[Vorbis-dev] Probleme with bitrate when encoding small files

[Jean-Francois Guay]

Hello,
We found a problem that we are not sure who is responsible.

Here is what we found:
On some small file (around 1 sec) if we ask to compress the file at
128kb/s
On an encoder compiled with Microsoft visual 7.1 it generate a file at
160kb/s (that is the behaviour we expect because of the short length of
the file)
On same encoder compiled with Microsoft visual 8.0 with the same
parameter it generate a file at 87kb/s and the sound quality is not
really good (but the sound quality is coherent with the bitrate
effectively used)

>From what we have found to date it's look like it's related to rounding
error from float to int conversion but it's just an impression we have
not pin pointed the exact problem. We tried out different compilation
option on VC8 to reproduce the behaviour of VC71, but without success.

So we don't know if it's a Microsoft bug or if the problem is on the
side of vorbis lib or on our side. But what we are sure is that we send
the same encoding parameter to both version of the encoder.  Presently
we use libvorbis 1.0, it's in our plan to update to version 1.2 in the
future but for the short term we would prefer to stay with 1.0.

What we are interested, is if someone that have access to an encoder
compiled with VC71 and VC8 and can reproduce the same result as we have,
it would confirm that the problem is not with our implementation of the
encoder. Also if someone have a hint where to look in vorbis code to try
to understand what's happening it's would be helpful.

You can download the file that reproduces the problem by ftp at:
privateftp.ubisoft.com
login: 50579-tg-ogg
password: JZgkeypC

ftp://50579-tg-ogg:JZgkeypC@privateftp.ubisoft.com

Original file
small_sound.wav

Oggvorbis file with VC7 compiler
small_sound_VC71_good.ogg

And oggvorbis file with VC8 compiler
small_sound_VC8_bad.ogg


Thanks

--
Jean-Francois Guay
Sound Engine Team programmer
Ubisoft