Patent GB-849682-A (GB)
Title: Bandwidth reduction system
IPC: H04L25/493
Filed: 1957-09-30
Abstract: 849,682. Television. TECHNlCOLOR CORPORATION. Sept. 30, 1957 [Oct. 1, 1956], No. 30508/57. Class 40(3). The bandwidth requirement for the transmission of television signals is reduced by deriving therefrom binary electrical signals representative of discrete amplitude levels and of the duration of such levels these signals being stored at the rate at which they occur and then "read-out" at a uniform rate and transmitted as a multiplexed pulse train. As shown in Fig. 1, video signals from source 10 occupying a bandwidth of 4 mc/s. are supplied to a pulse coding device 14 to which is also supplied sampling pulses of 8 mc/s repetition rate from a generator 20. The device 14 operates in known manner to produce a code group of (in this case four) binary digits representative of discrete amplitude (i.e. brightness) levels in the input video signals and, in the arrangement shown, the transmission of one binary digit only is considered the apparatus employed being repeated for each binary digit in the output of device 14. In operation, a detector 16 supplies an output signal whenever the single binary digit considered changes from one binary representation to the other (i.e. a "one" or "zero") and this signal is employed as one input to a series of coincidence gate circuits 20A - 20E which receive as a second input the "pattern" condition of respective ones of a series of "flip-flop" circuits 18 forming a counter chain triggered by the pulses from generator 20. On the occurrence of a change in the binary output from 14 this "pattern", which represents the duration of the brightness level just terminated, is transferred via the gates to the (stationary) "writing" beams of a series of storage tubes 24A - 24E. At the same time, the output signal from 16 also conditions a further coincidence gate 20F to pass the binary digit which was present prior to the change (this being "held" by delay line 26) to the (stationary) "writing" beam of a further storage tube 24F. If no change has occurred in the output from 14 by the time the counter "flip-flops" 18 are completely "filled" the pulse output of the last "flip-flop" is effective via resistor 23 to open all the gates and allow such condition to be stored. When storage has been effected in all the tubes the "writing" beams are "stepped-on" by means of a horizontal deflection signal from a generator 30 under the control of the output signal from detector 16 this signal being delayed in 34 for a time sufficient to allow of storage being effected. In this manner the two possible brightness values of the video signals are stored in binary form in tube 24F and the duration of such values stored in a binary code pattern in tubes 24A - 24E. On the assumption that each complete picture has approximately 10,000 two-value brightness changes, and considering a 525 line picture, the storage pattern in each tube consists of 100 lines each containing 100 storage positions and the horizontal "resetting" and vertical "stepping" of the "Writing" beams of the tubes is therefore effected under the control of the original horizontal synchronizing pulses from generator 12 which are supplied to the horizontal and vertical stepping generators 30 and 32 respectively via a ¸ 5 circuit 36. The stored information is then "read-out" at a constant rate under the control of sweep generator 40 which supplies signals to the parallel connected deflection circuits of the "reading" beams of all the storage tubes and, after multiplexing in 44 and combination with synchronizing signals in 46, is supplied to a transmitter 48. At a co-operating receiver, Fig. 2, a multiplexer 54 is employed to produce six simultaneous outputs from the received sequence of signals and to supply them to the "writing" beams of storage tubes 56A - 56F. The digital information thus stored is simultaneously "read" out and supplied to respective registers 64A - 64F the "pattern" of the first five of which is representative of the duration of a particular brightness level and the condition of the last one, 64F, respresentative of the binary value of that brightness. When the "pattern" of the registers 64A - 64E corresponds to the "pattern" of the output of a counter chain 78, similar to the counter chain 18 of Fig. 1 and operated synchronously therewith, a comparator circuit 72 supplies an output signal, via a delay circuit 74, to control the "stepping" of the "read-out" deflection generators 66, 68 and, via a further delay circuit 76, to the "readout" guns of the storage tubes which turns the beams "on" for a short interval to allow the next stored digital information to be transferrcd to the registers. From the commencement of each transfer period, the binary digital signal representative of brightness, which is derived from register 64F, is effective, via the decoder 60, to maintain the corresponding brightness level in the picture reproducing tube 62. This level then follows any binary variation in the condition of the register 64F. In both the transmitter and receiver the use of the reflected binary code instead of the regular binary code reduces the magnitude of possible errors and an arrangement for converting a conventional binary code to a reflected binary code is described in connection with Fig. 3 (not shown). Instead of employing electron beam storage tubes other storage devices such as magnetic tapes (Fig. 4 not shown) or photographic film (Figs. 5 and 6 not shown) may be employed. Reference is also made to the use of storage devices employing Xerographic and electrographic recording.