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Quantization using the Masking Threshold

  The main reason for using a psychoacoustic model for audio compression is that given a masking threshold tex2html_wrap_inline1060 , the amplitude at that frequency may be quantized with a step size proportional to tex2html_wrap_inline1060 . The quantization can be seen as introduction of noise with power proportional to tex2html_wrap_inline1060 :

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The quantization error can then easily be adjusted to be lower than the masking threshold, and thus become inaudible.

In the implementation of the coder, the psychoacoustic model is adjusted using only a quantizer (i.e no coding) with step size tex2html_wrap_inline1060 on every transform coefficient. The model is adjusted until the effects of the quantization is inaudable. After this process is finished, the actual audio coding and quantization is developed. This way, the coding is kept separate from the psychoacoustic model. Thus, when I start to design the coder, I can be certain to get perceptually perfect data independent of coding method.



Bosse Lincoln
Sat Mar 7 16:27:43 PST 1998