Two-Voice
The Oberheim Two-Voice may have started out as just an incomplete Four Voice, with the space for two of its SEM voices left empty (and sold at half the price). In 1976, it seems, Oberheim built a dedicated case for their Two-Voice. The 3-octave 37-note (C- C) duophonic keyboard was originally (in the first 100 or so) a Pratt-Reed keyboard similar in design and function to the one found on the ARP 2600. It had a Transpose knob rather than the switch pair as on the later models. It was labeled "16 - 8 - 4 - 2", much like the Minimoog's octave switches. There was also a two position slider switch labeled "2nd Voice". When engaged, it did the same kind of "highest/lowest note" priority response as you would get out of the "duophonic" keyboard of the ARP 2600. Later models used a keyboard which was the same mechanically, but had a different motherboard, and of course the left hand control panel was replaced with its new digital electronics they got from E-Mu. Portamento can be set for either or both SEMs, along with basic pitch. The Two-Voice also offers a switch to choose which SEM plays first in duophonic mode.
Attached to the keyboard of most Two-Voices is an 8-step 2-voice sequencer (the "Mini- Sequencer") with a voltage-controlled clock and the ability to use sequencer voice 2 or the keyboard output to change the clock rate, letting you set up rhythms. Dual- concentric knobs give you control over SEQ 1 (for SEM 1) and SEQ 2 for (SEM 2). There is also a noise source and a sample-and-hold. Either SEM can be controlled by voltages from its sequencer, the sample-and-hold and keyboard voltages. Two-Voices which do not have this sequencer often have a joystick controller in its place.
As for the SEMs, each has a "pair of oscillators [which] offer a choice of sawtooth or variable-pulse waveforms, the pulse width variable from 10 percent narrow to 90 percent narrow via a 50 percent "square" shape. Pulse width can be used as a modulation destination in a rotary-controlled contest with oscillator frequency (i.e., pitch). Mod sources include the LFO, which is preset to a smooth, vibrato-inducing triangle wave; the second envelope generator; or an external controller. The envelope generators themselves offer only attack, decay and sustain parameters, with decay effectively doubling up as release. The filter can be switched between its various modes, and there are rotary controls for cutoff frequency and resonance. Again, the filter can be modulated positively or negatively by the LFO, envelope 1, or an external source."