advice for creating special ALSA driver or plugin

Hi ALSA developers.

I'm completely new to ALSA developing, so I'm being lazy and ask for advices.

I have a USB recording device for four 16bit/48Khz channels, "encoded" in a stereo 16bit/96Khz track; because it is a USB audio recording device, so no special recording software is required, but a (proprietary Windows) converting software is provided (that I can't use).

Output channels are alternated at the sample level: L: 1313131313, R: 2424242424, etc...

So basically the driver/plugin would alternates between input samples of a stereo track to output 4 channels (at half the input sample rate).

Is there a code base I could study and adapt to start my simple project?

Thanks for your attention.

Marc