noise while making a phone call: hardware or software?

Michael Shiloh michael at
Fri Jan 4 20:00:41 CET 2008

Thanks everyone for your thoughts on this. From your descriptions I 
agree, this seems to be a hardware issue.

Furthermore, this seems to be Bugzilla Bug 883, which is assigned to 
Tony, our hardware guy. I'll follow up on this with him. Obviously we 
want to make sure this isn't present on GTA02.

I will also copy the ideas that came out of this discussion into the 
bug. Richard, Lorn, Joerg, would you rather do this yourselves?


joerg wrote:
> 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
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