Patent GB-1490423-A (GB)
Title: Apparatus for reproducing recorded colour video signals
IPC: H04N9/84
Filed: 1975-01-30
Abstract: 1490423 Television SONY CORP 30 Jan 1975 [5 Feb 1974] 4132/75 Heading H4F The invention is concerned with reproducing television signals having luminance and chrominance components composed of line and field intervals recorded in respective areas of successive parallel tracks on a magnetic record medium in H-alignment, the chrominance components in adjacent areas of successive tracks having (sub) carriers of a first phase during alternate field intervals and first and second phases alternately at the line rate during intervening field intervals. and being recorded by a pair of transducer heads having gaps with different azimuth angles so that when reproduced by means of similar heads and with appropriate correction of phase, cross-talk signals may be eliminated by means of a comb filter. Fig. 2 shows two transducer heads HA, HB carried by a rotating disc 2, Fig. 3 shows the different azimuth angles of the respective gaps GA, GB, and Fig. 4 shows the pattern of successive recorded tracks TA, TB made in the direction 3 by the respective heads HA, HB as the tape 2 moves in the direction 4. Fig. 1 shows one embodiment of the invention in which transducer heads HA, HB similar to those of Fig. 2 scan a recorded magnetic tape such as shown in Fig. 4, and supply the reproduced signals to an amplifier 16. A H.P.F. 18 selects the luminance signals and supplies these via a limiter 19 and frequency demodulator 20 to an adder 21. A L.P.F. 17 selects the chrominance components and supplies these via a phase-splitter 22 the opposite phase outputs of which are supplied to respective contacts of a switch 23. In the recording of the tape (Fig. 5, not shown) the chrominance components are frequency shifted from the chrominance subcarrier frequency fS to a frequency fA and recorded such that during alternate fields (tracks TA) the carrier/A has one phase whilst during intervening fields the phase of the carrier is alternated at line (horizontal=H) frequency between the one phase and the opposite phase (see Fig. 6A) and in reproduction the switch 23 is controlled by a switching waveform SC (see Fig. 6B) to remove the phase alterations after which a frequency converter 24 supplied with a frequency fS+fA restores the carrier to the normal chrominance sub-carrier frequencies fS. Because of the manner in which the signals are recorded on successive tracks cross-talk components are then eliminated by a comb filter 25 the output of which is combined with the luminance components in adder 21 to create the recorded signals. The frequency fS+fA and the switching waveform SC are produced as follows. An oscillator 31 operating at frequency fS feeds a frequency converter 33, a phase comparator 35 and via a phase shifter 38, feeds a second phase comparator 37. A gate 34 passes the synchronizing "burst" in the output of frequency converter 24 and supplies it, as second input, to phase comparator 35 the output of which controls an oscillator 32 operating at frequency fA. This frequency is supplied as second input to the frequency converter 33 which produces the frequency fS+fA for the frequency converter 24. The phase comparator 37 which also receives the "burst" output of the gate 34 produces a negative output when the two inputs (i.e. the "burst" signal and the frequency fS) are coincided in phase and a positive output when the two inputs are of opposite phase. These two conditions occur, respectively, when the switch 23 selects the positive phase (output of 37 negative) and the negative phase (output of 37, positive) and when the latter condition exists the positive output of gate 37 causes a flip-flop 39 to reverse its condition. Flip-flop 39 also receives a line (horizontal signal) PH which causes it to reverse its condition at every line interval H (see Fig. 6B) and its output forms one input to an AND gate 42. As a second input AND gate 42 receives the output of a mono-stable multivibrator 41 which is controlled by an input pulse SPG derived from a generator 40 responsive to the angular position of the (rotating) transducer heads HA, HB. The output of multivibrator 41 consisting of a rectangular wave of vertical (field) period is "low" (i.e. "O") when head HA scans the tape in, e.g., the odd fields and "high" (i.e. "I") when head HB scans the tape in the even fields. The output SC of AND gate which is, thus, as shown in Fig. 6B, then controls the changeover of switch 23 to produce the required input signals to the frequency converter 24. If, in the operation of the arrangement described above, the relative recorded positions on the tape of the horizontal synchronizing signals on adjacent tracks should be shifted slightly, but sufficient to produce a relatively high level of cross-talk, the reversal of flip-flop 39 may get out of the correct phase and to prevent this the output of the comb filter 25 (instead of the output of frequency converter 24) is supplied to the gate 34 (Fig. 13, not shown). The invention is described in the context of NTSC signals but PAL type signals may be employed with suitable modification of the circuitry.