TB-303 and Polyrhythms

The lasting love and affection for Acid music is as evident as ever. Part of what people love about it is no doubt the hard, squelchy sound of the Roland diode-ladder, analog filter. That filter is a defining characteristic of acid as a genre.

But the real heart of the sound, the thing that gives it a lot of “play”, and joy through repetition, is the unity of percussion and melody created by using the filter with the TB-303 style sequencer. The TB-303 is often used as sort of a polyrhythmic instrument, where rhythms kind of emerge and disappear as the musician turns the tiny aluminum knobs.

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These subtle variations are afforded by a few different mechanics. The most important is probably the Accent modifier, and the way it affects the filter and gain. You can set any step to be an accented step, and then you use the Accent knob to change the amount of the Accent. If you set it low, the Accented notes are almost the same as the regular notes , and the whole thing blends together a bit, in terms of the filter cutoff and volume.

As you raises the Accent, those accented notes will start to stand out more, and punctuate certain parts of the sequence. This can drastically change the rhythm of pattern, and of course changes the timbre of each note more dramatically. That’s because Accented notes raise the filter cutoff and the volume, for that note, as well as changing the Filter Envelope decay. The Env Mod knob also changes the rhythm of the pattern and percussion of each note, by increasing the slope of the filter envelope. So it makes every note, excluding slide notes, punch harder out of the mix. The sliding notes fade into the background, and the Env Mod and Accent create a counterbalance of focus between different sets of notes. These mechanics blur the line between melody and rhythm.

So you can have a kick drum beating away, and explore many subtle polyrhythms, and textural variations, with just a few knob twists. This is what leads to the drone-like quality of a good acid song, and raises our tolerance for the looping, repetitive nature of the music.