Moog Arturia

[Ermelindo Klatt]

Nextcomes the Minimoog's fabled filter. This is a single 24dB-per-octave low-pass device, with controls for cutoff frequency and 'Emphasis' (or, as we'd now usually describe it, resonance). There's also a knob to set the amount of the ADS (Attack, Decay, Sustain) envelope applied to the cutoff frequency. A Decay On switch determines whether the Decay time is also applied as a Release time. The final filter controls are three switches. Two of these provide keyboard tracking at rates of 0, 33, 67 and 100 percent, while the third applies modulation, provided by the Controllers section, or not.

The triangle waves exhibit a far greater difference in tone, with Minimoog V being much the 'woodier' of the two. But now we come to the mixed triangle/ramp wave. This is audibly not the same waveform on Minimoog V as it is on my Minimoog. The difference is so significant that I have produced the two oscilloscope traces overleaf to demonstrate the point.

While I had the oscilloscope running, I decided to check the other waveforms. The Minimoog V 's front-panel graphics are correct, with ramp (rising sawtooth) waves depicted for Osc1 and Osc2. Nonetheless, these settings produce falling sawtooth waves (the same wave but with its phase reversed). Likewise, the ramp setting on Osc3 produces a sawtooth, and the sawtooth setting produces a ramp! This means that the software is calculating the waveforms correctly, but that the phase of the output is inverted. It probably makes no difference at audio frequencies, but it's still not clever.

On the other hand, it's only right to compliment Arturia on the addition of the user-definable pulse widths mentioned above, and on the ability to adjust the shape of the triangle wave and the triangle element within the triangle/ramp waveform from saw-like to ramp-like. There's also the welcome addition of chromatic tuning. Hold down the Shift key while adjusting Osc2 or Osc3 tuning, and the pitch changes in precise semitone steps.

If you have access to a copy of Minimoog V, try the following experiment. Set oscillators 1 and 2 to the same waveform and precisely the same pitch. Move to the Mixer, and balance the levels of Osc1 and Osc2. Now, switch Osc1 'On', and the other four sources 'Off'. Make sure that the filter is wide open. Next, play a note in the mid-range and switch on Osc2. A slightly louder sound with a different timbre is generated, and it's not hard to hear that this is because Osc1 and Osc2 are somewhat out of phase with one another. Well... that's correct. When, as on the original Minimoog, you have free-running oscillators, this is bound to happen. Now switch off Osc2, and then turn it on again. You'll obtain a new timbre, quite different from the previous one. Now try again. A third timbre emerges (or even silence if the two waveforms are completely out of phase with one another). However, if you hold the note indefinitely, the timbre never changes. There's no oscillator drift, just rock-solid waveforms separated by a random phase difference determined when you switch an oscillator on. There is a way to change this in monophonic mode: the Detune knob that normally shifts Unison voices with respect to one another in polyphonic mode acts as an oscillator drift control in mono mode, introducing an additional random drift (undocumented in the manual). A knob setting somewhere in the middle works best for the sustained notes.

One of the key and distinguishing attributes of the Minimoog is the nature of its filter resonance. This diminishes with lower frequencies, and is important in creating the 'punch' of the Minimoog, because lower resonance at low frequencies means that more bass passes to the output as you play down the keyboard. In contrast, the Arturia filter self-oscillates down to 21Hz, and the amplitude of this signal is greater at low frequencies than in the mid-range! It doesn't, therefore, emulate the Minimoog's filter.

I also noticed zipper noise. If you set the Emphasis to maximum and use the cutoff frequency control to sweep the frequency of the resulting signal, you obtain obvious zippering. At first I was not concerned by this because, if you hold the Shift key while adjusting many of the knobs you obtain finer movement, which should eliminate the problem. Indeed, when you hold Shift and turn the knob slowly, matters are much improved. But move the mouse rapidly and the zippering reappears. This means that the user interface is being polled too slowly by the underlying algorithm, and/or that Arturia have not interpolated between values to provide a smoother response.

Nevertheless, I have some positive things to say about the filter. Firstly, if we ignore whether it's supposed to sound like that of the Minimoog, and evaluate it in purely sonic terms, it's rather good, with a rounded and warm character that many users will like. Secondly, unlike many digital filters, it requires no input from the oscillators or noise generator to initiate self-oscillation, so you can play it as an oscillator if you wish.

Unfortunately, Minimoog V does not do this. So, to measure its envelope response, I fed some white noise into the mixer, opened the filter fully, and set the VCA contour generator to produce as instantaneous an envelope as possible. The resultant click lasted about eight milliseconds. A similar measurement from my Minimoog reveals a duration of four milliseconds. However, more important than the total duration is the Attack time. On Minimoog V, this is about 1.25ms. On my Minimoog, it is closer to 1.5ms. The slightly superior figure on Minimoog V will ensure that it produces the same snappy attacks as the analogue synth. However, when you play quickly and non-legato (ie. triggering every note) on a Minimoog, the envelopes are reinitialised from the points to which they have dropped during the previous release.

If you play a typical ADSD envelope (as shown right) this means that each successive contour 'climbs upward' if you play quickly, as shown in the second diagram. This is a critical element in the playing characteristic of the Minimoog, but Minimoog V does not emulate this, and its envelopes respond as shown in the third diagram, below. The detrimental effect of this is not subtle; imagine the difference between the loudness and brightness of a sound contoured by filters and amplifiers responding to the curves in the last two graphs shown.

The Modulation Mix knob works as you would expect, and using Osc3 as an LFO to add simple vibrato to Osc1 and Osc2 works as on the Minimoog. In contrast, generating FM sounds by modulating at audio frequencies does not, although the differences could possibly be corrected by changing the amplitude of the modulation signal, or by replacing some of the aged components in my 30 year-old synth. Either way, the Minimoog and Minimoog V sound different, but I'm not too concerned by this.

If you look at the External In on/off switch on Minimoog V, you will see that it has a third position: 'Out'. This allows you to 'virtually' feed Minimoog V 's output back into its own External Input, and is included to replicate the old trick beloved of many Minimoog aficionados whereby, if you feed the Low output back to the External Input and monitor with the High output, you get a slight but very pleasant thickening of the sound. If you feed back the High output and monitor with the Low, the feedback dominates at all but the lowest External Input volumes, and the synth takes off into the nether regions of sonic wobbledom. Minimoog V emulates the latter, and you can create some radical timbres if you set up the oscillators, filter and external input correctly. This sounds different to how it does on my Minimoog, but this point doesn't concern me too much; of the three Minimoogs I've owned, all have all responded to the High feedback loop differently from one another, so you could say that Minimoog V is just another variant.

But the more I introduced filter resonance, the easier it became to distinguish between them. If I then used different waveforms or introduced multiple oscillators, it became simple to identify which synth was which. And when I stopped playing individual notes in isolation, and started to play riffs and melodies, the incorrect envelope generation of Minimoog V made it obvious which was which.

This was when I also started to notice that the timbre would jump slightly after every handful of notes played. Playing rapidly with the Detune knob (ie.drift) set to zero, and while watching the waveform on an oscilloscope, I could see that the relative phases of the oscillators would occasionally jump from one state to another. As expected, there was no change when I held a single note indefinitely, just a quantised jump after a bunch of notes were played. This makes soloing an odd experience, and may be even more disconcerting for players who use their Minimoogs primarily for bass work. I also noticed Minimoog V generating random clicks when I played quickly with instantaneous (ie. Attack = 0, Decay = 0) envelopes. These are not the Attack On and Release Off clicks of the original synth, but digital clicks generated in the software as it fails to handle the rapid envelope settings.

So what if we ignore the pretty picture on the screen, and judge Minimoog V as a synth in its own right? The inaccuracies in the emulation are then irrelevant. Considered like this, what we have is a fat-sounding, interesting, and sonically imposing synthesizer that can sound excellent, albeit with a couple of operational quirks and bugs that need correcting.

As a bonus, you can direct a single controller to as many functions as you wish, making complex timbral changes possible using a single knob, slider, wheel or sequenced MIDI continuous control message. I like this.

Furthermore, if your host application supports automation, the panel controls of Minimoog V will respond to this. However, cable changes for the Glide on/off and Decay on/off pedal inputs (see the box at the end of this article) cannot be automated.

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