[speex-dev] speex_denoise on non-microphone noise (static ?)
tgrand at canvaslink.com
Thu Sep 18 11:25:15 PDT 2003
I'm not sure how speex_denoise() works, but my solution to this problem
is fairly straightforward and works well in practice (in a simple VoIP
program.) Note that I am also an amateur and am making up some
terminology here... but I've tested my approach on several noisy
environments with low-quality mics and it does work well.
Problem: There's usually annoying noise present in a recorded signal,
which can be attributed to interference from the PC and background noise
in the user's environment (fan noise is common.)
Assumptions: The noise is regular with respect to time - that is, it's
steady and ongoing. It tends not to change significantly in frequency
or amplitude over a long period of time.
Solution: Subtract it from the signal. In order to do this, there are two
steps required: 1) Build an average profile of the noise. This is done
infrequently and [currently] triggered by the user. 2) Subtract the
profile from the signal. This is done before encoding the signal with
1) Building the profile: I build an "average profile" of the noise in the
frequency domain. So the user is prompted to be quiet for a few seconds,
the noise is recorded, and it's run through the FFT. The signal is
averaged (in the frequency domain) over those few seconds to get a "general
idea" of the noise characteristics.
2) Removing the noise: Before encoding some recorded sound with Speex, I
run the sound through the FFT. Then it's a simple matter to loop over the
frequency spectrum and subtract the noise profile. At this point it's
also easy to do simple filtering, so I do a high-pass filter around 200Hz
or so to get rid of unwanted bass. Then, I do the inverse FFT to get the
sound back into the time domain, and I encode it with Speex. The noise
is *greatly* reduced. The difference can really be dramatic.
Drawbacks: Not all noise is regular over time, and this approach is a
manual, non-adaptive process (for now anyway...) One person I talk to
has a monitor which causes annoying noise only when the screen is mostly
white. So sometimes I end up asking them to rebuild the profile.
Well, I'm not sure if this is helpful or not, but it works for me.. :)
<p>Tongbiao Li (tli at viack.com) wrote:
> The problem started with speech detection. Speech sections are detected
> well. However, once in a while non-speech sections are also marked as
> speech. The root was finally traced down to microphone static noise.
> Then I pulled the microphone out. Our system still records noise. To
> isolate the problem, I wrote a small app just to open the device and
> record raw samples, calls speex_denoise() and outputs both sample sets.
> The noise is still there, with level fluctuating with gain level, unless
> "All mute" is chosen.
> In the case when NO microphone is plugged in, speex_denoise() smoothes
> the signal and produces smoother (and even amplifies the signal) speech
> like signals. It seems that speex_denoise( ) is very sensitive to
> static noise.
> For regular speech COMBINED with microphone static (or more precisely,
> the static detected at the microphone plug, or noise from inside the PC
> ... someone help me out here), the noise samples do get suppressed
> compared to speech samples.
> One observation: many noise sequences seem to have a signature of sharp
> Anyone have a solution of supressing this type of static?
> Thank you.
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