GB1187536A - Processor for Helium Speech - Google Patents

Abstract

1,187,536. Vocoder helium speech processor. STANDARD TELEPHONES & CABLES Ltd. 28 Aug., 1968, No. 41143/68. Heading H4R. A processor for helium speech, e.g. from divers, comprises a vocoder with analysis channels arranged over one frequency range e.g. 0 to 6 kHz. and synthesis channels arranged to reproduce the speech in a compressed frequency range, e.g. 0 to 3 kHz., in which individual excitation signals are derived for a number of channels or groups of channels in order to improve the intelligibility. Fig. 3 is a block diagram of one channel of the processor and includes an analysis filter 13, amplifier 14, and envelope detector 15, the output from which is low pass filtered in 16 to serve as the spectrum function control signal for a synthesis modulator 17, and also fed to a threshold detector 19, and band-pass filter 20, to pick out the excitation frequency, which is infinitely clipped by clipper 21, the peaks detected by 22 and the negative-going edges used to trigger the excitation pulse generator 23, the output from which is applied to synthesizer modulator 17. The negative-going edges of the clipped signal are used to trigger the pulse generator rather than the positive to provide a small delay to compensate the delay in the spectrum analysis channel, and the energy level of the pulses is arranged to be maintained constant regardless of frequency by varying the pulse amplitude. As described each channel may use an excitation signal derived in the channel, as in Fig. 3, or the six highest frequency channels may be provided with individual excitation signals while the remaining channels use a common excitation signal derived from the lowest frequency channel. Circuits are described for the analysis channels, Figs. 4 and 5 (not shown).