furnace/doc/7-systems/c64.md

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# Commodore 64
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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.
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very popular in Europe and mostly due to the demoscene, which stretched the machine's limbs to no end.
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two versions of aforementioned chip exist - 6581 (original chip) and 8580 (improved version with working waveform mixing and somewhat more consistent filter curves).
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# 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.