[speex-dev] Frozen upper spectrum in WB VBR CNG

[Tom Grandgent]

Hi,

I've been using Speex in my voice-over-IP program on Win32, in 
wideband (16kHz) mode.  I just starting using VBR recently and 
have run into something that might be a problem within Speex:

If someone hasn't spoken for a little while, and the bitrate drops 
to very low, sometimes the high half of the spectrum becomes frozen 
with a looping sound.  The bottom half of the spectrum is always 
ok.  (I have a linear spectral analysis view that makes it very easy 
to see this behavior.)  The frozen sound is usually much louder than 
the background noise and sounds like a strange buzzing.  It seems 
like it could be a frame from recent speech activity.  Occasionally 
(like every few seconds) the sound will change to something 
different, but still frozen.  As soon as the speaker starts speaking 
again, the problem goes away and doesn't come back until another 
period of silence.

The problem seems to happen more often at lower VBR quality 
settings, but I was having a conversation with someone at quality 
8.0 and it still happened fairly frequently.  It seems especially 
easy to trigger by clicking the mouse near the mic.  The noise is 
much more noticible with headphones than speakers, even though it 
is definitely much louder than the normal background noise.

Now, to describe the audio data I'm sending through the codec...  
There's always some background noise but it's kept at reasonably 
low levels, and is what you would expect to pick up with a mic in 
a typical room with a computer in it.  I do a high-pass filter to 
reduce bass below around 400Hz before encoding.  (Without the filter, 
if there is significant bass background noise, VBR struggles and 
the quality is very poor.)

Any ideas as to what the problem might be?  Has anyone tried 
doing VBR in wideband mode in a VoIP application...?

Thanks,

Tom
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