JPS4999203A - - Google Patents

Abstract

1436440 Vocoders; delta modulators INTERNATIONAL BUSINESS MACHINES CORP 15 Nov 1973 [29 Dec 1972] 52961/73 Headings H4R and H3H [Also in Division G4] The pitch period of voiced speech is determined (e.g. for vocoding and speech/speaker recognition) by sampling, quantizing and coding the analogue speech signal by means of an adaptive delta modulator 10 which for each sampling interval produces one delta bit and one sign bit which are each fed to respective current and time-shifted recycling stores 11, the binary representations of features of the data (e.g. 1st and 2nd derivatives, equivalent to slope and curvature of the data) are compared 12 according to a predetermined key to control a counter 13, the interval between the beginning of comparison and regaining the initial count corresponding to the pitch period. The delta and sign bits are generated (Fig. 4) from the audio input 14 by quantizers 16, 18 sampled at rate f s (20 kHz). The effective step size of quantizer 16 is automatically adapted to the speech by multiplying 24 the delta stream by a signal 23 representing the average level of speech over a previous short interval and subtracting 15 the product from the speech. This reduces the redundancy of delta bits for large amplitude speech and improves resolution for small amplitude speech. The step size motor signal may be transmitted with the data. Each store defines a "window" (25, Fig. 1, not shown) longer than the expected pitch period and feeds the current and time-shifted bit streams in time multiplex to the extractor 12 where two or three successive bits of both the delta and sign streams are examined to evaluate whether the slope and curvature of the stream is positive, negative or zero (Figs. 5, 6, not shown). The results of the examination are then compared, one slope bit with one curvature bit according to the predetermined key (Fig. 7, not shown) to produce zero or decrement commands to the counter if either bit of a pair is zero or the bits are of opposite sign respectively. If the bits are the same, the product of the current and time-shifted sign bits determines incrementing or decrementing commands. The counter reaches a peak of similar count to that at the start when the end of the pitch period is reached. Alternatively or additionally, the third and fourth derivatives may be used in the examination and comparison operation.