furnace/papers/doc/7-systems/soundunit.md

# tildearrow Sound Unit

a fantasy sound chip, used in the specs2 fantasy computer designed by tildearrow.

it has the following capabilities:
- 8 channels of either waveform or sample
- stereo sound
- 8 waveforms (pulse, saw, sine, triangle, noise, periodic noise, XOR sine and XOR triangle)
- 128 widths for the pulse wave
- per-channel resonant filter
- ring modulation
- volume, frequency and cutoff sweep units (per-channel)
- phase reset timer (per-channel)

# effects

- `10xx`: set waveform
  - 0: pulse wave
  - 1: sawtooth
  - 2: sine wave
  - 3: triangle wave
  - 4: noise
  - 5: periodic noise
  - 6: XOR sine
  - 7: XOR triangle
- `12xx`: set pulse width (0 to 7F)
- `13xx`: set resonance of filter (0 to FF)
  - despite what the internal effects list says (0 to F), you can use a resonance value from 0 to FF (255)
- `14xx`: set filter mode and ringmod
  - bit 0: ring mod
  - bit 1: low pass
  - bit 2: high pass
  - bit 3: band pass
- `15xx`: set frequency sweep period low byte
- `16xx`: set frequency sweep period high byte
- `17xx`: set volume sweep period low byte
- `18xx`: set volume sweep period high byte
- `19xx`: set cutoff sweep period low byte
- `1Axx`: set cutoff sweep period high byte
- `1Bxx`: set frequency sweep boundary
- `1Cxx`: set volume sweep boundary
- `1Dxx`: set cutoff sweep boundary
- `1Exx`: set phase reset period low byte
- `1Fxx`: set phase reset period high byte
- `20xx`: toggle frequency sweep
  - bit 0-6: speed
  - bit 7: up direction
- `21xx`: toggle volume sweep
  - bit 0-4: speed
  - bit 5: up direction
  - bit 6: loop
  - bit 7: alternate
- `22xx`: toggle cutoff sweep
  - bit 0-6: speed
  - bit 7: up direction
- `4xxx`: set cutoff (0 to FFF)