Sequential Circuits Prophet VS. Courtesy of Matrixsynth.
A hybrid analog-digtal synth produced by Sequential Circuits in the mid-1980s. It is notable as the first synth to implement the vector synthesis concept. According to a history published by Perfect Circuit, the idea for the VS originated with Sequential employee Chris Meyer, who was thinking about the wave scanning method used by the PPG Wave 2.2, and wondered what would happen if the interpolation method were to be two-dimensional, interpolating between four waveforms, instead of PPG's method of linear interpolation between two waveforms. From this, the basic concept of the Prophet VS arose.
Voice Architecture[]
The voice architecture centered around a set of four digital oscillators for each voice, each having a VCA that controlled that oscillator's contribution to the result. Both manual and automated methods were provided for controlling the combination. These were wavetable oscillators; the synth came loaded with 96 waveforms in ROM, plus 32 RAM locations for user-created waveforms. Most of these waveforms were complex, although there was a basic sine wave and sampled white noise, plus an all-zeroes "silence waveform". Each oscillator was individually tuneable, and its frequency could be modulated separately from the others. The four oscillators were mixed by a four-input "vector mixer", which was the crux of the vector synthesis method. The vector mixer is described in the next section.
The rest of the audio portion of the voice architecture was conventional, including a four-pole, low pass VCF, a VCA, and a panner that allowed notes to be spread across the stereo outputs. The control signals consisted of two LFOs and three envelope generators which were actually five-segment universal event generators, allowing the user to select both the rate and end point for each segment. Additionally, the final segment implemented Sequential's alternate release mechanism, which allowed the user to choose a "primary" and "alternate" rate setting. The alternate rate went into effect when a foot switch plugged into a rear panel jack labeled "ALTERNATE RELEASE" was pressed. Several looping options were available that could loop over any of the first four segments during the sustain phase, forwards, backwards, or alternating. One of the envelope generators was tied to the control input of the VCA; the second was tied to the cutoff frequency input of the filter, but could also be routed to modulate oscillator frequency. The third was allocated to the vector mixer and is described in the next section. The synth implemented a not-fully-interconnected form of a modulation matrix that allowed the LFOs, keyboard velocity and aftertouch, and several other sources to be routed to various destinations.
Vectors and Vector Modulation[]
The digital oscillators were named in the manual using the letters A through D. The mix of the four was conceptually controlled by a two-dimensional vector mixer, represented as a square graph in the user's manual, with a "mixing point" placed somewhere on the graph. When the mixing point was in the center (the "origin"), all four oscillators mixed at an equal level. As the mixing point moved towards one corner, the level of the oscillator represented by that corner increased, and the levels of the other three decreased. Centering the mixing point on one side equally mixed the two oscillators represented by the adjacent corners, while excluding the other two. Many combinations were possible by positioning the mixing point within the graph. The graph was constructed with oscillators A and C along one axis, and oscillators B and D along the other.
The vector mixer was controllable through three mechanisms. The most straightforward, and visually striking, was to use the joystick on the panel. This was a physical representation of the vector graph, with an oscillator name silkscreened on the panel at each corner of the joystick's movement square (which was rotated 45 degrees so that the four corners would be at the cardinal points). Moving the joystick directly controlled the vector mixer. However, an automated means of controlling the mixer existed in the form of the mixer envelope generator. Like the others, this was a five-segment universal event generator with various looping options, having a rate and endpoint setting for each segment. However, the endpoints, rather than being single values, were X-Y coordinates on the vector graph. (When programming the envelope generator, the end points were programmed by moving the joystick to the desired point for each segment.) Additionally, a number of modulation sources, including the LFOs, could be routed to either the A-C or B-D axis of the miser.
Programming and Performance Controls[]
Patch programming took place via a semi-one-knob interface. In most cases, each major function had its own selection button on the panel, but entry of continuous data values was via a data entry slider. A two-line, alphanumeric LCD interface helped. A special edit mode allowed the performer to create new waveforms to be stored in RAM, by mixing existing waveforms in various ways. Patch memory consisted of 100 RAM patch locations plus 32 RAM locations for user-created waveforms. A memory cartridge slot accepted RAM cartridges which provided the same memory as described above. ROM cartridges containing additional waveforms (96) were also available.
Basic performance controls consisted of a five-octave, C-to-C keyboard with velocity and aftertouch. Conventional pitch and modulation wheels were provided; curiously, the patch wheel had a maximum range of +/- a perfect fifth (7 half steps). The one other performance control was the vector joystick.
Split and layer modes were available for the keyboard. In the split mode, the synth was bitrimbal; the lower portion of the keyboard played one patch, and the upper portion played another. The VS had a unique method for choosing the patches for the two halves: a parameter in each patch was the number of a "linked patch". When the split mode was chosen, the left half of the split loaded and played the linked patch, while the right half continued to play the selected patch. Both the number of the linked patch, and the keyboard split point, were stored with the patch. Split mode allocated four voices to each side of the split. In layer mode (caled "Double" mode), pressing a key played two voices, layering a voice sounding the selected patch with a voice sounding the linked patch. (Layer mode, of course, made the synth 4-note polyphonic, since each note played consumed two voices.)
The VS was provided with a very powerful arpeggiator. This included the ability to "latch" an arpeggiation and recall it later, making it a sort of sequencer. A latched arpeggiation could be added to and then stored with a patch. Turning on a certain keyboard mode transformed the high C key on the keyboard into a "rest" key, allowing rests to be added to an arpeggiation. The arpeggiator could be driven by either an internal clock, or by incoming MIDI Clock.
MIDI[]
The Prophet VS featured an extensive MIDI implementation. It was one of the first synths to transmit Note On messages with velocity. The aftertouch, pitch wheel and mod wheel all transmitted the proper messages, as did program changes. It received all of the basic message types, but received Channel Pressure, Key Pressure, and Expression Pedal were treated the same as Modulation. Patches and user-built waveforms could both be dumped and loaded via Sysex. A quirk was that, for note-off events, instead of sending a Note Off message, it sent a Note On with a velocity of zero. This was permitted by early versions of the standard, but is not implemented properly by some synths. The synth supported all four of the MIDI channel modes, including Mode 4, which allowed an external source to control each voice individually.
Split mode allowed the performer to select a second MIDI channel for transmitting notes played on the left portion of the keyboard. Additionally, in split mode, receiving MIDI message on the second channel played the linked patch. Unlike playing from the keyboard, MIDI was not confined by the actual split point; both halves could play any note in the MIDI note number range. A peculiarity of the Double keyboard mode was that it caused all performance messages to be sent twice, once on each channel.
The joystick position was transmitted and received using the General Purpose Controller 1-2 continuous controller types, one for each axis. The "ALTERNATE RELEASE" foot switch sent the Sustain Pedal controller type, and receiving either the Sustain or Hold 2 controller messages was treated the same as using the foot switch.
All patch parameters were settable through MIDI using the Non-Registered Parameter mechanism (NRPM). (The Prophet VS was one of the first synths to implement this.) If it was enabled in the MIDI setup, the synth sent NRPM whenever a parameter was changed on the panel.
The MIDI implementation contained a provision allowing the performer to load waveform data from any external source, using the MIDI Sample Dump Standard messages. The data had to meet specific requirements, which were detailed in the manual.
Production History[]
The Prophet VS was the last synth model produced by Sequential Circuits. It was in production for less than a year before Sequential went under in 1987. Perhaps 250 were produced. Working examples are very rare and valuable now.
Most of the Sequential engineering team that designed the VS was hired by Yamaha after that company bought most of Sequential's assets. They went to design two other well-regard synths based on the vector synthesis method: the Korg Wavestation, and the Yamaha SY22.