[Mp4-tech] HE-AAC downsampled synthesis filterbank

[Andreas Schneider]

Hi Andy,
this is no trick. It was a typo in the specification that has been 
corrected by ISO/IEC 14496-3:2005/Cor.4:200X. As the 200X suggests, this 
corrigendum is not yet published.
Kind regards,
Andreas
mp4-tech-bounces lists.mpegif.org wrote on 25.09.2023 14:25:14:
> Hi folks,
> 
> I read the ISO spec 14496-3 Amd 1 on downsampled complex synthesis QMF
> filter. On page 67, it is said the filterbank coefficients are defined
> as (1/64)*exp(i*pi*(k+.5)*(2*n-127)/64).
> However, when I turn to the ISO reference, in ct_polyphase.c, the code
> defines the coefficients like:cos( PI / 64.0 * (k+0.5) * (2*l-127.5)
> ).
> 
> My question is why the spec uses 127 while its reference code
> implementation uses 127.5. I don't understand this 0.5 trick. Can any
> one explain this for me? Thanks in advance.
> 
> 
> -- 
> Regards,
> Andy Quan
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--
Andreas Schneider
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