Patent GB-1497865-A (GB)
Title: Magnetic recording and/or reproducing apparatus
IPC: H04N9/79
Filed: 1974-12-24
Abstract: 1497865 Colour television recording SONY CORP 24 Dec 1974 [28 Dec 1973 (2)] 55837/74 Heading H4F In an arrangement for recording PAL colour television signals on a plurality of parallel tracks on a magnetic tape the chrominance signals are converted to chrominance signals of the NTSC type (i.e. the line frequency alternation of the phase of the R-Y signal is arrested) by means of a suitable converting signal that also shifts periodically in polarity or frequency so that the recorded chrominance signal in each line interval in each track, when played back, are free of crosstalk picked up from adjacent tracks. Fig. 11 shows one embodiment in which, from composite PAL signals at input 48, a comb filter 56 separates the chrominance sub-carrier f s  and via a "burst" gate 79 synchronizes a sub-carrier frequency oscillator 81 the output is supplied to a frequency converter 64. Additionally, an oscillator 63 synchronized by the horizontal synch. signals from separator 59 and operating at an integral multiple of the horizontal frequency fh supplies its output at frequency fc as a second input to frequency converter 64. The output of the latter after filtering by B.P.F. 82 consists ofthe frequency fs+fc and this is supplied to a second frequency converter 83 which has as second input a frequency f s -f h /4. Converter 83 produces an output which, after filtering in L.P.F. 86 consists of fc-fh/4 and this is further converted in a third frequency converter 87 having fs as second input to a frequency band including fs-fc+fh/4 and fs+fc-fh/4 which are selected by respective band-pass filters 88, 89 and comprise the signals to be combined with the chrominance sub-carrier fs during alternate line (horizontal) intervals for arresting the phase reversal of the R-Y signal to produce an NTSC type chrominance signal on a sub-carrier of frequency f c -f h /4. The supply of the signals during alternate line intervals is effected by switch 97 operated by the waveform A from line rate flip-flop 96 (see also Fig. 12) but, to ensure freedom from cross-talk, as indicated above, the polarity of the signals is periodically reversed by means of inverter 98 and a second switch 99 operated by waveform D (Fig. 12), the reversal actually occurring during alternate line intervals in alternate fields. The output of switch 99 is then combined in a further frequency converter 57 with the chrominance sub-carrier fs and supplied via BPF 58 to a mixer 54 for combination with the luminance component YFM from carrier frequency modulator 52 and the combined signals, Fig. 3 (not shown), are fed to recording heads 21, 22 (see also Fig. 4) for recording on magnetic tape 29 the operation of which is synchronized in known manner. In a play-back arrangement for signals recorded by the system of Fig. 11, a switch 119 (Fig. 14) removes the polarity reversal introduced by switch 99 (Fig. 11) to produce a standard NTSC chrominance signal which is converted back to the original sub-carrier frequency fs by means of frequency converter 122 supplied with a second input [fs-(fca-fh/4)]. The output of converter 122, fs is then supplied via B.P.F. 123 to a comb filter 124 which produces an output S s (n-1)=Sin(W s t+#) i.e. one of the phases [for line (n - 1)] of the PAL chrominance sub-carrier. This is supplied to one contact of a switch 126 the other contact of which receives the PAL chrominance sub-carrier phase for line (n) = Sin(W s t-#) i.e. the oppositely phased (R-Y) chrominance signal. Switch 126 is operated at horizontal frequency and supplies the two inputs alternately to mixer 117 in which they are combined with the luminance signal (after frequency demodulation in 114) to recreate the original composite PAL signal. Because of the action of inverter 121 and the switch 119 together with the action of the comb filter 126, (which subtracts signals of line n from signals of line n - 1) cross-talk signals are eliminated. In a second embodiment (Fig. 15, not shown) the PAL chrominance signals are converted to NTSC type by means of two frequencies which differ by an odd-multiple of ¢ the line frequency and are respectively operative during alternate field intervals. Thus, the chrominance components which are recorded on adjacent tracks in successive field intervals are in interleaving (frequency) relation and therefore, during play-back (Fig. 16, not shown), cross-talk may be eliminated by a comb filter.