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ARP Quadra: Part 2

Keyboard Synthesizer (Retrozone) By Gordon Reid
Published May 2002

In Part 1 we finished with the question: 'What is the Quadra? Is it a turkey of the first order, or a misunderstood and overlooked masterpiece?' This month, we'll conclude with a closer look at the instrument itself to find the answer.

Before looking at the sound-generating sections themselves, let's start by examining the brain of the ARP Quadra: a primitive Intel device called the 8048. More than just a microprocessor, we should perhaps call this an integrated microcomputer because it incorporates an on-board ROM containing the programming needed to control the Quadra. At first sight, this seems like a good idea, but it has a big drawback. If the processor in a Z80-based synth goes 'phffzzzt!' you can always remove the fried chip and insert a replacement, so long as there's no other damage. However, if the processor in a Quadra dies, you can only replace it with an 8048 programmed for a Quadra — and how many of those do you think exist today?

ARP Quadra.Photo: Richard Ecclestone

Setting aside this difficulty, I should also point out that in today's world the 8048 is significantly less intelligent than a washing machine. The most obvious consequence of this is that, despite ARP's claims to the contrary, the Quadra is not a programmable synthesizer. Sure, it has 16 memories, but its 64 bytes of memory (yes, 64 bytes!) can't store the parameter values that determine each patch. Let me explain...

The processors used in hybrid analogue/digital synthesizers can store and respond in real time to dozens of parameters, while simultaneously scanning the keyboard and generating real-time LFOs and envelopes. Ignoring other factors, the power of the central microprocessor determines how well a given synth does this, so the more recent the model, the quicker and smoother its digital facilities should be. However, there is a threshold below which a processor is incapable of carrying out these tasks: and the 8048 falls way short of the mark. This means the Quadra is an idiot, capable only of scanning its keyboard to see what you're playing, controlling a handful of performance facilities, and storing the on/off status of the 40 switches on the control panel.

To understand how limiting the last of these is, let's consider parameters such as attack rate, sustain level, or filter cutoff frequency. On an analogue synth without memories, each of these has a theoretically infinite number of possible values; but when you add digital circuitry to scan and record them in memory, the number of possible values is determined by the number of 'bits' used in the storage process. If, for example, the cutoff frequency is stored in five bits it has two to the power of five possible values, which is 32. Considering that a single bit can only be on or off, and nearly all the Quadra's parameter memories are 1-bit, this means that instead of storing a value the memories store only an on/off status associated with each parameter. In other words, the memory determines whether a given control is active or not. And, to make matters worse, not all of the Quadra's controls have these switches, so an already primitive system isn't even comprehensive.

You may ask, 'what happens when a parameter is off?', and to answer this, take a look at the Quadra's control panel. You'll find that 21 of the on/off switches are linked graphically to 21 of the faders, and, looking more closely, you'll see that 17 of these faders have small red dots drawn alongside them. These dots tell you the preset value used when the fader is off — which is to say, when it's disconnected from the parameter it otherwise controls.

You might think this system is ridiculous and, in my opinion, you'd be right. However, it's surprising how much mileage you can get from these memories, switching one or more of the Quadra's four synthesizer sections in and out of the patch. The trick is to find a combination of fader settings that allows you to jump between dissimilar but still usable patches when you change memories. This isn't trivial, but it's entirely possible if you're prepared to compromise a little.

Having got that major obstacle out of the way, let's take a look at each of the Quadra's four synthesizers in turn. We'll start with the Strings, which could hardly be more basic...

More Reliability Issues

In Part 1 I noted some of the reliability problems encountered on the Quadra, but here's another: the membrane switches in the control panel have a finite lifespan, and when they die there's no way to resurrect them. Cannibalisation is your best bet, but that requires that you find another Quadra with a functional panel, which isn't very likely. Furthermore, I've heard rumours that there were two versions of the panel and the board that drives it, making it possible that, even if you find a potential replacement, it will be incompatible.

Apparently, one fabled Quadra owner has built a substitute panel using sheet metal and modern micro-switches, which must have been a labour of love and resulted in Frankenstein's Quadra. I'd love to see a photo of this — are you out there, Dr F?

The Strings

The underlying sound generation of the string section is provided by the top octave divide-down technology used in cheap organs, electric pianos, and ensemble keyboards throughout the 1960s and '70s. This means that a single master oscillator is divided down into 12 frequencies (one for each note in the octave), and then further divided in factors of two providing the frequencies for each of the notes on the keyboard. However, instead of having 48 (or even 96) oscillators covering the four octaves over which the Strings are playable, there's just one! Moreover, only a square wave is produced by the oscillator, so the Quadra requires a wave-shaper to provide the sawtooth waves used for conventional string ensemble sounds.

From the player's point of view, the Strings section covers the top four octaves of the Quadra's five-octave keyboard, offering two pitches (4-foot and 8-foot, which can be played individually or in unison) with variable attack and release. And that's it — just two octave settings and an AR envelope!

Unfortunately, that really is it because there's only a single envelope and VCA for the entire string section, and unlike the Logan String Melody II, for example, the Quadra is unable to articulate notes individually. In fact, this problem is so severe that if you program a sound with a fast attack and a long release and then play two chords in quick succession, the first chord swells up slightly with the second before dying away. Not too distressing if you play two inversions of C major, but it's a different story if you play F# major after Cmaj7. On the positive side, though, many ensemble keyboards produce a continuous 'swishing' noise whether you're playing them or not. The Quadra doesn't suffer from this because when you stop playing, the VCA gain drops to its minimum, closing the signal path.

However, to concentrate on these strengths and weaknesses is to miss the point. The Quadra's lush string sound, created by ARP's revered chorus ensemble (called a "phasor"), is a classic — not surprising, given its pedigree. The Quadra's predecessors (the Solina and ARP String Ensemble, both built by Eminent and imported from Holland, and the Omni and Omni II) dominated the mid-'70s and remain objects of desire to this day.

The Quadra's Filter & The Continuing Mystery Of ARP Filters

The Quadra incorporates two ARP 4075 filters: one for the Lead synthesizer, and one for the Polysynth section. The 4075 is a self-oscillating 24dB/octave device introduced by ARP in 1976, which became the company's standard filter for most of its instruments thereafter. These include Odysseys with model numbers from 2810 to 2823, Axxes from 2310 to 2323, the Omni and Omni II, the ProDGX and the Avatar, in addition to the Quadra itself.

The advantages of the 4075 are that it offers low noise and distortion, sounding much like a Moog filter at low resonance settings, and, unlike many filters, it passes an input signal at high resonance. The disadvantage is that, due to a design miscalculation, it has a maximum cutoff frequency of around 12 kHz, but fortunately all is not lost. If you're a bit of a whiz with a soldering iron, you can increase the cutoff frequency by replacing four resistors. If you're prepared to take the risk on such a fragile machine, I suspect that you'll improve the sound of the Quadra considerably by doing so.

The Electric Bass & Strings Bass

To the left of the Strings is the Bass synth, which can be played using the bottom two octaves of the Quadra's keyboard. There are two sub-sections: the first is the Electric Bass and, as the name suggests, this produces something akin to an electric or synthesized bass sound. It's based on a single oscillator but, thanks to another frequency divider, offers 8-foot and 16-foot octave settings. A 'fine tune' fader is provided, together with a 'Down One Octave' switch that allows you to play deep bass at 16-foot and 32-foot pitches. The VCO itself produces a simple sawtooth wave, and this is used unmodified for the 8-foot setting. However, a wave-shaper converts the sawtooth to a pulse wave for the 16-foot setting, although ARP's documentation is inconsistent on this point. Sometimes it suggests both the 8-foot and 16-foot pitches are pulse waves, but elsewhere it states the 8-foot waveform is the unmodified sawtooth.

The mixed 8-foot and 16-foot waves pass down a simple VCF-VCA signal path, and an AD envelope (with variable Decay) controls the pulse width of the waves, the filter cut-off frequency, and the VCA gain. There's also a slider that allows you to control the filter resonance, which is slightly weird since the documentation says the Bass section uses a single-pole (6dB/octave) filter. If I remember my electronic theory correctly, you can't have a resonant single-pole filter — there's something odd going on here.

The second sub-section is the Strings Bass, and this too offers 16-foot and 8-foot settings, incorporating wave-shaping and PWM of its 16-foot output, fine-tuning fader and a Down One Octave switch. However, the Decay and Resonance controls do not affect it. Instead, the output from the Strings Bass is controlled by a dedicated volume fader before passing to the String section's chorus generator, and then through the Strings' VCA, which is, of course, subject to the Strings' AR contour generator. This means that you can extend the attack time of the Strings Bass, but not its preset release.

The Quadra's designers may have hoped this arrangement would allow you to detune and mix the Strings Bass with the Strings themselves to create fat, synthesized bass sounds, but, if so, they'd have been disappointed. Low stringy sounds, yes. Fat basses — not a chance. And, while you can use the Electric Bass to create a wide range of plucked bass sounds, the use of a single oscillator for all four of the footages in the String and Electric Bass sections ensures the classic sounds of, for example, Moog Taurus pedals are totally out of reach.

The Lead Synth

Next we come to the Lead synthesizer, which covers all five octaves of the keyboard, or just the top three if the Bass section is active. The manual proclaims that, "if not contained in the Quadra... its features would rival those of other lead synths in its class", but I'm not sure I agree. To be fair, the sparse controls hide a wealth of clever ideas, but many important facilities are missing, such as a noise generator, adequate modulation, and truly independent oscillators. Still, rather than concentrate on what's lacking, let's see what survived.

There are two detunable oscillators, and you can further offset these in semitones using a novel approach requiring you to 'write' an interval using the keyboard. Each oscillator produces sawtooth, square, and 10 percent pulse waves, with PWM for the last two of these. Unfortunately, the pulse width is modulated only by the envelope, so there's no cyclic PWM for 'chorus' waves. Furthermore, there's only one waveform selector for both oscillators, so you can't mix sawtooth and pulse waveforms.

The keyboard generates two monosynth pitch CVs, so, like the ARP Odyssey, the Quadra offers a two-voice mode allowing one oscillator to track the highest note played, while the other tracks the lowest. This is not true duophony because there's only one signal path, but it can still be very useful.

A dedicated LFO provides vibrato, but this is always delayed by a preset amount. I tend to use delayed vibrato, or control its amplitude using pressure sensitivity, so this doesn't bother me; but I expect that there are some players who will find it very annoying. The other pitch control is Trill, and this does what you would expect: it trills between two user-defined pitches. However, setting this up is arcane, and you'll never stumble across the method by accident.

The output from the oscillators passes through a self-oscillating 24dB/octave filter (see box) that tracks the keyboard accurately at 1V/octave. There are cutoff frequency, resonance, and envelope 'amount' controls, but no variable keyboard tracking or LFO filter modulation. And this brings us to the envelope generator itself — one of the weirdest parts of the Quadra.

The Attack is simple with its conventional attack rate. The Decay isn't quite so easy. If the 'Sustain = Low' switch is on, the Decay slider acts conventionally, and the Sustain Level is preset to approximately 20 percent of the Attack level. But if the 'Sustain = Low' switch is off, the decay time is preset, and the Decay slider sets the Sustain Level between 0 and about 75 percent of the Attack level. The envelope generator holds the Sustain level (set by the Decay slider) for a preset time of around 1.5 seconds, and then enters a second attack to climb back to 100 percent of the peak level, where it stays until you release the keys, before falling at the rate set by the Release slider. ARP described this as an ADSASR generator, and it turns out that its shape (known classically as sforzando) is ideal for brass patches.

The Lead synth has a number of other goodies tucked away. For example, the VCOs have independent portamento speeds, and played monophonically these provide a range of interesting effects. When played duophonically, they allow you, for example, to slew the melody while jumping between notes in the counterpoint. The keyboard also offers single and multi-triggering, plus pressure sensitivity that affects either the pitch or the brightness and loudness. It's a shame you can't control the LFO depth this way and, since the Quadra lacks pitch and modulation wheels, a joystick, or a ribbon, I reckon ARP should have been more generous with its aftertouch.

Finally, as far as the Lead synth section goes, there's a basic arpeggiator with 'up' and 'up & down' modes. There's no way to select the number of octaves over which the arpeggiator plays (it's just the notes you hold across the keyboard), but you can combine the arpeggiator with the Trill feature to generate some interesting effects.