Amiga Mod

What it is

Paula, the sound chip in a Commodore Amiga, plays four channels of 8-bit PCM straight out of memory with no CPU involvement, two hard-panned left and two hard-panned right. Karsten Obarski wrote Ultimate Soundtracker for it in 1987 and the .mod file format that came out of that ran the demoscene for a decade. Four channels means four voices total, and two of them are already spoken for by a bassline and a lead, so the drums get whatever is left. That is why mod drum parts are so sparse: not taste, arithmetic. Producers stole a bar of a break, chopped it into a kick, a snare and a hat, and rebuilt it a hit at a time, which is the technique that fed straight into jungle a few years later on the same machines.

What it sounds like

Eight bits, hard panned and a little distorted. The sample rate ceiling on Paula is about 28kHz, so anything above 14kHz is simply gone and hats arrive as a dull tick rather than a hiss. Samples were traded on floppies at whatever length would fit in 512K of chip RAM, so a kick might be 4KB and audibly truncated. Pitch is transposition, so a sample played high gets shorter and brighter and one played low gets long and muddy, which is why mod toms sound like the same drum in three sizes. The stereo image is extreme, with no centre at all, and most people fix that on a modern release by narrowing to about 60 percent.

Key elements

• Four voices, totalBass, lead and two left over. Two drums can sound at once and no more, so a mod drum part is written by deciding what to leave out.
• Volume in sixty-four stepsThe tracker volume column runs 0 to 64, so every accent lands on an even number when mapped to MIDI. Coarse, but not as coarse as a chip.
• Speed and tempoSpeed 6 at tempo 125 is the ProTracker default: six ticks per row, four rows per beat, 125 beats a minute. Nearly every classic mod starts there.
• Hard panningChannels 1 and 4 go left, 2 and 3 go right, with no pan control. A drum on channel 2 is in your right ear and stays there.
• Effect commandsRetrigger, note delay and sample offset are single hex digits in the pattern. A drum roll is a retrigger command, not eight typed notes.

How to get them

• Budget the channelsTwo tracks for music, two for drums, and enforce it. Mute anything that breaks the rule rather than working around it, because the rule is what produces the sound.
• Downsample everythingResample every drum to 22kHz and reduce to 8-bit with no dither, then low-pass at 12kHz. The hats go dull and that is correct.
• Chop a break by handTake one bar of a funk break, cut it at the transients into three or four one-shots, and rebuild the pattern from them rather than looping the bar.
• Pitch instead of layeringGet a tom from the same snare sample played five semitones down. Transposition changes the length as well as the pitch, and that is a feature here.
• Pan hard, then pull backPrint the drums fully left and right the way Paula does, then narrow the bus to about 60 percent on the master so it survives headphones.
• Set the volume columnWrite accents as values out of 64 and only use whole numbers. Doubling those gives the velocities in the grid below, all of them even.

Reference points

Purple Motion, Skaven, Jogeir Liljedahl, Lizardking, Nemesis, Moby of Future Crew fame, and the Future Crew and Triton demo catalogues. Named as reference points for the sound, not endorsements.