[Gta04-owner] Echo cancellation again

Dr. H. Nikolaus Schaller hns at goldelico.com
Thu Mar 29 13:48:03 CEST 2012


Am 29.03.2012 um 13:14 schrieb Radek Polak:

> Hi,
> it seems now that last piece missing for telephony on GTA04 is good echo 
> cancellation.
> 
> The software solution (gta04-gsm-voice-routing) is currently not working very 
> good for echo cancellation. It has to be adaptive, so that it works in noisy 
> environments and it needs some more tuning.
> 
> Anyways - new GTA04A04 users will probably want to use the hardware sound 
> routing. So i wonder how GTA04 will do echo cancelling if the audio goes 
> through wires. Will do modem do it?

I think it should do...

> I have tried to play with various PCM profiles (using the _OPCMPROF
> ) command, but it did not cancel the echo. Is it supposed to work only for  
> audio transfered over wires? Did someone with GTA04A04 tried audio in GSM 
> calls using the HW voice routing?

HW voice routing needs modifications in the ALSA drivers so that the McBSP
vs. TPS tristate can be switched. It may be sufficient to introduce one more
binary control for amixer in the gtm601.c ALSA SoC driver that calls the
appropriate tristate control functions.
That is something for our kernel experts.

> Btw please notice, that echo cancelling is must have. You have nice sounds, 
> but for the other the echo is really annoying. People will hate you for this 
> ;-)

I have not found time to do such experiments. So it means that the
landline side is hearing herself as an delayed echo?

This means that the signal received through the modem should be delayed
and subtracted from the signal from the microphone is sent? And the
delay time and intensity should be adaptive?

Maybe, the OPCMPROF does not have enough delay to compensate for
the delay introduced by our software sound forwarding. Or we have to
tweak more filter settings.

Maybe we should consider to switch off the AGC of the microphone signal path.

Nikolaus



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