Patent GB-904036-A (GB)
Title: Improvements in or relating to apparatus for recording and reproducing electrical signals
Inventor: NUTTALL THOMAS CAYTON
IPC: G11B15/14
Filed: 1959-09-24
Abstract: 904,036. Controlling record tapes; television. BUSH & RANK CINTEL Ltd. Sept. 26, 1960 [Sept. 24, 1959], No. 32482/59. Class 40 (2). [Also in Groups XXXV and XL (b)] In multi-head apparatus for recording and reproducing an electric information signal, means are operative during periods when two heads are in operative relation to the record medium to cause that head which is about to record the said signal to record a supervisory signal, and to cause that head which has just ceased recording said signal also to record an identical supervisory signal and means operative when said recorded information and said supervisory signals are being played back to reduce any discontinuity in the information signals reproduced by successively operative heads. A recorder has 4 rotating heads on a member 5, Fig. 1, and a gated signal applied thereto, so that only one of two heads driven from each of bus-bars 13, 14 will be in operative relation to the tape at any one time. A known pattern carried on rotating member 19 reflects light from a source to a cell 20 whose amplified square-wave output, Fig. 2 (A), is differentiated at 22, Fig. 2 (B), rectified at 23, 24 and applied to pulse generators 25, 26 and 27, 28. Pulse generator 25 is triggered by positive-going spikes, Fig. 2 (B), to yield pulses 2C the duration of which equals the interval between the leading edge of the supervisory pulse which is to precede a signal trace recorded by heads 1 and 3 and the beginning of the trace. Pulse generator 26, triggered by negative going spikes, generates pulses whose duration equals the time between that spike and the termination of the signal trace recorded by heads 1 and 3. Pulses from generators 27, 28 are of like duration but are triggered by spikes of reverse polarity yielding waveforms 2E, 2F. The pulses from generators 25-28 are differentiated and rectified to yield spikes corresponding to the trailing edges of the pulse 2G, H, J, K. The spikes from 29 and 30 are applied to a bi-stable circuit which controls the positive and negative going excursions of the output signal (waveform 2L) to control gate circuits 11, 12A. Similarly, signals from 31, 32 control a bistable circuit 34 whose gating signal output 2M controls gates 11A, 12. Supervisory pulses are generated thus. The control signal 2A is differentiated to form a waveform like 2B. The negative-going spikes are inverted and combined to yield a train, Fig. 2 (N), which is applied to a delay circuit 37 to delay applied signals by a time equal to the desired interval between the supervisory pulses which precede and follow two consecutive signal traces, and form a train 2P. The delayed train 2Q is then applied together with 2N to yield a train of pulse pairs 2R. These are applied to a pulse modulator 39 the output of which is applied to the gate circuits 11A, 12A. Thus the signals applied at 13, 14 are of the form 2S, 2T. Each head thus receives a signal for a period which overlaps the periods in which the signal is applied to the preceding and following heads. In the play-back condition the output of the four heads is applied through gate circuits 45-48, Fig. 3, so that the signal from only one head at a time appears at 49, 50. The output signals are applied to a monitor and amplifier and appear at 59. The signals are also limited at 60, demodulated at 61 and after amplification are fed to a device 63 which develops correcting signals. A photo-cell 20 provides a signal for a gate signal generator, and a further cell 20A provides a signal having half the periodicity, to ensure that signals J, M (Fig. 4, not shown), are in correct phase. The train of pulse pairs delivered by amplifier 62 is separated into trains of equi-spaced pulses. An error signal is derived the amplitude and polarity of which correspond to the magnitude and sense in an error in the positions of the transducer heads upon the record. Other known errorcorrecting systems may be incorporated, e.g. to control the capstan motor. In the embodiment described, phase changes will take place owing to lack of mechanical identity among heads. To avoid pulses due to this appearing at the output separate demodulators for successively operative heads are used. The outputs of the two demodulators are compared to derive an error voltage which can correct the characteristic of one demodulator, Fig. 6 (not shown). Optically generated timing pulses may be developed having a differentiated characteristic by employing a rotating prism 108, Fig. 7. By arranging a further photo-cell 109 behind a slot in a diaphragm 111, which allows light from lamp 101 to reach cell 109 at an appropriate interval a double pulse is generated.