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).

the 6581 has a hardware quirk which produces a DC output with its intensity being regulated by the global volume register. this register can be abused to produce a crude, virtual fourth 4-bit PCM channel. Furnace supports this with the "Commodore 64 (SID 6581) with software PCM" system. the later 8580 chip fixed this problem, making such PCM nearly inaudible; while other PCM playback methods have been invented, Furnace does not support them at the moment.

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 and 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 and 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.
  • this effect only exists for compatibility reasons, and its use is discouraged.
  • 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)
• 20xy: set attack/decay.
  • x is the attack.
  • y is the decay.
• 21xy: set sustain/release.
  • x is the sustain.
  • y is the release.
• 22xx: pulse width slide up.
  • xx is speed. if it is 00, the slide is stopped.
• 23xx: pulse width slide down.
  • xx is speed. if it is 00, the slide is stopped.
• 24xx: filter cutoff slide up.
  • xx is speed. if it is 00, the slide is stopped.
• 25xx: filter cutoff slide down.
  • xx is speed. if it is 00, the slide is stopped.
• 3xxx: set duty cycle. xxx range is 000 to FFF.
• 4xxx: set cutoff. xxx range is 000 to 7FF.

info

this chip uses the C64 instrument editor.

channel status

the following icons are displayed when channel status is enabled in the pattern view:

• channel is silent:
  • it's not
  • gate bit disabled
  • gate bit disabled and test bit enabled
  • test bit enabled
  • ch3off enabled in filter mode

chip config

the following options are available in the Chip Manager window:

• Clock rate: sets the rate at which the chip will run.
• Global parameter priority: change the priority of macros which control global parameters, such as volume and filter.
  • Left to right: process channels from 1 to 3. the last one to run macros will take effect.
  • Last used channel: process channels from oldest to newest (since last note). the one which had the latest note on will take a effect.
• Hard reset envelope: configure the envelope parameters used during the short reset before a note.
• Envelope reset time: set how long will the pre-note reset last, in ticks.
  • 0 disables reset, which prevents notes from triggering.
  • 1 is short, but may exhibit SID envelope bugs.
  • 2 is a good value.

the other options are for compatibility with old Furnace and DefleMask, so they won't be documented here.