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

[Alex Converse]

Ralph Sperschneider wrote:
> 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
>

Hi Ralph,
That makes a lot of sense. It initially read to me like the "decode 
spectral coefficients of embedded single_channel_element into buffer 
'cc_spectrum[]'" was a post condition/description of the code below. But 
I guess it's really more of a precondition.
Thanks for the help,
Alex Converse