Patent GB-730997-A (GB)
Title: Improvements in or relating to apparatus for reproducing recorded television or likesignals
IPC: H04N5/919
Filed: 1953-02-12
Abstract: 730,997. Multiplex signalling. MULLIN, J. T. Feb. 12, 1953 [Feb. 18, 1952], No. 3931/53. Class 40 (5). [Also in Group XL (b)] In a system for reconstituting television signals recorded on a record medium as a plurality of phase-displaced tracks each of which is representative of the signal to. be reproduced " sampled " at successive uniformly spaced intervals the samples on each track being intermediate those on each of the others so that the tracks jointly represent a continuous succession of equally spaced samples the signals derived from each track are first stored on storage means (individual to each track) which are subsequently discharged under the control of a train of phase-displaced pulses (derived from a timing wave common to all tracks) in the same order in which they are stored to provide a continuous succession of equally spaced samples equivalent to the original television signal. This method of operation permits a greater tolerance in the positioning of the reproducing heads (relative to the . positions occupied by the recording heads) since the storage allows the instants at which individual samples are taken to reconstitute the original signal to be precisely determined. Fig. 1, shows the general arrangement in which a number of reproducing heads 3, 3a ... 3n each engage respective ones of a plurality of parallel tracks on a moving magnetic tape 1, the track engaged by head 3 carrying a recording of an unmodulated timing wave of 165 Kc/s. and the tracks engaged by heads 3a ... 3n, each having recorded thereon a 165 Kc/s. wave, each half-cycle of which is modulated by a separate sample taken from the original television signal and each 165 Kc/s. wave being phase-displaced by 180‹/n. In operation the' phase-displaced signals-derived by the heads 3a ... 3n are supplied (via amplifiers 21a ... 21n)to decoding devices (which include the storage means) 23a ... 23n respectively which are conditioned successively by means of appropriate ones of a train of phase-displaced pulses, derived' from a delay line 19 and controlled by the 165 Kc/s. timing wave output of head 3, to supply the stored signal samples to a common output circuit 25 in continuous succession so as to reconstitute the original television signal, Fig. 2, shows details of one form the decoders 23a ... 23n may assume and comprises a pair of condensers 43, 431 which are charged to the peak value of the positive and negative half-cycles respectively of the sample modulated 165 Kc/s. wave, supplied from the appropriate track via conductor 27, under the control of a switching wave derived from the 165 Kc/s. wave via a ringing circuit 33 which is coupled to the circuit through amplifier 35 and inductances 37 and 29 the tuning of circuits 33 and 37 and the bias produced by elements 45, 47 (45<SP>1</SP>, 45) being such that condensers'43, 43<SP>1</SP> are charged during very short intervals when the switching wave reaches its peak value. The stored charges are then supplied to the output circuit 25 by means of a discharge circuit (similar to the charging circuit) comprising elements 49, 51, 53, 55 under the control of switching signals comprising positive and negative pulses of 0.1 micro-second duration which are derived by the differentiating action of the transformer formed by coils 51, 65 on the pulse train output of line 19 (Fig. 1), this train being fed thereto via amplifier 63 and cathode-follower 57. In a further arrangement allowing greater tolerance in the positioning of the reproducing heads, each decoder includes two pairs of condensers 43a, 43b (Fig. 4), the signal modulated (165 Kc/s.) wave being supplied (via lead 27) to each pair and the charging of which is controlled respectively by two 90-degree phase-displaced switching waves of half the frequency of the signal modulated wave (i.e. 82.5 Kc/s.). The switching waves are derived from the 165 Kc/s. wave via a ringing circuit 33 which excites an 82.5 Kc/s. oscillatory circuit 91, 93 from which the 0 degree phase is supplied, via amplifier 101a, to control the charging of condenser pair 43a and which supplies the 90-degree phase (derived by means of phase splitters 95, 97), via amplifier 101b, to control the charging of condenser pair 43b. In order to store the maximum amount of energy the biasing resistors 47a, 47b (equivalent to 47, Fig. 2) and the phases of the switching waves (which may be adjusted by means of condensers 39a, 39b) are adjusted so that charging commences when the signal modulated wave (165 Kc/s.) is passing through zero and terminates at the voltage peak of the wave. The charged condensers are then discharged in the appropriate order to supply the reconstituted television signal to the output circuit 25 by means of two 90-degree phase-displaced 0.1 micro-second pulse trains of 82.5 Kc/s. repetition rate produced by the differentiating action of coupled coils 105a and 117a (for the " a " side of the decoder) and coupled coils 105b and 117b (for the " b " side of the decoder) on two 90-degree phasedisplaced rectangular pulse trains derived from the unit 13 of Fig. 1, via separate multivibrators and delay lines (Fig. 3, not shown) and fed to the " a " and " b " sides of the decoder via valves 57a, 63a and 57b, 63b respectively (equivalent to valves 57, 63 of Fig. 2). With this form of decoder nearly two cycles of the 165 Kc/s. wave are available in which the discharge may be effected and this period may be increased by further sub-dividing the decoders. Fig. 6 shows an alternative embodiment in which the signal modulated (165 Kc/s.) wave is supplied via terminal 27 to an electronic switch 133 which is " closed " by pulses derived (from the 165 Kc/s. wave) in unit 131 and occurring at twice this frequency the signals passed by the switch, which comprises positive and negative peaks of the signal modulated wave, being fed via oppositely poled rectifiers to charge condensers 139 and 1391. These condensers retain their charge until discharged via switches 141 and 141<SP>1</SP> respectively under the control of short duration pulses occurring at the 0 and 180-degree phase angles of the 165 Kc/s. wave the output signals being developed in a circuit 25 in the same manner as the output signals in the arrangement shown in Fig. 2. The arrangement of Fig. 6, may replace that of Fig. 2 and may also form the individual elements of the sub-divided decoders described with reference to Fig. 4. An advantage of the decoder shown in Fig. 6 is a still further improvement in the position tolerance of the reproducing heads by reducing the charging period (by employing short duration pulses instead of comparatively broad crested sine waves) and consequently extending the period in which the discharge may be effected. Circuits for generating the charge and discharge switching pulses employed in the decoder shown in Fig. 6 are described respectively with reference to Figs. 7 and 8 (not shown). Specification 727,842 is referred to.