[speex-dev] Frozen upper spectrum in WB VBR CNG
jean-marc.valin at hermes.usherb.ca
Sat May 3 22:03:01 PDT 2003
Yep, you found a bug and here's the patch. It should solve your problem
(using VBR without DTX), but there many still be some tuning do to on
<p>diff -u -r1.118 sb_celp.c
--- sb_celp.c 18 Mar 2003 06:13:30 -0000 1.118
+++ sb_celp.c 4 May 2003 04:58:16 -0000
@@ -351,6 +351,8 @@
modeid = mode->nb_modes-1;
+ if (st->relative_quality<-1)
@@ -361,7 +363,7 @@
thresh = (st->vbr_quality-v1) *
- if (st->relative_quality > thresh)
+ if (st->relative_quality >= thresh)
<p><p>Le sam 03/05/2003 à 15:48, Tom Grandgent a écrit :
> Jean-Marc Valin (jean-marc.valin at hermes.usherb.ca) wrote:
> > > 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.
> I'm using 1.0. DTX is definitely turned off. (I tried turning it on
> and saw the bitrate go even lower, but the sound still sounds the
> same as when DTX is off.)
> > > 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?
> Ok. I reproduced the problem using speexenc and speexdec (the Win32
> binaries posted at http://www.speex.org/download.html) and have put
> my sample input, the encoded .spx, and a .txt with commands used here:
> http://www.grandgent.com/tom/temp/clickExample.zip (220KB)
> You will hear some background noise from the room and me clicking the
> mouse a few times. The first buzz is brought to a halt by the sound of
> a soda can being opened. Then there is a second buzz later on. It
> should be pretty noticible.
> This is kind of a dumb example but I think it will demonstrate the
> problem sufficiently. If you want, I can make an example involving
> conversation to prove that this problem is troublesome during typical
> > > 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.
> I think the bass was just too strong. My friend had his mic sitting
> on his computer case... :) At first I wondered why his voice sounded
> so strange. Then I had him turn on the filter on his end and it was
> perfect after that.
> > I think most VoIP applications so far have used CBR because it's more
> > predictable.
> Yes, I was pretty happy with CBR but it is kind of wasteful to be
> transmitting at full bitrate during silence. So, I decided to at
> least make VBR an option in my program, since Speex supports it.
Jean-Marc Valin, M.Sc.A.
Université de Sherbrooke, Québec, Canada
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