furnace/doc/4-instrument/c64.md

C64 SID instrument editor

the C64 instrument editor consists of two tabs: "C64" to control various parameters of sound channels, and "Macros" containing several macros.

C64

• Waveform: allows selecting a waveform.

  • more than one waveform can be selected at once. in that case a logical AND mix of waves will occur...
    • due to hardware flaws, the mixing is a bit weird and sounds different between the 6581 and the 8580.
  • noise is an exception. it cannot be used with any of the other waveforms.

• Attack: determines the rising time for the sound. the bigger the value, the slower the attack. (0 to 15).

• Decay: determines the diminishing time for the sound. the higher the value, the longer the decay (0 to 15).

• Sustain: sets the volume level at which the sound stops decaying and holds steady (0 to 15).

• Release: determines the rate at which the sound fades out after note off. the higher the value, the longer the release (0 to 15).

• Duty: specifies the width of a pulse wave (0 to 4095).

• Ring Modulation: when enabled, the channel's output will be multiplied with the previous channel's.

• Oscillator Sync: enables oscillator hard sync. as the previous channel's oscillator finishes a cycle, it resets the period of the channel's oscillator, forcing the latter to have the same base frequency. this can produce a harmonically rich sound, the timbre of which can be altered by varying the synchronized oscillator's frequency.

• Enable filter: when enabled, this instrument will go through the filter.

• Initialize filter: initializes the filter with the specified parameters:

  • Cutoff: the filter's point in where frequencies are cut off (0 to 2047).
  • Resonance: amplifies or focuses on the cutoff frequency, creating a secondary peak forms and colors the original pitch (0 to 15).
  • Filter mode: sets the filter mode. you may pick one or more of the following:
    • low: a low-pass filter. the lower the cutoff, the darker the sound.
    • high: a high-pass filter. higher cutoff values result in a less "bassy" sound.
    • band: a band-pass filter. cutoff determines which part of the sound is heard (from bass to treble).
    • ch3off: mutes channel 3 when enabled. not sure why is this part of the chip's design, but it is.
    • multiple filter modes can be selected simultaneously. for example, selecting both "low" and "high" would combine the effects of both, resulting in a bandpass filter.

• Absolute Cutoff Macro: when enabled, the cutoff macro will go from 0 to 2047, and it will be absolute (in other words, control the cutoff directly rather than being relative).

• Absolute Duty Macro: when enabled, the duty macro will go from 0 to 4095.

• Don't test before new note: this option disables the one-tick hard reset and test bit before a new note.

Macros

• Volume: volume sequence.
  • warning: volume sequence is global! this means it controls the chip's volume and therefore affects all channels.
• Arpeggio: pitch sequence.
• Duty: pulse width sequence.
• Waveform: select the waveform used by instrument.
• Pitch: fine pitch.
• Cutoff: filter cutoff.
• Filter mode: select the filter mode.
• Resonance: filter resonance sequence.
• Special: ring and oscillator sync selector, as well as:
  • gate bit:
    • set (1): key on. if previous state was 0 it triggers envelope start/restart; if previous state was 1, it does nothing.
    • reset (0): key off. if previous state was 1 it triggers envelope release; if previous state was 0, it does nothing.
  • test bit:
    • set (1): immediately mute channel
      • if the channel is a source of ring mod and/or hard sync, those stop working until the bit is reset.
    • reset (0): unmute channel and restore ring mod/hard sync.
• Attack: sets envelope attack speed.
  • if you modify attack speed when the envelope is in attack phase it immediately changes.
• Decay: sets envelope decay speed.
  • if you modify decay speed when envelope is in decay phase it immediately changes.
• Sustain: sets envelope sustain level.
  • if you modify sustain level when envelope is in sustain phase it immediately changes, although you can only go down. for example, 9-to-8 and 8-to-8 both work, but 8-to-9 immediately mutes the channel.
• Release: sets envelope release speed.
  • if you modify release speed when envelope is in release phase it immediately changes.