A Study of the ITU-T G.729 Speech Coding Algorithm ...

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MASTER THESIS
Datum - Date Rev Dokumentnr - Document no.
04-09-28 PA1
CELP-coding, the long-term predictor is also called an adaptive codebook. The adaptivecodebook
entries are segments of the recently synthesized signal [15]. The short-term
predictor accounts for the spectral envelope, i.e. the formants, and is the same type of
production filter used in vocoders. Figure 11 depicts how speech is synthesized in a CELPencoder
[27]. Thus, the fixed innovation comes from the fixed codebook which is then
filtered through the long-term production filter followed by the short-term production filter.
Another way to see this is that the excitation vectors from the adaptive and fixed codebook
are added to construct the excitation signal which is then filtered through the short-term
production filter.
Figure 11: Long-term prediction / Adaptive codebook
By subtracting the long-term and short-term predictions of the speech signal, the redundancy
of the signal is removed. The remaining signal is called the innovation or excitation
signal. A set of signals which approximates the possible excitation sequences forms the
codebook. Thus, a CELP- encoder algorithm consist of three stages:
1. Finding the best long-term and short-term linear predictors.
2. Filtering the predictor filters with all possible excitations and finding the sequence in
the codebook that minimizes the perceptually-weighted mean squared error. This
is an analysis-by-synthesis procedure. The perceptual weighting takes into account
how humans perceive sound or speech and is absolutely essential to the success of
CELP-coding.
3. Send information to the decoder about the long- and short-term predictor coefficients
and the binary codebook word corresponding to the sequence chosen from the codebook.
The decoder then looks up the excitation sequence from the codebook with the received
codebook word and drives the excitation sequence through the long- and short-time production
filters [13]. The CELP-coding principle is depicted in Figure 12 [8].
CELP Complexity
A CELP-coding algorithm is complex and therefore computationally demanding. Assuming
a CELP-coder that uses a 10-bit word as an index to the codebook, would give a codebook
consisting of 2 10 = 1024 excitation sequences. Applying each of these 1024 excitation
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