Patent GB-1267251-A (GB)
IPC: H04N9/898
Filed: 1969-04-21
Abstract: 1,267,251. Television. NIPPON VICTOR K.K. 21 April, 1969 [23 April, 1968 (3)], No. 20354/69. Heading H4F. Variations in the phase and frequency of the subcarrier, e.g. of an NTSC colour television signal reproduced from a video tape recorder, are corrected in a double heterodyne circuit by adding a signal of the correct subcarrier phase and frequency from a reference oscillator 17 to the chrominance part of the signal and subtracting a signal of the erroneous phase and frequency derived from an oscillator 24 triggered by the colour burst at the commencement of each line, the triggered oscillator 24 also being controlled as to its phase and frequency since it is normally tuned to the correct subcarrier frequency and would therefore suffer a progressive change in frequency and phase during a line scan time. In the embodiment of Figs. 6 and 7, which is a combination of two embodiments as in Figs. 1-4 (not shown), the triggered oscillator 24 is controlled by signals from an open loop coarse control circuit which comprises the phases of a reference oscillator 31 at line frequency and the horizontal line sync. pulses and also by a closed loop fine control circuit in which the phase of the burst signal and the triggered oscillator 24 are compared. The video signal from a magnetic head 14 is demodulated at 15, the luminance signal is filtered at 27 and the colour component including the incorrect subcarrier of frequency (f+#f) is separated by a band-pass filter 16 and mixed at 20 with a signal  of frequency (n + 1) f supplied by the reference oscillator 17 of frequency f, an n times frequency multiplifier 18 and a frequency converter 19, f being the correct subcarrier frequency. The colour burst passes through a gate 23 to trigger the oscillator 24 at the erroneous frequency (f+#f), whose output is mixed at 25 with a signal of frequency nf from converter 18 and then mixed at 22 with the filtered output of the mixer 20 to give a chrominance signal with a subcarrier frequency f, which is recombined at 26 with the luminance component of the video signal. In the coarse control circuit, the horizontal line sync. signal is separated from the luminance signal at 29 and its phase compared at 30 with the phase of the 15À75 kHz sawtooth oscillator 31. A phase error detector 32 controls the oscillator 24. In detail, Fig. 7, the line sync. pulse is supplied to a gating pulse generator 34 so that a gate circuit 35 removes a part of the sawtooth wave from oscillator 31, the magnitude of the D.C. component of this part being dependent on the phase error. The D.C. component is held during a line scan by a circuit 36 and is converted into a change in electrostatic capacity by a variable capacitor 38 to alter the frequency of oscillator 24. A part of the output of circuit 36 is fed back to oscillator 31. In the fine control circuit, Fig. 6, the line sync. pulses from 29 operate a gate 50 so that the output of oscillator 24 sustains oscillation of a ringing oscillator 51. The phases of the oscillator 51 and the burst signal from 23 are compared at 52 and simultaneously a line sync. pulse is supplied to the detector 52. The output of the detector 52 is sustained during a line scan and fed to a tank circuit of oscillator 24. In detail, Fig. 7, the phases of the oscillator 51 and the burst signal are compared at 56 and the output therefrom is gated by a circuit 58 during the burst signal under the control of a monostable multivibrator 59 which is supplied with line sync. pulses. The output of gate 58 is held constant at the peak value of the detected error during a line scan at 60 and supplied to a variable capacitor 62. The change in capacity is fed back to oscillator 24 to alter its natural resonance frequency.