Chrono Trigger/Music Notation and Commands
If you want to disable particular channels in a CT SPC file, you can hex edit bytes 01F4 and 01F5 in the SPC file. Those are 00F4 and 00F5 in the SPC700 memory space, representing the communication port with the 65816. Put F4 at 01F4 and a bitmask of the channels to disable at 01F5; F4 05 disables sound channels 0 and 2.
These are the commands I've found. F4 gets the command byte; F5 and F6 are xx and yy respectively. I haven't found anything using zz (F7). Many commands cause playback to stop regardless of parameters; most of these are likely crashes and some are the code going into a wait state.
Check List of Sound Effects for a list of sound effects and IDs.
18 - play sound effect xx = effect ID yy = panning for sound effect 00 = left 80 = middle FF = right 19 - play sound effect; same as 18? 80 - volume control xx = number of ticks in which to fade out 00 = now FF = about 7 seconds yy = volume 00 = off FF = normal 81 - volume control; same as 80? 85 - song raw speed (tempo and pitch together) xx = time to slide to desired setting 00 = instant FF = about 7 seconds yy = raw speed modifier 80 = slowest 00 = normal 7F = fastest 86 - song tempo xx = time to slide to desired setting 00 = instant FF = about 7 seconds yy = tempo modifier 80 = slowest 00 = normal 7F = fastest 87 - song pitch xx = time to slide to desired setting 00 = instant FF = about 7 seconds yy = tempo modifier 80 = lowest 00 = normal 7F = highest 88 - event control xx = event enable 00 = no event 01 = event will occur next time mark passes, if one exists F4 - channel disable xx = bitmask of channels to disable F5 - playback disable xx = whether to enable 00 = enabled 01 = disabled F6 - max tempo enable xx = whether to enable 00 = disabled 01 = enabled F8 - disable single channel xx = channel number F9 - enable single channel xx = channel number FC - unknown effect on channels xx = channel bitmask to enable effect FE - seems to switch to some kind of download mode FF - reset xx = what to do 01 = unknown effect FF = full reset; jumps to SPC's boot ROM other = no effect
I do know from the assembly code that 10-1F, 80-8F and F0-FF are the only valid command ranges. Commands 84, 8A, 8B, 8C, 8D, 8F, F7, FA and FB have blank handlers so do nothing. I found the dispatcher for 8x and Fx but not 1x yet. 00 on the F4 port means no command as a special case.
Not sure how the APU handles it, but the preprocessor in the CPU code can handle a much larger range. I'm not sure how many of these commands will get stored to the APU, but the section of code from C70907 to C70912 stores E0 to special registers 2140 and 2141 (which correspond to F4 and F5 respectively in the APU). In order to get to this code, the command issued to the preprocessor must be 70, with the first parameter being different from 7E1E11 (not sure what that is). The preprocessor then sends the APU the FE command first, and then E0.
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