[Freeswitch-dev] Custom format module

I'm creating a custom format module for an audio stream that cannot be opened by any of the existing format. I've used mod_tones as an example, my format module supports read only and is generating the tones itself. Does the read function have to return slin format data or can it return alaw or another format, if so how do I specify the format, I see a format field in the stream handle but cannot see where it is used in fs core? Thanks, Simon