[M4IF Technotes] Re: AAC: PNS scale factors

[Ralph Sperschneider]

John Cox wrote:
> Dear Ralph
> 
> I'm glad that you agree that the standard and reference stuff don't
> match at the moment.  Your fix is probably optimal.  However, in my case
> where I am implementing a fixed-point decoder, perfoming a square root
> on a general number is hard, whereas the individual sample scale factor
> can be pulled from a small lookup table (after all there are only about
> 32 availible values of size).  I don't know how important having the
> correct energy in any given frame is vs. having the correct energy over
> time - has anyone done any tests?

Dear John,
I have been informed from our DSP specialists that for fixed point 
implementations there is a less complex algorithm available to derive 1/sqrt(x) 
directly (just multiplications shall be required in this case).
Having the correct energy in any band seems to essential according to our 
experience.
> Ideally there would by synced 'random' number generators in the encoder
> and decoder so the encoder knows exactly what to expect, in which case
> there would be no need for the final renormalization.

I guess you agree with me that this is a theoretical approach only. ;-)
Best regards,
Ralph
> Many thanks
> 
> John Cox
> SJ Consulting
> 
> 
> On Mon, 23 Jun 2023 19:04:10 +0200, you wrote:
> 
> 
>>Dear John,
>>
>>the reference software comes with the following code:
>>
>> norm = 1.0 / sqrt( size * MEAN_NRG );
>> for (i=0; i<size; i++) {
>>   spec[i] = (Float)(random2( state ) * norm);
>>   nrg += spec[i] * spec[i];
>> }
>> s = 1.0 / sqrt( nrg );
>> for (i=0; i<size; i++) {
>>   spec[i] *= s;
>> }
>>
>>Due to the second rescaling, the first scaling is obsolete. Thus, MEAN_NRG can 
>>have any non-zero value, the output will always be the same.
>>
>>Beside this, the rescaling formula in the standard seems to be wrong, and the 
>>formula used in the software seems to be correct.
>>
>>However, the algorithm using MEAN_NRG does not assure that the energy per band 
>>fits the requirements, since the delivered random values (its a finite number) 
>>might have another mean energy. Therefore, the second rescaling has been added 
>>in the software.
>>
>>Subsequently, we tend to propose a correction of the pseudo code in the standard 
>>as follows:
>>
>>nrg=0;
>>gen_rand_vector( &spec[g][b][sfb][0], size );
>>for (i=0; i<len; i++) {
>>  nrg += spec[g][b][sfb][i] * spec[g][b][sfb][i];
>>}
>>/* avoid division by zero */
>>sqrt_nrg = sqrt (nrg);
>>if (sqrt_nrg <= 0.0f ) {
>>  sqrt_nrg = FLT_MIN;
>>}
>>scale = (2.0^(0.25*noise_nrg [g][sfb]))/sqrt_nrg;
>>
>>/* Scale random vector to desired target energy */
>>for (i=0; i<len; i++) {
>>  spec[g][b][sfb][i] *= scale;
>>}
>>
>>The subsequent text needs than to be adopted as follows:
>>"The function gen_rand_vector( addr, size ) generates a vector of length <size> 
>>with signed random values. A suitable random number generator can be realized 
>>using one multiplication/accumulation per random value."
>>
>>What do you think about this?
>>
>>Best regards,
>>
>>Ralph
>>
>>John Cox wrote:
>>
>>>Hi
>>>
>>>Thanks for the pointer.  If those reference wav files are definitive
>>>then the published standard is wrong and the reference software is
>>>right.  The scale factor should read:
>>>
>>>scale = 1/sqrt(size * MEAN_NRG);
>>>
>>>Many thanks
>>>
>>>John Cox
>>>
>>>
>>>
>>>>Dear John,
>>>>
>>>>I don't have an answer to your first question.  But for the reference wav 
>>>>files, you can fetch them from:
>>>>ftp://mpaudconf:adif2mp4@ftp.iis.fhg.de/guests/mpeg4audio/incoming/testSequences/mpeg4audio-conformance/referencesWav/
>>>>
>>>>Regards,
>>>>Eddie
>>>>
>>>>At 04:09 PM 6/18/2003 +0100, John Cox wrote:
>>>>
>>>>
>>>>>Hi
>>>>>
>>>>>The standard says (ISO/IEC 14496-3:2001(E); 4.6.13.3 page 174) that the
>>>>>initial scale factor for PNS values is:
>>>>>
>>>>>scale = 1/(size * sqrt(MEAN_NRG));
>>>>>
>>>>>however the reference software from the ISO site (both refSoft &
>>>>>rewrite) seems to calculate it as:
>>>>>
>>>>>scale = 1/sqrt(size * MEAN_NRG));
>>>>>
>>>>>Which is "correct" and what have other people done?
>>>>>
>>>>>I've found the PNS reference streams AL18*.mp4 & AL19*.mp4 (though not
>>>>>AL09,10,11) but no corresponding .wavs to check against - do they exist
>>>>>anywhere?
>>>>>
>>>>>Many thanks
>>>>>
>>>>>John Cox
>>>>>SJ Consulting
>>>
>>>
>>>
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> 
> 
> 
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Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
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