Intense Tech #17 — The Joys of Noise (Defense Mechanism)

A deep dive into lsdj's noise channel by defense-mechanism (intense-tech): how the shape byte maps onto the LFSR's octave and divisor, short-loop versus long-loop mode, how S, P, C and transposes act on shapes, the random-mute quirk, randomised crashes with Z, a noise kick recipe and turning noise into a pulse wave with retrigs.

Key claims#

• The noise channel is a linear-feedback shift register generating "a series of 1-bit pulse waves each with random width"; its clock is divided by a divisor ("except for the highest digit, which actually doubles the frequency") and then by a power of two that sets the octave. In LSDj the first digit of the noise shape is the octave, the second the divisor.
• Noise shape chart — first digit: F highest octave, E one lower … 2 thirteen lower, 1 and 0 no sound. Second digit: F/7 × 2, E/6 × 1, D/5 ÷ 2, C/4 ÷ 3, B/3 ÷ 4, A/2 ÷ 5, 9/1 ÷ 6, 8/0 ÷ 7. Digits 0–7 select short-loop mode (half-length waveform: "more tonal, ringing, metallic tones"), 8–F long-loop ("less pitched and more akin to white noise").
• Before 8.6.0 the octave of the phrase note selected the first digit (octave 5 = the instrument's digit; octave 6 before 5.1.6); the pitch class had no effect. Since 8.6.0 shapes can be typed directly in phrases.
• Short-loop pitches form a "negative harmonic series" (the base frequency, near a C, divided by whole numbers): not exact, but closest to C, F, A♭ and D. Example: a noise-only demo by they/them.
• S adds per digit: S01 on shape 4F gives 40 (not 50), SFF gives 3E; S is additive across successive commands. A transpose (phrase or table) changes the whole byte — 4F + 01 = 50, + FF = 4E — and only while its table row runs, reverting afterwards.
• P and C on noise add their value to the shape: C alternates base and base + value every tick (FF + C01 → FF, F0, …), P keeps adding (FF → F0 → F1 → F2 …); CMD/RATE slows them. Because they ignore loop modes and the real frequencies, smooth sweeps need hand-made S tables.
• Random mute: "there's a chance of 1 out of 256, or 0.4%, that the noise channel will go silent if it switches from long-loop mode to short-loop mode" (second digit 8–F → 0–7). The instrument's S MODE (S CMD in earlier versions) set to STABLE instead of FREE allows short → long but not long → short.
• Random crashes: Z after the shape randomises each digit independently, so the variation stays in one loop mode and octave — first digit + 0…4 (capped, since wrapping past F to 0 is silence), some rows with a first digit of 0, second digit F for a completely random divisor (credited to an Exploring Chip Tunes video).
• Noise kick: base shape 95; table row 1 transposes 10, so tick one sounds A5 (move the TSP to change the attack alone); then 93, 91, 90; then 80, 70, 60 so the second digit never wraps back to F; S MODE STABLE keeps it in short-loop mode. "Noise kicks sound great when they are LOUD!"
• Retrig → pulse: switching the noise on and off rapidly with R makes a pulse wave; depending on the version R00 retrigs once or every tick (in 8.9.3 R01 retrigs every tick); loudness via a shorter LENGTH or ADSR; the pitch depends on the R rate, the tempo (tick length) and the shape; R80–R8F, "the super-fast retrig", ignores tempo and LENGTH.
• .exe's NOIZE JAMS uses only the noise channel; its save file (LSDj 8.5.1) is shared with the article.

Practical takeaways#

Read a noise shape as octave + divisor, stay in one loop mode (STABLE) for drums, sweep with S tables rather than P, and remember that the second digit's 0–7 half is where the pitched, metallic tones live.

Notable quotes#

"Sometimes it goes silent for no reason."

Pages touched#

instrument-design · lsdj · game-boy-apu · pitch-slide-and-portamento · retrigger · randomization-and-probability · chord-arpeggio · intense-tech · defense-mechanism