Patent GB-597647-A (GB)
Title: Improvements in synchronising arrangements for electric television and facsimile systems
IPC: H04N7/08
Filed: 1945-03-17
Abstract: 597,647. Television ; pulse modulation ; pulse amplifying circuits. STANDARD TELEPHONES & CABLES, Ltd., CHATTERJEA, P. K., and HOUGHTON, L. W. March 17, 1945, No. 6799. [Classes 40 (iii) and 40 (v)] A synchronising arrangement for a combined picture and sound television transmission system combines means for generating a train of regularly repeated pulses of identical form having discontinuities introduced at specified intervals in the pulse repetition period each discontinuity affecting not more than two consecutive periods. The pulses are modulated with the sound signals and are transmitted over a channel different from that used for the picture signals. A pulse train, Fig. 1, consisting of line synchronising pulses a and frame synchronising pulses b is applied to terminal 1, of the arrangement of Fig. 2, which comprises a unit 2, introducing a time delay equal to the period T of the line synchronising pulses a, a unit 3 which is arranged to pass pulses fed to it from terminal 1 only when pulses from unit 2 are simultaneously applied, and a unit 4 which passes pulses to terminal 6 only in the absence of a pulse from unit 3. Hence a pulses are obtained at 5 and b pulses at 6. A suitable circuit for unit 3 is described with reference to Fig. 3 (not shown). Fig. 4 shows another arrangement for separating the pulse train of Fig. 1 which differs from that shown in Fig. 2 in that unit 3 is omitted and consequently both line and frame synchronising pulses appear at terminal 5, frame pulses alone appearing at terminal 6. The interval t, which defines the position of the b pulses between two consecutive a pulses, may be varied to provide additional facilities such as to control the mean brightness of the reproduced image. A further pulseseparating circuit shown in Fig. 5 is similar to that of Fig. 2 except that the delay introduced by unit 2 is now t and unit 22 is similar to unit 4 of Fig. 2. Unit 4 of Fig. 5 is either a subtracting or adding circuit and is arranged to cancel pulses which arrive simultaneously from terminal 1 and from unit 22. Fig. 6 shows another arrangement in which unit 2 introduces a delay t, the delayed pulses being added in unit 4 to those fed directly from terminal 1. The output from 4 thus consists of a series of a pulses followed, after delay t, by shadow pulses of equal amplitude except when a b pulse occurs in which case the amplitude of the b pulse is doubled. An amplitude limiting device 23 is arranged to pass b pulses only to terminal 6. A practical embodiment of Fig. 6 is described with reference to Fig. 7 (not shown). Fig. 9A, shows a synchronising pulse train for use when double interlaced scanning is employed. The a, b, and b2 pulses may be separated by the arrangements of Fig. 2 or Fig. 5 if unit 2 is made to provide two delay paths equal to T/4 and 3T/4. Fig. 8 shows an arrangement for separating the line and frame pulses of the train shown at Fig. 9A. A pulse generator 34 supplying rectangular pulses of period T and duration 3T/4, Fig. 9B, to two adding circuits 30, 31 (similar to 4 of Fig. 2), is excited by line synchronising pulses obtained from 32 and 2. The waveform applied to 32 is shown at c, Fig. 9 and 32 is designed to limit somewhere above zero voltage so that it transmits all a pulses but omits b pulses. A pulse train, similar to Fig. 9B, but in the positive sense, is applied to 31 which produces an output as shown in Fig. 9D. The limiter 33 is designed to operate at a level above the top of the a pulses so that only b pulses reach terminal 6. Another method of dealing with the pulse train of Fig. 9A, is shown at Fig. 10 in which 35 is a device which transmits an applied pulse as a positive or negative pulse according to the value of a controlling potential applied thereto. The characteristic curves of a pentode valve, Fig. 11 (not shown), indicate how this condition may be obtained. The necessary controlling potential is supplied by the pulse generator 34 and is such that 35 is conditioned to emit a positive pulse for each a pulse and a negative pulse for each b pulse, these two sets of pulses being passed by the amplitude limiters 32, 33 designed to pass positive and negative pulses respectively. A modification applicable only to sequential scanning consists in the periodic omission of a single line pulse to effect frame synchronisation. A pulse train of this type may be separated by the arrangement of Fig. 2 or Fig. 12, the latter being similar to Fig. 10 with the unit 34 omitted. During the intervals between frame pulses, line pulses are fed to 35 from terminal 1 and from unit 2 which introduces a delay T. The pulses from 2 condition 35 so that all line pulses emerge with the same polarity and are passed by 32 to terminal 5. On the arrival of the next pulse following the omission of a line pulse there is no corresponding pulse from 2 and 35 now operates to give a pulse of opposite polarity to the normal line pulse. This pulse which is used for frame synchronising is passed to terminal 6 by 33. Details of Fig. 12 are given in Fig. 13 (not shown). An arrangement for generating a pulse train similar to Fig. 1 or 9A, or of the type in which frame synchronisation is effected by the periodic omission of a line pulse is shown in Fig..14. Two generators 43, 44 generate a train of saw-tooth waves, 43 at line frequency (or a multiple thereof) and 44 at frame frequency, a link circuit 45 locking the two frequencies together. Line and frame scanning voltages are available at 50 and 51 respectively. The outputs of 43, 44 control the pulse generators 46, 47 the pulses from which are combined in 48, which is an adding or subtracting circuit depending on whether frame synchronisation is effected by the addition or omission of a pulse. The sound signals are carried by the synchronising pulses as amplitude or pulse-duration modulations. A complete system for sound and picture transmission with the picture signals and modulated synchronising signals transmitted over separate channels is described, Fig. 15 (not shown.) Specifications 442,333, 527,310 and 593,482 are referred to.