# Microchip AY8930

a backwards-compatible successor to the AY-3-8910, with increased volume resolution, duty cycle control, three envelopes and highly configurable noise generator.

sadly, this soundchip has only ever observed minimal success, and has remained rather obscure since.
it is best known for being used in the Covox Sound Master, which didn't sell well either. It also observed very minimal success in Merit's CRT-250 machines, but only as a replacement for the AY-3-8910.

emulation of this chip in Furnace is now complete thanks to community efforts and hardware testing, which an MSX board called Darky has permitted.

# effects

- `12xx`: set channel duty cycle. `xx` is a value between 00 and 08.
  - `00`: 3.125%
  - `01`: 6.25%
  - `02`: 12.5%
  - `03`: 25%
  - `04`: 50%
  - `05`: 75%
  - `06`: 87.5%
  - `07`: 93.75%
  - `08`: 96.875%
- `20xx`: set channel mode. `xx` may be one of the following:
  - `00`: square
  - `01`: noise
  - `02`: square and noise
  - `03`: envelope
  - `04`: envelope and square
  - `05`: envelope and noise
  - `06`: envelope and square and noise
  - `07`: nothing
- `21xx`: set noise frequency. `xx` is a value between 00 and FF.
- `22xy`: set envelope mode.
  - `x` sets the envelope shape, which may be one of the following:
    - `0: \___` decay
    - `4: /___` attack once
    - `8: \\\\` saw
    - `9: \___` decay
    - `A: \/\/` inverse obelisco
    - `B: \¯¯¯` decay once
    - `C: ////` inverse saw
    - `D: /¯¯¯` attack
    - `E: /\/\` obelisco
    - `F: /___` attack once
  - if `y` is 1 then the envelope will affect this channel.
- `23xx`: set envelope period low byte.
- `24xx`: set envelope period high byte.
- `25xx`: slide envelope period up.
- `26xx`: slide envelope period down.
- `27xx`: set noise AND mask.
- `28xx`: set noise OR mask.
- `29xy`: enable auto-envelope mode.
  - in this mode the envelope period is set to the channel's notes, multiplied by a fraction.
  - `x` is the numerator.
  - `y` is the denominator.
  - if `x` or `y` are 0 this will disable auto-envelope mode.
