Implement Audio Processing Unit

[taylus]

Game Boy sound hardware:

• http://bgb.bircd.org/pandocs.htm#soundcontroller

Sound in MonoGame

• https://github.com/taylus/monosynth

Boot ROM sound initialization code starting at $000c:

    ld hl, $ff26
    ld c, $11
    ld a, $80
    ld (hl-), a        ; $ff26 = $80 (turn on sound by setting bit 7 sound on/off register)
    ld ($ff00+c), a    ; $ff11 = $80 (set pulse channel 1 to 50% duty cycle, aka square wave)
    inc c
    ld a, $f3
    ld ($ff00+c), a    ; $ff12 = $f3 (tweak channel 1's envelope to decrease in volume (I think?))
    ld (hl-), a        ; $ff25 = $f3 (output sound channels 1 and 2)
    ld a, $77
    ld (hl), a         ; $ff24 = $77 (set both sound outputs to max volume)

And the code that later plays the da-ding sound:

• Per bgb, register C is $13 at the time this runs
• Per bgb, register E is $83 at the time this runs

addr_0080:
    ld a, e
    ld ($ff00+c), a    ; $ff13 = $83 (set channel 1 frequency low bits)
    inc c
    ld a, $87
    ld ($ff00+c), a    ; $ff14 = $87 (set channel 1 frequency high bits + etc)

[taylus]

Info from The Ultimate Game Boy Talk (33c3)

The $FF00 - $FFFF address space, look at all those sound registers (21:54)

Sound controller info starts at 24:10:

4 voices x 5 registers each (roughly):

• Control
• Frequency
• Volume
• Length
• Sweep

• T = trigger bit (on/off)

• L = length bits (turn off after N sec, where N = (64-L)*(1/256) seconds)

• F = frequency bits

  • Frequency is an 11 bit unsigned number F (i.e. 0 <= F < 2048)
  • For the pulse channels, the frequency of the sound = 131072 / (2048 - F) Hz
  • This makes for a frequency range from 64 Hz - 131072 Hz w/ a distribution like this:
  • For the wave channel, the frequency of the sound = 65536 / (2048 - F) Hz
  • This makes for a frequency range from 32 Hz - 65536 Hz w/ a similar hyperbolic curve
  • Humans can generally only hear from 20 - 20,000 Hz (source)

• V = volume bits

• D = duty cycle bits (shape of pulse wave) https://gbdev.gg8.se/wiki/articles/Gameboy_sound_hardware#Square_Wave

Wave channel can play "any" sound wave. The 16 bytes in registers W0-W15 are 32 entries of 4-bit samples (values 0-F):

Pulse wave duty cycle options:

Volume sweep effect (going down sounds like normal instruments, going up sounds dreamy?) Frequency sweep (sound effects use this more than music)

https://www.littlesounddj.com/lsd/index.php for composing music on the GB itself