Patent GB-1477466-A (GB)
Title: Magnetic recording and/or reproducing apparatus
IPC: H04N9/84
Filed: 1974-07-26
Abstract: 1477466 Colour television SONY CORP 26 July 1974 [31 July 1973 18 Aug 1973] 33180/74 Heading H4F To reduce the crosstalk between the chrominance components recorded in adjacent tracks on a magnetic tape the chrominance component is modulated alternately in a predetermined order on two subcarriers whose frequencies or polarities differ, a comb filter being used in the reproducing apparatus to remove any crosstalk components. Crosstalk between the frequency- (or phase-) modulated luminance components recorded in adjacent tracks is reduced conventionally by recording adjacent tracks with heads having different azimuth angles. In a first embodiment, Figs. 3, 7, the luminance and chrominance components are separated by a low-pass filter 54 and a comb filter 61, Fig. 3, respectively, the luminance component frequency modulating a carrier at 57 and being passed via a mixing circuit 59 to recording heads 86, 87. The chrominance component is frequency converted at 62 to have a subcarrier frequency fca during alternate field periods and a high subcarrier frequency fcb during the intervening field periods, the signals of one field period being recorded on each track on the tape, and the modulated subcarriers being of lower frequency bands than the frequency modulated luminance component. The difference between the frequencies fcb and fca is an odd multiple of half the line frequency fH, and the frequencies fcb and fca may be fc + fH and fc # fH respectively, fc being 44fH. The frequency converter 62 is fed with signals from a frequency converter 63 supplying signals alternately at frequencies fc + fs + fH and fc + fs - fH, fs being the original subcarrier frequency of the chrominance component. The frequency converter 63 receives a signal at frequency fc from an oscillator 68 and a signal from a switch 72 operated at the end of each field period and receiving signals from oscillators 73, 74, providing signals at frequencies fs + fH and fs - fH respectively. Field synchronizing pulses are recorded by a transducer 78. The played-back signal is separated into luminance and chrominance components by high-pass and low-pass filters 102, 108, Fig. 7, respectively. The chrominance component is fed to a frequency converter 109 which receives a signal at frequency fc and a signal from a switch 72 controlled by a flip-flop 114 receiving signals from transducer 78 and from a field sync. separator 69, the switch 72 receiving signals at frequencies fs - fH and fs + fH from oscillators 117, 118 respectively. The frequency converter 109 receives from frequency converter 63 a signal at frequency fs + fc - fH when the played-back chrominance component has a subcarrier frequency fc - fH and a crosstalk component from the adjacent track of subcarrier frequency fc + fH. The output of frequency converter 109 thus has a subcarrier frequency of fs which is passed by comb filter 112 and a crosstalk component of subcarrier frequency fs - ¢fH which is attenuated by comb filter 112. In the intervening fields, the frequency converter 109 receives from frequency converter 63 a signal at frequency fs + fc + fH, the played-back chrominance component having a subcarrier frequency fc + fH is again frequency converted to subcarrier frequency fs, and the crosstalk component of subcarrier frequency fc - fH is frequency converted to subcarrier frequency fs + ¢fH which is again attenuated by comb filter 112. In an alternative embodiment (Figs. 11, 12, not shown) the recorded subcarrier frequencies are 50¥fH and 36fH, the two signals fed to switch 72 in both the recording and playback apparatus being derived from one oscillator directly and via a frequency divider and frequency multiplier, the output of switch 72 being fed via a frequency converter and frequency divider to the frequency converter 62 or 109. In this embodiment the crosstalk components at the output of frequency conconverter 109 are at frequencies which are attenuated both by band-pass filter 111 and by comb filter 112. In an alternative arrangement, Figs. 14, 15, modulated subcarriers of opposite polarities are recorded alternately. Modulated subcarriers of opposite polarities are provided at the outputs of a balanced modulator 133 receiving a signal at frequency fc from an oscillator 134. The two outputs of modulator 133 are selected by a switch 72 so as to provide a recording pattern as shown by the larger arrows in Fig. 15, all lines in alternate tracks 139 being recorded with one subcarrier polarity, alternate lines 147, 149 in the intervening tracks 140 being recorded with the opposite subcarrier polarity. The playback apparatus (Fig. 17, not shown) includes a balanced modulator (133) receiving the separated chrominance component and a reference signal and providing opposite polarity outputs at frequency fs to the two inputs of a switch (72) whose output is connected to comb filter 112. During playback of track 139 this switch (72) is held in one position; the combfilter 112 includes a one-line delay line and the signals in track 139 from adjacent line periods add whereas crosstalk components from adjacent line periods in track 140 cancel. During playback of track 140 the switch (72) is operated at the end of each line period effectively inverting the signals in alternate line periods so that the reproduced signals again add and the crosstalk components cancel in the comb filter. The apparatus may be modified for recording PAL signals by providing alternate subcarriers whose frequencies differ by an odd multiple of one quarter the line frequency or by recording in alternate tracks, two lines with one subcarrier polarity and the following two lines with the opposite subcarrier polarity.