense;

The waveform described possesses the advantages that all the positive automatic gain control pulses are equal in length so that design of a rapidly acting automatic gain control circuit at the receiver is facilitated and it is not necessary to arrange for blacking out the frame pulse at "the receiver.

While in the waveform described the line synchronizing pulses have been shown to be shorter than the automatic gain control pu1ses,,they may if desired be of the same length or longer than the automatic gain control pulses. In either of these latter cases, the suppressing pulses may be obtained merely by applying the line synchronizing pulses to a delay network.

What I claim is:

1. In a television system wherein line and frame synchronizing impulses of the same polarity are transmitted between sets of video signals, and wherein a signal of opposing polarity is transmitted after each line synchronizing signal, the method which comprises the steps of separating the synchronizing pulses, delaying the pulses by an amount differing from their duration, broadening the pulses to a breadth slightly greater than the width of the pulses having an opposing polarity thereto, and mixing the pulses in a negative sense with the video signals.

2. In a television transmitting system wherein automatic volume control signals are transmitted at a time adjacent the transmission of line synchronizing signals which are of opposite polarity thereto, and in which frame synchronizing signals network means serially connected, each of said delay network means having a diode connected substantially across the terminals thereof.

ERIC LAWRENCE CASLING WHITE.

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