Patent GB-929889-A (GB)
IPC: H04N21/438
Abstract: 929,889. Pulse-code modulation systems. WESTERN ELECTRIC CO. Inc. May 25, 1960 [June 15, 1959], No. 18457/60. Class 40 (5). To avoid " wandering zero" and loss of timing information when transmitting information such as television signals which have relatively long periods in which the signal does not vary appreciably, every alternate code element of the train representing successive components of the signals is replaced by its binary counterpart before transmission. Transmitter.-Amplitude samples from a television camera 1, Fig. 1, are supplied to a cathode-ray tube mask-type coder 10 providing a parallel output in reflected binary code on leads 13-1 to 13-7. The code elements are subjected to different delays in networks 17 and are then applied to gating circuits 20-1 to 20-7. A 10 Mc/s. source 6 controls the sampling of the television signal every 0.1 Á.sec. and also supplies gating circuits 20 both directly, and via a divide-by-two circuit 24. The circuits 20 are controlled so as to invert alternate signals applied thereto from the respective blocking oscillators 15-1-15-7. There thus appears on output terminals D 1 -D 7 , seven simultaneously occurring code element trains which are displaced in time with respect to each other and alternate code elements in each of which are inverted. Terminals D 1 -D 7  are connected to different carrier transmitters. Receiver.-The separated simultaneously occurring code element trains appear on terminals D 1 -D 7 , Fig. 5, and the basic 10 Mc/s. timing signal is derived by blocking oscillators 31-1 to 31-7 and unit 32. The 10 Mc/s. timing signal from pulser 42 is applied to gating circuits 40 both directly and via divide-by-two circuit 41 so as to restore the original respective train of code elements on each of the channels. A conversion to the natural binary code next takes place in comparators 44, and the reason for differentially delaying the trains of code elements will now be apparent. Since the most significant digits of the natural and reflected binary codes are the same this digit is immediately available to determine the next digit of the natural code by comparison with the incoming corresponding digit of the reflected code. The remaining digits are differentially delayed to allow the respective other comparisons to take place. Delay networks 47-1 to 47-7 restore simultaneity of the seven digits representing each amplitude sample. Constant current switches 49-1 to 49-7 and weighted resistors 32R-R together with low-pass filter 50 complete the decoding process and supply the reconstituted signal to reproducer 52. The operation of converting the modified element trains to natural binary code may be performed by a single unit, Figs. 6, 7 (not shown). Synchronizing.-Three methods are described and these form the subject of Specification 929,890. In the first method, each synchronizing pulse of the television signal occupies about 50 coding periods and the peak level of the pulse is represented by the group 0000000. If the circuit 41 is of incorrect phase, units 40 develop the inverse code groups, and the occurrence of large numbers of the group 1111111 is detected and causes switch 80 to be opened for one clockpulse period. In the second method, since the digit of second highest significance in the reflected code is " 1 " for the centre of the signal amplitude range, this digit is more likely to be " 1 " than " 0," and the receiver is arranged to adjust the phase of circuit 41 when this digit is developed as a " 0 " more frequently than as a " 1 " over a considerable period of time. In the third method, one code group is not used in representing the signal amplitude, and thus can only occur at the receiver when circuit 41 is incorrectly phased, detection of the " forbidden " code group initiating phase adjustment of circuit 41. Specifications 663,872, 679,725, 694,030, 724,460 and 857,615 also are referred to.