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From: Dirk Bell on 3 Sep 2009 14:55
On Sep 3, 1:28 am, "createdon2003" <createdon2...(a)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.

Dirk Bell
DSP Consultant
From: Vladimir Vassilevsky on 3 Sep 2009 15:08


Dirk Bell wrote:

> On Sep 3, 1:28 am, "createdon2003" <createdon2...(a)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







From: createdon2003 on 4 Sep 2009 01:52
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
From: Vladimir Vassilevsky on 4 Sep 2009 09:32


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
From: steveu on 4 Sep 2009 11:59
>
>
>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

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