Jupiter-4 Compuphonic
The Jupiter 4 Compuphonic (known to most of us as just the Jupiter 4) is a four-voice polyphonic synthesizer with an arpeggiator and a 49-note (C-C) keyboard. It has 8 user-programmable memory locations and 10 preset patches with names like "Piano" and "The Force". The buttons for patch changes are in the front of the keyboard, underneath the keys. Each voice consists of a single VCO with sawtooth, square, or square with Pulse Width Modulation (PWM) and a suboscillator that can be switched on or off. There is also a noise generator that can be switched on and off. Like the suboscillator, it has no level control.
The Jupiter 4 can be put into mono mode which stacks all 4 oscillators on each note, creating a very thick, although monophonic, synth. The filter section consists of a high pass filter and a low pass resonant filter. The Jupiter 4 has two ADSR envelopes, one for the filter which can be inverted, and another for the VCA. It also has one LFO, switchable between sine, square, ramp up and ramp down waveforms. The LFO can be routed to the VCO, the VCF, the VCA, or PWM. Although some say that the LFO rate on the Jupiter 4 is the slowest of any production keyboard, it also can modulate up into the audio frequencies (around 100 hz), making it very flexible. Only the filter can be modulated by a sample and hold function of the LFO labeled as "VCF mod" in the filter section.
The Jupiter 4 has a built-in chorus, a single on/off switch labeled "ensemble". Its arpeggiator is a fixed range running the length of the keyboard. However, there is a click input to sync the arpeggiator to an outside source. The click input can also be used to trigger filter modulation. This input and a cv input for the VCF were the only interface available with the Jupiter 4. It has no keyboard cv/gate input. It is possible to overdrive the Jupiter 4's VCA. When you do this a little light comes on to let you know.