Patent GB-1398928-A (GB)
Title: Voice or analogue communication system employing encoding techniques
IPC: H04B1/66
Filed: 1972-07-06
Abstract: 1398928 Signalling ADAPTIVE TECHNOLOGY Inc 6 July 1972 [9 Aug 1971] 31734/72 Heading G4H Analogue information is transferred from one station to another by sampling the analogue information at the transmitting station, transmitting a station identification at a time corresponding to the sampled value, and at the receiving station reconstructing the analogue information. In one embodiment, a north end unit drives a north transmission line which links a plurality of shift registers in series, each shift register being in a respective station. A south end unit drives a south transmission line which links a plurality of further shift registers in series, each register being in a respective one of the stations, so that the south line links the stations together in the opposite order from that defined by the north line. Each end unit uses an oscillator and counters to establish a sequence of transmission periods on its line, each period consisting of 134 subperiods, each of 10 bits. The oscillator of one end unit is frequency controlled via a phase-lock loop from the line fed by the other end unit and thus from the other oscillator 128 of the sub-periods are for transmission of analogue information, 4 are for a "handshaking" operation (for "request service", "acknowledge", "my station indentification is", and "terminate" respectively), one is for a sync pattern, and one is for a "boxing" operation (not described). The end unit applies the appropriate bit pattern to the line in each sub-period, this being a pattern meaning "empty" except in the case of the sync and boxing sub-periods, the sync sub-period receiving the sync pattern, and the boxing sub-period receiving a period count. The sync pattern is detected at each station when it is present in the shift register and is used for synchronizing communication on the line involved. Any station can communicate with any other, the "handshaking" operation being used by the initiating station to inform the other that communication is required and inform it of its (the initiating station's) identification. At the initiating station, a circulating stored station identification register is loaded from a keyboard with the identification of the other station, and at the latter station the corresponding register is loaded with the communicated identification of the initiating station. Communication is two-way, the north line being used for one direction and the south line for the other. The analogue information communicated is speech, done as follows. At the sending station, the speech waveform from a microphone is passed through a delay and then sampled at a rate determined periodically in accordance with the maximum significant frequency present in the speech at the delay input. The delay is to give time for this frequency determination on a segment of the waveform so that the sampling within each segment can be done at the rate appropriate to it. The lengths of the segments and correspondingly the delay could also be varied, e.g. in dependence on the latest sampling rate. When the sampling rate is changed, the new rate is communicated to the receiving station, either as analogue information is sent, every tenth period reserved for communicating the rate, or alternatively one or more extra subperiods in a normal period could be provided to send the rate or to specify that the analogue information portion of the next period will be used for this purpose. The amplitude of each sample of the speech waveform is digitized, the resulting 7-bit code being stored in a send buffer having space for 8 such codes. Alternatively, delta modulation, slope or predictive encoding could be used. The codes are taken in the order of storage and compared with a subperiod counter respective to the line being used, except that the count is passed to the comparator via a circuit which XORs each bit with a corresponding bit of a Z-number, this being 7 least-significant bits of the identification of the destination station. The Z-number may alternatively be changed according to an arithmetic permutation or at random. On equality in the comparison, if the current sub-period is "empty", the identification of the destination station is inserted into the sub-period on the line, and the code is deleted from the send buffer. The reason for the use of the Z-number is so that different pairs of communicating stations will have different assignments of voice amplitudes to sub-periods, so that the channel will not be overloaded by the more frequent amplitudes when a number of different pairs of stations are using the channel concurrently. The receiving station detects its identification, replaces it with the "empty" pattern, and gates the current count of the sub-period counter respective of the line, via a circuit which XORs each bit with a corresponding bit of the Z-number, to a receive buffer (capable of holding 8 codes) feeding (at the appropriate rate), a D/A converter, filter and audible output transducer. The filter time-constant may be controlled in accordance with the decoding rate. A random noise clock source may replace the oscillator in each end unit. The A/D converter in the sending station may have gain or attenuation stages to prevent the range of the converter being exceeded. These stages may be amplitude-dependent giving dynamic range compression or expansion, or this may be done in the converter itself. It may be logarithmic. The stations could be in a closed loop. The send and receive buffers in a given station may be part of the same memory addressable for read/write by any one of four counters (send read, send write, receive read, receive write), the two send counters being compared and the two receive counters being compared to detect buffer full/empty conditions. Flipflops distinguish full from empty. When either of the buffers is full and further write-in is required in that buffer, the oldest code stored in that buffer is discarded. Each station may be replaced by common equipment dealing with a respective group of 9 stations on a time-division basis.