Oberheim introduced the Synthesizer Expander Module (SEM) in 1975 to complement the DS-2 sequencer and enable a user to play one synthesizer while the DS-2 played a sequence on another. The SEM featured a two-pole filter that could operate as a lowpass, high-pass, bandpass, or band-reject filter, giving it a different sound than the filters of the Moog or ARP filters popular at the time. By combining multiple SEM modules with a digitally scanned keyboard and two-channel voltage-controlled sequencer, the company next introduced the Two-voice TVS-1. The Four-voice FVS-1, was available with a rudimentary programmer module, capable of storing and recalling sound settings and, in 1977, the Eight-voice EVS-1 combined a Four-voice with an external module of 4 additional SEMs. These were among the first commercially available polyphonic synthesizers.