Patent GB-1140595-A (GB)
Title: Colour correction system for video tape recorders
IPC: H04N11/14
Filed: 1966-09-29
Abstract: 1,140,595. Colour television signal recording. RADIO CORPORATION OF AMERICA. 29 Sept., 1966 [22 Oct., 1965], No. 43666/66. Heading H4F. In the magnetic recording and reproduction of an N.T.S.C. signal, a subcarrier demodulation or phase correction signal is provided by combining with the recorded signal a reference signal (termed the "ART" -added reference transmission-carrier) which is derived as shown in Fig. 1 by synchronizing an oscillator 2 to the N.T.S.C. "bursts", shifting the output at 3 by 57 degrees to bring it to the I phase and then applying it to be combined with the N.T.S.C. signal in adder 6 by way of a phase shifter 4 and gate 5 whereby the reference signal is inverted in phase in alternate lines and also blanked during the intervals between lines. In the reproducing apparatus, Fig. 3, the N.T.S.C. subcarrier and reference signal are selected by filters 22 and delayed by one line interval at 24 to make available simultaneously the signals from two successive lines. An adder 29 adds the signals to produce the reference signal alone, whilst a subtractor 30 derives the difference of the signals to produce the N.T.S.C. subcarrier alone. The separation is achieved since the N.T.S.C. subcarrier has the customary odd multiple relationship to half line frequency and hence is inverted in phase in successive lines, whilst the reference signal is brought into phase in successive lines by the periodic phase inversion. The separated reference signal is limited in 33 and then applied via a phase inverter 34 and line frequency switch 35 to establish a signal of appropriate phase to operate colour decoder 36 to which the separated subcarrier from subtractor 30 is applied. The complete N.T.S.C. signal together with the reference signal is taken from before filter 22 and applied through a subtractor 37 to remove the subcarrier signal and a notch filter 38 to remove the reference signal whereby there remains only the luminance signal Y. The correct operation of adder 29 and subtractor 30 in separating the reference and subcarrier signals depends upon the absence of phase distortion between the signal in successive lines. In accordance with a feature of the invention such errors are corrected by a phase detector 27 which responds to the signals in successive lines obtained from before and after delay 24 and derives an error signal which is applied via a correction circuit 28 to operate a phase shifter 25. In a first construction for this phase error correcting system, Fig. 4 (not shown), the phase detector responds throughout the line interval and produces an error signal determined by the subcarrier signal but modified by a spurious signal due to the reference signal. The system includes a subtractor circuit which is arranged to remove the spurious signal. In a second construction, Fig. 6 (not shown), the phase detector is arranged to respond to the colour synchronizing "bursts" in successive lines, the "bursts" being applied first to synchronize crystal oscillators which provide signals lasting throughout the line intervals. In a third construction, Fig. 7 (not shown), the "bursts" are compared in the phase and funish an error voltage which is arranged to persist throughout the line interval.