Patent GB-1263982-A (GB)
Title: Method and system for recording sampled signals on a continuous recording medium
IPC: G11B20/02
Filed: 1969-02-18
Abstract: 1,263,982. Television. WESTINGHOUSE ELECTRIC CORP. 18 Feb., 1969 [8 March, 1968], No. 8758/69. Heading H4F. In an arrangement for recording a signal on a continuous loop storage medium having first and second storage states, e.g. a magnetic drum or disc, a clock signal having an integral number of cycles of equal width alternating between first and second polarities is recorded on the medium to provide a timing reference for recording the signal. Means are provided for sampling the signal, adjacent samples being recorded as pulses in the opposite states of the recording medium, and the amplitude of the samples determining the position of an edge of the pulse in relation to a corresponding cycle of the clock signal. The arrangement may be used in the system disclosed in Specification 1,263,981 for transmitting a slow scan video signal over a low bandwidth channel, for recording the received narrow bandwidth signal as a series of vertical sweeps and reading it off as a series of fast scan horizontal sweeps to reconstitute the video signal in conventional form. General description.-As shown in Fig. 1, the narrow band signal at 12, which may be from a slow scan TV camera or a signal received over a transmission line, is supplied to a sample and hold circuit 20 which supplies signal samples to a pulse modulating circuit 50 under the control of a timing and delay circuit 21. The clock signal A is recorded around track 32 of a magnetic drum and via playback head 46 controls the modulator 50. Additional sync. signals are recorded on tracks 33 and 37, the signal from track 33 being frequency-divided at 22 and compared at 24 with frame sync. signals derived from circuit 18 to control the speed of motor 26. The width modulated pulses from modulator 50 are recorded on track 34 and read out via head 47. The output of head 47 is differentiated at 82 and compared in known manner at 86 with a sawtooth reference wave derived from the clock signal A to produce amplitude modulated pulses which are filtered at 88 and supplied to a cathode-ray tube 42. Switching circuits 38, 40, co-operating with tracks 34, 36, may be provided for interlacing purposes. Modulating circuit.-The clock pulses A, Fig. 6, are differentiated at 54, Fig. 2, inverted and differentiated at 58, 60 and rectified 56, 57 to provide pulses B which are supplied to a counter 62. After a predetermined number of pulses B (equivalent to the period T of the horizontal line of a fast scan signal) a bi-stable 64 is operated to open a gate 66 which passes a sampling pulse C. After a predetermined number of pulses C equal to one rotation of the drum 30, the counter provides one picture element delay before the next sample is recorded during the next revolution of the drum so that the pulses 1 1  to 1 N , 2 1  to 2 N , &c., Fig. 4, representing the vertical sweeps, do not overlap and erase each other, the time T being equal to the horizontal scan period of the signals 1 1  to M 1  &c. read out from track 34. Pulses C reset the bi-stable 64 and are also supplied to a monostable circuit 68 controlled by the input signal to produce width-modulated pulses D the leading edges of which coincide with the edges of corresponding clock pulses A. Pulses D are supplied to gates 72, 70 controlled by pulses A and inverted pulses A respectively and operating monostables 74, 76 respectively whose outputs E and F are combined at 80. By this means in the output G adjacent pulses are inverted in polarity and are of constant width and the leading edge of each pulse is spaced from the edge of the corresponding pulse A a distance corresponding to the amplitude of the input signal. The signal G is recorded on the drum 30 with sufficient amplitude to saturate it to either state depending upon the polarity of the applied pulse. Fig. 7 shows the signals recorded on the drum 30 during a plurality of revolutions. Line a represents clock signal A and during the first revolution waveform b will be recorded on track 34, i.e. waveform G of Fig. 6. On the next revolution pulses c adjacent those already recorded will be recorded but will be of opposite polarity and in overlapping relationship d, but will retain the significant information, i.e. the position of the leading edge of each pulse in relation to an edge of the corresponding clock pulse. Waveform e indicates the signal recorded after the M revolutions required to record a slow scan frame. In a modification, Figs. 9 and 10 (not shown), the track 34 is pre-recorded with the inverse of waveform A this allowing the signal samples to be recorded in any desired sequence.