[FFmpeg-devel] [Issue 664] [PATCH] Fix AAC PNS Scaling

[Alex Converse]

On Tue, Oct 21, 2008 at 1:29 PM, Alex Converse <alex.converse at gmail.com> wrote:
> On Tue, Sep 30, 2008 at 11:25 PM, Alex Converse <alex.converse at gmail.com> wrote:
>> Hi,
>>
>> The attached patch should fix AAC PNS scaling [Issue 664]. It will not
>> fix PNS conformance.
>>
>
> Here is my understanding of the current situation:
> 1) PNS is part of the MPEG-4 AAC-LC object type
> 2) PNS is part of the most commonly used family of 14496-3sp4
> profiles, the AAC family of profiles (the AAC profile, the HE-AAC, and
> the HE-AACv2 profile)
> 3) PNS is *broken* in FFMPEG svn [Issue 664], *regardless of per band scaling*
> 4) (For better or worse) PNS requires per band scaling in the latest
> standard (ISO/IEC 14496-3:2005)
> 5) PNS is a low bitrate tool
> (<http://www.hydrogenaudio.org/forums/index.php?s=&showtopic=34652&view=findpost&p=305762>,
> <http://www.hydrogenaudio.org/forums/index.php?s=&showtopic=53340&view=findpost&p=478068>)
> 6) ffaac is enabled by default in FFmpeg
>
> It seems counterproductive to let this broken situation remain while
> arguing the merits of square roots and other normalization schemes.
>
> I propose we:
> A) Fix PNS using one sqrtf() per band (aac-pns-scale-v3.diff)

Apologies, I attached the reversed diff. The patch attached to this
message is what I actually propose.

> B) People interesting in a non-inner-most-loop optimal 1/sqrt can duke
> it out afterwards. Alternative normalization schemes (like
> pregenerating noise) can also be explored by interested parties
>
> This will allow users to immediately stop using broken PNS
> accidentally (6,1, 2, 3) while the computational hit of one sqrtf per
> band (A) should be mitigated by the decreased complexity in decoding a
> low bitrate stream (5).
>
> Thoughts?
>
> [...]
>
> --Alex
>

--Alex
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