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'black box', echo and Voip

Started by johnstokes June 12, 2009
I have a practical question about echo cancellation and Voip.

I have made a 'black box' which contains analogue electronics of a phone
earpiece and mic.  However i have problems when connected to Voip.  This
black box is plugged into a major manufacturers Voip device + PC which
converts the analogue voice into a Voip signal for input then across the
Voip network. However When a far end caller using PSTN, calls 
through the Voip box and into my black box he can hear substantial echoes
of his voice. 


I know that the Voip will add delay to the system thus any impedance
mismatches in system such as my black box will cause echoes.
I have tried to alter the impedance of my black box but still echoes occur
at far end caller using PSTN.

I can place a DSP with adc and dac between my black box and Voip box to
perform some echo cancellation. However would this work at that position?
or would i need to place the DSP echo canceller somewhere else on
network? 
Hope this diagram helps. 
					------------------------------------------------------------------	
		      |                 |	     	     |		
//////////	/////////////	|\\\\\\\\\\\\\\\|     ////////////
PSTN call------>PC + Voip box	|   position of |
//////////	//////////////	|echo canceller | <----	Black box
			      	|    DSP ?      |	analogue voice
			      	/////////////////     /////////////
			      		|		     |
			      	     
					-------------------------------------------------------------------

Thanks
John Stokes
>I have a practical question about echo cancellation and Voip. > >I have made a 'black box' which contains analogue electronics of a phone >earpiece and mic. However i have problems when connected to Voip. This >black box is plugged into a major manufacturers Voip device + PC which >converts the analogue voice into a Voip signal for input then across the >Voip network. However When a far end caller using PSTN, calls >through the Voip box and into my black box he can hear substantial
echoes
>of his voice. > > >I know that the Voip will add delay to the system thus any impedance >mismatches in system such as my black box will cause echoes. >I have tried to alter the impedance of my black box but still echoes
occur
>at far end caller using PSTN. > >I can place a DSP with adc and dac between my black box and Voip box to >perform some echo cancellation. However would this work at that
position?
>or would i need to place the DSP echo canceller somewhere else on >network? >Hope this diagram helps. > ------------------------------------------------------------------ > | | | >////////// ///////////// |\\\\\\\\\\\\\\\| //////////// >PSTN call------>PC + Voip box | position of | >////////// ////////////// |echo canceller | <---- Black box > | DSP ? | analogue voice > ///////////////// ///////////// > | | > > ------------------------------------------------------------------- > >Thanks >John Stokes >
You should position your echo canceller between the Voip box and the Black box as shown in your diagram. Even though the echo delay caused by the black box is likely short, the voice quality that the PSTN user hears can be very poor due to the long delay through the Voip network (can be tens to hundreds of milliseconds). When the delay is this long even a very small echo sounds annoying. The G.168 spec from the the ITU-T has a lot of info on echo cancellers if you are interested: http://www.itu.int/rec/T-REC-G.168/en Other than the newest (pre-published) version, the spec can be downloaded for free. Its too bad your voip box doesn't have an echo canceller built into it. Shawn Stevenson http://www.linkedin.com/in/sestevenson
>>I have a practical question about echo cancellation and Voip. >> >>I have made a 'black box' which contains analogue electronics of a
phone
>>earpiece and mic. However i have problems when connected to Voip.
This
>>black box is plugged into a major manufacturers Voip device + PC which >>converts the analogue voice into a Voip signal for input then across
the
>>Voip network. However When a far end caller using PSTN, calls >>through the Voip box and into my black box he can hear substantial >echoes >>of his voice. >> >> >>I know that the Voip will add delay to the system thus any impedance >>mismatches in system such as my black box will cause echoes. >>I have tried to alter the impedance of my black box but still echoes >occur >>at far end caller using PSTN. >> >>I can place a DSP with adc and dac between my black box and Voip box to >>perform some echo cancellation. However would this work at that >position? >>or would i need to place the DSP echo canceller somewhere else on >>network? >>Hope this diagram helps. >> ------------------------------------------------------------------ >> | | | >>////////// ///////////// |\\\\\\\\\\\\\\\| //////////// >>PSTN call------>PC + Voip box | position of | >>////////// ////////////// |echo canceller | <---- Black box >> | DSP ? | analogue voice >> ///////////////// ///////////// >> | | >> >> ------------------------------------------------------------------- >> >>Thanks >>John Stokes >> > >You should position your echo canceller between the Voip box and the
Black
>box as shown in your diagram. Even though the echo delay caused by the >black box is likely short, the voice quality that the PSTN user hears
can
>be very poor due to the long delay through the Voip network (can be tens
to
>hundreds of milliseconds). When the delay is this long even a very
small
>echo sounds annoying. The G.168 spec from the the ITU-T has a lot of
info
>on echo cancellers if you are interested: >http://www.itu.int/rec/T-REC-G.168/en >Other than the newest (pre-published) version, the spec can be
downloaded
>for free. > >Its too bad your voip box doesn't have an echo canceller built into it. > >Shawn Stevenson >http://www.linkedin.com/in/sestevenson > > >
Hi So essentially i have to model the impulse response of the box and use this as the weights of an adaptive filter. Since these weights adapt to the input can i initially simply model the impulse (weights) as a simple impulse (for example)? thanks john