furnace/doc/7-systems/c64.md

# Commodore 64

a home computer with a synthesizer-grade sound chip of which people took decades to master. three oscillators with four selectable waveforms, ring modulation, oscillator sync, multi-mode filter and ADSR envelope.

very popular in Europe and mostly due to the demoscene, which stretched the machine's limbs to no end.

two versions of aforementioned chip exist - 6581 (original chip) and 8580 (improved version with working waveform mixing and somewhat more consistent filter curves).

# effects

- **`10xx`**: change wave. the following values are accepted:
  - `00`: nothing
  - `01`: triangle
  - `02`: saw
  - `03`: triangle and saw
  - `04`: pulse
  - `05`: pulse and triangle
  - `06`: pulse and saw
  - `07`: pulse and triangle and saw
  - `08`: noise
- **`11xx`**: set coarse cutoff. `xx` may be a value between 00 to 64.
  - **this effect only exists for compatibility reasons, and its use is discouraged.**
  - use effect `4xxx` instead.
- **`12xx`**: set coarse duty cycle. `xx` may be a value between 00 to 64.
  - **this effect only exists for compatibility reasons, and its use is discouraged.**
  - use effect `3xxx` instead.
- **`13xx`**: set resonance. `xx` may be a value between 00 and 0F.
- **`14xx`**: set filter mode. the following values are accepted:
  - `00`: filter off
  - `01`: low pass
  - `02`: band pass
  - `03`: low+band pass
  - `04`: high pass
  - `05`: band reject/stop/notch
  - `06`: high+band pass
  - `07`: all pass
- **`15xx`**: set envelope reset time.
  - this is the amount of ticks the channel turns off before a note occurs in order to reset the envelope safely.
  - if `xx` is 0 or higher than the song speed, the envelope will not reset.
- **`1Axx`**: disable envelope reset for this channel.
- **`1Bxy`**: reset cutoff:
  - if `x` is not 0: on new note
  - if `y` is not 0: now
  - this effect is not necessary if the instrument's cutoff macro is absolute.
- **`1Cxy`**: reset duty cycle:
  - if `x` is not 0: on new note
  - if `y` is not 0: now
  - this effect is not necessary if the instrument's duty macro is absolute.
- **`1Exy`**: change additional parameters.
  - `x` may be one of the following:
    - `0`: attack (`y` from 0 to F)
    - `1`: decay (`y` from 0 to F)
    - `2`: sustain (`y` from 0 to F)
    - `3`: release (`y` from 0 to F)
    - `4`: ring modulation (`y` is 0 or 1)
    - `5`: oscillator sync (`y` is 0 or 1)
    - `6`: disable channel 3 (`y` is 0 or 1)
- **`3xxx`**: set duty cycle. `xxx` range is 000-FFF
- **`4xxx`**: set cutoff. `xxx` range is 000-7FF.