Send signal to a bus, crossfading with previous contents.
xfade is a level for the crossfade between what is on the bus and what you are sending. The algorithm is equivalent to this:
See the Server Architecture and Bus helpfiles for more information on buses and how they are used.
The index of the bus to write out to. The lowest numbers are written to the audio hardware.
An Array of channels or single output to write out. You cannot change the size of this once a SynthDef has been built.