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Digisound 80 [Retrozone]

Modular Synthesizer By Paul Nagle
Published January 2000

The kit format meant that Digisound modules appear in a variety of guises. The top row contains the system built by Dave Robinson (see 'Construction Time Again' box), using home‑made panels, while the two lower rows are custom black‑faced modules. Beneath the ARP sequencer is the polyphonic keyboard controller.The kit format meant that Digisound modules appear in a variety of guises. The top row contains the system built by Dave Robinson (see 'Construction Time Again' box), using home‑made panels, while the two lower rows are custom black‑faced modules. Beneath the ARP sequencer is the polyphonic keyboard controller.

British company Digisound's affordable modular synth may not have the cachet of a Moog or an ARP, but — as Paul Nagle explains — they were innovative, well designed, and sounded great... Additional material by Dave Robinson.

In February 1980, Charles Blakey launched "the synthesizer you've been waiting for" in Electronics Today International magazine. In his opening article, he stated that any hardwired synthesizer inevitably suffered from "compromises in terms of the capabilities of the instrument." He set out, therefore, to provide an affordable, flexible synthesizer in modular form, the parts of which could be constructed as stand‑alone units or as 9 x 3‑inch panels in a larger case. This instrument he called the Digisound 80 Modular Synthesizer.

Of course, once people get a soldering iron in their hands, anything can happen, and the Digisound 80 Modular appeared in many guises, from the typical 'white‑faced' kit (as supplied by Blakey) to many variations which used the published schematics merely as a starting point. With this in mind, I won't be trying to write the definitive work on all things Digisound. Instead, I simply hope to show that this synth deserves its place up there with the VCS3 and the Wasp as one of the most important British electronic instruments ever conceived.


The powerful VCO is based on a Curtis chip, and offers the rarely‑found 'soft sync'.The powerful VCO is based on a Curtis chip, and offers the rarely‑found 'soft sync'.

For many years I have been the custodian of a modest twin‑oscillator Digisound, which I used to use mainly to make 'spacey' noises. Ironically, perhaps, I started to extend those duties after an encounter with a modern virtual analogue: Clavia's Nord Modular. With my enthusiasm rekindled, and a passing regret for all the synths I discarded cheaply in the '80s, I started to explore all things analogue once more. This culminated in the recent purchase of a large Digisound system, complete with eight‑note polyphonic keyboard, four oscillators, eight VCAs, three VCFs, six envelope generators, four LFOs and more.

My original instrument incorporates Digisound's own printed circuit boards into a 44‑inch‑long wooden box, with each cream‑coloured metal front plate individually cut and hand‑labelled. The latest addition to my studio, however, is quite different, containing more heavily customised modules and home‑made circuit boards. I guess it's closer in appearance to the separate free‑standing modules from the original articles in Electronics Today International (and Electronics & Music Maker). And unlike my original synth, it's black. In common with most Digisounds, patching is carried out using minijack connectors. Compared to the quarter‑inch jacks used by Moogs this may seem like a compromise, but it was one shared by other classic instruments from ARP, Roland and others and, for me, the minijacks have proved very reliable. The sockets are all laid out beneath the controls, leaving the generously proportioned knobs far more accessible than those of today's (more compact) modulars from Doepfer, Analogue Systems and so on.


A Digisound 80‑7A State‑variable Filter module: this one has the original white panel.A Digisound 80‑7A State‑variable Filter module: this one has the original white panel.

An earlier Digisound modular, the 20000 Series, used the (now very rare) SSM chips, but this was superseded by the Digisound 80 system which relied almost exclusively on those from Curtis Electronics. Modules in the Digisound 80 range included:

  • 80‑1A Power Supply.
  • 80‑2 VCO.
  • 80‑3 LFO.
  • 80‑4A Voltage Controlled Mixer.
  • 80‑5 Processor.
  • 80‑6L/B/H/P Low‑Pass, Band‑Pass, High‑Pass, Phase‑Shifting Filter.
  • 80‑7A State‑Variable (multimode) Filter.
  • 80‑8 Basic ADSR.
  • 80‑9A Dual VCA.
  • 80‑10 Voltage‑Controlled Envelope Generator.
  • 80‑11A Dual Ring Modulator.
  • 80‑12 Noise Generator With Sample & Hold.
  • 80‑13 External Input Module.
  • 80‑14 Stereo Power Amplifier.
  • 80‑15x various keyboards/accessories.
  • 80‑15E X‑Y Controller (joystick), Octave Shift, Touch Trigger.
  • 80‑16 Dual Resonant Filters.
  • 80‑17 Reverberation Unit.
  • 80‑18 Multi‑function Envelope Generator.
  • 80‑19 Dual VCLFO.
  • 80‑20A/B Waveform Multiplier (designed, I believe, by David Ward‑Hunt of Wavestar).
  • 80‑21 Voltage Controlled Digital Oscillator.
  • 80‑22 Patcher.
  • 80‑23 Quad LFO.
  • 80‑24 Polyphonic Keyboard Controller.
  • 80‑C9 Voice Card.

Let's take a look at some of them..

Module 80‑2 VCO

The polyphonic keyboard's controls and interfaces.The polyphonic keyboard's controls and interfaces.

At the heart of a Digisound modular beats the classic Curtis CEM 3340 oscillator chip. This has graced such fine instruments as Roland's SH101, Oberheim's Xpander and Sequential's Prophet 5 and Pro One, and provide a rich, stable oscillator with a variety of features and control, whilst considerably reducing the number of components and therefore the complexity of construction. The Digisound VCO features no less than six simultaneously available waveforms. The first four (ranging from 0 to 10 Volt output levels) are saw, variable pulse, sine and triangle. The final two also produce sine and triangle, but are available as ‑5V to +5V outputs. Why is this range shift useful? Well, remember that in a modular system, there are no restrictions on the connections you can make, and it's quite typical to use oscillators as modulation sources. There are controls for pulse width, coarse tuning, fine tuning and an 'off' switch to deactivate the coarse knob. This coarse knob itself was occasionally built using a stepped octave selector, but it's more usual to see a smooth pot covering the full 10‑octave range. The VCOs remain reliably in tune and yet are warm and powerful with a delightful lower end.

Another notable feature of the VCO is its oscillator sync which, as well as hard sync in both positive and negative incarnations, has the rarely‑found but very desirable soft sync. The Digisound manual tells me that "soft synchronisation causes premature reversal of the waveforms from the slave VCOs with the result that their oscillation period is an integral multiple of the pulse period of the master VCO." Having used it for many years I still cannot always predict how it's going to act, but I've learned to expect mystical tricks as the frequency of the slaved oscillator jumps through hoops to attain bizarre, almost quantised, harmonic changes. Alternatively, with a slight frequency adjustment, it will lock perfectly. For my purposes, it adds a richness to any multi‑oscillator patch that goes far beyond the traditional analogue tricks of detune or PWM. Soft sync requires a negative input pulse of ‑5V, so it can be switched on and off automatically, for instance by using an envelope to vary the level of the incoming waveform so that synchronisation is achieved only during the attack and ceases during the sustain phase of a note.

Finally, linear and exponential frequency‑modulation inputs and attenuator knobs complete the impressive set of features. The 80‑3 Voltage Controlled LFO is equally well endowed (except there is no 'off' switch for the coarse tuning, sensibly enough). In fact it is identical to the VCO but with a frequency range of 0.01Hz to 5kHz (this range can be adjusted too), making it surely one of the most versatile analogue LFOs ever made!

80‑6 And 80‑7a Filters

The filter module seen most commonly is the 80‑6 24dB/octave VCF — and what a fine filter it is! According to how it is built, it may function as a low‑pass, high‑pass, band‑pass or phase‑shift filter. In its low‑pass configuration, it has a warmth and 'roundedness' which I consider second only to the classic Moog modular VCFs, and its voltage‑controlled resonance input gives it the edge on many of its contemporaries. Other than that, there's remarkably little to say; it has two signal inputs, two control inputs, coarse and fine cutoff pots and a very sweet resonance. A later design, the 80‑7A State‑Variable Filter, featured optional 12 or 24dB operation for low‑, high‑ or band‑pass filtering, plus a notch (band‑reject) filter. This is a very versatile module, its band‑pass mode being my personal favourite.

80‑12 Sample & Hold Plus Noise Generator

This module has three sound sources: white noise, pink noise and a deeper 'low' noise which is sometimes referred to as red noise. If noise 'colour' has always confused you, a brief explanation may be in order. White noise consists of an even distribution of audio energy across all frequencies — every frequency in the audio spectrum is present at the same level. In pink noise, on the other hand, the energy is distributed logarithmically rather than linearly across the frequency spectrum (ie. the audio energy is distributed evenly across octaves, rather than frequencies), making it less bright than white noise. As the noise colour shifts towards the red end of the scale, it loses more and more of its high frequencies until it becomes a deep, low rumble. If the noise has its low frequencies progressively removed, it is referred to as 'blue'.

Noise and sample and hold are popular bedfellows, because any S&process needs a source from which to sample a voltage. The random mixture of frequencies that noise provides is ideal, although you aren't restricted to using noise as the source. The principle is simple: feed a source into the module, set the speed at which samples are taken and direct the output to another module. This is typically used to create random pitch or filter fluctuations. The speed is governed either by the module's own internal clock or from an external source such as an LFO, or a clock from a drum machine.

80‑8 ADSR

The envelopes have a useful Manual trigger switch so you can start them without need of a keyboard. My own envelope modules are mostly the basic 80‑8 ADSR, plus a couple of AR types. A later model of envelope, the 80‑18 Multi‑function Dual Envelope Generator made several significant advances including an Automatic mode and a Damped mode. This latter may have been developed with the polyphonic ALPHADAC controller in mind. It changes the shape of the envelope into an ADRR — a sharp initial attack, brief decay, a longer decay and then a final short release as the gate off occurs. Releasing the note thus has the same action as applying a damper pedal on a piano. The envelopes have seperate gate and trigger inputs, so applying an external trigger while the gate signal is still present will restart the attack cycle and thus create multi‑peak contours. The Automatic mode is useful if you wish to trigger envelopes from non‑keyboard events such as LFO blips or the short pulses of a clock signal. It allows the envelope to generate a complete ADR cycle, something not normally possible because envelopes enter the release stage as soon as the gate signal ends (key off). Times quoted are in the range of 2 milliseconds to 10 seconds and, indeed, my Digisound envelopes are very snappy.

80‑5 Processor

The Processor is an important module with several functions. Firstly it contains a 'lag processor'. If you pass an oscillator's pitch‑control voltage through that, you can achieve the classic portamento effect as the incoming steps are 'smoothed'. You can pass audio signals through it too, achieving an effect similar to a simple low‑pass filter (which is exactly what the lag processor is). Four attentuators are provided, which serve as signal distributors or simple mixers for inputs which lack them. Two of these are configured as Subtractor/Inverters. An inverter reverses the polarity of incoming signals, so a +5V signal is transformed to ‑5V. This has all kinds of applications, such as producing negative envelope curves or, in conjunction with an LFO and the dual VCA module, stereo tremolo. Finally, the subtractor subtracts up to 10V from the input voltage and also inverts the signal in phase.

80‑9A VCA

In most hardwired synths, the VCA is always the output stage. Here, it's far more versatile, carrying audio or control signals. Twin VCAs are provided for stereo operation, with both linear and exponential CV inputs. A footswitch connected to the stop input instantly closes down the VCA, which can be a life‑saver in live use! A Quad VCA was available too, and was a much simpler affair with four signal inputs, outputs and control inputs plus gain knobs to adjust each input level. My system now has 10 VCAs in all but you can never have too many.

I still cannot always predict how it's going to act, but I've learned to expect mystical tricks as the frequency of the slaved oscillator jumps through hoops to attain bizarre, almost quantised, harmonic changes.

Keyboard Controllers

Digisound's keyboards were pretty advanced for their time. They were digitally scanned, accurate and could be readily interfaced with a microprocessor for sequencing, composing and polyphonic use. My system's 'round robin' keyboard is driven by a Z80 processor and supports up to eight notes of polyphony (though I only have six oscillators) as well as providing mono, unison, and key‑cycling modes. Voltage from the X‑Y joystick can be routed to control any aspect of the instrument with an immediacy yet to be equalled by a modern synth's LCD‑based modulation matrix. Overall tuning was adjusted from here, as was portamento and key transpose, with bizarre tunings also on offer. My previous controller had a series of LEDs which created patterns according to the key held down: many regarded this display as the highlight of my stage show.

Digital Modular

Some of Digisound's later modules showed just how forward‑thinking the company was. Long before MIDI was introduced, Charles Blakey was thinking of a digital interface along the same lines, and before sampling became a commercial reality, Digisound had made an 8‑bit sample‑playback module complete with multisamples. This used compression techniques, had a looping facility and could replay 32 sounds. A Digital Wave Generator module, similar to those found on Korg's DW‑series synthesizers, reproduced a variety of single‑cycle waves. Indeed, the sample module's developer, Graham Gerard, reflects that he approached "most of the big companies" with his sampling idea but was told it would never work because the sample couldn't be transposed naturally. Graham's answer — multisampling — was deemed too complicated, and it was left to Digisound to take up the baton. Perhaps the great Japanese sampler industry could have been a great British industry, but the death of Charles Blakey and subsequent winding‑down of Digisound meant that very few of these modules ever made it into the marketplace (though the technology eventually found its way into 'talking' lifts!). Another module from that time was the 19‑inch rack‑based PK1, which was a pitch‑to‑voltage conversion module with integrated envelope follower.

The now legendary ALPHADAC 16 microprocessor was connected between the controller keyboard and the synth modules. It allowed up to 16 voices in polyphonic mode, split keyboard operation, arpeggiation, plus real‑time and step‑time recording. Its 16‑key pad could reputedly bring in various programs in real time, with several processes available simultaneously. Amongst the programs were a four‑channel recording system (with 448 notes per channel), a 16‑voice polyphonic version, and a 4‑channel 'Not Real Time' polyphonic composer/sequencer. Charles Blakey started to explore 'voice cards' — complete synthesizers using the CEM 3372 chip, with battery‑powered memory for stored patches. Another rare module, Blakey's Patcher, was a central programming point for connections and an early attempt to incorporate programmability into a modular synth (Korg's PS3200 being another, quite different, example).

A Good Buy?

The Digisound contains all the ingredients of a classic modular synth. Although modules could be purchased ready‑made, its origins as a series of magazine projects reduced its status in the eyes of some. It also meant a considerable variation in construction quality, appearance and even the functionality of any system — but such things are common among analogue modulars. Some of the more obsessive analogue fans look down their nose at the Digisound because it 'cheats', using Curtis chips rather than many discrete components. My advice to them is to get out more — and I don't just mean to the nearest train station. A well‑made Digisound should last for years: they are still relatively easy to repair, and with the unexpected appearance of a large supply of Curtis oscillator chips (see Chip Shop box), you could even start building more modules if you wanted. Indeed, the Doepfer A100's 'high end' oscillators use the same chips, some of which were salvaged from (what I hope were) deceased synths. The Digisound interfaces well with modules and sequencers from other companies. The low output impedance and high input impedance allow one output to drive several inputs without overloading or loss of voltage. The design works very well and means you can control several oscillators from a single voltage source without needing to resort to CV buffer modules — some modern modulars please take note!

Long before MIDI was introduced, Charles Blakey was thinking of a digital interface along the same lines, and before sampling became a commercial reality, Digisound had made an 8‑bit sample‑playback module.

I see Charles Blakey as one of the great unsung heroes of synthesizer development, inspiring a wide range of people who couldn't otherwise afford it to take up electronics and analogue synthesis. He had the vision to incorporate digital elements (including microprocessors) into his analogue instruments well before such things were common. Whether this small company would have continued to flourish had he lived, we will never know, but the design of the Digisound modular has stood the test of time remarkably well and should continue to delight for years to come.

Construction Time Again

I used to get Electronics Today, and saw the Digisound modular kit advertised over several months. The first module I built was the VCO: I got that, plus the power supply and all the parts from Charles's shop in Blackpool. I didn't even get the standard white front plate, so I could save some money for more modules. Having seen his, I had some idea what they should look like, so I took a flat, cream‑coloured metal sheet and sawed it up into the right size, drilled it, and put the knobs on. When I'd built the VCO — which was quite easy — I switched it on, took an output from the sine wave generator and plugged it into my stereo. Nothing happened. I thought, "bugger, it doesn't work".

My amplifier, a 180 Watt per channel Crimson job with a big meaty power supply, was also home made — and really loud. I also had some enormous Badger speakers. The voltages seemed right, so I turned up the amplifier, but I still couldn't hear anything. At this point my wife, who was upstairs, started hammering on the floor and shouting "turn that bloody thing off!" I ran out to see what was the matter, looked up and the lampshades were shaking in the hallway and the letterbox on the front door was flapping violently. I ran back, took my speaker covers off and the cones were flying backwards and forwards. What I hadn't done was turn the oscillator's frequency up, and it was so low that it was shaking everything in the house!

After that, I built module after module. When you looked at the cost of the components and circuit board it somehow seemed really cheap, yet in 1981 the whole system cost me about £500 to put together, which was anything but cheap for the time! I remember Charles Blakey as a polite, short‑haired bloke in a shirt and tie who reminded me of a headmaster — the sort of guy you'd expect to be into electronic music.

I expected to have to buy a keyboard, but one of the first things Charles said to me was "you don't need a keyboard to play it". At the time I built the Digisound, I was guitarist in a band who were into Hawkwind and Roxy Music. We used the modular for sound effects, and I soon found out why Charles Blakey said you didn't need a keyboard. As I was no keyboard player in any case, we used it to provide effects (bombs, whistles and so on) rather than solos. Sometimes we'd play along with it — set up a patch and start it going, then join in. In the early days of construction I'd just play the oscillator rather like a theremin. Eventually, though, I did make the keyboard — a fine, digitally scanned thing with a trigger and joystick. The trigger was handy because you could use it to fire the envelopes at any time rather than use the keyboard.

If you build something yourself, you have to understand it fully. Modern synths, on the other hand, give you so many features that you will never really spend enough time with them to know them thoroughly. As I built each component of the modular, I tried every possible combination of connections. I learned to create sounds and routings with my small but growing instrument that I would never have hit on if presented with a large, complete system. The synth has worked reliably for almost 20 years now and Paul still gigs it occasionally — not bad for a kit! Dave Robinson

Chip Shop

Recently, a large collection of Curtis chips has been uncovered, and is now offered from the web site of American modular creators Synthesis Technology. This collection includes a large number of 3340 VCO chips plus 3320 VCFs, 5530 Sample and Hold and others. It seems that finding CEM3310, 3312 and 3330 (ADSR and Dual VCA) chips might be rather more of a challenge though.

Check availabilty (and even order on‑line) at Synthesis Technology's website: You might even find some SSM chips there too!