[Speex-dev] Notch Filter in AEC

QianBin cola_tin at 163.com
Wed Mar 3 22:54:51 PST 2010

Steve Underwood,

	Thanks for your patience.


>On 03/03/2010 10:22 PM, QianBin wrote:
>> Hi,
>>     But in fact, it really affects the voice quality. One of my tester says, "Is your mouth far way from the mic?"
>>     Could you explain why we should cut 200hz below?
>We already said it affects the quality when the voice is compressed. Are
>you asking why that should be?
>Even with a simple form of lightly lossy compression like alaw/ulaw, a
>big low frequency waveform can push the coder into using coarse steps,
>so the more interesting energy in the middle of the voice band is coded
>with those coarse steps - i.e. poorly.
>Low bit rate codecs, like G.729 or speex, usually apply their own fast
>rolloff filter for low frequencies - e.g. G.729 rolls off its input
>hard, with a turnover at 140Hz - their whole coding methodology tends to
>break down without that.
>The telephone network normally rolls off hard below about 300Hz, so this
>actually perfectly normal voice for a phone call.
>A second reason to remove the bass frequencies is if you don't you can
>get nasty echoes if the audio is sent to the PSTN. Analogue PSTN
>interfaces don't always remove the bass frequencies, and digital
>interfaces (BRI/E1/T1) never do. If you sent those frequency through an
>analogue segment of the network, the hybrids can do horrible non-linear
>things that the echo cancellers cannot handle. To protect their own
>stability, echo cancellers may insert their own bass filter when then
>switch on, but its poor practice to rely on that. Its better not to
>throw the bass frequencies at them.
>>> The notch filter is specifically designed to cut below 200 Hz when 
>>> working in narrowband. In wideband, the cutoff is more around 50 Hz. The 
>>> reason is that in narrowband operation (irrespective of the codec), 
>>> you're not really supposed to have anything below ~200 Hz, but a lot of 
>>> people forget that.
>>> 	Jean-Marc
>>> On 2010-03-03 02:40, brant wrote:
>>>> Hi,
>>>>     The notch filter in AEC is only used to remove DC signal, and the time of convergence is not important, right?
>>>> If so, I think preset value of notch_radius is too small, and it causes noticeable distortion(freq<  200hz cut).
>>>> There is a picture in attachment to show signals under different radius in time-domain.
>>>>    By setting notch_radius to 0.999 for all sampling rates, I found better voice effect(distortion), while AEC still
>>>> working fine.
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        cola_tin at 163.com

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