Mark Taylor wrote:
> So the filterbank is only in layer-3 because it was left over from
> layer-1 & layer-2? It does seem strange to do a Hann windowed DCT (the
> filter bank) and then process the output of that DCT with another
> DCT (as I understand it, the MDCT is a windowed DCT with a fancy
> overlapped orthogonal window).
>
> And what about the FFT for the psy-model? If, as in MPEG4, the
> whole frame is processed with a MDCT, couldn't these MDCT spectral
> coefficients be used for the psy-model, rather than requiring
> an additional FFT? (and anyone know why an FFT is needed
> for this stage, when everything else was a DCT? Is it important to have
> sines and cosines for the psy-model?
>
> ....
>
> Mark
In the encoder I�ve seen (MPEG1/2 and AAC), the psychoacoustic analysis is done
separately
from the rest of the encoder. In effect this psy-model, I thing, have been developed
separately using the result of �general� test done previously by Ehmer (1959)or
Hellman(1972) (this are examples, there are other people..).
The use of a FFT is explained, because although the MDCT is a frequency mapping , it
doesn�t give the same �energy� result as the FFT. So, to have a good (�precise�)
psy-model
is better to use the FFT-transformation and, after that, to adapt the output masking
threshold to the use of the MDCT transformation.
The MDCT is used, because (although an overlap of 50%) it give only � �frequency
value�-result respect an FFT-transformation: the �frequency mapping� transmission of N
samples needed the transmission of N-MDCT-frequency-values (with the use of the 50%
overlap!).
Davide
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Davide Manetti Phone: +41 32 718 34 25
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Electronics and Signal Processing Laboratory
Institute of Microtechnology
University of Neuchatel
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