[vorbis] Should Speex VBR Introduce Distortion?

[J C Fitzgerald]

Hi All,

I've run into a small hiccup in encoding my audios with Speex.  When I
encode audience laughter and applause with 'speexenc' (version 1.0.1),
the result is quite acceptable... until I enable VBR.  Then it distorts
horribly.

My understanding of VBR is that it frees the encoder to vary the number
of bits emitted to better maintain the quality requested, and so I would
have expected, if there were to be distortion, that it would have shown
itself with VBR turned off.

I thought that VBR turning on VAD might be persuading the encoder to
interpret the sound as background noise, but experiments with VAD in CBR
mode seem to my ear to add only a very small amount of distortion over
non-VAD CBR mode and certainly nothing like the VBR level.

I don't believe it is the playback that is at fault as it handles the
CBR mode Speex file, the same source Vorbis encoded and the original
RIFF files just fine.  It is possible that Speex requires such a high
bit rate to represent the audio at these times that it swamps the player,
but I would not have expected it to be much higher than what would have
been required in CBR mode.

So, am I looking at an encoding artifact here, or is there a less naive
answer?

TIA,
John.
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