[Mp4-tech] Re: [Audio] CCE buffers

[Ralph Sperschneider]

Alex Converse wrote:
> Dear Experts:
>
>
> I'm having trouble understanding part of the AAC CCE procedures. From 
> 14496-3 section 4.6.8.3.3:
> ""
> spectrum_l(idx, domain)    Pointer to the spectral data associated 
> with the left channel of the
>                           channel_pair_element() with index idx. 
> Depending on the value of
>                           "domain", the spectral coefficients before 
> (0) or after (1) TNS decoding are
>                           pointed to.
> ""
>
> ""
> decode_coupling_channel()
> {
>   - decode spectral coefficients of embedded single_channel_element
>     into buffer "cc_spectrum[]".
>
> ...
>
>     couple_channel( cc_spectrum,         spectrum_l( 
> cc_target_tag_select[c], cc_domain ),         list_index++ ) );
>
>
> ...
> }
>
> couple_channel( source_spectrum[], dest_spectrum[], gain_list_index ) {
> ...
>     dest_spectrum[g][b][sfb][i] +=         cc_gain[idx][g][sfb] * 
> source_spectrum[g][b][sfb][i];
> ...
> }
> ""
>
>
>
> Now besides obvious the extraneous parenthesis on couple_channel(), 
> the procedure seems to reading the output buffer, modifying it and 
> witting over the input buffer!!! Am I missing something here or did 
> the buffers get swapped in error in the spec?
>
> Thanks,
> Alex Converse
>
>
Dear Alex,
I checked your observations. With regard to the extraneous parenthesis 
you are definitely right. However, wrt. source and destination I think 
the standard is correct:
Here, the destination is derived (in place derivation):
dest_spectrum[g][b][sfb][i] += cc_gain[idx][g][sfb] * 
source_spectrum[g][b][sfb][i];
Thus, the target channel is mixed with the coupling channel, using 
cc_gain as attenuation factor.
Therefore, the relationship:
source_spectrum = cc_spectrum = the spectrum of the coupling channel
destination_spectrum = spectrum_[rlm] = the spectrum of the output channel
seems to me correct.
Best regards,
Ralph
-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 6167
Fraunhofer IIS                  | Fax:   +49 9131 776 6099
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