Sequential Circuits Prophet-600
Article from Electronics & Music Maker, April 1983
The instrument is certainly made to be portable — it's light, very low profile, measuring 37½ x 12 x 4ins (L x D x H), and yet well constructed from black 'mottled' stove-enamelled finish metal. Slightly less attractive wooden end pieces complete the instrument — some polish and wood less prone to splintering would be an improvement here. Still, it won't make any difference to the sturdiness or playability of the 600.
The main panel is logical and easy to use, with synthesiser programming sections clearly marked, from left to right, for Poly-Mod, LFO-Mod, Unison Track, Oscillator A, Oscillator B, Mixer & Glide, Filter, Amplifier, Master Tune and Volume.
At the far left of the panel is the new mylar membrane touch section with numerical keypad and large two-digit red LED display for program selection. Other touch pads operate tape storage, tune, plus Sequencer 1 and 2, and arpeggiators, with Speed control knob for both these. The familiar pitch and modulation wheels are provided next to the keyboard for left hand operation, and the standard size keyboard plays easily and smoothly. If anything, the mechanics of these wheels are as simple as possible, yet accurate enough for their purpose.
The rear panel connections are straightforward, with standard jack sockets for external (close-to-make) footswitch, or 5-15V positive going trigger from a drum machine etc. There's mono audio out (which also provides a stereo signal out sufficient to drive a pair of headphones at 600 ohms minimum for monitoring), a filter voltage control input for increasing frequency cutoff (needs 0-10V DC e.g. from the SCI model 840 pedal), cassette interface in and out, MIDI 5-pin DIN in and out, line voltage and neat 'press-to-release' fuse holder, plus Europlug socket for separate mains cable with power on/off switch.
I'll briefly mention here that the MIDI interface will slave 600's together to all work from one keyboard or home computer with RS 232 serial output.
The mechanical controls use low cost knobs and slider switches, with some slight movement of knob spindles apparent. Four rotary controls did not fully utilise their range — the two frequency pots for the oscillators stopped at the highest pitch from 7-10 on the dial, and the pulsewidth controls for these oscillators turned off at 2 and 8 when a little more range, especially at the midpoint for selecting a square wave, would have been desirable.
Access to the circuitry is quickly done by removing 4 side screws and lifting up the hinged main panel. This may be necessary for the owner to do from time to time, as instructions are given for adjusting the 12 oscillators and filters by simply linking 2 wires and tuning the 18 presets (shown on the right hand board in the base). This only need be done if oscillators have drifted so far off, the micro can't pull them back and this will probably only happen if you move the instrument around a lot without the necessary care. The check procedure neatly uses 6 of the LEDs in the touch switches to help you get it right.
Four large PCBs are used to hold the circuitry — two for the main panel control connections and logic, and, in the base next to the transformer, there's the main micro board holding Z80A, EPROM, RAM, interface and PSU, which is hard-wired to the second PCB containing the 12 synthesiser voices.
The Prophet-600 contains a micro system based on the Z80A running at 4MHz with hardware basically similar to the Prophet-5 and -10 computers. However, as mentioned, there is new software generation of the envelopes that produces dynamic CVs at 3 sample/holds for filter, amplifier and oscillators. This is why no gate signals appear in the synthesiser. The filter envelope used to set the filter timbre contour and the Poly-Mod modulation of Osc A is also the computer at work. Similarly, the LFO is digitally summed into control destinations, e.g. the 12 separate oscillator CVs, two PW CVs, or 6 filter frequency CVs, along with the Pitch and Mod wheels. This greatly improves the overall stability of the instrument.
Generally, the circuitry has been simplified by the use of Curtis CEM chips, including the new CEM 3372 which has VC mixer, filter and amplifier in one IC for each voice. Pulse waveform switches are gone, and the PW CV drives the duty cycle to DC (which may account for the limited pot control range).
CEM 3360s take care of VCA balance and the Tune function now includes the filter's resonance setting as well. The only presets are for scaling the 12 oscillators and the 6 filters.
With power off, the non-volatile battery supplies the two RAM chips holding the 100 sound programs, sequences and some protection circuitry, which also removes any odd noises when turning the instrument off. The software is held in one EPROM.
The system clock runs at 8MHz and is divided to give 4MHz for the Z80 micro, 2MHz for the Tune Timer and 500kHz for the MIDI. The computer generates 'maskable' interrupts and its basic scan routine checks envelopes, LFO, sample/holds, LEDs, Pitch and Mod wheel, plus one other control knob.
Incidentally, it was necessary to check the wiring connectors inside the instrument and push them firmly in following the shipment from Holland.
Pot motion is detected by comparing current and previous scan values, while the DAC 71 makes a 7-bit approximation (8-bit for pitch wheel). The oscillators are CEM 3340 based, feeding to the CEM 3372. An 8x8 diode matrix sets keys pressed, with further 8x3 and 8x2 matrices for the mylar control panel and synth programming switches respectively, all into a 4514 chip.
On switch-on, the Tune switch LED lights while initial tuning of the oscillators takes place. After a few seconds, the tune LED goes out and the instrument becomes playable, jumping into Preset mode, shown by the Preset LED. Any of the 100 'factory-loaded' program sounds can now be dialled up using the touch numerical keypad 0-9 called Program Select. Two numbers are required from 00-99 and are shown on the display, with an 'opt out' after the first number entry if you make a mistake.
The Tune switch is used occasionally (more during the first 30 minutes) to bring the oscillators back to the pitch. The time it takes to do its checking is increasingly reduced as the pitch stabilizes. Of course, the instrument cannot be played while you re-tune, but previous settings are restored afterwards, including any unrecorded editing you've been doing.
A quick check of the output Volume setting and the Master Tune (adjustable +/-1 semitone) will also be necessary with audio mono out connected to mixer or amplifier.
Of course, the aim of this instrument is to offer complete programmability for you to create your sounds, rather than stick with factory presets (some people do, amazingly enough!). So, once you've set up the programming section you can 'edit' a stored sound or create a new one at any time. This allows you to prepare 'libraries' of 100 programs for your performance or studio work which can then be 'saved' on a mono cassette recorder for later retrieval. Two Sequencers will play your polyphonic music and two types of Arpeggiator increase the instrument's scope greatly.
Five touch buttons with LED indicators built-in are used for storing program sounds and your prepared sequences of music played from the keyboard.
The cassette interface provided will save information on cassette in 3 ways; it will save all 100 programs in one go, save one group of 10 programs, or save both sequences.
As with most cassette saving operations, some consideration has to be given to the particular recorder used, as well as the quality of the cassette. My own tests were with cheap mono recorders and a stereo portable that I use for other micros, and saving, verifying and loading were all done successfully. The operating manual gives useful suggestions to help you with this.
Saving data is basically the same for programs and sequences. The 'Record' pad is pressed on, followed by the 'To Tape' pad, unless you're saving a bank of 10 programs or sequences. Then you insert another instruction between — either pressing one Program Select for the group of 10 programs to be saved, or Seq. 1 or 2 pads for both sequences to be saved together.
A simple verify check can be made using the 'From Tape' pad — if its LED blinks you know you've got a tape error and you should try again. Otherwise, a tune routine will be followed by normal operation again. It's not possible to jump out of the 'To Tape' mode — you have to wait the time it takes (up to 30 seconds or so for the whole 100 programs or full sequence).
In this way, you can build up tapes for programs and tapes for sequences, best done for relatively small numbers of files on C15 cassettes.
Loading follows the same kind of procedure, using 'Record' and 'From Tape' pads with the same insert for a program group or sequences.
A real-time polyphonic sequencer is provided on the Prophet-600 for recording bass line, short riffs or ostinatos, and melodies. It remembers exactly your keyboard performance using up to 6 notes each time and its playback speed can be changed from ¼ to 4X real-time using the 'Speed' control. Playback speed can be programmed freely and stored with the sequence on tape.
A footswitch can be used to stop both recording and playback and functions are simply done with 'Record' and 'Sequence 1' or 'Sequence 2' pads. The footswitch is useful for fixing the sequence endpoint on the following beat, as the sequence will automatically loop on playback.
It works extremely well, 'catching' everything played, but the total number of notes shared by Seq 1 and Seq 2 is 400. If you make one too long, it will 'steal' notes from the other banks once the limit is reached — this can be a nuisance if you've spent some time getting a perfectly played sequence in 1, followed by another masterpiece in 2, only to find that 1 has been chopped off!
Speed control is always internal and cannot be operated from a drum machine, for example, even with MIDI in operation.
While the sequence plays back, you can still use the keyboard and this offers some very effective 'solo with sequence' playing.
The arpeggio feature is one sure way of livening up your performance and the 2 arpeggio functions here can be running automatically while you play the keyboard at the same time. Actually, as with the sequencer, the result is really achieved by swapping between notes, but it's nevertheless an interesting option. Two types of arpeggio can be set to play around the notes held on the keyboard — Up/Down, which plays held notes from the lowest to the highest and back continuously, or Assign, which repeats notes in the order played. The latter in particular makes very good live sequence patterns and both types can be latched by pressing the 'Record' pad. However, you cannot continue to add notes once latched which restricts the sequence length to your physical finger stretching.
Arpeggio speed can be adjusted and either a footswitch or external clock/trigger can synchronise with a drum machine or other timed event processor. This is done by turning the Speed control fully anticlockwise, to receive the external pulse, although unusual effects can be obtained by sharing pulses at centre speed settings. Because the external trigger goes into the footswitch, the Sequencer will not play back correctly if a trigger is currently being input, say, from a recent arpeggio passage.
The performance wheels at the left of the keyboard affect all 6 synthesiser voices at once. Voice assignment is performed by the instrument's microcomputer, so the most recently played keys take priority ('last-note priority'). Operating the pitch wheel, which is centre-notched to fix normal pitch, will raise or lower all voices at the same time over a maximum major 3rd (4 semitone) interval up or down.
The Mod wheel introduces LFO modulation, setting a maximum 3rd jump using a square wave or a 5th jump with a triangle wave source. Both are essential for the performance of modern music to add variety through tonal and pitched nuances.
Besides playing polyphonically, 3 types of unison modes can be programmed using the 'Unison Track' switch. Normal Unison gives the big monophonic sound of all 12 oscillators on a single note (the lowest played). It also changes the synth from multiple to single triggering — only restarting a new envelope once all keys have been released. Single Voice Unison produces a thinner sound using just one voice per note. Tracking Unison is an important feature and latches the chord played, to consequently transpose the same interval shapes from any single (root) note played. Fast parallel note runs can be achieved, especially using this with the Arpeggiator. Even more exciting is the facility to record this 'Tracking Chord' as part of the sound program.
A programmable Glide function gives fully polyphonic portamento between successive notes at variable rates. Some unpredictability occurs here as the 6 voices go to each new note in rotation. This means that repeated sliding between the same notes at different ends of the keyboard quickly ceases until new notes are introduced, and the poly portamento from 6 low notes will reach new middle notes before new high notes. The effect can be spectacular even so.
The only important non-programmable controls are the two wheels (although the Mod-Wheel's destination and source wave is programmed), while all the switches and controls necessary to establish a sound program on recall (through program select) are programmable.
In addition, a sound can be further edited at any time by simply turning or switching the appropriate control. Of course, you can't see from the panel what settings were made to create a preset on later recall, so you have to go through a methodical switch/knob checking to readjust the sound. The editing process lets you take a preset sound and change it completely or go to Manual mode where all the controls show the actual settings for the sound produced. If any editing is done to a program (by moving a switch or knob), a decimal point will appear between the two digits on the display. You can cancel the editing easily by redialling the program number, or you can store a newly created (or edited) sound by pressing 'Record' followed by the program selection number from 00 to 99.
A little care has to be taken, otherwise you could destroy an important program — it's good practice to save a program group or set after a time to guard against errors.
The centre and right hand main panel controls let you create or edit sound programs, with each function acting on each synthesiser voice identically. Although many of the controls are digitally synthesised, they eventually end up controlling the following analogue devices.
Oscillator A provides frequency adjustment over 4 octaves in semitone steps. Sweeping the Frequency control may produce uneven step jumps, so you won't often use it as a performance effect, but precise pitch setting over 16' 8' 4' and 2' can be achieved. A Sync switch locks this oscillator to Oscillator B in 'hard' sync for exciting harmonic timbre changes. There are sawtooth, triangle and pulse waveshapes available, the latter with square approximately at centre setting (but you will have to listen carefully to find it). Each wave can be switched in or out together or separately.
Oscillator B is the second oscillator for each voice and has the same 4-octave Frequency control, but with a Fine control as well for detuning. Waveshapes are the same and both oscillators can be balanced from the mixer control knob. Glide can also be set as mentioned.
From the mixer, the oscillator signals pass through a 24dB low pass filter with standard cut-off frequency, resonance and envelope amount for the ADSR. Maximum attack, decay and release times are around 9 seconds. A noticeable drop at the end of the decay and release was heard, but is not very significant. There's also a keyboard filter tracking switch with full, ½ or off setting. It's used to maintain a consistent brightness of tone over the keyboard.
High settings of the Resonance control will turn the filter into a pure sinewave oscillator and it will track the keyboard substantially better than most instruments for some 3 octaves.
The signal chain is completed by a VCA with ADSR controls as in the filter section and a Master Volume control for making sure program sounds are all at a constant level during selection.
Two modulations sources are available — LFO-MOD and POLY-MOD. The more common Low Frequency Oscillator can be programmed with an 'Initial Amount' control to constantly modulate up to 3 'destinations' — the frequency of oscillators A and B, their pulsewidth, or the filter cut-off. The frequency can be set for triangle or squarewave output (for smooth or sudden changes in pitch and timbre). Alternatively, with Initial Amount at zero setting, the Mod wheel can take over control instead. Both can work together but still give max pitch change of a major 3rd with a square wave.
The Poly-Mod section opens up the synthesising by making polyphonic pitch sweeps using the filter envelope as a modulation source for Osc A pitch. When used with oscillator Sync on, rich timbre changes can be made. The section also uses Oscillator B for modulating both Oscillator A pitch and/or the filter. Poly-Mod lets you create ring-modulation effects like bells, chimes, percussion as well as rich timbre changes, weird Tomita-style voices, pan flutes, growls and so on.
SCI say that this is the first commercial synthesiser with MIDI. Besides allowing 600s to be slaved together, it offers control from a home computer through a serial interface. The software in the 600 also lets you decide such things as enabling or disabling a program select number on one instrument to another, Pitch and Mod slaving, and dumping programs between instruments. In 16 bytes of program data all the necessary programming information can be transmitted to set up a sound program.
With a maximum retail price tag of £1625.00 inc. VAT, the Prophet-600 represents a state-of-the-art machine that 'looks to the future' with the MIDI interface. With several very good polyphonic instruments now available, you'll have to make your own comparisons from this information herewith the competition before you decide to buy.
The Prophet-600 is easily programmed from the analogue style controls, although a tuning reference would be useful to keep your frequency adjustments correct (at present a cross check between presets or external oscillator is necessary). The quality of its sounds leaves little to be desired and its synthesising capabilities are well demonstrated in the 100 program samples supplied, especially through the provision of dual oscillators and EGs. The new membrane switching is fine in use, although some functions have no LED indication. You'll have to wait for the Prophet t8 if you want touch and pressure keyboard control and I've already noted other omissions in the review. Certainly, a program advance feature would have helped a lot in selecting program sounds.
For many electro-musicians, the Prophet-600 will become the ideal reasonably low cost polyphonic, unlikely to become redundant because of its MIDI link — so now's the time to learn to use a home micro or you'll be left behind!
A list of U.K. dealers is given on page 59. Please contact your local dealer for further information or write to: SCI, (Contact Details); or SCI, (Contact Details).
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Review by Mike Beecher
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