[Gta04-owner] Echo cancellation

Dr. H. Nikolaus Schaller hns at goldelico.com
Mon Feb 20 12:34:10 CET 2012


Am 20.02.2012 um 11:03 schrieb Radek Polak:

> Hi,
> i have started working on echo cancellation in the gsm voice routing program 
> [1].
> 
> Why do we need this? If you try e.g. QtMoko v40 you will notice that the 
> speaker and mic volumes are very low during call. I have been experimenting 
> with higher volumes, but then the other party starts hearing herself which is 
> very annoying. And current low volumes are usable only indoors. Btw same 
> problem was on Freerunner. Freerunner has special AT commands for this and it 
> was all handled by modem firmware then.

You can also play a little with the sound profiles of the Modem:

AT_OPCMPROF=0	Handset
AT_OPCMPROF=1	Headset
AT_OPCMPROF=2	Speakerphone
AT_OPCMPROF=3	Bluetooth Headset

This appears to modify the sidetone, noise suppressor and AGC
behaviour.

AFAIK, our sound codec (TPS65950) also has an AGC.

> So I have added two new defines in the voice routing program:
> 
> USE_WALKIE_TALKIE_AEC
> USE_SPEEX_AEC
> 
> If you enable first one, it will use simple algorithm which lowers mic volume 
> and increases speaker volume when the other party is speaking and does the 
> opposite when you are speaking.
> 
> Second define uses speex library which has echo cancellation algorithm for VOIP 
> calls. You can find some info about it here[2].
> 
> I have not tested it very much yet, but the walkie talkie algorithm seems to 
> work quite good. Using speex does not work for me yet - maybe i am doing 
> something wrong.
> 
> I am leaving home until Sunday so i will not work on this now.

Nikolaus



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