I'm starting developing a project, where we will need to use both fmradio and microphone at the same time (and do some mixing on those two streams to output it to headphones).

Is it technically possible on n900?

Replying to my own question, yes it is technically possible.

I wold however ask for some in this topic. What I'm trying to get is an application (or two separate apps, doesn't really matter) that will be able to output two streams (from fmradio and mic) separately (so some operations may be performed on them).

So far I'm trying to user fmradio and maemo-recorder to achieve this.
Running fmradio and setting Input Select (in alsamixer -c0) to Digital, let me hear both fm output and mic at the same time. The problem is that those are the same stream for the application, so I cannot manipulate them separately. With this setting, however, maemo-recorder is able to record only mic (without fm stream). What stuns me is that both fmradio (giving both mic and fm) and maemo-recorder (giving just mic) use pulsesrc in gstreamer.

Any ides how this (two separate streams) can be done?

The FM radio chip supports two audio routes, DAC and I2S. Maybe it's possible to catch the stream somewhere before it's routed into pulseaudio. But I'm just guessing here.

Another guess: make use of stereo if possible. Left channel mic input, right channel FM radio input, for instance. If you manage to get it that way, it should be possible to separate the streams again. At least the FM radio allows you to fine-tune which channels you want to hear in the alsamixer.

Thank @pycage, you have been of really great help (also outside tmo).
Is there no real documentation about this? How exactly the sound architecture looks like and how it can be accessed? It's all about reverse engineering once again?

:| i did try hunting this but i also can't find any documentation - sorry hop

This is simply amazing... Once more the lack of documentation strikes back...

Here's the source code of the fm radio kernel driver.

https://patchwork.kernel.org/patch/22851/

Ok, I am able to produce stream with fm on one channel and mic on the other. I have found deinterleave which of course doesn't have proper documentation. Any suggestion how this can be used in Python (i.e. how to use it to get two separate streams)?