[Ffmpeg-devel] [PATCH][RFC] ac3enc window generation

Michael Niedermayer michaelni
Thu Apr 20 19:46:03 CEST 2006


On Thu, Apr 20, 2006 at 04:48:14PM +0000, Benjamin Larsson wrote:
> >so the solution would be not to round the values independantly but
> >to choose the pairs so that their summed squares are <=1 and the
> >error is minimized
> >so simply try a few pairs surounding the rounded one and choose the
> >best
> >
> >  
> >
> I thought about this but trying diffrent pairs surrounding the rounded
> value could result in the window beeing non smooth. And I'm pretty sure
> that a non smooth window is more worse then a small rounding error. So I
> think the choise is between flooring or round to nearest.

well, do you think that trying round up / round down pair wise (down, down)
(up, down) (down, up) would be non smooth too? if so then why do you think
that the normal rounding / flooring is more smooth? they should have a
systematic error too (using a simple line as very crude model for our
window and rounding/flooring it to 16bit would result in nasty periodic 
errors ...)

and about the decission between rounding vs. flooring i dont feel like i
understand the MDCT/windowing and its reversibility and such well enough to
decide it
maybe you could elaborate on where that overflow happens in case 
x(i)^2+x(i+n)^2>1 ?
isnt that at the decoding side already? in that case i dont think
there a problem if they add to larger then 1 as due to quantization and
rounding errors in the (I)MDCT the decoder must deal with too large samples



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