Patent GB-1197124-A (GB)
Title: Improvements relating to Processing Colour Television Signals
IPC: H04N9/89
Filed: 1968-05-20
Abstract: 1,197,124. Colour television. AMPEX CORP. 20 May, 1968 [22 May, 1967], No. 23945/68. Heading H4F. A colour burst switching sequence signal is developed from a colour television signal by providing a reference signal, e.g. from a gated oscillator 3, Fig. 1, in fixed phase relation to the line synchronizing pulses of the television signal, and comparing the phase of the reference signal with that of the colour burst of the television signal, e.g. as extracted from the television signal by a shaper or limiter circuit 2, to develop a representative signal varying in amplitude at the colour burst frequency. The frequency of th representative signal is then multiplied to produce the burst switching sequence signal at the line frequency of the television system, e.g. by applying the representative signal to a differentiator 5, a full-wave rectifier 6, an LC filter 7 and a clipper 8. A bi-stable switching circuit 9 may optionally also be provided to receive the square wave output from the clipper 9 and to respond to the line synchronizing pulses from the television signal via a harmonic eliminator 10, to effect synchronization of the square wave output with the line synchronizing pulses. Fig. 3 shows a magnetic tape reproducing system using the apparatus of Fig. 1, which appears as the switching sequence detector 1 incorporated in a fine correction circuit 31. The system comprises a multiple head down which reads the television signal recorded on a tape 13, the drive motors 18, 25 for the drum and tape respectively both being controlled by a servo control system 23. A switching network 28 receives signals from the multiple heads and recombines them to reconstitute the composite television signal, which is fed to a demodulator and process amplifier; and thence through a coarse colour phase correction circuit 30 to the fine correction circuit 31. The three inputs for the detector 1 are all derived from an input burst stripper 32, which also provides the colour burst signal to a gate 35 which selects particular parts of the burst as samples of the phase thereof. The switching sequence detector 1 output is supplied to a reference waveform generator 38, which is phase-locked to a reference synchronizing source of subcarrier frequency, to control a switching mechanism and change the phase of the colour burst reference waveform from the generator 38 between the NTSC and counter- NTSC phase. A phase comparator 40 compares the phase of this reference waveform with that of the colour burst signal obtained through the gate 35, and develops an error signal controlling a control signal driver 42 which operates a variable delay line 43 to provide precise adjustment of the time base of the television signal output. The output from the detector 1 is also fed back to the demodulator and process amplifier 29, and to the servo control system 23. The output from the video amplifier is thus compatible with the PAL system whereas in the absence of the switching sequence detector 1 the reproducing system is adapted to operate the NTSC system.