Digital Signal Processing Reference
In-Depth Information
10.2 Standard VAD Methods
In order to exploit the advantages of silence compression, a number of VAD
algorithms have been proposed, some of which have been selected by stan-
dards organizations including ITU-T, ETSI, and TIA/EIA. ITU-T released
G.729 Annex B (G.729B) [1] and G.723.1 Annex A (G.723.1A) [2] as exten-
sions to the 8 kb/s G.729 [3] and 5.3/6.3 kb/s G.723.1 [4] speech coders for
performing discontinuous transmission (DTX). ETSI recommended GSM-FR,
-HR, and -EFR VAD methods for European digital cellular systems [5-7].
Recently, ETSI released two more VADs, adaptive multi-rate VAD option
1 (AMR1) and option 2 (AMR2) [8], with a view to using it in UMTS
(the third generation mobile communications). The North American stan-
dards organisation, TIA/EIA, released two VADs one for IS-96 [9] and the
other for IS-127 [10] and IS-733 [11] (the VADs suggested for IS-127 and
IS-733 have the same structure). Table 10.1 shows standard VADs classi-
fied in terms of the input features mainly consisting of subband energies
and the spectral shape. For example, the TIA/EIA VADs use a small num-
ber of subbands whereas the IS-96 VAD uses the overall signal energy.
The IS-127 and IS-733 VAD, on the other hand, decomposes the input sig-
nal into two subbands only. Traditionally, ETSI VAD methods have been
based on a more accurate spectral shape of the input signal. The reason
behind this is that the energy of the predictive coding error increases when
the spectral shapes between the background and input signal mismatch
(i.e. when speech active). However, in the recent standard for AMR, ETSI
adopted two kinds of VAD algorithms both of which are based on the
spectral subband energies rather than the more accurate spectral shape.
The ITU-T VAD standards, G.729B and G.723.1A, conduct the detection
using four different features including both the spectral shape and subband
energies.
Table 10.1
Classification of standard VAD
methods depending on input features; the val-
ues in parentheses indicate the number of spec-
tral subbands
Main features
VAD
Spectral shape
GSM-FR, GSM-HR, GSM-EFR
Sub-band energies
IS-96 (1), IS-127 (2), IS-733 (2)
AMR1 (9), AMR2 (16)
Others
G.729B, G.723.1A
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