GB1172080A - Improvements in or relating to Vocoders - Google Patents

Abstract

1,172,080. Vocoder synthesizers. INTERNATIONAL BUSINESS MACHINES CORP. 29 Nov., 1966 [1 Dec., 1965], No. 53266/66. Addition to 1,087,305. Heading H4R. In an analysis-synthesis telephone system of Specification 1,087,305, in which digital signals are transmitted specifying the excitation function and the amount of energy in each of a number of spectrum analysis bands, the energy in which is sampled at a frequency equal to the excitation frequency, the receiver includes two digital to analogue converters which simultaneously process signals corresponding to predetermined respective parts of the spectrum of the speech to be reproduced. Fig. 1 shows the synthesizer of Specification 1,087,305 in which incoming digital signals are split in input device 2, the aggregate function component being fed to register 6 and the excitation function component to register 3. The digital signals representing the excitation function period are fed into a counter 4, to set it to a count corresponding to the complement of the period, and the counter is stepped on by the output of pulse generator 5 to produce an excitation function pulse when the counter reaches its full state. During unvoiced sounds the output of generator 14 is enabled to supply random pulses since pulses derived in the normal way would be at too low a frequency. The digital signals representing the aggregate function are fed from register 6 via digital to analogue converter 7 to the inputs of gates 8 1 to 8 n which are enabled by the distributer 11, which is fed with the excitation function pulses, so that excitation function pulses, modulated in amplitude according to the appropriate spectrum channel energy levels, are fed to the spectrum channel filters 9 1 to 9 n from the outputs of which the spectrum channel signals are combined and fed to output amplifier 10. In the modification according to the present invention, the aggregate function register 6 comprises, as shown in Fig. 3, a device R in which groups of twelve digits are stored representing four characters, each expressed in three digit code giving the amplitude, quantized into 8 levels, of the energy levels in four spectrum channels. The four characters are fed into two digital to analogue converters 18 i alternately in pairs, i.e. digits L 1 , L 2 and L 3 fed via AND gates A 1 , A 2 and A 3 and OR gates O 1 , 0 2 and 0 3 , into converter 18 1 while digits L 4 , L 5 and L 6 are fed via A 4 , A 5 and A 6 and O 4 , O 5 and O 6 into converter 18p. The analogue output of the converters is fed, as shown in Fig. 7, for 18 i , into a current generator 7 i which produces an output current, S, having a level corresponding to the digital input to converter 18 i . The current S is applied in common to the gates 81, 83 ... 8 15 which are rendered conductive in turn by the output of counter 11 so that the particular value of current supplied by the digital to analogue converter is fed to the appropriate one of the spectrum reconstitution channels. In addition, pulses at excitation frequency are fed to the gates 8 1 to 8 15 from the units 19 i where, by means of the logic circuits shown, the width of the excitation function pulses is changed in accordance with the voiced/unvoiced switching signal so as to reduce the energy content of the higher frequency unvoiced excitation pulses. Fig. 8 shows the circuit of a gate 8 and filter 9, the gate comprising transistor T 8 to which the output of the distributer 11 is fed together with the voiced/unvoiced logic circuit output so that the transistor only allows transistor T 9 to conduct when a signal is present from the distributer 11 and for a time dependent on the voiced/unvoiced logic circuit output applied over line 16 i . The value of the current fed through T 9 to charge C p will depend on the signal applied to the base of T 9 which is the output signal from the digital to analogue converter 18 and current generator 7. Thus the spectrum channel filter L p , C p and L s , C s receives pulses at excitation frequency of a level determined by the received digital aggregate function information. The pulse from all the spectrum channels from both the analogue to digital converters are combined in an output amplifier 10 (Fig. 7). Each operation of the distributer 11, which is triggered by each excitation function pulse to enable each spectrum function channel in turn, is also arranged to initiate the feed of spectrum function signals into the register 6 so that synchronism is maintained between the information fed into register 6 and the opening of the appropriate ones of gates 81 to 8 16 . During unvoiced sounds the signals fed to the distributer will be at a higher frequency than the original spectrum sampling frequency and so unvoiced sounds will be compressed in time.