[vorbis-dev] Observations about the floating point data in vorbisfile

[Beni Cherniavsky]

On 2003-02-05, Segher Boessenkool wrote:

> Beni Cherniavsky wrote:
>
> >>Not all lossy codecs have overshoot.  It would be really great
> >>if Vorbis could deal with this (yes, I know it's a hard problem).
> >>
> > The other ones should be called "strictly amplitude-lossy", I presume? :-)
>
> I'm slow today, it took me a few seconds to get the joke.
>
> > More seriously, how do they solve this?
>
> They don't use an fft/dct/overlapped-dct based filterbank, but a
> filterbank with better properties.  After you've got that, you
> only need to make sure not to round up some high-amplitude
> coefficients.
>
All right, that a bit beyond my dsp knowledge :-)...

> > Detect the case and locally attenuate?
>
> That's what we'd have to do to Vorbis to make it better in
> this regard, I think.  I couldn't come up with an algorithm
> that wasn't computationally intractable, though (I didn't
> look really hard, of course, only a few days; and if there
> is such a solution, it will of course be obvious after all :-/).
>
> > I can't think of something that will be better than applying a limiter
> > in the decoder, which has the benefit of allowing experimentation and
> > avoiding the distortion alogether when decoding to lower volume...
>
> The best and easiest solution is to use a more-than-16bit
> D/A converter.
>
> > > > This problem generally only happens (at least in ways that matter
> > > > - i.e. could be audible) with peak-to-peak inputs, which are a bad
> > > > idea anyway.
> > >
> > > Why are full-range inputs a bad idea?
> > >
> > Because they are not handled <wink>.
>
> You said they are a bad idea *anyway* -- the rest I understand, yes :)
>
That was Michael Smith, I think.  But even if you fix Vorbis, there is
always a chance that some processing after it (e.g. the player's EQ) will
clip.  So I had the habit of limiting my files to 98% at max anyway (the
number is not based on any real data).


-- 
Beni Cherniavsky <cben at tx.technion.ac.il>

If somebody builds a time machine he can gateway the Internet to itself
with a time offset.  I wonder what implications that would have...
--- >8 ----
List archives:  http://www.xiph.org/archives/
Ogg project homepage: http://www.xiph.org/ogg/
To unsubscribe from this list, send a message to 'vorbis-dev-request at xiph.org'
containing only the word 'unsubscribe' in the body.  No subject is needed.
Unsubscribe messages sent to the list will be ignored/filtered.