Hybrid frequency-domain coding of speech signals

Abstract

A new method of coding speech signals is introduced and its application to the low-bit-rate transmission of speech is examined. The new method, which is referred to as hybrid frequency-domain coding (HFDC), is based on a combination of sub-band coding and transform coding techniques and thus involves block coding.In one version of the HFDC scheme, a fixed data rate is used. In this version, blocks of speech signal are coded using a fixed number of bits. The technique is of relatively low complexity and results of subjective tests indicate that the quality of speech produced by the technique at 7.2 kbit/s is comparable with that produced by 19 kbit/s continuously variable slope delta modulation (CVSD); and that at 9.6 kbit/s, it is comparable with 23 kbit/s CVSD.In a further application of the HFDC scheme, a variable bit rate technique is used which takes account of the nonstationary nature of speech. Block coding is again used but. the number of bits allocated is made to depend on the characteristics of the block. Subjective tests have shown that. improved speech quality can be obtained at the expense of slightly increased end-to-end delay.Quadrature mirror filters are important in sub-band coding and a new method of designing these filters is described. The effects of finite-word-length on the performance of systems using quadrature mirror filters are considered and formulae are derived relating to the bounds and variances of the errors due to analog-to-digital conversion; quantization of the filter coefficients; roundoff in arithmetic computations and the quantization associated with compression encoding. The results obtained from extensive simulations are shown to be in excellent agreement with the theoretically derived values.Various other issues relating to the design of the HFDC scheme are considered. These include the analysis of errors resulting from the use of the block-floating-point fast Walsh-Hadamard transform, and the automatic classification of speech into voiced, unvoiced and silent intervals. Last but not least, I would like to express my thanks to Dr.Tay Eng Soon for his support and encouragement; and to the Ministry of Defence for granting the DSO scholarship and the paid study leave. 5