[Ffmpeg-devel] Re: BUG: AC3 encode volume is low

Michael Niedermayer michaelni
Sat Apr 8 15:41:38 CEST 2006


On Sat, Apr 08, 2006 at 08:59:26AM -0400, Justin Ruggles wrote:
> Hello,
> Pierre Marc Dumuid wrote:
> > Hi Matthieu,
> > Re: http://mplayerhq.hu/pipermail/ffmpeg-devel/2006-April/010010.html  
> > As I said, I wasn't on the mailing list, (I took a peek on the logs to 
> > see if any discussion had resulted)  Please CC me any discussion, 
> > (though I will take a peek)
> > 
> > Regarding there being a discussion, these discussions are quite old... I 
> > read them, and didn't completely understand them, though I get the 
> > feeling that nothing got committed to fix the problem.  Is the problem 
> > unfixable?  (because a poor definition of a formula).  If so, maybe a 
> > message should be displayed indicating the attenuation.  Alternatively, 
> > a command argument could be used such as --use_dodgy_ac3_formula...
> IMO, neither ac3's MDCT formula nor FFmpeg's implementation of it is
> dodgy.  

i agree here

> You're right, though, that there was never any consensus on a
> resolution to the issue and no fix was ever committed.  I still haven't
> changed my opinion on the matter, although there is one thing that I am
> not 100% sure about...
> Why does the MDCT output to a 32-bit int instead of 16-bit?  Isn't it
> just a signed 15-bit fixed-point implementation of the formula in the
> spec?  I can't find where the output coefficients would need to be more
> than a 16-bit signed integer.

and thats what scares me, i dont know ac3 very well, but i do know that a
N point decorrelation style transform will generally need log2(sqrt(N)) 
bits more at the output to maintain the same precission

> >From what I can tell from the MDCT code, and from the fix15() function,
> the output coefficients should have a range of -32767 to 32767.  Thus,

i dont think this is true

and as the patch is based on apparently wrong assumtations its rejected



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