[gta02-core] Audio rework

Werner Almesberger werner at openmoko.org
Tue Jun 23 22:57:48 CEST 2009

Rene Harder wrote:
> We keep the individual ECNs, in this way it's easier to have an overview
> of the changes.
> In the ECNs we shortly describe the reason/problem and refer to the ECN
> with the name "global audio rework".
> How does that sound?

Sounds great.

> Maybe we should stay as it is right now or are there any problems with
> the current configuration?

As far as I know, there are no unresolved problems with the jack
insertion detection logic.

> I could not find a way to solve the biasing problem with a common ground
> for headphone and microphone. So i guess we need to stay with the dc
> blocking capacitors.

So it seems :-(

> These resistor will increase the resistance in the signal path and
> improve this situation a little bit.

Ah yes, that would make even more sense.

> Do yo know how much it will decrease the channel separation 10, 20 or 60 dB?

That's too deep in analog land for me to know, sorry :)

> Do we have any data about that microphone, e.g. part number, vendor etc.

It should be a Knowles SPM0208HE5:
http://media.digikey.com/pdf/Data Sheets/Knowles Acoustics PDFs/SPM0208HE5 Rev B.pdf

> I do not have a lot of information what the cause of this problem
> was/is. I think it would be better to find and solve the source of the
> problem.

As far as I remember, the story went something like this:

- JK4401 acts as an antenna receiving the GSM's RF we emit

- the protection diodes of U4401 rectify the RF, thus acting as an
  AM demodulator

- the demodulated AM creeps via R4408 into MICBIAS, overlaying the
  microphone signal

- signal gets amplified, etc.

I'm sure Joerg will set me straight if I missed any additional twist
in this tale :-)

- Werner

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