Patent GB-1441169-A (GB)
Title: Recording and playback apparatus
IPC: G11B27/02
Filed: 1973-08-20
Abstract: 1441169 Controlling tape transport in video recorders PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 20 Aug 1973 [23 Aug 1972] 39214/73 Heading G5R A video tape recorder has means for changing the signals fed to a servo 5 controlling a capstan 1 in order to ensure that a new recording will follow an existing recording without substantial irregularity occuring in the vertical synchronising pulses which are recorded, or in the transport; wherein during playback of the existing recording the servo 5 compares the recorded synch pulses with reference pulses 13, e.g. from the mains: but prior to switchover to the new recording, the reference pulses 13 are replaced by the synch pulses of the new recording, so that the recorded pulses become synchronised therewith: and after switchover the recorded pulses, being no longer available, are replaced by a constructed pulse train having the same phase by means of a device 17 which, prior to switchover, repeatedly measures the phase of the recorded pulses with respect to pulses from a capstan tachogenerator 6 and stores the last-measured value, the measurements ceasing at switchover, and circuitry 26, 18 which after switchover utilises the tachogenerator pulses in generating a pulse train which is phase-shifted therefrom by the stored last-measured value; the pulses recorded on the tape after switchover being those of the new recording, or the generated pulse train. The circuitry 26, 18 comprises a comparator 18 which gives a pulse when its two inputs 24, 25 are equal, and a signal generator 26 connected to one comparator input 25 and providing an increasing output which is periodically reset 51 to zero by each tachgenerator pulse. The generator may be digital, i.e. a counter 48 counting presses 47, or analogue, i.e. a ramp generator, Figs. 3, 4 (not shown). The phase measuring and storage device 17 is connected to the other comparator input 24, and also includes a signal generator 41 providing an increasing output: preferably this generator is also reset by each tachogenerator pulse, and is stopped by each recorded synch pulse (the necessary switching being performed by a bistable multivibrator 38), the value attained being stored by the counter (digital form) or by a capacitor (analogue form) until the next resetting tachogenerator pulse. The two signal generators have equal counting rates/slopes so that after switchover, when generator 41 is stopped, generator 48 output will attain equality with the stored value in generator 41 at the point at which the next recorded synch pulse would have occurred, and comparator 18 issues a pulse. In the analogue form, there may be one common signal generator (57), Fig. 3 (not shown). In alternative arrangements, Figs, 4, 5 (not shown), the phase measuring and storing device includes a signal generator with a much higher slope/counting rate than the other generator, and which is reset by the synch pulses, so that equality of the two outputs is achieved shortly after each synch pulse: the equality output pulse from the comparator is then used to stop the faster generator.