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I have discovered a AMK bug.

When I generate a SPC (norom) with the following file (minimal test case),
#amk 1

#3 v255 o4
@24 >c4 c4<

The first note is the wrong octave, but the second is correct octave.

If I do this,
@24 o4 [>c4 c4<]2

or this,
@24 o4 > [c4 c4]2

the first note of each loop is wrong.

But if I do this,
@24 o4 > c4 [c4 c4]2

only the note before the loop is wrong.

I don't know if the bug is present in the music data inserted into the ROM.
Not exactly a bug. Due to how the engine works, percussion instruments (from @21 to @29) will always play at a given pitch (@24 plays o4c for instance). But this only affects the first note after the instrument definition, which is why we usually redefine them before every note (which does not increase insert size however).

If you want to play these at whatever pitch you want, try making a custom instrument with e.g. @24 as a base:

	@24 $FE $6A $B8 $07 $00
Where do I get the default hex values of original instruments?
What is the correspondence between the tuning hex values and the pitch?
If you scroll down a bit here you can see all of them.

I'm not really sure about the corrispondence between tuning values and the actual pitch though.