Creative Tools: ProTracker’s Four-Channel Tricks

A convincing ProTracker arrangement could sound considerably busier than its four channels suggested. The trick was deciding which sounds needed to play simultaneously — and which could take turns.

The classic four-channel MOD format stores patterns with 64 rows. Each position in the grid occupies four bytes containing a sample number, a note period and an effect command with its parameter. One complete pattern therefore takes just 1,024 bytes, excluding the samples.

Those samples account for much of a module’s size. A loop allows a short section of recorded sound to sustain a note without storing a long recording. The loop boundaries matter: an abrupt discontinuity can introduce an audible click.

Effect commands add movement without requiring a fresh sample for every variation. The arpeggio command 047, for example, rapidly alternates between the base note and pitches four and seven semitones above it. On C, that produces a C–E–G sequence: a chord-like effect using one channel rather than three simultaneous notes.

Another useful command is 9xx, which starts playback partway through a sample in steps of 256 bytes. It allows different sections of one recording to be triggered independently.

There are trade-offs. An arpeggio sounds different from a sustained chord, while replacing a sound on a channel interrupts whatever was playing there before. Good arrangements account for those interruptions.

The result is worth examining with the pattern editor open. A busy drum part may share one channel between several samples, leaving the others available for bass, melody and accompaniment. The apparent extra capacity comes from the arrangement.

Recommended reading: For the wider story of the Amiga and its rivalry with the Atari ST, explore 68000: THE FLAME WARS.