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
For each subframe the LSP-coefficients are interpolated and converted to LPC-filter coefficients.
Then, for each subframe, the excitation signal is constructed by adding the adaptive
and fixed-codebook vectors multiplied with their respective gain. The excitation is then filtered
through the LPC- (or short-term) filter. Finally the reconstructed speech is further
enhanced by a post-filter.
The signal flow of the decoder is displayed in Figure 16 [16].
3.6 G.729A
Figure 15: Decoding principle of the G.729
The G.729A is a reduced complexity version of the G.729, i.e it requires much less processing
power. The G.729A is bitstream compatible with the G.729. Thus, the speech can
be encoded with an G.729 encoder and decoded with a G.729A decoder, or vice versa.
One important note though, the synthesized speech of the G.729A is not as good as
from the G.729 in some cases. The G.729A has a MOS of 3.7, which is not toll-quality,
compared to the G.729 which has a toll MOS of 4.1. Therefore, one might question the
suitability of using the G.729A in a ToIP system which is intended to provide a high quality
service. However, according to [1] the G.729A only suffers in performance during multiple
tandem encoding (encoding and decoding the signal repeatedly) and is not a reason for
anyone to not use the G.729A compared to the G.729.
The G.729A has only some small differences compared to the G.729. The differences
in the encoder and decoder are stated in the following two sections. The reader interested
in detailed changes in the C source code should refer to document [17] and [31].
3.6.1 Encoder Differences Between G.729 and G.729A
The modules which have been simplified are:
• Perceptual weighting filter
• LP to LSP transformation
• Search for adaptive-codebook delay
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