Furnace tracker — C64 (SID) system and instrument documentation

Two pages of the manual of furnace, a multi-system tracker (GPL-2.0 repository), describing how it drives the sid. Furnace is not one of this wiki's focus tools; the pages are kept as a second, independent description of the same chip behaviour — waveform AND-mixing, the envelope bug and its "envelope reset", the 6581 DC-offset digi channel — in a different vocabulary: macros instead of tables, xxx effect values instead of table bytes, "envelope reset" instead of hard restart.

Key claims#

The chip, as Furnace describes it#

• The C64 has "a synthesizer-grade sound chip of which people took decades to master": three oscillators with four selectable waveforms, ring modulation, oscillator sync, a multi-mode filter and an ADSR envelope. Two versions: the 6581 (original) and the 8580, an "improved version with working waveform mixing and somewhat more consistent filter curves".
• Selecting more than one waveform gives "a logical AND mix of waves"; "due to hardware flaws, the mixing is a bit weird and sounds different between the 6581 and the 8580"; noise "cannot be used with any of the other waveforms".
• The 6581 "produces a DC output with its intensity being regulated by the global volume register", which "can be abused to produce a crude, virtual fourth 4-bit PCM channel" — Furnace's system variant Commodore 64 (SID 6581) with software PCM. "The later 8580 chip fixed this problem, making such PCM nearly inaudible"; other PCM methods exist but are not supported.
• Ring modulation: "the channel's output will be multiplied with the previous channel's" (the datasheet's triangle-only condition is not mentioned). Oscillator 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."
• Filter: cutoff 0–2047; resonance 0–15 "amplifies or focuses on the cutoff frequency, creating a secondary peak"; modes low, high, band and ch3off, combinable — "selecting both 'low' and 'high' results in a bandstop (notch) filter". Its aside on ch3off: it "was originally planned for usage with two registers where program could read current oscillator and envelope outputs … but who wanted to sacrifice one channel out of three! so aforementioned was just done in software, and the feature was never used" (an opinion; the datasheet and PRG pages in this wiki document that use).

Envelope reset — Furnace's hard restart#

• Chip config: Hard reset envelope — "configure the envelope parameters used during the short reset before a note"; Envelope reset time in ticks — "0 disables reset, which prevents notes from triggering. 1 is short, but may exhibit SID envelope bugs. 2 is a good value."
• Effects: 15xx set envelope reset time — "the amount of ticks the channel turns off before a note occurs in order to reset the envelope safely"; 00, or more than the song speed, means no reset. 1Axx disables the reset for the channel.
• Instrument: Don't test before new note — "disables the one-tick hard reset and test bit before a new note".
• A/D/S/R macros change the running envelope immediately in the matching phase, with one hardware rule: in the sustain phase "you can only go down. for example, 9-to-8 and 8-to-8 both work, but 8-to-9 immediately mutes the channel."

Effects (C64 system page)#

The 10xx values are the SID waveform nybble (triangle 1, saw 2, pulse 4, noise 8, sums for combinations) without the control bits — an observation from the table, not a statement of the docs.

Instrument editor and macros (C64 instrument page)#

• Fields: Waveform (multi-select); Attack, Decay, Sustain, Release 0–15 ("the bigger the value, the slower the attack"; sustain = the level held); Duty 0–4095; Reset duty on new note ("only useful when using relative duty macro"); Ring Modulation; Oscillator Sync; Enable filter; Initialize filter with Cutoff, Resonance and Filter mode; Absolute Cutoff Macro ("the cutoff macro will go from 0 to 2047, and it will be absolute … rather than being relative"); Absolute Duty Macro (0–4095); Don't test before new note.
• Macros: Volume — "volume sequence is global! this means it controls the chip's volume and therefore affects all channels"; Arpeggio (pitch sequence); Duty; Waveform; Pitch (fine); Cutoff; Filter mode; Resonance; Special — ring and sync plus the gate bit (set: "key on. if previous state was 0 it triggers envelope start/restart; if previous state was 1, it does nothing"; reset: "key off. if previous state was 1 it triggers envelope release") and the test bit (set: "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: "unmute channel and restore ring mod/hard sync"); Attack, Decay, Sustain, Release (each takes effect at once in its own phase).
• Chip config also has Global parameter priority for macros that touch 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"; and a Clock rate setting. Other options are DefleMask/old-Furnace compatibility and undocumented.
• Channel-status icons in the pattern view: silent / gate bit disabled / gate off + test on / test bit enabled / ch3off active.

Practical takeaways#

Furnace confirms, in its own words, the three facts every SID tracker builds on: combined waveforms are an AND and differ by chip; a note needs a short gate-off with a special ADSR before it (two ticks is the safe value, one may bug, zero stops notes from triggering at all); and the 6581's volume register leaks a DC level that can carry 4-bit samples. Its "last used channel wins" filter policy is a third option beside SID-Wizard's leftmost-channel rule and its "latest instrument takes over" (filter-programming). Its sustain-only-goes-down rule explains why raising the sustain register mid-note silences a voice.

Notable quotes#

"0 disables reset, which prevents notes from triggering. 1 is short, but may exhibit SID envelope bugs. 2 is a good value."

"if you modify sustain level when envelope is in sustain phase it immediately changes, although you can only go down."

Relevance#

An independent cross-check for hard-restart, adsr-envelope, sid, pulse-width-modulation and filter-programming, and the source of the furnace cheat sheet.

Pages touched#

furnace · sid · hard-restart · adsr-envelope · pulse-width-modulation · filter-programming · chord-arpeggio · ring-modulation-and-sync · chip-samples · commodore-64 · instrument-tables · sidechain-pump