Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 332

9.11 Acoustic Noise and Channel Error Performance
Robustness to background noise and channel errors is an important factor for
any practical speech-coding algorithm. The speech coders designed formobile
andmilitary communication applications frequently encounter acoustic noise
and channel errors. The background noise may be suppressed before the
encoding process using a noise preprocessor [54]. However, this involves
additional complexity and delay, which may not be desirable for mobile
communication applications. Therefore the speech-coding algorithms are
expected to produce intelligible synthetic speech even in the presence of
background noise. Generally, AbS coders perform better than parametric
coders under noisy background conditions. This inherent robustness of AbS
coders is due to their waveform-matching process. The error minimization
process attempts to synthesize the input waveform regardless of its contents.
The model parameters estimated by the parametric coders may not be
accurate when the input speech signal is corrupted with noise. Inaccurate
model parameters may severely degrade the synthetic speech of a parametric
coder.
Channel errors are usually divided into two classes: random errors and
burst errors. A speech-coding algorithm should provide a reasonable output
even if a small proportion of the received bit stream is incorrect due to random
bit errors. Robustness against random channel errors can be increased by
means of index assignment algorithms [55, 56], through proper quantizer
design, and by adding redundancy into the transmitted information [57, 58,
59]. Unequal error protection techniques may be applied to provide a higher
degree of protection to the most sensitive bits. For example, in CELP coders,
the spectral envelope parameters are the most sensitive to errors, followed by
the fixed codebook gain, the adaptive codebook index, the adaptive codebook
gain, the sign of the fixed codebook gain, and the fixed codebook index [60]. In
the case of sinusoidal coders, the gain is the most sensitive to errors, followed
by the voicing, the pitch, the spectral envelope parameters, and the spectral
amplitudes [61].
In the case of burst errors, error detection schemes are used to classify each
frame of received bits as usable or unusable. A similar problem encountered
in packet voice communication systems is lost packets due to transmission
impairments and excessive delays. In order to reduce the annoying artifacts
due to lost frames, concealment techniques based on waveform substitution
can be used [62]. The burst errors may also be converted to occur in a
more random fashion using interleaving techniques. The performance issues
specific to a hybrid coding algorithm are the robustness of the classification
algorithm under acoustic noise and the channel bit error performance of the
coding mode; otherwise, the performance of hybrid coders will be similar to
either ACELP or harmonic coding.