Jan at 12:48, 22 March 2005

2005-03-22T12:48:57Z

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The encoded [[MDCT]] coefficients are grouped into scalefactor bands, allowing an efficient coding. Noise shaping introduces as much [[quantize|quantization]] noise as the [[masking]] for a particular band allows.

It does this by adjusting the [[quantize|quantization]] stepsize. The larger the stepsize, the more [[quantize|quantization]] noise you'll get. In Mpeg layer3 the global gain defines the largest stepsize to use. The scalefactors are used to reduce the stepsizes for the special needs of the scalefactor bands.

While there already is an inherent noise shaping in the non-linear [[quantize|quantizer]] it is usually not sufficient to achieve acceptable audio quality. To improve the subjective quality of the coded signal the noise is further shaped via scalefactors. The way the scalefactors are working is the following:

Scalefactors are used to amplify the signal in certain spectral regions (the scalefactor bands) to increase the signal-to-noise ratio in these bands. Thus they implicitly modify the bit-allocation over [[frequency]] since higher spectral values usually need more bits to be coded afterwards. Like the global [[quantize|quantizer]] the stepsize of the scalefactors is 1.5db.

To properly reconstruct the original spectral values in the decoder the scalefactors have to be transmitted within the bitstream.

Whenever a scalefactor band is amplified, it will force the next [[quantize|quantization]] to use more bits for that band. This will result in more bits used to encode the [[MDCT]] coefficients in that band, and thus less [[quantize|quantization]] error. That is why bands with audible distortion are amplified. However, it will also result in less bits for the unamplified bands. But these bands had a [[quantize|quantization]] error less than the allowed [[masking]], so hopefully they can tolerate a little more noise. The whole procedure is designed to allocate the bits to the bands which need them the most.

Scale factor - Revision history

Jan at 12:48, 22 March 2005