Sound effects

fortISSimO does not include a built-in sound effect engine. However, it has functionality to cooperate with any sound effect engine you want (here is one): if a channel is “muted”, then fortISSimO will never access any of that channel’s registers; this leaves it available for any other code, such as a sound effect engine!

A channel is considered “muted” if its corresponding bit is set in hUGE_MutedChannels; bit 0 controls CH1, bit 1 controls CH2, bit 2 controls CH3, and bit 3 controls CH4¹. (The constants hUGE_CHx_MASK are available (with x between 1 and 4) for your convenience.)

While a channel is “muted”, all of its effects are processed, but any writes to hardware registers are discarded. This means that “global” effects, such as 5xx, 8xx, Fxx, etc. are still applied properly.

When a channel is un-“muted”, fortISSimO waits until a new “full” note (with instrument) is played on it to resume; this strategy avoids playing any corrupted sounds by accident, but can cause a channel to remain muted for a long time depending on the song’s structure.

Wave RAM

The wave channel needs one extra precaution: if wave RAM is written to while CH3 is “muted”, fortISSimO must be informed by setting hUGE_LoadedWaveID to the constant hUGE_NO_WAVE. This will force it to reload wave RAM the next time a note is played on CH3.

Stereo

Not only is a “set panning” (8xx) effect processed even on a muted channel, as explained above, its argument is also written in full to NR51. This can interact poorly with sound effects, since it can alter the panning of a channel not meant for the sound driver at that time.

To remedy this, fortISSimO supports the FORTISSIMO_PANNING tunable. It should designate an address that panning info will be written to (in the usual NR51 format, since rNR51 is its default value). That address must be in HRAM, though.

You can set this to the address of a variable in HRAM, and implement “mixed” panning yourself. For example, running the following code right after hUGE_TickSound:

ldh a, [hUGE_MutedChannels] ; Assuming that all muted channels are used for SFX.
; Duplicate the lower nibble into the upper nibble.
ld c, a :: swap a :: or c
ld c, a

ldh a, [FORTISSIMO_PANNING]
or c ; Force all SFX channels to be centered.
ldh [rNR51], a

…or if you want stereo SFX:

ldh a, [hUGE_MutedChannels] ; Assuming that all muted channels are used for SFX.
; Duplicate the lower nibble into the upper nibble.
ld c, a :: swap a :: or c
ld c, a

ldh a, [FORTISSIMO_PANNING]
ld b, a

ldh a, [hSfxPanning]
; "Bit mux": for each bit, if it's set in `c`, use the bit from `a`, otherwise from `b`.
xor b :: and c :: xor b ; Basically per-bit `c ? a : b`.
ldh [rNR51], a