G.726 is a speech coding and decoding
recommendation of ITU that provides 4 Kbps speech bandwidth switchable
at 40, 32, 24 or 16kb/s. It specifies an ADPCM (Adaptive
Differential Pulse Code Modulation) codec system to achieve the
overall compression ratio of 2.8:1, 3.5:1, 4.67:1, or 7:1 for
14-bit linear data at 8 kHz and 1.6:1, 2:1, 2.67:1, or 4:1 for
8-bit compounded data at 8 kHz.[45]
G.726 also includes other ITU recommendations
such as G.723, which is the subset of the 3, 4, and 5 bit versions
of G.726, and G.721, which is the 4bit version of G.726 (32 Kbps).
ADPCM
(Adaptive Differential Pulse Code Modulation) codec:
ADPCM is a codec that instead of
quantising the speech signal directly, like PCM, it quantizes
the difference between the speech signal and a prediction that
has been made of the speech signal and produces the PCM values
of the differential. Then if the prediction is accurate, this
difference will be quantised with fewer bits than the quantisation
of the original speech samples. Adaptive prediction and quantisation
is used in this codec to adapt to the changing characteristics
of the speech. (Figure 2-22) [46]
Figure 2-22 ADPCM
encoder
The decoder operates in the counter
fashion (Figure 2-23).
Figure 2-23 ADPCM
decoder
