Vorbis dev - Feb 2003 - Observations about the floating point data in vorbisfile

Hello.

I noticed that when reading data with ov_read_float(), you can get
values outside [-1..1] when the stream is encoded at lower quality, but
with higher quality, the values trim down to inside [-1..1].
Looking at the plot from -q10, the data from ov_read_float seems
clipped.

I've made some plots of encoding the start of Rammsteins Feuer from the
xXx soundtrack (it's a pretty loud soundtrack, but max is still max.. ).
http://www.ing.umu.se/~stric/tmp/vorbis/
These are plots of data read with ov_read_file, printf()'d and fed into
gnuplot. The x axis is sample number, counting from sample 900k
(left,right,left,right,...)

Looking at ov_read(), it clips the data to [-1..1], but I have encoded
files where samples go up to 1.45 which I suppose isn't right.

Both 1.0 and 1.0rc3 show similar stuff.

/Tomas

-- 
Tomas Ögren, stric@ing.umu.se, http://www.ing.umu.se/~stric/
|- Student at Computing Science, University of Umeå
`- Sysadmin at {cs,ing,acc}.umu.se
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Tomas Ogren <stric@ing.umu.se> said:
> Hello.
> 
> I noticed that when reading data with ov_read_float(), you can get
> values outside [-1..1] when the stream is encoded at lower quality, but
> with higher quality, the values trim down to inside [-1..1].
> Looking at the plot from -q10, the data from ov_read_float seems
> clipped.
> 
This is correct. As a lossy codec, vorbis does not guarantee that the output
will be the same as the input (obviously) - and part of that is that the
approximations may produce out of range output. ov_read() performs clipping,
as you noted, in order to deal with this. If you use ov_read_float(), it's
up
to you to deal with the values produced appropriately.

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. 

Mike

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