Sequential Prophet X

The Sequential Prophet X combines analogue and sample‑based synthesis in one enormously ambitious and flexible instrument.

A keyboard synthesizer that marries digital oscillators and sample‑based sound generation to an otherwise analogue signal path controlled by digital contours and modulators isn’t a new departure for Sequential. Launched in 1985, the Prophet 2000 fits that description, albeit with the ability to record samples as well as replay them. So what impact have the technological advances of the past 33 years had on the concept? Is it just a case of higher bandwidth, longer wordlengths, gigabytes instead of kilobytes, and a 21st‑century GUI instead of an ’80s parameter access editing system, or does it run much deeper than that? Let’s find out.

Oscillators & Instruments

The Prophet X is an analogue/digital hybrid with analogue filters and audio amplifiers but everything else implemented in the digital domain. It offers up to 16 voices, each comprising two digital oscillators and two sample‑based ‘Instruments’, each with either one or two 24dB/oct resonant low‑pass analogue filters (depending upon the voice mode chosen) and stereo audio amplifiers, and each shaped by four LFOs and four contour generators.

The dual oscillators are much as you might expect, although their waveshaping can be unusual. For example, shaping the sine wave appears to add the second harmonic with the same polarity (‑ve shape) or inverted polarity (+ve shape). When shaping the sawtooth wave, a second waveform is again added and the amount of Shape determines its amplitude and relative phase. Shaping the pulse wave, which is square at its centre detent, results in PWM and the usual range of swept or chorused sounds. The fourth wave is SuperSaw, and shaping this introduces and then increases the amplitude of up to five additional components for an ensemble effect. (The manual says that increasing the shaping increases the amount of detune, but that’s wrong.) The relative position of the detuned SuperSaw partials is different when the shape is +ve from when it is –ve, offering two tones of ensemble.

You can tune and fine‑tune the oscillators independently over an exceptional range of nine octaves, which is equivalent to an analogue oscillator reaching from 64’ to 1/8’. You can also disconnect either oscillator from the keyboard, which was a programming trick much used on the Prophet 5. Once disconnected, you can then leave that oscillator at a fixed pitch or modulate it from elsewhere, which suggests many interesting possibilities.

There’s a welcome lack of aliasing, even at high oscillator frequencies, and this seems to be the consequence of an algorithm that tells the oscillators not to generate harmonics above 20kHz. You can obtain partials above this frequency if you utilise sync or engage in a bit of naughty audio frequency modulation (or FM, as we usually call it) but in normal use that’s not going to be an issue.

Other facilities include ‘slop’ (which adds pitch drift and also results in a slightly different pitch each time that you retrigger a note) and you can choose whether the oscillators are free‑running or are re‑initialised when a voice is initiated. The former is more authentic for analogue‑iness, but the latter can be useful for ensuring that sounds are consistent from one note to the next.

The final oscillator facility is glide, of which there are four modes. At its fastest, glide is all but instantaneous; at it’s slowest, the rate is around five seconds per octave, and you can set the rate independently for each oscillator, which is an effect that I like very much.

If there’s an omission in this area, it’s the lack of sub‑oscillators. Sure, you can combine up to four oscillators in a Program, so this is perhaps not the issue that it would otherwise be, but it would have been nice to generate sub‑octaves without using up an oscillator to achieve the same result.

The two Instruments per voice are sample‑based oscillators that draw upon 150GB of 16‑bit/48kHz multi‑samples supplied by 8DIO and held on the internal SSD. The supplied library is huge, not just in terms of the amount of memory used, but also in terms of the number of instruments provided. These include 11 ambience instruments, many of which have different but related sounds under various keys, 51 basses, 94 brasses, 93 choirs, 55 cinematic effects (again, many with different sounds under different keys), 17 orchestral effects, 33 ethnic instruments, 40 guitars and related instruments, and more... and yet more, all organised into 17 Types. Some of the Instruments have just a single layer while others have two or more velocity‑switched layers, and there will be no specified limit to the number of layers available when Sequential introduces the software that will enable end‑users to import their own samples. (This is currently scheduled for delivery in December 2018.)

Despite the absence of a waveform display to show what’s happening, I lost many hours of my life selecting and editing Instruments. You can set the start and end points of playback, and choose whether the sample plays forward or backward. You can also determine the start and end points of a loop (with a crossfade that helps to make the results smoother) and slide this forward and backward within the sample to find the optimum position. There are three loop modes: Regular, Pitched (which calculates the length of the loop as a function of the note number and root pitch so that you can use it to create new waveforms), and Sync (which loops in sync with the synth’s master clock). Sadly, there’s no alternate mode (forward/reverse/forward...), and you can only loop during the Sustain phase of a sound; looping during the Release phase in not possible so you have to be careful not to design a sound that’s curtailed unnaturally when you release the key.

If you fancy a trip back to the 1980s, you can create Programs from a single sample within an Instrument by selecting Sample Stretch and pressing the key under which the wanted sample lies. (If the multi‑sample is also multi‑layered, you will need to hit the key at the velocity that triggers the sample you want.) This maps the selected sample across the whole MIDI key range. I find this to be both incredibly limiting and amazingly creative. By the way, you can apply glide to sounds generated this way, which, for what I hope are obvious reasons, you cannot for multi‑sample‑based sounds.

Other Instrument parameters allow you to tune them across a 10‑octave range, switch key follow on or off, and switch velocity sensitivity off so that only the highest‑velocity layer plays, no matter how quickly you depress the keys. There’s also a dedicated dual‑mode filter called Tone. There’s nothing special about this, it really is just a tone control for attenuating either high frequencies or low frequencies to help the sound sit more comfortably in a composite patch or a mix, but it’s more useful than you might imagine. Instruments can also bypass the main filter section (which we’ll come to shortly) which allows you to create Programs that combine filtered oscillators with unfiltered samples. To help balance the sources, you can boost the Instruments’ amplitudes by up to 9dB.

New sample libraries are also promised. Loading these will be straightforward: find a USB2 or USB3 memory stick, format it on the Prophet X, transfer it to your computer and load the library on to it, then return the stick to the synth, load the data and power cycle to register the new content. Once the procedure has been completed, the library will be stored within an additional 50GB of SSD storage and the underlying samples will be accessible in the User Group. The Programs that make use of these samples will then appear in banks A1 to A4 (512 Programs in all), which co‑exist alongside the 512 permanent factory Programs and the 512 user memories.

As I was writing this review, the much‑delayed library of raw samples for a forthcoming 40th Anniversary Prophet 5 library was completed and, although the factory Programs hadn’t yet been programmed, 8DIO sent me a copy. Once these samples were installed, it didn’t take much tweaking of the filters and contour generators to start hearing the Prophet 5 come to life with 16‑voice polyphony, velocity and pressure sensitivity, and much more than the original synth could deliver. Two pianos are also in preparation: the Yamaha CP70 and an unspecified Grand Piano with up to 10 Layers per note. It would be nice to think that 8DIO are also working on packs containing things such as the original libraries for the Emulator 1, the Fairlight, and even the Korg DSS1 and Ensoniq Mirage because I think that these would be ideal for the Prophet X.

Measuring 976 x 343 x 109 mm, the Prophet weighs in at a healthy 10.8kg.

The Signal Path & Modulation

The four sound sources are mixed before being passed to the synth’s analogue low‑pass filters. The Mixer section offers more than is immediately apparent: you can determine each sound source’s position within the stereo field when in eight‑voice mode, and you can reduce the wordlength (hacking) and sample rate (decimation) of all the sources. Set the hack to four, and you reduce the wordlength to 12-bits; set it to eight and you reduce it to 8-bits. Now reduce the sample rate... If you want everything to sound gritty, crunchy and aliased (in other words, vintage digital) this is how you do it.

There are 16 24dB/oct low‑pass filters in the Prophet X, allocated in two ways depending upon the voice mode chosen. If you stick with the default eight‑voice mode there are two signal paths for each voice — one to the left and the other to the right of the stereo image — and each contains a filter, thus limiting the polyphony to eight voices. If you choose the 16‑voice mode, there’s a single signal path so you can have up to 16 voices.

At this point, allow me to jump ahead of myself and say that I like the sound of these filters very much. In fact, I liked them very much even before I found out that they are based upon a new filter chip (the SSI2144) that is itself based upon the revered SSM2044 filter that was used in the Rev 1 and Rev 2 Prophet 5s. When I discovered this, I must admit that I gave myself a hearty pat on the back because it confirmed what my ears were telling me.

Controlling the filters is straightforward. The filter section offers knobs for input drive, cutoff and resonance, the last of which will invoke filter oscillation at high values. Key tracking is also provided, with 100‑percent tracking available at the knob’s clockwise extreme. In eight‑voice mode, an additional parameter allows you to offset the cutoff frequencies of the filters lying in the left and right channels and this makes it possible to do things such as sweep the upper frequencies of a sound from side to side while leaving the lower frequency components largely unaffected. I like this effect very much; it adds width and movement without creating a hole in the middle of the soundstage.

A dedicated filter contour generator is provided, which can be applied with either +ve or –ve polarity. A velocity on/off switch then allows you to control the gain of the contour using key velocity should you wish. From the front panel this contour appears to be a standard four‑stage ADSR but, when you invoke the appropriate menu, you find that there’s also a Delay stage before the Attack so it’s actually a five‑stage DADSR. Furthermore, you can make the DAD stages loop until you release the key. At its fastest, this loop can reach into the audio band, so there’s a lot of potential to be uncovered here.

After the filter section, the signal reaches the audio amplifier section, which boasts the same loop‑able five‑stage contour generator, bi‑polar amount control and velocity on/off switch as the filter contour. It also offers a Program volume control that allows you to balance the levels of your Programs (which is vital when you play live) and a Spread control that spreads the voices across the soundstage whether you are in eight‑ or 16‑voice mode.

Two further contour generators share a single set of controls, and two buttons determine which you’re adjusting. Like the filter and amplifier contours, four knobs determine the ADSR parameters, with another for the bi‑polar amount and a switch for velocity on/off, and the menus again add the Delay phase to make them DADSR contours with DAD looping if desired. A sixth knob allows you to determine the destination for each of them.

The Prophet X also incorporates no fewer than four LFOs per voice, each generating five waveforms (including ‘random’ for sample & hold type effects) with Wave Reset and Phase functions that allow you to switch between free‑running and a key sync’ed mode in which the LFO is reinitialised at a user‑defined phase when you press a note. There’s also a Slew function that acts as a low‑pass filter on the LFO waveforms, reducing all of them to an approximation of a sine wave when set to maximum. Their frequency ranges are impressive, extending from roughly one cycle every 45 seconds to 500Hz, the latter of which makes them suitable for audio rate AM and FM effects. Alternatively, you can synchronise them to the synth’s master clock. Each LFO can be directed to a destination chosen within its own menu.

In addition to the dedicated destination paths found elsewhere on the synth, the Prophet X offers a modulation matrix comprising 16 slots with 28 sources (including the dedicated contours) and 87 destinations (the manual says 88, but one of those is ‘off’) and it’s capable of applying any source to any destination or combination of destinations with either positive or negative polarity. Modulation sources and the amounts of modulation can themselves be the destinations of other slots, so it’s straightforward to do things such as apply an LFO to a destination while its frequency is being swept up and down by a contour, or to create new and curvier contours by using one to shape the stages of another. If this sounds a bit fiddly, it isn’t; in most cases, you just hold down the Source button and adjust the appropriate control to assign the source, then hold down the Destination button and adjust the appropriate control to assign the destination. Even if the parameters you want are found only in the menus, it’s still easy to make assignments.

Layers, Effects, Arpeggios & Sequences

The Prophet X’s 16 voices are contained in either one or two Layers called A and B. The sound programmed in a given Layer is what we would traditionally call a patch and, in addition to this, the Layer incorporates a unison option, an effects section, an arpeggiator and a sequencer. You can edit the Layers independently or simultaneously, copy sounds from one to the other, and import a Layer from any Program into the one that you’re currently editing.

How the sounds are played is then determined by the various on/off combinations of the 16 Voice button together with the Split, Stack and Edit B Layer buttons. By default, both the 16‑voice and eight‑voice modes play only Layer A (or only Layer B when the Edit B Layer button is On) but splitting and layering is also possible. In 16‑voice mode, layering reduces the polyphony to eight notes; in eight‑voice mode, it drops to a meagre four notes.

There are two effects units in each Layer, each offering synchronisable delays of up to one second, reverbs, a high‑pass filter, distortion, chorus, flanging, phasing and a rotary speaker effect. All but the high‑pass filter have four parameters — the wet/dry mix, plus three parameters specific to the type of effect — and all of the parameters are destinations in the modulation matrix. Even when using the single‑channel 16 Voice mode, the chorus, flanging, phasing, rotary speaker and reverb effects remain in stereo, so you can add width and movement to your sounds without losing half of your polyphony. I also have to tip my hat toward the high‑pass filter; since the cutoff frequency, resonance and mix are all destinations in the modulation matrix, the Prophet X offers powerful band‑pass filtering even though this is not apparent from the front panel.

The arpeggiator and sequencer in each Layer are mutually exclusive (you can only use one at a time) and both are driven by MIDI Clock or the onboard clock with rates ranging from 30 to 250 bpm and 12 ratios ranging from bpm /2 to bpm x12. The arpeggiator offers five modes (up, down, up/down, random and ‘as played’) over one, two or three octaves, with optional repeats of one or two notes per step. You can use the Hold button with this, and determine whether new notes are added to the existing arpeggio or whether they initiate a new one. The sequencer is not much more complex. It offers up to 64 steps, each of which is up to six‑note polyphonic, and you can play along to it if you have sufficient polyphony available. Recording is simple; enable Record and play, adding ties and rests between notes as desired. When you have everything as you want it, just press play and transpose the sequence in real‑time if you wish. If you’re not happy with your creation you can edit it, correcting individual notes and their velocities as required. It’s simple, and it works.

The final Layer facility of note is a Unison mode that can incorporate up to eight or 16 voices depending upon the voice mode selected, and with a user‑defined amount of detune between them. You can also select the key priority — low, high and last, each with legato and multi‑triggering modes — so all of the usual monosynth playing responses are available. As you might expect, the sounds generated by an (up to) 64‑source monosynth can be huge; the manual suggests “speaker rattling” and it’s not wrong. Unison mode also offers a Chord Memory function — just play the wanted chord and press the Unison button to memorise it. I recreated Moog’s classic I/IV/V lead synth patches, and the results could be excellent.

In Use

It’s tempting to think of the Prophet X as akin to a Prophet 12 with four extra voices and sample playback capabilities. To an extent, that’s appropriate. Both combine four digital sound sources per voice with dual analogue filters per voice, both offer four LFOs per voice, both offer four contour generators per voice, both incorporate 16‑slot modulation matrices, and both are based around a 61‑note semi‑weighted keyboard that is both velocity‑ and pressure‑sensitive, with performance control provided by two assignable wheels, two assignable Touch Sliders, and a selection of pedal/CV inputs. There’s a strong family resemblance too, notwithstanding the new synth’s smart black case. There are differences, of course, but as a fan of the Prophet 12, I quickly felt at home with the controls and menus of the Prophet X. Nonetheless, it took me a little longer to learn to love it. Let me explain why...

One of my first experiments involved recalling the Basic Program (a single sawtooth wave with the filter wide open) from the Global menu, selecting 16‑voice mode and stacking Layers A and B. I tuned Layer B upward by seven semitones for a classic ‘fifth’ patch, then closed the filter a little, increased the resonance a little, and added a touch of drive on both Layers. Next, I increased the amplifiers’ Attack and Release settings in each Layer to make the sound ‘speak’ a little more gently and applied a touch of chorus and reverb to just Layer B. It took no more than 30 seconds, but the results were gorgeous — I was instantly transported into Prophet 5 territory, but with arguably greater depth. (That’s heresy, I know, but since I have a fully functional Rev 1 Prophet 5 just a few feet away from me I feel able to make that statement.) This was because the settings of the two Layers were subtly different from one another and only one was passing through effects, so the patch sounded richer than having two oscillators passing down a single VCF/VCA/effects signal path. I then went on to programme pads, brass and more, and very much liked what I was hearing. Quality analogue sounds just pour out of the Prophet X.

Next, I decided to try to create some workstation‑style sounds using the Instruments alone. This wasn’t so promising because I started with the pianos and, despite the evident quality of individual samples, the switching between velocity layers was way too obvious for my taste. I had heard great things about 8DIO samples, but this proved to be an issue with many of the Instruments, some of which also exhibited audible looping. I was considerably less impressed than I had expected.

At this point, I was beginning to have mixed feelings about the Prophet X, but then I discovered Type 15 / Instrument 2: VS Wave Tables which holds all of the Prophet VS’s waveforms distributed under 97 MIDI keys from C‑2 to C7. The VS holds a special place in the hearts of many players, not just because it introduced vector synthesis to the world, but because it was and remains a superb instrument capable of some wonderful sounds. The Prophet X emulates it very well. You can select any of the VS waves as an Instrument, then use Sample Stretch to distribute it across the whole keyboard. Layers A and B offer the VS’s full complement of four oscillators and, although there’s no joystick, you can use the Touch Sliders to emulate X‑Y movement, using one to sweep North‑South and the other East‑West. With appropriate modulators directed to the Sliders, you have a full emulation of vector synthesis. Sequential haven’t marketed the Prophet X as a recreation of the Prophet VS so I didn’t feel the need to perform an A/B test of the two synths, but a few hours spent programming VS sounds demonstrated that the results were excellent, with a very different character from the vintage analogue sounds that I had already programmed.

It was while experimenting with the VS waves that I realised why I hadn’t been falling in love with the other multi‑samples. It came about by accident; I had recalled an earlier patch and accidentally added one of the choral Instruments to it. The result was stunning, so I isolated the multi‑sample to listen to what it was adding to the sound. To be honest, I didn’t think that it was an exceptional sample but it worked perfectly when combined with the other sources. My inner light bulb lit, and I realised that I was finally beginning to ‘get’ the Prophet X. The sample library hasn’t been designed for use as a ROMpler, but as a different basis for synthesis and sound design. (Had I taken the trouble to study the literature before plunging in, I might have understood this from the start!) This also explained why there are so many variations — and sometimes rather odd variations — of some sounds; being able to replace a sample with a related one in a composite sound could prove very useful when trying to home in on that sound, whatever that might be.

I now started to combine the samples and oscillators in different ways, sometimes combining two samples to create new textures, sometimes creating new timbres from the underlying samples, sometimes modulating the loop parameters to obtain weird results, sometimes distributing a single sample across the whole keyboard, sometimes modulating the Instrument/oscillator mixes... and much more. The results could be stunning and it became clear that the Prophet X offers a much wider palette than any previous synth bearing the Prophet name. But this still wasn’t the end of its capabilities.

One of the advantages of digital oscillators is that you can obtain consistent cross‑modulation from note to note as you play up and down the keyboard. So I have to mention the FM sounds that I obtained by using the modulation matrix to route the outputs from any of the sound sources (including the filter) to the frequency control inputs of any of the others. This means that each Layer offers a wide selection of algorithms, and I could even stack the Layers for greater sonic complexity and depth. With a bit of planning, the results could be very rewarding.

Oh yes, and then there were the LA Synthesis patches that I started to investigate, creating short attack snippets using the samples and following them with analogue‑style waveforms using the oscillators. The results that I obtained had a different character from the Roland D50 and its brethren, but the technique remains as valid and as interesting as it ever was.

If all of this programming sounds like it was a bit tricky, it wasn’t, and I was by now whizzing around the Prophet X as if I had used it for years. Of course, none of this would have mattered if, having programmed it, I didn’t enjoy playing the Prophet X, but I did. The keyboard is expressive, the wheels and Touch Sliders gave me lots of control and, despite being compact and remarkably light, the whole thing felt solid and robust. Sure, it’s not cheap, but there was a feeling of quality about the whole experience.

Teething Problems

When the review unit arrived, it suffered from a number of teething problems, most obvious of which were the digital clicks that could be generated at the starts and ends of notes. I was tempted to think that these were all consequences of its remarkably rapid contour generators, but I could also obtain them with slower contours so there was clearly something deeper going on. In addition to this, some of the effects algorithms reduced the amplitude of the sound as the effect Mix was increased from zero. So I contacted Dave Smith at Sequential and we fired a number of emails back and forth, after which he undertook to update the operating system to try to eliminate them (the problems as well as my annoying emails).

Whatever Dave and his team did, it worked. Less than 48 hours later (that’s a fantastic response from a small company!) I was able to update the synth’s operating system and DSP code and the incidence of clicks was greatly reduced. I could still obtain them with the Attack and Release at zero, but that was as it should be... a compliment to the speed of the contours as they snapped open and shut. However, I could also obtain them with slow Release times when voice stealing — which entails the killing and almost instant re‑initiation of a complete synthesizer voice — occurred. I suspect that Sequential have already adjusted this to reduce the severity of any clicks generated, but there’s still some way to go to eliminate them completely. As for the effects... they now worked as they should. If you already own a Prophet X you should definitely upgrade it now, and when the latest OS versions became available.

Sadly, this upgrade didn’t cure another problem that I had detected. When using Unison, a thump can occur at the start of a note. This may again be a consequence of the voice allocation mechanism, so it should be curable. But until it is, you’ll have to approach Unison with care. This is a shame because it can sound superb.

Finally, I had noticed during my initial tests that stepping through the oscillators’ waveforms while a key was depressed resulted in different waveshapes each time that I went round the loop. This is probably of no great significance because the correct wave is generated when the next note is played, but I understand that the bug will be squashed in the next OS revision anyway.

Conclusions

The Prophet X is not an alternative to a conventional ROMpler or digital workstation; it has capacious sample memory and it offers drum sounds and effects, but its limited polyphony and its even more limited multi‑timbrality (well... bi‑timbrality, to be precise) mean that it’s an altogether different type of instrument. Mind you, it took me a while to appreciate this and to complete the transition from slightly underwhelmed to greatly impressed. But once I had gotten to grips with it, I realised that it’s a hugely powerful and expressive synth that can sound wonderful and is both simpler to use and sonically deeper than it appears to be. Mind you, we won’t discover its limits (or the lack of them) until the sample import software becomes available. It’s not for everybody — apart from anything else, the hefty price tag takes care of that — but for those with deep pockets, this is a flagship synthesizer that will be creating new and interesting sounds for many years to come.