Microphone level & GSM-noise "buzz" issue

Erland Lewin erland at lewin.nu
Wed Oct 29 21:15:29 CET 2008

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

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?

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