Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 345

excitation memories. However in Figure 9.40b, the next frame after decoding
an ACELP frame as harmonic is also harmonic. Hence, the error propagates
into the next frame, due to the harmonic interpolation process.
The LPC filter may propagate the errors, when the filter response is highly
resonant. However the bandwidth expansion of the LPC coefficients ensures
that the LPC impulse response dies awaymore quickly. Therefore all themode
errors are localized and the output does not become unstable in the presence
of mode errors. This is mainly due to the independent memory initialization
procedures of the coding algorithm when switching between the modes.
The white-noise excitation mode always sets the previous gain equal to the
present one when switched from a different mode. The harmonic excitation
mostly depends on the received harmonic parameters when switched from a
different mode; only the amplitude quantizer memories are initialized using
the previous excitation vector. The LTP buffer is refreshed, regardless of the
mode, with the latest excitation vector.
9.11.3 PerformanceImprovementUnderChannel Errors
During the experiments described in the preceding sections, the robustness
to mode-bit errors was improved by limiting the LTP gain to 1.2 and using
the same set of bits to transmit the LSFs of all the modes. The encoder and the
decoder cannot synchronize the random number generators at the presence
of mode-bit errors. This affects the performance of the LTP when switched
from white-noise excitation. However the exact content of the white-noise
excitation has no significance and can be represented by any noise excitation
vector. Therefore, the performance of the LTP was also improved by always
reinitializing the LTP buffer to a fixed stored noise excitation vector when
switching to ACELP from the white-noise excitation.
The robustness to mode-bit errors can be further improved by using error
detection and correction techniques. If a mode error is only detected and
not corrected, the concealment techniques based on waveform substitution
can be used to reduce the resulting annoying artifacts [62]. The decoded
parameters and the synthesized waveform may also be used to detect mode
errors. As can be seen in Figures 9.38, 9.39, and 9.40, mode errors generally
result in sudden changes in the waveform shape and the signal level, which
are unusual for speech signals. Moreover certain mode patterns are more
common than the others, e.g. for many speech utterances, ACELP to harmonic
and back to ACELP occur, while the silence segments before and after are
synthesizedwith thewhite-noise excitation. The transition fromwhite noise to
harmonic mode is extremely rare, since generally the onsets request ACELP.
Consequently in order to assist in detecting mode errors, one can limit the
possible switching combinations.