noise while making a phone call: hardware or software?

[joerg]

Am Fr  4. Januar 2008 schrieb Richard Reichenbacher:
> From what I can tell the noise comes and goes.  When I'm in a good signal
> area I really don't hear it but when I'm in a bad signal area it's very
> loud. 
So this is clearly RF-interference. GSM-modem RF-power is high in a situation 
where rcv-signal-strength is low, and vice versa. So obviously antenna of GTA 
is feeding the well known 200Hz hum to to some audiopath. Unused "open" audio 
inputs are especially prone to RF interference, and often shortcircuit 
(grounding) the input pins of unused signal paths can help. Also it's a very 
good idea to disable / level to 0 all currently unused inputs.
To test you may attach an external antenna. 
Touching single leads with a 2'-4' short insulated wire - e.g. a needle - will 
proof which path is sensitive, by suddenly increasing level of hum.
However there's no use to test this way, as long as we don't even have 
schematics. I just can't believe there is any NDA on simple circuit diagrams, 
in the end it's just a more legible version of the free sold PCB itself. So 
*which* secret might be protected by not disclosing the circuit diagram?!

> I asked Lorn Potter what he thought and he said it was probably a 
> software issue.  Although I'm not too sure because I get the same sound if
> I set the phone next to my computer speakers.  So maybe the case is that
> the microphone is too close to the GSM modem or the connection from the
> microphone to the board isn't shielded well enough.
Well, the way it understand things, there must not pass any audio from 
microphone to speaker of device. If it does, mixer setup is incorrect.
GSM noise in GTA's speaker either is induced on path from D/A converter to 
audio amplifier (or any "open" secondary path leading to amp input, e.g. from 
am unused mixer input), or the amp or D/A power supply is unsufficiently 
decoupled from GSM-modem transmitter (this might be tested by connecting some 
elcos to power pins of amp and D/A).

cheers
jOERG