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
Then, for each subframe analysis-by-synthesis is performed to retrieve the CELPparameters
for the best synthesized signal.
In Figure 13, the excitation signal consists of contributions from both, the adaptive and
fixed codebook. The adaptive codebook then simulates the pitch, and therefore voiced
sounds. The fixed codebook simulates unvoiced sounds. In reality, the adaptive codebook
is an adaptive filter through which the fixed excitation signal is filtered. The following two
sections deal with the fixed and adaptive codebook in detail.
3.3.1 Fixed Codebook
The fixed codebook has an algebraic structure and is called interleaved single-pulse permutation
(ISPP) design [13]. Each 40-sample fixed codebook excitation vector has only
four pulses, denoted i0, i1, i2 and i3, with, as previously mentioned, two possible amplitudes:
+1 and -1. The fixed codebook is displayed in Table 4. Each excitation codevector
is a sum of four pulses, given by
c(n) = s0δ(n − m0) + s1δ(n − m1) + s2δ(n − m2) + s3δ(n − m3) n = 0, ..., 39 (15)
where δ(n) is the unit impulse at time instant n.
Table 4: G.729 fixed codebook
Pulse Sign Positions
i0
i1
i2
i3
s0 : +
− 1 m0:0, 5, 10, 15, 20, 25, 30, 36
s1 : +
− 1 m1:1, 6, 11, 16, 21, 26, 31, 36
s2 : +
− 1 m2:2, 7, 12, 17, 22, 27, 32, 37
s3 : +
− 1 m3:3, 8, 13, 18, 23, 28, 33, 38
4, 9, 14, 19, 24, 29, 34, 39
The fixed codebook is searched in four nested loops, one for every pulse. However, to
reduce complexity, the fourth loop is not entirely searched since a full search in the fourth
loop would produce slightly higher speech quality, but also require high computational effort.
Since the four excitation pulses are unable to have the same position, the search is
named conjugate-structured.
As can be seen in Table 4, the index to the fixed codebook consists of two parts: sign
and position. Each pulse requires one bit per sign, thus, a total of 4 bits is needed to
encode the signs. For the first three pulses, i0, i1, i2, 3 bits are needed to encode the pulse
positions. For the fourth pulse, i3, 4 bits are required. This yields a total of 13 bits to index
the pulse positions. Thus, altogether 4 + 13 = 17 bits are needed to transmit the fixed
codebook index (without gain), which in fact is done once every subframe.
3.3.2 Adaptive Codebook
The adaptive codebook, which in fact is an adaptive pitch filter is given by
P (z) =
1
1 − βz −T
(16)
32 (78)