GTA02 Audio issues

[joerg]

Am Di  4. März 2008 schrieb Graeme Gregory:
> I have been working on the problem of echo in GSM calls and I seem to
> have boiled the issue down to a hardware problem. There is I think the
> possibility there is a mechanical problem with the case design where
> audio from the handset speaker ends up entering the handset mic far too
> easily causing echos.
> 
> I suspect this issue also affects GTA01, its just no-one used that as a
> phone much.
> 
> I have attached the private email thread that occurred to lead up to
> this issue. It would be useful if more people had ideas.

Try to physically insert a switch to one of the 2 wires leading to earpiece. 
So you may switch on and off audio while not changing mixer setup.

Be aware there may be echo cancelers/suppressors in GSM-provider's gateway, 
making it hard to get reproducable results on real cellphone calls.

I'm note quite sure about to wich extent this may be a common problem to be 
found on many cellphones, which probably is handled very often by by a echo 
suppression on board the cellphone itself by simply reducing mic sensitivity 
with increasing earpiece level and vice versa lowering level with increasing 
input.
Some tests with a scope at mic and playback of a standard leveled testtone may 
give an idea for different devices.

In the end, when nothing else succeeds, there might be the possibility to 
implement a fullsize echo cancelation algorithm, which may work pretty good 
on this short delay between audio out and echo on input and very constant and 
defined parameters of the loop.

Mechanically, the speaker must be decoupled both for mechanical vibration from 
the case as well as any air transmitted sound getting inside the case. 
Backside of the membrane capsuled volume, frontside sound through a rubber 
tube to the case outlet. Same for mic. (sorry when telling old storries, have 
no NEO to compare how it's done now. Just felt i should mention it)



cheers
jOERG