Composing in SID Factory II, Part 4 — Instruments (Chordian)

A tutorial blog post by chordian on making instruments in sid-factory-ii: what the ADSR bytes do, why hard restart exists, the four waveforms and their combinations on old and new chips, the noise lock-up and the test bit, and how the wave table works — ending with an "oldskool snare drum" recipe. Written hands-on: every claim comes with values to type and a song to play.

Key claims#

• ADSR: each of attack, decay, sustain, release is a 4-bit nibble. Examples on instrument bytes: 04 69 = instant attack, short decay 4 to sustain 6, long release 9 (an audible volume bump at the start); 00 a0 = instant everything at sustain A, "louder"; 9b 2a = slow fade-in, slower decay to a quiet 2, slow release. Note-off "drops everything it's doing … and immediately starts fading it out, regardless of what volume level the attack or decay timers had actually reached" — with long attacks the notes "never really get started".
• Hard restart: a sequence of short notes kept gated until the next one makes "the ADSR stumble here and there"; some values alleviate it. Hard restart "works by gating off and resetting the ADSR values a few ticks before the next note triggers"; most SID Factory II drivers do it "exactly two ticks before". Example: a 15-tick note plays 13 ticks, then the restart gates off and applies the HR ADSR — possibly "a tiny bit staccato", but "the next note always triggers perfectly". Enable it with bit $80 of the instrument's third byte; the HR table pair defaults to 0F 00 — "I have never had to change it".
• Waveforms as wave-table bytes with gate: 11 triangle ("closest … to the sine"), 21 sawtooth, 41 pulse ("the only waveform that can be modulated. Great for leaders"), 81 noise. Combinations: 31 tri+saw "virtually inaudible" on the 6581, thin on the 8580; 51 tri+pulse "the most used combined waveform since it's clearly audible on both"; 61 saw+pulse almost inaudible old, organ-like new; 71 all three virtually inaudible old, thinner than 31 new. Chip history: 6581 (1982) — combined waveforms were never intended by bob-yannes; 8580 (1986, white 64C) — more responsive filter, much improved combining. SID Factory II defaults to the 8580.
• Noise lock-up and the test bit: combining noise with another waveform (91 and up) produces silence and locks the noise generator so that 81 stays silent afterwards, until the test bit is set. The instrument flag $10 (third byte) sets the test bit briefly at the start of every note ("also known as the oscillator reset") — "Setting the test bit has no downsides."
• Pulse is inaudible until a pulse width is set; minimal pulse table 84 00 00 / 7f 00 00.
• Wave table: one row per tick from the instrument's wave index until a 7f xx loop row; repeated rows slow the alternation ("Repeating lines in the wave table can often make instruments sound more pleasant"); second byte = semitone offset (0c = +1 octave) — but slide/portamento/vibrato only work while the offset is 00, so leave the sustained rows at 00; offsets that leave the 8-octave frequency table sound "somewhat random". Values 80–df are static notes taken straight from the frequency table, unaffected by the played note — for a static hi-hat before a bass note, or a fully static drum.
• Snare recipe: 81 ca (noise, high, one tick) · 41 a8 · 41 a4 (pulse at two medium static pitches — a "thunk" slide) · 80 ca (noise, gate off) · 7f 08 (loop on the gated-off noise). Gating off inside the loop sends the drum into release even when the sequence holds +++; every note key gives the same drum. Keeping notes gated with +++ packs smaller than --- rows. Advice: keep wave-table rows gated on until the looping part, because gate on/off across rows gives "a wobbling volume that you can't always trust".
• SID Factory II facts in passing: F10 file selector, Ctrl+1/2/3 toggle tracks, Alt+i/w/p instrument/wave/pulse tables, Shift+q… play an instrument, Ctrl+Enter jumps from an instrument's table index to the table, Shift+Enter opens a bit editor for a flag byte; Vincenzo's two SID Factory II video tutorials on YouTube (StrayBoom).

Practical takeaways#

Three rules worth carrying to any SID tracker: enable hard restart and leave its ADSR at 0F00; always set the test bit on the first frame; put the pitch offsets only on the short attack rows of a wave table so slides and vibrato keep working. The snare recipe is the same shape as the classic wavetable drum in wavetable-programming.

Notable quotes#

"Hard restart was invented to defeat this ADSR bug."

"Setting the test bit has no downsides. If you want, you can always leave it enabled."

Pages touched#

sid-factory-ii · chordian · hard-restart · adsr-envelope · sid · wavetable-programming · instrument-design · ring-modulation-and-sync · pulse-width-modulation · bob-yannes · octave-bass · semitone-math · melody-writing · diatonic-chords · sound-effects