Patent GB-1208356-A (GB)
Title: Magnetic recording and playback system
IPC: G11B15/467
Filed: 1968-09-20
Abstract: 1,208,356. Automatic speed control. SONY CORP. Sept.20, 1968 [Sept. 20, 1967], No.44831/68. Heading G3R. In a magnetic recording and reproducing system comprising rotary heads, for successive tracks skewed relative to the lengthwise direction of a transported tape, alignment of the heads with successive tracks during reproduction is obtained by comparison of a reference signal generated in accordance with the rotational position of the heads with the envelope of the normal recorded (e.g., video) signals, thus rendering a separate control signal unnecessary, simplifying the construction of the recorder and allowing fuller use of the width of the tape for normal recording. The invention relies on the fact that incorrect head positioning relative to the tape causes the reproduction from skewed tracks to commence (and end) earlier or later with respect to the rotational cycle of the head Fig. 1 (not shown). Three basic embodiments for video recording are schematically described. In the first, Fig. 2, six tracks per field are recorded by two rotary heads 5a and 5b driven across tape 1 by a motor 11. Horizontal synchronizing pulses are separated from video signal Sg by circuit 12 and fed to circuits 13a and 13b alternately gated by flip-flop 16 under control of pulses from magnetic generator 14 via amplifier 15. A flip-flop 17 is operated by the first horizontal sync. pulse from gate 13a and reset by the first sync. pulse from gate 13b, its output being fed to circuit 3 for switching the signal Sg between heads 5a and 5b. On playback the heads are respectively connected to amplifiers 18a and 18b, and part of the signal from amplifier 18b (for example) is fed to an envelope detector 22 whose output is applied to a gate 23 which receives an inhibiting signal from a mono-stable multi-vibrator 24 synchronized with heads 5a and 5b by the pulses from generator 14. Thus the output of gate 23 is proportional to the amount by which the envelope from 22 precedes the inhibit signal from 24, and this is fed via D. C. amplifier 25 to a braking device 26 which controls the speed of motor 11. Thus the signal representative of the mean position of the start of signals reproduced by head 5b is held in a substantially constant relationship with the signal representing the angular position of the heads. In a second embodiment, Fig. 5 (not shown) each field is recorded in a single track on tape 1. A vertical sync. signal is separated and applied to a flip-flop which causes switching of the signal between heads 5a and 5b and also feeds a differentiation circuit (29) whose output (K), Fig.3 (not shown) is fed to a gate circuit under control of short pulses (L) from the head-position signal generator 14. The output of this gate, which varies with the relative position of the differentiated signals (K) and pulses (L), is fed to the braking device 26 controlling the speed of the heads 5a and 5b so that the vertical sync. signal occurs at the beginning of each magnetic track. On playback the differentiation circuit (29) is switched to receive the envelope signals from detector 22 and these are then gated by pulses from generator 14 and fed to the braking device 26 to control the speed of the heads in a similar manner. In the third embodiment Fig. 6 (not shown) the signal Sg is simultaneously applied to the two heads 5a and 5b which are arranged to provide slight overlap such that a portion of the signal is recorded both at the end of one track and the beginning of the next, a phase comparator (27<SP>1</SP>) being used to control the braking device 26 during recording by comparison of vertical sync. signals with pulses from generator 14. On playback the overlap of the envelopes of the signals from the two heads is detected by an AND circuit (35) whose output is applied to two gates (36a and 36b) providing positive and negative outputs under control of the headposition signals from generator 14. Since the overlap changes its position relative to the angular position of the heads when these are misaligned with the magnetic tracks, the positive and negative outputs of the two gates (36a and 36b) are not then equal and their combination in an adding circuit (38) provides a positive or negative resultant which is amplified (at 39) and fed to the braking device 26 for head speed control. Only one head, or more than two, may be used. The tape speed may be controlled in addition to, or instead of, head speed, and direct motor control may be used instead of a braking device.