Microphone level & GSM-noise "buzz" issue

Joerg Reisenweber joerg at openmoko.org
Thu Oct 30 10:25:23 CET 2008

Am Mi  29. Oktober 2008 schrieb Erland Lewin:
> Here is some speculation on the GSM buzz issue:
> I was doing some testing today, calling my SIP number connected to my server
> and recording the audio.
> Firstly, I have never been able to get a strong signal (high sample values)
> at the receiving end when calling from the Freerunner, despite trying
> various ALSA state files.
> Today I tried holding the Freerunner flat in front of my face (as opposed to
> holding it with the earphone beside my ear. I got a significantly stronger
> signal at the other end with the microphone orientation like this. Not only
> did I get a stronger signal, but also a weaker background GSM buzz. My guess
> is that this is because the ALC (Automatic Level Control) of the Wolfson
> chip decreased the gain of the microphone amplifer whereby the GSM buzz also
> decreased.
> Is the signal from the microphone in general is so low that the Wolfson chip
> needs to run at max gain (and still doesn't manage to get a very strong
> output signal)? And that this high gain makes the GSM buzz so loud (despite
> being fairly low on the input)?
> Is there anything that could be done to increase the signal from the
> microphone? It seems to me like this might improve audio quality,
> transmitted audio level, and decrease GSM buzz. Is there a bias voltage that
> can be increased? Could the mic be replaced with a more sensitive one? By
> the way, is there a data sheet for the microphone?
> Comments?

there is no ALC involved inside Wolfson, on standard GSM-handset.state 
Your observations make perfect sense though, assuming the ALC is done on 
network side (or *inside* calypso chipset).
Alas there's very obviously nothing we can do via mixer settings to improve 
this situation.


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