Patent GB-1317541-A (GB)
Title: Speech transmission system
IPC: G10L19/18
Filed: 1971-04-01
Abstract: 1317541 Speech communication system STANDARD TELEPHONES & CABLES Ltd 1 April 1971 8382/71 Head ingH4R In a speech transmission system non-fricative sounds, e.g. those in the frequency range 200 to 2000 Hz are transmitted by p.c.m. while fricative sounds, e.g. those above 2 KHz are transmitted as statistical information on the signal waveform which is used to control fricative noise generation at the receiver. As described the statistical information is based on the Markov statistics and the high frequency components of the speech are clipped and sampled at a clock pulse rate and the probabilities of the following pulse being a "1" after various different combinations of the prior four pulses is determined by feeding the input lipped speech into a shift register 20, Fig. 2, connected to feed the first four bits of the signal stream to an N- bit decoder 21 which produces an output on a respective lead in accordance with the particular sequence of 4 bits in the register 20. Each lead is connected to a respective counter A, which indicates the number of occurrences of that particular 4 bit combination, and to a gate G which also has an input from the fifth bit position of register 20. The output from gate G feeds a counter B which thereby indicates the number of times the particular four bit combination is followed by a "1", When counter A reaches a predetermined count the contents of counter B are transmitted to a register C, counters A and B are reset, and a flag flip-flop F is set to indicate readiness for reading. When all the flip-flops F are set the settings of registers C indicate the relative probabilities of the particular sequences of digits which probabilities are transmitted as the statistical information in respect of the fricative sounds. In addition the energy level of the fricative sounds is determined and the amplitude transmitted by delta modulation so that the amplitude of the fricative speech output at the receiver may be modulated in accordance with the amplitude of the signals at the input to the transmitter. At the receiver, Fig. 3, the probabilities received from the transmitter are entered in circulating registers 30 which are shifted at a rate CN where C is the clock pulse rate at the transmitter and N + 1 the number of digits in the groups in the probability determination. The outputs from the probability registers are gated through to a comparator 33 in accordance with the particular combination of the previous four digits received as determined by the N-bit decoder on the stages of the output shift register 32. The selected probability count from the appropriate one of registers 30 is fed to the comparator 33 to be compared with an equi-probable random number from noise generator 34. The gating on the input to flip-flop 35 is arranged so that it is set to a "1" condition only with unequal inputs. The probability number is fed to the gates in the comparator least significant digit first and the flipflop final state will correspond to the position of the most significant "1" that appears in one number and not the other and the final digit at the end of comparison of the two signals is fed into the register 32. In modifications the probability counts may be taken on transitions from one polarity to the other in the clipped speech and in addition the number of probability counts may be reduced by assuming equal probabilities for some particular binary series. Alternatively the statistics may be gathered in respect of run lengths.