Patent GB-1243437-A (GB)
Title: Video signal identification circuit
IPC: H04N5/50
Filed: 1969-07-08
Abstract: 1,243,437. Television. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 8 July, 1969 [11 July, 1968], No. 34298/69. Heading H4F. The presence of both the line and field synchronizing pulses in a video signal is arranged to produce a control magnitude for switching- on associated television apparatus whilst in the absence of either line or field pulses (or of both) a switching-off control magnitude is produced. The apparatus may be, e.g. a television receiver or the transmitter of an unmaned television station. In one embodiment (Figs. 1 and 2), the video signal (Fig. 2a), is supplied via terminal 2 to a separator circuit 1 which operates to separate the synchronizing pulses from the picture information and to suppress all but the first pulse 9 of the pulses occurring during the field synchronizing period. The separated waveform (Fig. 2b) is then supplied to a sawtooth generator 11 of the type in which the maximum voltage to which the saw-tooth capacitor 12 can charge may be increased, above that determined by the supply voltage, by the voltage developed across a (feed-back) capacitor 13, this latter voltage being a function of the rate at which sync. signals (which trigger the generator) are received. The output sawtooth from generator 11 (Fig. 2c) is then fed to a threshold circuit 22 the parameters being such that circuit 22 produces an output during each field period only when all of the line and all of the field sync. pulses are present in the video signal and this output, after inversion in 27, is employed to trigger a monostable multivibrator 28 the natural period of which is equal to about one-half the field period. Thus the criterion for a complete video signal is the production of a pulse output at terminal 32 having a repetition frequency equal to the field frequency and this waveform is then utilized to operate switching equipment associated with the television apparatus to be controlled. Fig. 2c, shows the output from the saw-tooth generator when a complete video signal is present and illustrates that, during the field period only, such output (21) is sufficient to exceed the threshold level 24 set in circuit 22. For other conditions, e.g. line sync. pulses or field sync. pulse alone or none present the output from generator 11 is insufficient to exceed the level 24 so that multivibrator 28 is not triggered and switching-on of the television apparatus does not occur or switching-off if it was previously switched-on. Noise pulses, in the absence of any sync. pulses, also do not produce switching function either because they do not result in an output from generator 11 exceeding the threshold 24, e.g. when they have a low repetition frequency or, if they have a repetition such that an output from the generator 11 exceeding threshold 24 is produced, the triggering of multivibrator 28 by each input pulse (i.e. at a rate exceeding its natural period) maintains it in its metastable condition so that the field frequency pulse waveform is not produced. This action, and the normal triggering action due to an incomplete video signal, is enhanced by the action of a feed-back circuit 33 which, when the multivibrator 11, 28 is triggered to its metastable condition, operates by substantially short-circuiting resistor 16 in generator 11 so that the mean valve of the sawtooth (and thus the peak amplitude) is made to approach the threshold level 24 and thus the production of trigger pulses for the multivibrator is enhanced. In a second embodiment the sawtooth generator comprises a Miller integrator circuit, the feed-back circuit operates to adjust the threshold level of circuit 22 and the video waveform comprises a positively modulated signal (Figs. 3 and 4, not shown).