I would like to create a DSP plugin which takes an input of 8 channels (the 7.1 speaker mode), does some processing then returns the data to 2 output channels. My plan was to use setspeakermode to FMOD_SPEAKERMODE_7POINT1 and FMOD_DSP_DESCRIPTION.channels to 2 but that didnt work, both in and out channels were showing as 2 in my FMOD_DSP_READCALLBACK function.
How can I do this?
You cannot perform a true downmix in FMODEx using the DSP plugin interface. The best you can do is process the incoming 8ch data, then fill just the front left and front right parts of the output buffer leaving the rest silent.
Setting the channel count to 2 tells FMOD your DSP can only handle stereo signal, setting the count to 0 means any channel count.