The term sequencer has been associated with voltage-controlled analog synthesizers. It refers to a module that can be programmed to store the waveform of the DC voltages used to control the voltage controlled oscillator. The sequencer may receive its control voltage waveform from the keyboard or other voltage control source. This made it possible to program and play a series of notes at different speeds or with different synthesizer sounds. Buchla's synthesizer was the first to include a sequencer that would soon become a standard accessory in any voltage-controlled synthesizer. Before MIDI was introduced in 1983, sequencers were the most popular method of storing the note flows. They were usually used to provide consistent rhythms and harmonic lines that could be repeated while other notes were played freely at the same time. The sequencer music has become synonymous with the steady, trance-like rhythms that characterized the music of artists such as Tangerine Dream, Kraftwerk, Isao Tomita and Klaus Schulze.