Two years ago we took a look at Soulsby's Atmegatron.
Since then this reprogrammable 8-bit synth has dropped in price (by over £100). Not only that, but the range of official guises that the unit can inhabit is now six.
The intervening time has also seen an ever-expanding range of 'small and interesting' boutique synth boxes being produced to satisfy all manner of audio appetites.
The Atmegatron Odytron Special Edition is one of these, and is painted in an alternative livery and legending appropriate to the task. Under the hood though, things remain the same. The Atmegatrons are based around Atmel's ATmega328P 8-bit microcontroller, used by the Arduino Uno development system.
Arduino/Genuino is an Open Source reprogrammable single-board microcontroller. It is both cheap, and (relatively) easy to program - you can even build your own board from scratch. Fully functional, ready-built boards are available for under £20.
Central to Arduino is its ability to interface with the outside world (given the right components and programming). This makes it the perfect candidate for building MIDI tools and simple audio devices that can be programmed via a USB interface.
Given that the Atmegatron is based around a repurposed 8-bit chip that it relies upon directly for sound production, it is no surprise that the results are very much of the lo-fi digital variety. This was especially true of the original Atmegatron, which had a distinctively chiptuned sonic hue. This was only enhanced by use of the arpeggiator - a classic trick employed by early computer game music pioneers to overcome limited polyphony.
The Odytron, though still an 8-bit synth, takes on an analogue flavour. This time the internal software is employed to create a simple dual-oscillator, duophonic synth - duophony in synth parlance usually alluding to the ability to play two notes simultaneously, but with both processed through a single filter/amplifier section.
This obviously has limitations, but also creates some interesting possibilities born out of this apparent deficiency.
As far as overall architecture and signal flow is concerned, the starting point is the two DCOs. Each is capable of generating a Saw or Square wave, which are selected via the functional dial and knob (both with integrated push-button switch and internal status LED).
"It includes a programmer cable plus overlays for many of the other synth engines it can conjure up."
The front panel has two independent DCO Frequency controls - DCO1 covers 0 to -12 semitones and DCO2 from 0 to +12. These allow for subtle detuning or more extreme intervals that become very useful when the various modulation capabilities are thrown into the mix.
When playing two notes, Oscillator 1 has lowest note priority, and Oscillator 2 the highest. The oscillators are not really DCOs in the most commonly-used sense. We would be far more inclined to call them digital oscillators. They are, after all, created inside a chip and then processed further inside the chip - they are most definitely not discrete circuits.
The mix levels of each oscillator can be adjusted independently, and mixed with either white noise or a ring modulation output. The manual describes ring mod as a Boolean Logic operation (XOR), though from a sonic perspective it is easier to understand by stating that the output is based around the sum and difference of the two input frequencies.
Varying one (or both) oscillator frequencies allows the creation of new tones - from harmonic purity to clangorous nastiness. Both oscillators can have their frequency modulated by a sine or square wave LFO (there's a dedicated knob to set the LFO speed/frequency) and Sample & Hold (random) generator or the four-stage ADSR envelope.
Also, DCO2 can be sync'd to DCO1 and allows control over the pulse width of its square wave (if selected). The pulse width can be modulated by the LFO sine wave or aforementioned ADSR.
Next we have the DCF (we're not going to quibble any further about the naming!). This is a digitally generated low-pass filter and comes in two flavours - green and red - selected via the front panel DCF Type button.
If you want the full 8-bit oscillator sound in all its glory, you need to turn the filter off. The green filter was a little too tame for our liking, though it is a useful additional tone-shaping tool.
Red would certainly be our first choice. It is a shame that the cutoff range doesn't extend a little higher, as there were times when we were itching to push that resonant bump up an octave. In fairness, the manual makes it clear that, due to constraints of the 8-bit filter algorithms, neither filter will self-oscillate and that cutoff range is restricted.
The lack of self-oscillation is perhaps less of an issue, as the red filter does dig into the signal in a pretty decent way. Following the low-pass filter there is a simple 6dB-octave high-pass filter with variable cutoff. This is only really for final tone shaping, as it cannot be modulated in any way, and lacks a separate resonance control.
That covers the bulk of the digital synthesis options, though it is worth mentioning that there is a second envelope generation onboard that can be used to modulate filter cutoff independently of the main ADSR.
Portamento/glide is also available. This function dial slot is also used for saving patches. Unfortunately, as the function button is employed as a fetch filter selector on this version of the Atmegatron, the saving procedure is a little different to the original (and all other software variants). This is one of the downsides to the re-configurable architecture. It certainly adds to the confusion when swapping between the Atmegatron's various guises.
Overall though, the Odytron is perhaps the easiest to understand - if only because it is based around the familiar language of 'traditional' analogue synthesis.
Finally, once the signal leaves the digital domain, it passes through a Bass Boost circuit (a basic 0-6dB low-shelf EQ) and on to the Volume pot. We found the Odytron Special Edition more engaging and clear to use than the original.
The original Atmegatron had some functions that were selected from numerical lists. This brought chance changes to the fore but did slow things down. The simpler Odytron approach means some functionality and flexibility is lost, but you can always load up the unit with the earlier software.
It should be stressed that what remains is the lo-fi digital nature of the synthesis. Aliasing is ever-present and, unlike hybrid digital-analogue synths, is never hidden with judicious additional filtering.
Also, at times, a finer resolution of modulation depth control would have been welcome. That said, you would not be buying a unit such as this if the limitations of 8-bit computing weren't your cup of tea.
We still have reservations regarding the longevity of some of the controls, but this doesn't detract from the look of the synth or its appeal. This is especially true given that, at £225, the Odytron is much better value than the original on release, and includes a programmer cable plus overlays for many of the other synth engines this little box can conjure up.