Digital Speech: Coding for Low Bit Rate Communication Systems, 2nd Edition

9

Multimode Speech Coding

9.1 Introduction

Harmonic coders extract the frequency-domain speech parameters and speech is generated as a sum of sinusoids with varying amplitudes, frequencies and phases. They produce highly intelligible speech down to about 2.4kb/s [1]. By using the unquantized phases and amplitudes, and by frequent updating of the parameters, i.e. at least every 10 ms, they can even achieve near transparent quality [2]. However this requires a prohibitive bit-rate, unsuitable for low bit-rate applications. For example, the earlier versions of multi-band excitation (MBE) coders (a typical harmonic coder) operated at 8kb/s with harmonic phase information [3]. However, harmonic coders operating at 4 kb/s and below do not transmit phase information. The spectral magnitudes are transmitted typically every 20 ms and interpolated during the synthesis. The simplified versions used for low bit-rate applications are well suited for stationary voiced segment coding. However at the speech transitions such as onsets, where the speech waveform changes rapidly, the simplified assumptions do not hold and degrade the perceptual speech quality.

Figure 9.1 demonstrates two examples of harmonically-synthesized speech, Figure 9.1a shows a stationary voiced segment and Figure 9.1b shows a transitory speech segment. In both cases, (i) represents the original speech, i.e. 128 kb/s linear pulse code modulation, and (ii) represents the synthesized speech. The synthesized speech is generated ...