Sine-Wave Amplitude Coding at Low Data Rates

Abstract

An analysis/synthesis system based on the sinusoidal speech model has been developed [1]. In that system, the sine-wave amplitudes and frequencies are located by searching for the peaks of the magnitude of the short-time Fourier transform (STFT) of the input speech. The phases are computed from the real and imaginary parts of the STFT at the measured frequencies. The frequencies on successive frames are matched, used in a cubic phase interpolator and applied to a sine-wave generator. Each sine wave is amplitude-modulated by the linear interpolation of the matched sine-wave amplitudes. At a 10 ms frame rate, this system produces speech that is perceptually indistinguishable from the original [1]. Since it is not possible to code all of the sine-wave parameters at low data rates, a system has been developed that codes the sine-wave frequencies by fitting a harmonic set of sine waves to the input waveform using a modified mean-squared error criterion [2], and codes the phase information implicitly using a voicing adaptive transition frequency to provide for a mixed voiced/unvoiced phase excitation model [3]. Provided a postfilter is used at the synthesizer to attenuate the noise in the formant nulls, the speech synthesized by this system is of quite high quality having achieved a DAM score of 63.0 in the uncoded mode. Since the fundamental frequency can be coded using ≈ 7 bits and the voicing measure can be coded using ≈ 3 bits, then the possibility exists for good speech quality at low data rates provided the sine-wave amplitudes can be coded efficiently. In this paper the zero-phase, harmonic analysis/synthesis system and the post-filter design methodology will be described and then the various techniques that have been examined for coding the sine-wave amplitudes will be discussed.

This work was sponsored by the Department of the Air Force.