GSM-noise "buzz" issue / ways forward

Joerg Reisenweber joerg at
Mon Sep 22 18:04:35 CEST 2008

Am Mo  22. September 2008 schrieb Andy Green:
> Somebody in the thread at some point said:
> | Is there any way to switch the audio path to some file output?
> | i.e. can I (potentially) record "buzz" without an active GSM connection
> | ( while irradiating the phone from an external HF source )
> | would be cheaper and easier to set up that way.
> Werner pointed out yesterday to a guy asking the same thing that
> /usr/share/openmoko/scenarios/capturehandset.state
> should be the thing.
> As far as I know, nobody yet tested this with buzz and so confirmed that
> the effective buzz problem actually comes at the MIC inputs to WM8753.
I actually did in TPE, with arecord piped thru STDOUT->ssh-> aplay on PC
instead of aplay on PC, you could use "cat >file.wav"
Ah, right: result was we get noise when applying RF from external phone.

> | <someone>
> |>  Well maybe luckily we *might* eventually find a magic place for a
> (33pF) C
> |>  somewhere near Wolfson chip, to rework GTA02 out in the field this way.
> |>  Anyway I'd rather expect this magic fix to be found by try and
> error, rather
> |>  than probing on a setup that's completely changed by mere applying a
> probe to
> |>  it.
> |
> | Could someone with the device open place 100Ohm resistor in series with
> | 3/4pF from  the mic line to GND and test it? ( instead of the ferrite
> bead.
> |
> | Is the trace in the top layer of the pcb or is it burried?
All traces shielded between GND and VDD planes, for 99% of their length 
(obviously doesn't apply for pads)

> I don't have gerber layer data, but the bead was placed in series with
> headphone jack p4, it should be possible to try.
> But I don't have 3.. 4 pF caps lying around, I order some for tomorrow.
> -Andy

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