Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 273

9
Multimode Speech Coding
9.1 Introduction
Harmonic coders extract the frequency-domain speechparameters and speech
is generated as a sum of sinusoids with varying amplitudes, frequencies and
phases. They produce highly intelligible speech down to about 2.4 kb/s
[1]. By using the unquantized phases and amplitudes, and by frequent
updating of the parameters, i.e. at least every 10ms, 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 8 kb/s with harmonic phase information [3]. However, harmonic coders
operating at 4 kb/s and belowdo not transmit phase information. The spectral
magnitudes are transmitted typically every 20ms 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 using the split-band linear
predictive coding (SB-LPC) harmonic coder operating at 4 kb/s [4]. The
synthesized waveforms are shifted in the figures in order to compensate
for the delay due to look-ahead and the linear phase deviation due to loss
of phase information in the synthesis. The SB-LPC decoder predicts the
evolution of harmonic phases using the linearly interpolated fundamental
frequency, i.e. a quadratic phase evolution function. Low bit-rate harmonic