TwinVQ
To increase coding efficiency for coding of musical signals at very low bitrates,
TwinVQ based coding tools are part of the General Audio coding system in MPEG-4
audio. The basic idea is to replace the conventional encoding of scalefactors
and spectral data used in MPEG-4 AAC by an interleaved vector quantization
applied to a normalized spectrum [6][7].
The rest of the processing chain remains identical as can be seen in Fig.
2.
Fig. 3 visualizes the basic idea of the weighted interleaved vector quantization (TwinVQ) scheme. The input
signal vector (spectral coefficients) is interleaved into subvectors. These subvectors are quantized using vector
quantizers.
Figure 3: Weighted interleaved vector quantization
Twin VQ can achieve ahigher coding efficiency at the cost of always creating a minimum amount of loss in terms
of audio quality. Thus, the break even point between Twin VQ and MPEG-4 AAC is at fairly low bitrates (below 16
kbit/s per channel).
