The TB-303 was a synthesiser/sequencer produced by the Roland corporation in 1982 that had a crucial role in the development of contemporary electronic music. The TB-303 was originally sold to guitarists for bass accompaniment when practising alone, a role in which it failed miserably. It wasn't until the mid- to late-1980s that DJs and musicians in Chicago found a use for the machine in the context of the newly developing house music. DJ Pierre's "Acid Traxx" is widely acknowledged to have been the first track to incorporate the sounds that have come to define the sound the TB-303 is known for today. The well-known "acid" sound can be produced with a TB-303 by playing a melody while changing the cutoff frequency, envelope modulation and accent controls on the filter.
The unit used a half cosine wave (i.e. from 0 to 180 degrees repeating) or a square wave (which is actually not really a square but an alternation between a regular saw and a saw upside down) (switchable between the two) and used a lowpass filter with a rolloff of -18 dB per octave.
It also featured a 'simple' step-time method for entering note data into the programmable sequencer. This was notoriously difficult to use, and would often result in entering a different sequence than the one that had been intended - some users also take advantage of the quirky fact that when the batteries are removed for a certain period, patterns that are programmed in memory begin to vary in random ways - one of the factors that helped to create the randomish "Acid" sound.