NL7307871A - - Google Patents

Abstract

1412518 Vocoders SIEMENS AG 6 June 1973 [6 June 1972 13 June 1972] 26955/73 Heading H4R In a digital channel vocoder for generating synthetic speech, pulses from a pitch frequency or noise generator, according as the " speech " is voiced or unvoiced, are duration (rather than amplitude) modulated with the spectral envelope parameter in the respective band-pass filter channel. As shown, the coded spectral envelope A, B, C for one channel is temporarily stored 1 for 20 ms. (equivalent to a TDM scanning period for the channels) and read-out under command of transfer pulse UP from an input 11 through a binary counter 41 with a device 31 which presents to zero all those stages of the counter 41 for which the output of a code converter 22, common to all channels, provides a one. Pulses at 5 MHz for unvoiced and 1 MHz for voiced speech, determined by parameter TS on line 23, pass to the converter 22 which generates a series ZJ of 7 pulses whose distance from a start pulse SJ derived from a pitch generator 24 or noise generator 25 is in each case doubled. The counter output via NAND gate 51 represents the duration-modulated output of that channel and resets the counter via gate 13 when all counter stages are at one. The synthesis unit precedes or follows a bandpass filter and the channels may include amplifier stages 91. Amplitude modulation of amplifier stages in the channels may be superimposed on the duration modulation. In one modification (Fig. 3, not shown) there is a code converter (2) for each channel instead of the central unit 22 and this generates a 7-bit spectral envelope code from a 3-bit code which may have been stored or transmitted, the presetting device (3) setting a seven-stage counter (4) according to the code converter output. Alternatively (Fig. 5, not shown) the spectral envelope code A, B, C may be fed to an IC selector (6) which, according to the code, selects the appropriate one of a succession of pulses of geometrically increasing spacing (corresponding to those from the central code converter above) to operate a flip-flop 7 to stop passage of the pitch or noise pulses SJ to the output, thereby duration-modulating them. Digital generation of the pitch frequency is preferred by successive binary division of a pulse sequence (TT0), each stage output feeding NAND gates also receiving the inverted sequence (TT0) and an element of the spectral code. The combined pulse train is divided (163) to bring it down to the desired rate and routed through a monostable (164) to provide regular pulses.