ARP Pro Soloist, courtesy of Matrixsynth
A preset synthesizer introduced by ARP Instruments in 1972. It was a monophonic unit with 30 presets, an aftertouch sensitive keyboard of three-octave span (C to C). The Pro Soloist had an internal architecture which used a form of digitally controlled oscillator and patch memory, both unusual and forward-looking for their time. The synth was a sales success for ARP, but it also possibly represents a missed opportunity.
The Pro Soloist replaced an earlier model called the Soloist, which had been one of ARP's first products after the 2500. The Soloist, like other small synths of the day, had been marketed as an adjunct to an organ, intended for both home and professional use. Its packaging was rather awkward; it had organ-like tabs which protruded from the front of the case for preset selection, which got broken off when used by touring musicians. It was also said that its conventional analog circuitry was not very reliable, and difficult to keep in tune. (Donald Fagan notoriously burned one after an early Steely Dan session.) Accordingly, ARP set out to fix those problems with improved packaging (the preset controls were moved to a tilted rear panel), and vastly improved and innovative circuitry for its era.
The Pro Soloist's voice architecture consisted of the aforementioned DCO, a set of fixed filters, a low pass VCF, a high pass non-voltage controlled filter, a set of fixed filters, a VCA, and two envelope generators. The parameters for all of these functional components were stored as digital values in ROM, which were converted by crude D/A converters to drive the circuitry when a preset was selected. This was done using discrete digital logic.
The DCO consisted of a high-frequency oscillatory which drove a digital counter, which in turn produced a train of pulse waves through a series of five pulse dividers, each producing an output at half the frequency of the previous one. This set of six pulse trains was mixed in specific proportions to produce an approximation of a sawtooth wave (see Walsh functions). Different presets used the sawtooth, one or more of the pulse waves, or both in combination, depending on the preset. An interesting aspect of the DCO is that although it was driven by an analog control voltage, that voltage was generated in the digital domain. ARP designed its own version of a scanning keyboard (different from the design patented by E-mu Systems) that produced a six-bit binary value for each key. This was used to index a ROM which contained the values of the control voltages to be output by each key, which was converted to an analog voltage by an A/D converter, which in turn controlled the VCO. Since the voltage values in the ROM were pre-computed exponential voltages, no exponential converter circuit was needed, eliminating the main source of tuning instability in the Soloist.
The selected DCO waveforms for a given preset were mixed and then sent through the fixed filters, in some combination according to the preset selected. These were tuned so as to help produce realistic tones for the presets that were labeled with names of real instruments, e.g., "cello", "bassoon", etc. The output of the fixed filters was then mixed and routed to the VCF and then the VCA, which were of conventional design. The VCA was under the control of either a ADSR envelope generator or an AR envelope generator depending on the preset chosen, that voltage was produced digitally. Some presets also had the VCF under control of the AR envelope generator. (Early production versions of the Pro Soloist used a version of the ARP 4035 VCF, which infringed on a Moog Music patent. After lawsuit threats from Moog, ARP replaced this with a VCF of its own design in later production, as it did for other models of the era.)
The aftertouch was routeable to several destinations, using panel controls separate from the presets. Since the Pro Soloist lacked a pitch wheel or mod wheel, the aftertouch performed the usual pitch bend and vibrato functions, as well as being routeable to VCF cutoff frequency. Additional controls provided some control over the output of the fixed filters. Vibrato was produced by adding the output of an LFO (which generated only a triangle wave) to the control voltage being fed to the DCO; the LFO speed was controlled by a panel knob. On a few percussive presets, it was also possible to have the LFO retrigger the envelope generators, so that the sound repeated at the LFO rate as long as the key was held down.
Selection of presets was under the control of 15 toggle switches on the front panel, plus an "upper/lower" toggle switch. Under each preset switch was shown the names of two presets, one above the other. If the upper/lower switch was in the "upper" position, the preset switch selected the upper of the two names, and vice versa. By this method, any of 30 presets could be selected. A priority circuit ensured that only one preset could be selected at a time; if several preset toggles were turned on simultaneously, the preset selected was controlled by the switch that was rightmost on the panel. This was both awkward and sometimes useful; at times, performers "cued" one or more presets by having several switches on, and then shutting off the rightmost one to select the next preset to the left. A portamento switch (in the middle of the preset switches, for some reason) turned on the portamento function, which rate was controlled by one of the sliders to the left of the keyboard.
Despite all this, the Pro Soloist would probably be a footnote in synth history had it not been for the efforts of Tony Banks, who used one extensively on Genesis recordings from 1973 to 1976. (His "In The Cage" solo, well known to progressive rock fans, was played on the Pro Soloist.) The advanced circuit design contained many of the building blocks of a voice-allocating polyphonic synth, and also some very forward-looking digital circuits in the DCO. And it is somewhat surprising that ARP never attempted to extend the design into that territory, which (with a proper microprocessor replacing the discrete logic, which would have been practical by about 1975) would have given them one of the first practical polyphonic synths. Instead, they did not explore this territory until much later with the Chroma, and the DCO design was never used in any other ARP product. (See the Welson Syntex for a contemporary of the Pro Soloist that used a similar DCO design. A scientific article describing the DCO designs of both the Pro Soloist and the Syntex is here.)
At least two versions of the Pro Soloist were produced, but they apparently differed only in cosmetic details. The panel on early units was notated by white graphics on a black background; later units switched to the black-and-gold graphics that were somewhat of an ARP standard in the mid-1970s. Some sources reference a "Soloist II", which apparently was a prototype that never went into production.
ARP Pro/DGX, with the unique red and green graphics for the preset names. Courtesy of Audio Fanzine.
The Pro Soloist was replaced by the Pro/DGX in 1977. This was very similar, but replaced the preset select switches with pushbuttons (and moved the portamento switch to the left), and also had some internal circuitry improvements, including replacement of some of the discrete logic with a microprocessor. The pushbuttons had LEDs to indicate which preset was active. There were 16 preset pushbuttons; another 15 presets, as indicated by "T" shapes on the panel, could be selected by simultaneously pressing two adjacent buttons. Early versions of the Pro/DGX had unique red and green graphics for the preset buttons; later ones adopted the late-1970s ARP "Halloween" orange-and-black standard. The case was also made stronger. The Pro/DGX remained in production until ARP folded in 1981.