GSM Call Audio Stream

Al Johnson openmoko at
Fri Dec 5 19:06:37 CET 2008

On Friday 05 December 2008, Esben Stien wrote:
> I don't understand where the audio from a GSM call comes from; is it
> wired at the hardware level?. In that case, is it also wired to the
> sound chip?.

The GSM analogue audio output goes to the RXP/RXN input pins on the audio 
chip. The Mono1 output from the audio chip goes to the GSM analogue audio 
input. The handset and wired headset mics use the Mic2 and Mic1 inputs 
respectively on the audio chip. The audio chip LOUT1 and ROUT1 outputs go to 
an external amp chip to drive either the speaker or the wired headset, while 
LOUT2 and ROUT2 drive the handset earpiece. The CPU connects to the sound 
chip via the HiFi PCM interface, while the Voice PCM interface is wired to 
the bluetooth chip PCM interface. The audio chip itself has extensive routing 
and mixing capabilities, so you can route most signals to most destinations, 
possibly at the same time, and possibly mixed with other signals. There is an 
annotated block diagram with the most of the blocks labelled with their alsa 
channel numbers at the link below.

> How can I get this stream?. There seem to be no interface for it.

You need to work out a suitable mixer setting. You probably want to mix the 
gsmhandset.state and voiphandset.state files, but it really depends on 
exactly what it is you want to do.

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