furnace/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)