It's actually for the Game Boy, but since the NES has fairly similar sound hardware I figured I'd just post here for maximum exposure to the correct audience (feel free to move it if you'd like mods).
Here's the basic specs:
- Each frame must have the same pattern on all channels.
- Each "row" can have one or more global effect that's executed before the channels are updated, such as tempo, master volume, or flow control.
- There's underlying support for per-channel effects, although the only one I have implemented now is octave up/down (which Famitracker doesn't seem to support, anyway?)
- Instruments, but all five sequences (volume, arpeggio, pitch, hipitch, duty) must have the same length/loop point. release points aren't supported.