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

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Chip Versions

Two versions of the chip exist:
  • 6581: Original chip with a hardware quirk producing DC output regulated by the global volume register. This can be abused to create a crude virtual fourth 4-bit PCM channel.
  • 8580: Improved version with working waveform mixing and more consistent filter curves. The DC output quirk is fixed, making PCM playback nearly inaudible.
Furnace supports the 6581 with the “Commodore 64 (SID 6581) with software PCM” system.

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Effects

  • 10xx: Change wave
    • 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 (00 to 64)
    • This effect exists for compatibility only; use 4xxx instead
  • 12xx: Set coarse duty cycle (00 to 64)
    • This effect exists for compatibility only; use 3xxx instead
  • 13xx: Set resonance (00 to 0F)
  • 14xx: Set filter mode
    • 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
    • Number of ticks the channel turns off before a note to reset the envelope
    • If xx is 0 or higher than song speed, envelope will not reset
  • 1Axx: Disable envelope reset for this channel
  • 1Bxy: Reset cutoff
    • x ≠ 0: On new note
    • y ≠ 0: Now
    • Not necessary if instrument’s cutoff macro is absolute
  • 1Cxy: Reset duty cycle
    • x ≠ 0: On new note
    • y ≠ 0: Now
    • Not necessary if instrument’s duty macro is absolute
  • 1Exy: Change additional parameters (compatibility only)
    • x selects parameter:
      • 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: Attack
    • y: Decay
  • 21xy: Set sustain/release
    • x: Sustain
    • y: Release
  • 22xx: Pulse width slide up
    • xx is speed; 00 stops the slide
  • 23xx: Pulse width slide down
    • xx is speed; 00 stops the slide
  • 24xx: Filter cutoff slide up
    • xx is speed; 00 stops the slide
  • 25xx: Filter cutoff slide down
    • xx is speed; 00 stops the slide
  • 3xxx: Set duty cycle (range: 000 to FFF)
  • 4xxx: Set cutoff (range: 000 to 7FF)

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Channel Status

When channel status is enabled in the pattern view, the following icons are displayed:
  • Not muted
  • Gate bit disabled
  • Gate bit disabled and test bit enabled
  • Test bit enabled
  • Ch3off enabled in filter mode

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Chip Configuration

Clock rate: Sets the rate at which the chip will run Global parameter priority: Change the priority of macros controlling global parameters (volume, filter)
  • Left to right: Process channels 1 to 3, last one takes effect
  • Last used channel: Process channels from oldest to newest; latest note takes effect
Hard reset envelope: Configure envelope parameters during the short reset before a note Envelope reset time: Duration of pre-note reset in ticks
  • 0: Disables reset (prevents notes from triggering)
  • 1: Short, but may exhibit SID envelope bugs
  • 2: Good default value

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