• Prophet T8
  • Prophet T8
  • Prophet T8
  • Prophet T8

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Prophet T8

Touch-Sensitive Polyphonic Synthesiser

The new Sequential Circuits touch sensitive programmable poly synth



Ever since the announcement that Sequential Circuits were working on a touch-sensitive version of the perennial Prophet 5, everybody has been waiting with baited breath to see the result of their endeavours. In the meantime, we have seen the highly successful Pro-One redefine the standards for the monophonic synth, and the 5 continue to sell steadily despite competition from all sides (a rare event in the professional keyboard market - where are all the other synths of five years vintage?).

But now, over two years later, after several changes in design philosophy, the T8 is finally being shipped to dealers. The first units arrived in Europe recently and we managed to take a good look at one at Sequential Circuits European Headquarters in Holland before they were dispatched to eager shops and awaiting customers.

The RRP of the instrument in the UK is £4,700 and as such represents a large investment for any keyboard player. So what features have they added to the basic Prophet to make it worth the price and how successful are these developments?


Keyboard



Obviously the most vital part of any touch sensitive synth is the keyboard. For years the quality of the keyboard has not been of over-riding importance on synthesisers as its function was merely selecting the notes to be heard and automatically triggering the envelopes. But when the keyboard becomes part of the expressive control of an instrument (as it always has been on a piano), then the quality and feel of the design assume a greater significance.

The keyboard on the T8, we are happy to report, fulfills all expectations. Wooden keys go back, 14" of 15" into the machine to give an authentic weight to the action. Here they tip a smaller wooden lever on which are focussed the optic sensors which time the velocity of the key-strike (see photo).

Timing the interval between the start and finish of the key descent is the most common method of determining the force with which the key is struck. Normally, two contacts are affixed to each key. One is broken when the key begins to move and the second is made when the key reaches the end of its travel. The harder the key is struck the shorter the time taken and so a harder strike is registered.

This was the system with which development began on the T8, but it was found to be too insensitive for the level of control SCI were aiming for, so a new system using the optic sensors was developed. These give a more accurate indication of key travel beginning and ending and the microprocessor times the interval in the usual way.

The keyboard is a delight to play, giving the light but responsive touch of an acoustic piano. The weighting of the key allows one to gauge the different speeds of keystrike required to obtain the different effects.

Below each key is a pressure sensor which serves to measure the amount of 'second touch'. This is a separate function which can be activated after the key has depressed. Pushing harder on any key means that various elements of the sound can be altered.

The optical sensors also fulfil a second role, that is timing the release of the key. This time can also be used to control various parameters. We will come onto all these various elements of the sound which can be varied by the velocity and pressure information later but first let us have a look at the basic controls of the instrument. The design is based very much on the original Prophet 5, so those of you who are already familiar with this old faithful may prefer to skip the next section.

Basic Format



The Prophet voice design is centred around 2 VCOs, 24dB Low Pass Filtering, a separate ADSR for both VCF (that is the filter) and VCA (amplifier) and a comprehensive modulation section. Particular features which have made it popular include Sync between the Oscillators, the fact that Osc B can be used as a second LFO to modulate the filter and the frequency or width of Osc A, as could the filter envelope. None of this has been lost on the T8, so those who have spent years familiarizing themselves with the nuances of the Prophet design will find it time well spent.

However, some of the features which were missing on the old Prophets have now been catered for. Polyphonic Glide, originally not included (because Dave Smith didn't see the point of it) is now standard. Originally Filter Tracking (to move the filter cut-off frequency in relation to oscillator pitch, thereby keeping the harmonics contained in the sound constant) was originally on or off. Now it is fully variable between 0 and 100%. Amounts of modulation can now be memorised as part of a patch whereas before they had to be introduced by the Mod wheel - an Initial Amount control sets the level of modulation before the wheels increase it up to maximum.

New Features



Besides eight-note polyphony, the T8 has several new features in synth programming. The envelopes feature two of these. The first is the ADR switch. By selecting this, the sustain time is automatically reduced to zero, so that when the decay reaches the sustain level, the envelope is automatically forced into the release phase. This is particularly useful for percussive envelopes, from pianos through the drum sounds.

The second innovation on the envelopes is the Second Release switch. This allows a second release time to be memorised for each patch, and activated by the foot pedal. This can then be used like the sustain pedal on a piano when not depressed a shorter release time can be programmed, when it is depressed a longer release can also be obtained. This feature can be programmed separately or together for both the filter and amplifier envelopes.

Another innovation is a Programmable Volume control which allows a sound to be stored at a particular level. Of course, this is relative to whatever setting Master Volume is on when the program is recalled. However, it does allow you to boost a quiet patch (for example if the filter cut-off is low) without changing the quality of the sound. Similarly louder patches can be moderated to balance their quieter neighbours.

Programmer



The T8 also has the Split/Layer feature which has been available on synths such as the OBXa, the JP-8, and the Synthex for some years now. This means that the keyboard can be split at any point and a different sound (with four voices unless Unison has been programmed) can be set up on either side of the split. Alternatively, each note played (up to four) can be used to trigger two different programs, effectively doubling voices. In addition, two programs being used in Split or Double mode can be stored together and selected as a Link patch, which makes complex set-ups quickly available.

The programmer itself actually stores 128 programs at any one time, selected as either a Left or a Right sound. If the switch marked Left is lit, then a new sound can be selected for that side by tapping a number between 11 and 88 (64 programs) on the keys numbered 1 to 8. Pressing the Right switch allows 64 different programs to be selected in the same manner.

In Single mode only the switch illuminated (left or right) controls the sound of the whole keyboard, whereas in Double both are heard at once. In Split mode, Left controls the lower voice, and Right the upper.


Velocity Parameters



These are controlled by four potentiometers at the top right-hand end of the panel. The first of these controls the effect of different strikes on the rate of Attack and Decay of the envelopes. The more to the right this control is set the more effect is exaggerated. For example, a fast strike will give an even shorter attack and decay, whereas a slow strike will result in a much longer attack and decay. Next to this is the knob which controls the effect upon the Release. As its value is increased, then the release times are more exaggerated in a similar manner to the Attack/Decay knob. This particular control 'goes one better' than a piano, where release rate cannot be controlled. The last two pots control the effect on the filter and amplifier envelopes respectively of the velocity effect. Both amounts can be set positive or negative allowing inverse responses to be programmed.

Although four controls may not seem many for the much-vaunted velocity sensing, don't forget the effects produced are to a great extent altered by the individual ADSR and Second Release settings of each envelope. The overall response is extremely flexible and can easily and quickly be changed to suit individual taste. We will see some of the effects that can be produced later, when we come on to look at the factory presets.

Pressure-Mod



The signal from the individual pressure pads under each key can be routed to modulate the frequency of Osc A and/or B, Pulse Width, Filter, Amplifier, LFO, Amount and/or Frequency. The routings for all these are to be found in the Pressure-Mod section at the top left-hand end of the panel. The amount knob allows you to increase the effect either positively or negatively. The various effects which can be obtained from pressure are as the follows:

Frequency A/B: 'bending' the pitch of either or both oscillators.
PW: changing the Pulse Width if a Pulse Waveform has been selected.
Filter: the Filter can be opened or closed.
Amp: the overall volume made louder or quieter.
LFO Amt: the amount of modulation from the LFO can be increased/decreased.
LFO Freq: the LFO modulation can be made to go faster/slower.

All these effects can be produced simultaneously or selectively, giving an extremely wide range of pressure-controlled performance options. Let us now look at some of the Factory Programs to see the use to which Product Specialist John Bowen has put these velocity and pressure controls.

(Click image for higher resolution version)

Presets



The manual comes with a handy reference sheet listing all the Factory Programs (Left and Right), which can be detached and laid on top of the synth until you are familiar with the presets. These are very logically laid out in groups of sounds for ease of location. The first column (all the programs ending with a 1) contains all the piano sounds, the second strings, the third brass, etc, with all the pitchless sound effects in the last row. Each pairing tells you whether the Left and Right sounds are designed to be used in Single, Split or Double mode.

Most synth manufacturers tend to put one of their impressive sounds on the first preset of bank 1, but even so left 1.1 on the T8 comes as a revelation. Below middle C, the sound is indistinguishable from a real piano. John Bowen has used the velocity to accentuate Osc A which is up 3 octaves plus a major 6th (an important harmonic on the piano when struck hard) from Osc B which provides the fundamental frequency. But the second release option allows the right pedal to work identically to a real piano sustain pedal.

Right 1.1, designed to cope with the top end of the piano is less successful (although streets ahead of most synths). However, we found that Right 7.7 (another split acoustic piano patch) to be more realistic and split with Left 1.1 proved most effective.

Other piano sounds on this bank include a fruity Honky Tonk (2.1 Double) and a couple of electric pianos, Left 4.1 which has the growl of a Wurlitzer and Left 5.1 with more of the delicacy of the Rhodes.

2.2 gives you a double string sound. By combining a velocity sensitive patch with a less responsive sound, it ensures that even the gentlest playing gives you a basic sound level (often a problem with string ensembles from a touch sensitive machine - some notes can get lost and give an unbalanced chord) but with extra emphasis on certain lines or chords available by either velocity or pressure controls.

2.2, 3.2 and 4.2 give you solo strings sounds, violin, viola and cello respectively. All have velocity controlling the imitation of bow speed and pressure bringing out higher harmonics and vibrato. By careful combination of these programs, it is possible to get a highly realistic string quartet. Other strings include Strings with Pressure Volume (Double 5.2) and Strings with Brass (Double 7.2).

Brass sounds are also available as solo or ensemble patches, varying from strident (Right 1.3) to mellow (Left 3.1). Sounds created by complex cross-mod, with Sync and ring mod type effects from Osc B (whose frequency can of course be controlled by pressure - giving performance control of complex modulations) or on 2.4 (called Shin-Shen), 3.4 (Harp), 5.4 (Sync Sweep), and 7.4 (rejoicing in the name of Polymod LFO to Pulse Width).

The 5s are all percussive sounds. Double 1.5 (called Vibalimba) changes from vibes to marimba if you hit it harder. There are a couple of very responsive Clavs (Left and Right 4.5), a totally unelectronic harpsichord (Right 6.5) which respond beautifully from the keyboard and a lovely tinkly Metallic 1 (right 7.5).


The next group of sounds show you some of the effects that can be created by Pressure: Chorus with Vibrato (1.6), Cross Fading (2.6), Pulse Width Mod (4.6) and Sound Effects (8.6).

What about 'fat' lead sounds, we hear you cry? For these we go to the 7s. Making use of the Unison/Track, Pressure and Velocity Sensing, they range from Lucky Man (left 3.7), a close copy of the Moog solo sound on the ELP track of that name, through Vibrato by Pressure (right 1.7), Sync Sweep by Pressure (Right 6.7), Bass with Pitch Bend (Left 1.7) to the best of all, Right 3.7, which has the lot, Pitchbend, Vibrato, Filter Bend, and LFO Mod, all brought in by pressure - the ultimate lead sound.

The 8s are all sound effects from Timpani, Bells, and Toms, which are powerful percussion options, to others with such names as Alien, Moonwaves, Cats under Pressure, Space Trek, Pressure Cooker, Pied Piper, and Newt Flute. These have to be heard for they are indescribable.

In the manual, it states that the presets should be regarded 'not as absolutes but as examples' and 'should serve as a starting point for you to create your own sounds through the editing process'. However, the Factory Programs are so good that we suspect that the same will happen as with the original Prophet 5 (4 out of 5 repairs came back with the original sounds untouched). Indeed, this is one of the best sets of factory programs ever provided with a synth, and will be a major selling point in itself.

Sequencer



The only new feature of the T8 which we have yet to mention is the Real Time Polyphonic Sequencer. This has a total capacity of 670 notes and will record up to eight individual sequences with a preset sound, velocity information and looping.

The T8 comes with some demonstration sequences already in. Particularly impressive are Number 1, a funky brass sequence, and Number 4, a double bass and flute duo which is incredibly realistic.

To create your own sequences, you merely press the record switch, sequence select and one of the number switches. The sequence doesn't begin to record until you start to play. A preset selected before you begin to play is recorded as part of the sequence. A loop point can be created by the foot switch which can also start and stop sequence playback, leaving the hands free to do the playing. The sequences are played back exactly as recorded, but they can be played four times faster or slower.

Internal Construction



The internal layout of the T8 is extremely concise. The 14 inch deep keyboard, with its single contact board underneath, sits on the base. Along the back of this runs the mounting with the optical sensors on it. The potentiometer circuit board is attached to the panel and lifts off with it. The rest of the instrument is on three circuit boards which sit across the main body on a shelf. These contain not only the eight voice cards but the main control board as well.

An increasing part of the design has been digitized, including the envelopes (hence their increased flexibility) but the essential sound components i.e. the oscillators and the filters, are still analogue. The T8 now uses two processors, the Z80 which does the keyboard scan and associated functions (velocity and pressure sensing) whilst the newer Z8000 handles the routings and presets, as well as the envelope generation and sequencing. The oscillators are still Curtis 3340s and the filters 3372s, as with the later revisions of the Prophet 5. The VCAs, which used to be 3280s, have now been replaced by 3360s.

Back Panel



The Audio Outputs provided are Mono or Stereo, Left and Right. In Double or Split Mode, the Left and Right assigned sounds (four voice channels each) are routed to their respective outputs, whereas in Single whichever of the two is selected is routed to both sides. Headphones (with a minimum impedance of 1200 Ohms per element) can be connected either through two mono jacks to Left and Right, in which case the sound will be in stereo or they can be put through the Mono socket using a stereo jack, giving a mono signal to both channels.

Footswitches can be used to control the operation of the Second Release (see New Features paragraph), Unison/Track or the Sequencer. A piano-type dual pedal comes as standard with the instrument which will operate any two of these functions at one time.

The Cassette Interface provides unlimited storage of programs (128 or 8 at a time) or sequences, so a library can be built up and reloaded at a moment's notice. Save/Load time for 8 programs is about 14 seconds or 94 seconds for 128 programs or 8 sequences.

The Record Enable/Protect switch allows presets and sequences to be overwritten or protected from accidental erasure, depending on the position.

The MIDI In/Out sockets allow the connection of an MIDI devices, giving access to program parameters as well as allowing external sequencing. Sequential Circuits are soon to be bringing out a sequencer using their MIDI Sequencer Interface unit with a Commodore 64. Alternatively, E&MM's Micro-Midi board (E&MM May 83) could be used in conjunction with a suitable personal computer and software.


Conclusion



The Prophet T8 is essentially a performer's instrument. The high quality wooden keyboard and quick programmer functions make it a superb performance machine which responds to the subtlest of playing techniques. Sequential Circuits designs have always been both easy-to-use and flexible. The T8 is no exception, and adds velocity-and pressure-sensing to the Prophet's repertoire.

Unfortunately, the RRP of £4,700 means that large number of aspiring players will not be able to afford this, but for those who can get the wherewithal together, it will indeed be money well spent.

For further information contact Sequential Circuits, (Contact Details).


Also featuring gear in this article



Previous Article in this issue

Steve Gray on the DX7

Next article in this issue

Yamaha PC-1000


Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.

 

Electronics & Music Maker - Dec 1983

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Review by Paul Wiffen

Previous article in this issue:

> Steve Gray on the DX7

Next article in this issue:

> Yamaha PC-1000


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