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Acoustic echo cancellation

Started by createdon2003 September 2, 2009
Hi all,

I am working on acoustic echo canceler for mobile telephony. I have two
doubts to clarify:

1) Avi perry in his book "Voice Quality Engineering in Wireless Networks"
has mentioned that although speech signal power is distributed almost
uniformly across the entire spectrum, the acoustic echo power concentrates
most of its energy in the  1700-2500 Hz range. Anybody having any idea of
the possible explanation.

2) I am searching a bulk delay estimation method (for AEC)that can be used
in the presence of heavy non-linear distortions.  These non linear
distortions are introduced by : GSM codec, Speaker, Mechanical/Acoustic
coupling. I will be very thankful if anyone can guide me through. 

Any good books that deal with Acoustic Echo Cancelers in wireless
network.

Thanks
Sankalp



createdon2003 wrote:
> Hi all, > > I am working on acoustic echo canceler for mobile telephony. I have two > doubts to clarify: > > 1) Avi perry in his book "Voice Quality Engineering in Wireless Networks" > has mentioned that although speech signal power is distributed almost > uniformly across the entire spectrum,
???? Average speech signal power has maximum at few hundred Hz and then rolls down towards the high frequencies with the rate about 10dB/oct.
> the acoustic echo power concentrates > most of its energy in the 1700-2500 Hz range. Anybody having any idea of > the possible explanation.
A misprint, perhaps. More likely, they meant 170...2500.
> 2) I am searching a bulk delay estimation method (for AEC)that can be used > in the presence of heavy non-linear distortions.
Run a traditional *.LMS algorithm over all span of the expected delay. Then look at the resulting impulse response; the bulk delay part will be seen clearly. To keep the computations reasonable, you can decimate the signal to the lower sampling rate.
> These non linear > distortions are introduced by : GSM codec, Speaker, Mechanical/Acoustic > coupling. I will be very thankful if anyone can guide me through.
Auch. I doubt you can achieve any good result if you have a vocoder in the EC loop. The compensation of the small nonlinearities is feasible; it is better done in the frequency domain EC. However I don't know of any practical EC system which employs the nonlinear compensation.
> Any good books that deal with Acoustic Echo Cancelers in wireless > network.
The EC loop is closed at the ends of the link; it doesn't matter what kind of network is in between. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
thanks for the reply...
> > >createdon2003 wrote: >> Hi all, >> >> I am working on acoustic echo canceler for mobile telephony. I have
two
>> doubts to clarify: >> >> 1) Avi perry in his book "Voice Quality Engineering in Wireless
Networks"
>> has mentioned that although speech signal power is distributed almost >> uniformly across the entire spectrum, > >???? > >Average speech signal power has maximum at few hundred Hz and then rolls
>down towards the high frequencies with the rate about 10dB/oct. > >> the acoustic echo power concentrates >> most of its energy in the 1700-2500 Hz range. Anybody having any idea
of
>> the possible explanation. > >A misprint, perhaps. More likely, they meant 170...2500.
Actually I have verified it through a real recording, but couln't find the reason. I am suspecting the mechanical coupling because of the poor casing..ne idea???..how does that coupling is modeled (Frequency response n all)..couln't find it anywhere..
> >> 2) I am searching a bulk delay estimation method (for AEC)that can be
used
>> in the presence of heavy non-linear distortions. > >Run a traditional *.LMS algorithm over all span of the expected delay. >Then look at the resulting impulse response; the bulk delay part will be
>seen clearly. To keep the computations reasonable, you can decimate the >signal to the lower sampling rate. > >> These non linear >> distortions are introduced by : GSM codec, Speaker,
Mechanical/Acoustic
>> coupling. I will be very thankful if anyone can guide me through. > >Auch. I doubt you can achieve any good result if you have a vocoder in >the EC loop. >The compensation of the small nonlinearities is feasible; it is better >done in the frequency domain EC. However I don't know of any practical >EC system which employs the nonlinear compensation. > >> Any good books that deal with Acoustic Echo Cancelers in wireless >> network. > >The EC loop is closed at the ends of the link; it doesn't matter what >kind of network is in between.
Sorry probably it wasn't very clearly written. I meant GSM network (Vocoders are present in both directions).
> > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.com > > >
Sankalp

createdon2003 wrote:


> Sorry probably it wasn't very clearly written. I meant GSM network > (Vocoders are present in both directions).
If you have vocoders in the loop, the EC is not feasible. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
>> the acoustic echo power concentrates >> most of its energy in the 1700-2500 Hz range. Anybody having any idea
of
>> the possible explanation. > >A misprint, perhaps. More likely, they meant 170...2500.
Actually I have verified it through a recording (not sure whether speaker phone was on/off)that acoustic echo's power was present mainly between 1700-2500Hz, but couln't find the reason. I am suspecting the poor acoustic isolation due to cheap casing..any idea???..how does that coupling is modeled (Frequency response n all)..couln't find it anywhere.. Sankalp
On Sep 3, 1:28&#2013266080;am, "createdon2003" <createdon2...@yahoo.com> wrote:
> >> the acoustic echo power concentrates > >> most of its energy in the &#2013266080;1700-2500 Hz range. Anybody having any idea > of > >> the possible explanation. > > >A misprint, perhaps. More likely, they meant 170...2500. > > Actually I have verified it through a recording (not sure whether speaker > phone was on/off)that acoustic echo's power was present mainly between > 1700-2500Hz, but couln't find the reason. I am suspecting the poor acoustic > isolation due to cheap casing..any idea???..how does that coupling is > modeled (Frequency response n all)..couln't find it anywhere.. > > Sankalp
You need to explain the system that you are trying to apply this to and at what point in the system you are planning on putting aoustic echo cancelling. Ex: Maybe you have a talker in a car or a room where the audio (original and echoes) are going into a system that is doing bandwidth compression on the front end. If you put the echo cancellation before the coder, then the distortion of the coder will have little effect. In other situations the distortion could have a great effect. So, explain please, in some detail. Dirk Bell DSP Consultant

Dirk Bell wrote:

> On Sep 3, 1:28 am, "createdon2003" <createdon2...@yahoo.com> wrote: > >>>>the acoustic echo power concentrates >>>>most of its energy in the 1700-2500 Hz range. Anybody having any idea >> >>of >> >>>>the possible explanation. >> >>>A misprint, perhaps. More likely, they meant 170...2500. >> >>Actually I have verified it through a recording (not sure whether speaker >>phone was on/off)that acoustic echo's power was present mainly between >>1700-2500Hz, but couln't find the reason. I am suspecting the poor acoustic >>isolation due to cheap casing..any idea???..how does that coupling is >>modeled (Frequency response n all)..couln't find it anywhere.. >> >>Sankalp > > > You need to explain the system that you are trying to apply this to > and at what point in the system you are planning on putting aoustic > echo cancelling. Ex: Maybe you have a talker in a car or a room where > the audio (original and echoes) are going into a system that is doing > bandwidth compression on the front end. If you put the echo > cancellation before the coder, then the distortion of the coder will > have little effect. > > In other situations the distortion could have a great effect. So, > explain please, in some detail.
He is talking about the local feedback cancellation in the hands-free type of thing, and his main concern is not the acoustic echo per se, but the direct coupling between the speaker and the mike due to the deficiencies of the housing. If the speaker and the mike are arranged poorly, there is only so much that you can improve by the DSP. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
may be it will be more clear by a diagram.

--far_end-------------------|ENC|-------|DEC|--------//
   |                                                 | speaker
   |                                                 \\
  AEC
   |                                                    LEM with
T_60~60ms
   |                                                  
--to far end----------------|DEC|-------|ENC|--------O microphone
                                                     |
AMR codecs are to be used for ENC_DEC, I am simulating the worst case
MR475.

Codecs introduce heavy non linearity but even then the frequency
components are moreover preserved. But what i have observed in a real
recording is that the acoustic echo was present only in the region of
1700-2500 Hz. Now, 
1)frequency response of the speaker/mic is not that bad for sure.
2)room reverberations doesn't cause this phenomenon.
3)doesn't make any sense to suspect codecs for it.

 The only thing I can suspect is that this acoustic feedback might be due
to the mechanical/acoustic coupling. What do you think ?? and if you have
an idea, do you think it can be modeled??.

Now that i have given the diagram (hope the situation is clearer now) can
you suggest any good material which can help me design a AEC for this
condition. 

and Thanks a ton to you guys for responding.

Sankalp

createdon2003 wrote:
> may be it will be more clear by a diagram. > > --far_end-------------------|ENC|-------|DEC|--------// > | | speaker > | \\ > AEC > | LEM with > T_60~60ms > | > --to far end----------------|DEC|-------|ENC|--------O microphone > |
> Now that i have given the diagram (hope the situation is clearer now) can > you suggest any good material which can help me design a AEC for this > condition.
You can not. The AEC is not going to work since you have vocoders in the loop. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
> > >createdon2003 wrote: >> may be it will be more clear by a diagram. >> >> --far_end-------------------|ENC|-------|DEC|--------// >> | | speaker >> | \\ >> AEC >> | LEM with >> T_60~60ms >> | >> --to far end----------------|DEC|-------|ENC|--------O microphone >> | > >> Now that i have given the diagram (hope the situation is clearer now)
can
>> you suggest any good material which can help me design a AEC for this >> condition. > >You can not. The AEC is not going to work since you have vocoders in the
>loop.
Vlad keeps saying this, and you keep ignoring him, so let add another voie to this...... Vlad is right. Echo cancellation is a process of system identification and subtracting the identified system model from the received signal. A low bit rate speech codec, like AMR, is heavily lossy. It does not preserve the signal nearly well enough to allow either the necessary system identification or the subtraction process to work. Steve