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

[Jean-Marc Valin]

> 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:

Are you turning on DTX in addition to VBR? Also, what version are you
using. As of 1.0, DTX is no longer implied by VBR.

> 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 only way the spectrum can be "frozen" is when DTX is on and the VAD
detects no speech. The idea is to reproduce the noise without
transmitting any data. Can you send me some samples (original and .spx)
so I can check what's happening?

> 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.

I guess I'll need to check that.

> 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.)

Strange, the wideband more is designed for the 50-7000 kHz band. Sure if
you lower the bit-rate a lot, there may be problems with the bass
though.

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

I think most VoIP applications so far have used CBR because it's more
predictable.

        Jean-Marc


-- 
Jean-Marc Valin, M.Sc.A.
LABORIUS (http://www.gel.usherb.ca/laborius)
Université de Sherbrooke, Québec, Canada

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