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Studio Electronics ATC1

Analogue Tone Chameleon By Paul Nagle
Published November 1996

A synth's filter plays a major part in defining its sound, and creates the particular character of the best‑loved vintage analogue models. Courtesy of its different filter cartridges, the ATC1 aims to be able to change its sound to suit your needs. Paul Nagle plugs in.

Regular SOS readers may recall my first impressions of the Studio Electronics Analog Tone Chameleon — or ATC1 — which were recorded in a preview of a pre‑production model in September's SOS. Since then, this colourful synth has had a software overhaul and now boasts the option of a 1U rackmounting filter bank, which can switch between up to four of its plug‑in filter cartridges. By the time this issue hits the streets, the ATC1 will also be properly on the market, which makes it time for the full SOS appraisal, though one or two software odds and ends were still being tied up as this review was completed.

To recap briefly for a moment, for anyone who didn't catch the preview: the ATC1 is a monophonic synthesizer module, filling two rack spaces and possessing a single knob which performs all editing functions. It has a standard quarter‑inch mono output, an external input to the filter, CV/Gate in/out, the obligatory MIDI In and Out (but no Thru), and stores 512 patches. Fifty pastel‑coloured membrane switches access sound parameters, and with two VCOs (Voltage Controlled Oscillators), two LFOs (Low Frequency Oscillators) three ADSR (Attack, Decay, Sustain, Release) envelopes, and a goodly range of modulation routings, you might assume this is just another analogue synth. But Studio Electronics, aware of the element which contributes most to the sound quality of such an instrument, have created plug‑in filter cartridges, which emulate the characteristics of various classic synthesizers.

Filter Bank

My initial reaction to the ATC1's filter cartridge system was based on two pre‑production cartridges, which I had difficulty telling apart. Happily, there are no such ambiguities with the finished versions, and I can report that each one exhibits the expected, wildly different charcteristics, transforming the synth significantly.

For the review, a prototype model of the filter bank was supplied, its only control being a simple four‑way switch to activate the desired module. I'm assured that the final product will feature a software‑controllable selector which will allow each patch to be associated with the correct slot — an excellent idea. Connected by a short cable (let's hope it's longer on the real thing), the filter bank takes its power from the ATC1 itself. There are no wall warts to contend with, just a proper internal power supply. I'm tempted to forgive the positioning of the on/off switch at the rear of the synth, because I suspect that few people switch on each component of their studio separately, and also because any ATC1 user with a couple of cartridges and no filter bank would need access to the rear anyway (you can still plug one filter cartridge at a time directly into the ATC1 rear panel, if you don't have a filter bank, though you have to power‑down to swap cartridges). With no cartridges present, the four gaping holes in the filter bank would have a tendency to collect dirt and foreign objects. This needs to be addressed when the design is finalised.

Once I'd extracted various stray bits of polystyrene packaging and filled the filter with cartridges, I quickly became hooked, jumping between the various filters. The Oberheim filter has a small switch to select low‑pass or high‑pass modes, but as the prototype filter bank makes this inaccessible, two identical cartridges were supplied for review, each set differently.

The first ATC1 filter modules are:

  • MINI: classic 24dB Minimoog filter.
  • 303: the techno‑classic TB303 filter.
  • SEM: Oberheim's 12dB Synthesizer Expander Module filter.
  • 2600: ARP 2600 filter (this didn't arrive in time for the review, alas).

The ATC1 is supplied with the Moog filter as standard — a sensible choice, since this is as warm, smooth and versatile as Desmond Lynam himself. I had no trouble programming some of the classic solo and bass Moog patches I remember from my Minimoog, and that modern synth manufacturers still strive to emulate.

The 303 filter was a revelation to me. I used to consider the 303 as more of a sequencer than a sound source but if my Basslines had sounded this good, I'd still have them today! There's no loss of bass presence when the resonance is fully whacked up, and the filter is punchy, a little dirty, and ideal for that squelchy bottom end. Using a single oscillator with either square or sawtooth waves, it is possible to come very close to the sound of the original Bassline, although the ATC1 can also do things the 303 never dreamed of.

Politest of all is the Oberheim filter, which has the additional feature of a high‑pass switch. I found that this one worked best for delicate solo voices, bubbly effects, or sequence parts, and reminded me of some of the Korg and Yamaha monosynths I've owned. Of the three, this is probably my least favourite, but the high‑pass setting adds the kind of thin, nasal twangs to your sound palette that no other cartridge can yet reproduce. It's just a shame that the alternative filter characteristic can't be switched in software.

Routing a suitably‑amplified external signal to the filter input was pretty rewarding, though there is no onboard mixer for this, and you need to provide a MIDI trigger to activate VCA and VCF envelopes before getting any output. I tried several of my other synths through the different filters, with worthwhile results, getting a great effect using an LFO square wave to add MIDI‑synchronised filter chops to some string pads.


To examine the ATC1's synthesis capabilities more closely, it has two Voltage Controlled Oscillators: each feature any combination of triangle, sawtooth and variable square waves. They are blended, along with a noise source, in a small mixer section.

An extensive modulation matrix (see box) provides access to almost everything you'd reasonably want to control, although some of the routings restrict you to a single target. For example, if you wanted to control the pulse width by the modulation wheel, you could only do this for a single oscillator. In practice, though, you soon begin to concentrate on the many things you can do, rather than the few you can't. A great bonus is the freely‑assignable third envelope, which can control a wide variety of targets, including resonance and the level of cross modulation.

Two Low Frequency Oscillators each offer six waveforms — triangle, square, sawtooth, reverse sawtooth, noise, and random. As if that wasn't enough, LFO2 can synchronise with MIDI clock — just the thing for those funky sample‑and‑hold filter burbles. Clock intervals can be saved in each patch, ranging from whole notes through to sixteenths, including triplets. I'd like to have been able to control the phase of the LFO waveform to help align it against a rhythm track — maybe in a future update?

Cross‑modulation provides some of the wilder sounds of the ATC1. Its function is to modulate the frequency of the filter or Oscillator 1 using Oscillator 2. To add further spice, Oscillator 2 has four modes:

  • Normal operation: keyboard controls oscillator pitch.
  • Hard synchronisation: to Oscillator 1.
  • Keyboard control off.
  • Keyboard control off, with sync to Oscillator 1.

The three envelopes are of the ADSR variety so beloved of traditional synthesists. Envelope 3 is freely assignable, so a number of intereresting uses spring to mind: delay or contoured vibrato is easily created, as are dramatic swirls of resonance, perhaps while Envelope 1 sweeps the cut‑off frequency in the opposite direction. By assigning Envelope 3 to the pitch of Oscillator 2, then hard‑syncing to Oscillator 1, you can create some incredible grungy sounds; modulate the envelope amount with velocity to vary the dirtiness, for some powerful Jan Hammer‑type solos. Envelopes 1 and 3 may both be inverted, and this compensates, in part, for the fact that all modulation amounts are positive. You can't, for example, use negative filter/keyboard tracking to make a patch get progressively more mellow going up the scale.

In Use

Listening to 512 patches is quite time‑consuming, especially when you multiply that number by the three filters I had to try. Classic analogue sounds are spread liberally amongst lesser patches, each of which took on a different persona as I cheerfully flicked from filter to filter. Strange noises, superb basses, and sweet solo voices were in great abundance but it wasn't long before I'd wiped dozens of them, to be replaced with my own versions. The ATC1 uses standard MIDI Bank Select protocol to access sounds beyond program location 128, and having so many locations available means that you can keep lots of interim versions without even the hassle of thinking of names for them. Interestingly, sounds do not cut off when program changes are received, so it is possible to switch seamlessly between patches that are subtle variations of each other — maybe having different envelope times or using different LFO waves.

Accessing parameters is simplicity itself. A push of each membrane switch reveals the current value of that parameter, displayed on the 3‑character LED, which then becomes 'live' for editing via the continuous knob. Some functions have shared switches and require two or three pushes to access, but this is still more convenient than most multi‑function knobs, whose position never represents all the shared settings. Most parameter edits are sent and received as MIDI control changes — whoopee!

Whilst being no replacement for dedicated controls, the ATC1's editing knob is comfortable, easy to use, and renders filter sweeps smooth and zipper‑free. It isn't the perfect tool for setting small increments, and suffers from a common problem of continuous dials: 'backstroke', where the value you set can sometimes decrement unexpectedly. Apart from this, though, it's a pretty good compromise and does reduce the number of moving parts, whilst keeping the cost down. The 3‑character display performs an adequate task of showing you what's going on, although some of its abbreviations are rather cryptic and occasionally inconsistent (Envelope 1 Amount is referred to both as E1A and EA1). I'd have liked the LFO waves to be named, rather than being simply numbered from 1 to 6, but this is one area where my ears came to the rescue long before I needed to reach for the manual. Speaking of which, the manual is currently quite slender but still manages to be pretty useful, even hinting at some features that are not yet implemented, such as a TB303 accent mode. There are, however, some important omissions, including a System Exclusive section, details on how to use the CV/Gate, and guidelines for using the filter input.


The Tone Chameleon combines raw analogue power with almost unparalleled flexibility. The filter bank is a convenient way of transforming the synth, but also acts as a kind of lure, tempting you to buy more cartridges.

During my time with the synth, I managed to crash it twice. Each time, the display reverted to the ATC startup message and my edits were lost. Another small annoyance was the synth's insistence on reverting to program 1 at each power‑up — not too friendly if you can't remember that number of the last patch you were working on. These things aside, having used the ATC1 for some time now, I'm not sure that I want to be without it. I'd probably be content with just a couple of cartridges initially, safe in the knowledge that I could choose a different filter at a later date. In these times of synthesizers for the masses, it's rare to be given the power to tailor your instrument at such a fundamental level.

In The Pipeline

As this review was being finished, genial Studio Electronics developer Tim Caswell was working on an ARP 2600 filter cartridge, and tying up the ATC1's CV/Gate software (I suggested that a 'gate detect' mode would be useful for Moog owners, so that the ATC1 could automatically cope with S‑Triggers). He was very receptive to my suggestion of allowing all front‑panel parameter changes to have an associated MIDI Controller, and promised that the finished manual would include the system exclusive codes required to initiate a remote patch dump. Encouragingly, he also agreed to consider new features, such as multiple targets for modulators, in future software upgrades. He wouldn't reveal which filter would come next after the ARP is finished, and he was probably bemused by my suggestions to recreate the Korg MS20 or design something new, such as a 48dB filter. Whatever comes next, it looks like an interesting future for the ATC1.

Coming down to earth for a moment, today's buyer needs to insist on the very latest ROM version to ensure that the ATC1's CV/Gate mode functions correctly.

ATC1 Features

  • Monophonic analogue synthesizer module.
  • 512 user‑programmable patches.
  • Two Voltage Controlled Oscillators.
  • Plug‑in filter modules.
  • Most parameter edits transmit and receive MIDI controllers.
  • External input to filter CV/Gate in and out.
  • Three ADSR envelopes.
  • Two Low Frequency Oscillators.
  • Single or multiple triggering (low or last note priority).
  • Portamento or auto‑portamento (glide up to or down from note).

Modulating Wildly

  • VELOCITY: Envelope 1, 2 and 3 amounts.
  • MOD WHEEL: VCF freq, Resonance, Osc 1 freq, Osc 2 freq, Osc 1 level, Osc 2 level, Osc 1 pulse width, Osc 2 pulse width, Cross Mod amount, Noise level, LFO 1 rate, LFO 1 depth, LFO 2 rate, LFO 2 depth, Env 1 amount, Env 3 amount.
  • AFTERTOUCH: As modulation wheel.
  • LFO1 & 2: Osc 1 Freq, Osc 2 Freq, Osc 1&2 Freq, VCF freq, VCF resonance, Osc 1 level, Osc 2 level, Osc 1 pulse width, Osc 2 pulse width, Cross Mod amount, Noise level, Volume.
  • CROSS MOD: Cutoff freq, Osc 1 freq or both.
  • ENVELOPE 3: Osc 1 freq, Osc 2 freq, Osc 1&2 Freq, Osc 1 level, Osc 2 level, Osc 1 pulse width, Osc 2 pulse width, Cross Mod level, Noise level, Resonance (Env 1 is hard‑wired to the cutoff frequency), LFO 1 rate, LFO 2 rate, LFO 1 depth, LFO 2 depth.
  • BENDER: Pitch, Filter or both.


  • Excellent analogue sounds.
  • 512 user patches.
  • Plug‑in filter system.
  • CV/Gate and external input to filter included.


  • Data knob not ideal for single increment editing.


A monosynth that not only sounds superb but can grow with you. If you feel in danger of getting bored, plugging in a new filter transforms the character of existing patches, whilst providing inspiration for new ones. If you are still looking for that elusive Moog or ARP synth, consider this an excellent alternative.