Patent GB-1318657-A (GB)
Title: Redundancy reduction systems and apparatus therefor
IPC: H04B1/66
Filed: 1970-12-15
Abstract: 1318657 Selective signalling WESTERN ELECTRIC CO Inc 15 Dec 1970 [18 Dec 1969] 59470/70 Heading G4H [Also in Division H4] In a redundancy reduction system, e.g. for a video telephone as described in Specification 1,218,529, or for the transmission in digital form of repetitively occurring framed data from a plurality of transducers, the number of bits necessary to transmit the location address of a transmitted sample in a group of samples is reduced by making the most significant bit transmitted that which has a value 1 and numbering the addresses from the end of the group of samples-e.g. the 9th and 13th samples in a group of 16 have addresses of 0111 and 0011 respectively and the addresses are transmitted as 111 and 11 respectively. In the embodiment shown samples in address groups of 16 are compared from frame to frame by means of frame memory 106 and selector apparatus 108. A sample detected to have undergone a significant change is fed as a four bit code amplitude word on lines A121, A122, A123, A124 to respective data flip-flops 305, 306, 307, 308 as well as being used to up-date memory 106. A transmit signal on line 250 reads the amplitude word into the flip-flops. The address of the sample to be transmitted, on lines 131, 132, 133, 134 is fed to and read into, data flip-flops 301, 302, 303, 304, the first sample in a group being indicated as 1111, and the last as 0000. The last sample in a group is forcibly transmitted for synchronism purposes by means of all-zeros detector 135, the output of which is coupled to transmit line 250 via OR gate 136. To reduce the address bits transmitted, one (0001) is subtracted from the address number at 201, this shifts the address one position towards the end of the group OR gates 215, 216, 217, 218 and control word flip-flops 221, 222, 223, 224 are connected such that if the least significant bit in the shifted address having a value 0, is the nth (n = 1 to 4), then flip-flops 30l-30n have state 0 and 30 n + 1-304 have state 1. The described circuit thus determines that of the four bits in the address of the next sample in the group, only the (n + 1th) to the 4th need be transmitted. This is summarized in the following table. Exclusive OR circuits 225, 226, 227 pass a 1 signal to stage 1, 2, 3 or 4, via AND gates 310, 311, 312, 313 if flip-flop 221, 222, 223 or 224 respectively is in the highest order 1 state-e.g. for control word 0111, stage 2 would be fed. If the control word is 0000 then inhibit gate 314 is uninhibited and stage 5 is fed. Each transmit signal generates eight very rapid (within a sample period) shift pulses, which shifts register 320 through all its stages. The 1 in stage 1, 2, 3, 4 or 5 is thus shifted down, enabling AND gates 341-348 in turn (if stage 1 is in the 1 state), and reading out the selected address bits and then the amplitude bits in serial form to buffer memory 351 and transmitter 352. At the receiver, a unique "start of transmission" pulse sequence is detected by 403 and opens gate 405. Bi-stable 407, 408 are cleared. The address and amplitude information is fed into and stored in memory 406. Each set of information is read out of memory 406 in turn into shift register 507 and thence into data flipflops 511, 512-518. The amplitude data is fed via gate 433 to up date frame memory 434 for display etc. in utilization apparatus 437 and the address word is fed to comparator 429 to bring local address generator 436 into synchronism. The pulses for reading out memory 406 and shifting register 507 are generated by a flag shift register 502 and pulse generator 505. The start of an address group is detected by all-zero detector 528.