Patent GB-854026-A (GB)
Title: Bandwidth reduction systems
IPC: H04N7/26
Filed: 1959-01-06
Abstract: 854,026. Bandwidth reduction systems. TECHNICOLOR CORPORATION. Jan. 6, 1959 [March 24, 1958], No. 487/59. Class 40 (4). [Also in Group XL (b)] A video signal is transmitted by separating the low-frequency (<¢ Mc.) components which are sent separately, and by sending difference signals indicative of the difference between the existing signal and its value a predetermined interval previously, said difference signal being quantized and transmitted only when said quantized difference is other than zero, together with a second signal indicative of the interval that has elasped since the last non- zero signal. The difference signal is sent by pulse code modulation and the interval is sent in binary form. Transmitter.-As shown, Fig. 1, the source of video signals (e.g. television signals) are fed to a low-pass filter 12 which passes all signal below ¢ Mc. and also to a subtraction circuit 14 arranged to subtract from the existing signal, the signal as delayed by a delay line 16 whose delay corresponds to one element of the picture. The difference signal is then fed to a quantizer and pulse code modulation encoder 20. This emits a binary signal indicative of the magnitude of the difference together with a further binary digit indicating whether the difference is positive or negative. The signal is then fed to a " run-length " encoder 22 consisting of a binary counter which has been counting the interval from the time of receipt of the last non-zero signal from the encoder 20. For each non-zero output from the encoder 20 the circuit substitutes two digits, the first indicative of the value of the non-zero output and a second indicative of the run length, These pairs are stored asynchronously and read out at a suitable speed for transmission at 18 together with the lowfrequency signal (which may also be sent in pulse code form). Receiver, Fig. 2. The coded signals, received over the lower receiver output, are fed to a runlength decoder 26 which establishes the time interval indicated by the second digit of a pair and at the end of it feeds the first number to the pulse code modulation decoder 28 for emission at the end of said time interval. Moreover, at the end of this time interval the next pair of digits is admitted to the run length decoder 28. The output from the decoder 28 is fed to a pulse shaper 30 for reconstituting the high-frequency components. Thus, as shown in Fig. 4, if curve 40 represents the original video signal, 42 represents the separately received low-frequency components, and 44 represents the output from the decoder 28, it is necessary to form therefrom a signal of the type shown at 46 for addition to that of 42 in order to reconstitute the video signal. Fig. 5 illustrates the pulse shaper in detail. The difference signals are fed to an integrating circuit 48 to produce a square-topped output. This is fed to a low-pass filter 50 whose output is subtracted from the square-topped signal to produce the required high-frequency output. The delay line 51 compensates for delay introduced by the filter 50. The highfrequency-signals are then combined with the low-frequency signals (which may require to be delayed slightly) in the adder 32 to produce the original video signal. Modifications.-Further band-width reduction may be obtained by introducing further subtraction circuits in which comparison is effected between successive lines or successive frames or both. Thus, as shown in Fig. 8, the subtraction circuit 72 is fed with the existing signals and those delayed by a period equal to the frame interval and the output of circuit 72 is fed to a further subtraction circuit 76 which emits the difference between these signals and corresponding ones existing a line interval previously. The output of circuit 76 feeds the subtraction circuit 14 of Fig. 1. At the receiver the output of the pulse shaper 30, feeds two adder circuits 78, 82 in tandem, each adder circuit having a recirculating input passing through delay networks whose delays are respectively that of a line duration and a frame duration.