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furnace/doc/4-instrument/c64.md
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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 logical AND mix of waves will occur.
    • 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.

  • Volume Macro is Cutoff Macro: turns the volume macro in the Macros tab into a filter cutoff macro.

    • volume control is global (affects entire chip output), hence the option.

  • 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.
    • this macro becomes Cutoff when the Volume Macro is Cutoff Macro option is enabled in the C64 tab.
  • Arpeggio: pitch sequence.
  • Duty: pulse width sequence.
  • Waveform: select the waveform used by instrument.
  • Pitch: fine pitch.
  • Filter mode: select the filter mode.
  • Resonance: filter resonance sequence.
  • Special: ring and oscillator sync selector.
  • Test/Gate: when on, the test bit is set, which mutes the channel.