Patent GB-1294264-A (GB)
Title: Automatic phasing of servo systems used in tape transport systems
IPC: G11B21/02
Filed: 1971-04-19
Abstract: 1294264 Automatic speed control AMPEX CORP 19 April 1971 [2 April 1970] 25669/71 Heading G3R In a transverse scan video tape recorder means is provided whereby during replaying the rotary heads 12-15 are automatically synchronized with the correct oblique tracks 21 on the tape and with a studio reference signal. In addition to the oblique tracks 21 the tape has recorded on it a control track 19 comprising an alternating signal of a frequency equal to the rotational velocity of the rotary head wheel 11 with superimposed frame synchronizing pulses. During playback, in an initial stage of synchronization, the frame pulses derived from track 19 by a head 26 are compared with a studio standard frame pulse signal (at 32) in a coarse framing control 28 which adjusts the phase of a capstan motor 37 operating in a closed loop speed control 34, 38 until the frame pulses coincide. At the same time the motor 41 driving the head wheel 11 is synchronized in phase and velocity with a vertical synchronizing signal (at 42) from the studio standard by means of a phase comparator 44 and a velocity feedback loop 52 which both receive signals from a tachometer 127, coupled to the motor 41, by way of a processing unit 54 and a selector 27. The unit 54 (which may be as described in Specification 1286434) converts the tachometer signal to eight separate signals each displaced at 45 degrees in phase from the next. During the initial stage of synchronization the selector 27 is switched, by an over-ride signal at 121, to select a fixed one of the eight signals for feeding to the phase comparator 44; this one signal is the one used to synchronize the tape during recording and should thus result in the head wheel scanning the correct tracks 21. If however the tape is replayed on another machine, the tape is stretched or the relationship between wheel 11 and control track head 26 is otherwise disturbed the heads on wheel 11 will scan the wrong tracks. To correct this error it is only necessary that the selector 27 select a different one of the eight tachometer signals to feed to the phase comparator 44. In order to measure the error, the time between the start of a vertical synchronizing pulse, replayed from the tape and separated from the video signal by a stripper 72, and the next following zero crossing of the A.C. control signal, derived from head 26, is measured by counting high speed clock pulses (e.g. the studio horizontal synchronizing pulses). The count is used to trigger one of eight gates (81-88), Fig.2 (not shown) each receiving one of the tachometer signals, the selected gate feeding its signal to the comparator 44. This brings the phase error of the headwheel to less than 45 degrees which is within the capture range of the remaining systems. Next the capstan servo 34 is switched (107) to receive a fixed one of the tachometer signals so that capstan and headwheel run in synchronism and finally the comparator 44 is switched (47) to receive the vertical synchronizing pulses replayed from the tape. This and a steady state phase error correction circuit 43, which may be as described in Specification 1294265, removes the final phase error from wheel 11.