Village BGM (7) slows down at the beginning.

I am 99% sure that this is a duplicate of issue #1. I will close this issue when I have made a proper patch for the game.

Some songs of Super Mario World SNSF has a slowdown at the beginning of song. They don't in usual gameplay, of course.

• Castle (Level #8)
• World Clear (Level #19)
• Bowser's Castle Emerges (Overworld #8)

Why does it happen?

TL;DR: The problem is caused by octave calculation routine and uninitialized note number. Apparently, it can be fixed by patching the octave routine. Many SNES games based on N-SPC driver have the kind of the same problem.

N-SPC sequences, which enables vibrato at the beginning of song and starts with a "rest" event, may cause notable slowdown.

1. N-SPC handles vibrato and tremolo, even though the channel does not utter a sound.
2. If the first note is a rest with vibrato, the driver will try to calculate the pitch from uninitialized note number (usually 0). It often makes the note number out of range (e.g. negative number).
3. At last, octave correction routine will cause slowdown. It repeats right-shifting the pitch value ridiculously.

; Get pitch from note number in A (with octave correction) to YA
sub_getPitch:
    mov   y,#$00
    asl   a
    mov   x,#$18
    div   ya,x
    mov   x,a
    mov   a,$12da+y
    mov   $16,a
    mov   a,$12d9+y
    bra   loc_octaveLoop

loc_decOctave:
    lsr   $16
    ror   a
    inc   x

loc_octaveLoop:
    cmp   x,#$06
    bne   loc_decOctave     ; Causes redundant repeat for unexpected note!

    mov   y,$16
    ret

Workaround (and side-effects)

I can provide a simple workaround for this.

loc_octaveLoop:
    cmp   x,#$06
    bcc   loc_decOctave
    ; bne (d0h) ==> bcc (90h)

Pros

• Rest with vibrato will no longer slows down the music.
• The patch size is only 1 byte. Surprisingly portable.

Cons

• Broken note issue: If a music sequence has an illegal note that its note number is below 0, the engine will play the note unexpected pitch (formerly it stops the sound instead). Unfortunately, that's not very rare mistake. I had seen the same error again and again.
  • You would want to replace those illegal notes to rest events.

I would recommend to make a separate patch as the secondary snsflib, then let only the problematic minisnsf to load the patch.