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.