Chapter 15. Emulating Classic Synths

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15.1 General Guidelines

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I think the most important thing really is just getting the routings right, if the signal flows through all the right things and you have all the right modulations available, you're going to be about as close as you can get and you ought to have a usable and fun synth in any case. Mixers are easily the most useful module, since they can gather all the modulation sources and set their levels prior to modulating pitch or whatever.

A few things that might not be totally obvious. For example, the Octave Cat is a 2 VCO synth with 1 LFO and 2 envelope generators (ADSR and AD). However, to emulate it, I had to build a 7 oscillator synth with 2 LFO's and 2 ADSRs, just so all the suitable signals were always present in the same way. But a synth like this is one heck of an instrument, especially if you break from some of the limitations of an accurate emulation (tune the oscs separately, add modulations etc).

It is useful to use master and slave oscs, and you should note a couple things that might not be obvious. A master will propagate its pitch modulations to its slaves, but not its FM modulations, so you have to FM modulate the slaves the same way with similar signal routings as the master. Also, when a master osc is sync'd it doesn't sync the slaves, so you have to do that routing too, though not all slaves have a sync input.

Some of the old monosynths would have made superior polysynths, which makes them fun to emulate. You can do "what-if" exercises, such as "what-if" my ARP Odyssey had a vocal tract filter.

Editor Conventions
I named all the interesting modules in this synth. If there are no symbols preceding the name, either I am indifferent about editing the parameters in this module in the editor, or the values are important enough that I exported their control to an external controller. If I put a '+' symbol before the name, that indicates a module that has values that I want to encourage myself to tweak in the editor, like significant or interesting modulations or routings that I didn't export. In the case of the switches a '+' usually indicates that I intend to try different routings, but the mix levels in the switch may be specifically set and I generally won't mess with those. If a module name is preceded by a '!', that indicates a structural element or logic setup that is carefully set (a necessary routing or a test) that I generally don't intend to mess with. In other words, these tweaking these modules is more likely to break the patch than to provide interesting modulations. I don't think I have any such discouraged modules in this Moctave Cat. In this document if I refer to the Cat I mean the Octave Cat, not the Moctave Cat.

Transient Generator section
The ADSR amounts are controlled by the sliders on my QS8 keyboard (my master controller), which are set up as controllers 12, 13, 91, and 93 respectively. The Attack/Release (AR) sliders are not exported. They can be set in the "+AR" module, which looks suspiciously like an ADSR envelope. The Cat's "ADSR repeat" function is not exported. It can be set in the editor between off and gated in the "+ADSR repeat" module. The additional non-gated auto-repeat functionality of the Cat can be achieved by decoupling the envelopes' gates from the voice gate and holding it high. The Cat's sample and hold switch is not exported, but it can be set in the editor with the "+S&H source" module. It samples either VCO1 or noise, and is triggered by the LFO.

Lower Left section
The duties of the pitch slider are relegated to the pitch controller. The Cat's octave shift button is not implemented as I have a big keyboard. The Cat's glide (portamento) slider is not implemented in the Moctave Cat; it should be there but I haven't figured out the right way to do it yet. The LFO frequency is exported on knob 1. The Cat's LFO supplies a square wave (this is also used to trigger the ADSR repeat if enabled) and a triangle. In the Moctave, I substituted a sine wave for the triangle, since it gives a better sounding pulse width modulation and smoother vibrato and otherwise doesn't matter on my Cat patches. I think the original Cat designers would have used a sine wave here if they could have generated it inexpensively.

VCO1 section
Coarse tuning is on knob 4, fine tuning is not exported but can done in the editor by tuning the VCO1 master osc and the 3 slaves. The slaves are not phased accurately to the real Cat which means you can't really do that goofy Cat note-sequencing thing accurately, but I've always thought that that's more interesting than useful. As long as the phases are wrong, I slightly detuned the slaves because I like that sound better. The Cat's VCO1 keyboard control switch isn't fully implemented. If you bump the Moctave polyphony down to 1 voice, then the Moctave emulates the Cat's mono mode, and if you turn off keyboard tracking in the "VCO1 pulse" module, it's like turning this switch off. I don't thing the Nord can really emulate the Cat's weird poly mode, since that would require a bit of logic processing on the uncooked keyboard status and the Nord really only forwards processed keys. The VCO1 modulation routing switches are not exported, but they can be set in the editor. The first switch selects LFO sine (my default), LFO square, or sample&hold as the modulator and is labeled "+VCO1 mod1". The second switch selects ADSR, the AR envelope, or VCO2 (default) as the modulator, and this one is labeled "+VCO1 mod2". The Cat has a single knob to select the amount of modulation for each of the mod sources, and these knobs are in the "VCO1 modamt" module and exported as knobs 2 and 5. The VCO1 mod amount module also mixes in a bit of weather modulation since my VCO1 is not entirely stable. I did not have enough free modules to accurately model the effects of weather on my Cat though! The Cat allows you to either set the amount of VCO1 pulse width modulation or to dial in the width of the pulse. This functionality is not exported but is found on the "+PulseMod switch" module. Unlike most of my switches, I do need to set the knobs on this module corresponding to the selected switch. Finally, VCO1 supplies 4 waveforms: a suboctave square wave (1 octave down), a sawtooth, a triangle, and a pulse. You set the mix level for each of these waveforms on knobs 3, 6, 9, and 12. These mix levels determine both the sound of VCO1 and the degree of FM modulation of each VCO1 waveform on VCO2 if you have dialed in FM on VCO2. (That's knob 11 and the default selection of switch 3 on the "VCO2 mod2" module for those of you who have read ahead...) Similarly, the levels of the VCO2 waveforms will affect the amount and sound of FM in this VCO1 section if you left the default routing alone and dialed up knob 5. By the way, if the knob assignments aren't obvious (and I don't expect that they should be yet) there's a simple organization to them that I will explain near the end ofthis document.

VCO2 section
VCO2 coarse tuning is exported on knob 7. VCO1 can be synced to VCO2 with the switch in the "+Sync VCO1" module, though this doesn't really sound like the Cat's sync, and I haven't looked into why. On the Cat, you can get a nice percussive sound by using AR as the main envelope and using a quick ADSR envelope with 0 sustain to modulate the synced pitch of VCO1. I haven't tried this yet on the Moctave Cat, though. This section largely mirrors the VCO1 section, so "+VCO2 mod1" selects LFO sine, LFO square, or S&H as a VCO2 modulator, and "+VCO2 mod2" selects ADSR, AR, or VCO1 as a modulator, and the mod levels are set in "VCO2 modamt" and exported as knobs 8 and 11. VCO2 supplies 3 waveforms: suboctave square, square, and saw, and their mix levels are exported as knobs 10, 13, and 16.

Filter section
I use the Nord's filter D for the Cat filter, since this one is closest to my Cat's sound. By the way, my Cat is the original model with discreet components, later Cats like the SRM have completely different filters. The Nord's D filter is the one that comes closest to the Cat's high end sizzle, though its frequency range doesn't go nearly high enough; The Cat filter has a magical sound as it emerges from super-audible frequencies that no digital filter that I have heard can match. Also the D filter has the closest behaviour in that magical hi-Q zone near oscillation, but again a digital filter and midi controller really can't do justice to an analogue filter and a pot. Finally, at hi-Q, the D filter thins the bass frequencies the least and again, this behaviour is closer to the Cat sound than the other Nord filters. There is a bummer to filter D, though; Its modulator input is control rate rather than audio rate, which cruds up the filter if you attempt to FM it, and filter FM is yet another key to the Cat sound. I don't know what to do about this yet. Note that I left an E filter in there as well; If you dial that one in with the "+Filter picker" module you can get a decent Minimoog-like bass if you dial the other parameters in right. The Cat switch for VCO1 audio out is not exported but can be set by turning off knob 1 in the "+VCF Input Mix" module; this turns off its audio without disabling its FM abilities. Filter keyboard tracking is not exported but can be set in the "VCF" module. Like the VCOs, there are two switches in the filter section for mod routings, and they are not exported. "+Filter Mod1" selects between LFO sine(default), LFO square, and S&H to modulate the filter cutoff frequency, and "+Filter Mod2" selects between ADSR, AR, and VCO1 (default) to do the same. On the Cat, you can get some great nasty but still musically useful sounds by FM'ing the filter frequency. A good modification for the Cat would be to use an envelope to just FM the filter frequency on the attack. The filter modulation amounts are set in the "Filter Mods" module, exported as knobs 14 and 17. This module also includes a non-Cat mod routing: the third mixer sets the amount that ADSR modulates the cutoff frequency and this level is assigned to the main modulation controller (my mod wheel). This allows me to use both FM and ADSR on the filter, which I always wanted on the Cat. Finally, filter frequency and resonance (Q) are exported on knobs15 and 18.

VCA section
Still reading? Well we're almost done since the Cat is almost out of knobs, and the Nord is completely out of knobs... The switch for amplitude level is not exported, but it can be set in the "+AEnv choose" module. The selections are ADSR, AR, and bypass, and by default I use the ADSR envelope to determine the output amplitude. On the Cat, you often use AR as the amp envelope when you are using ADSR for effects like a filter sweep or transient pitch weirdness. Bypass mode makes for a great drone. And for the final knob, I don't export the Cat's noise control but you can set it with knob 3 in "+VCF Input Mix". A worthwhile modification to the Cat would be to route the noise through an envelope, but that's not how the Cat does it.

Knob organization
There's a method to the knob organization that makes it reasonably easy to remember, especially if you're used to the real Cat. The first vertical block of 6 knobs is loosely dedicated to VCO1, the second block to VCO2, and the third block to the filter. The middle row is dedicated to modulators, so by default knobs 2 and 5 modulate VCO1 by the LFO and FM respectively, and knobs 8 and 11 do the same thing for VCO2, and knobs 14 and 17 do the same thing for the filter. The 4 knobs in the lower left select the output levels for the 4 VCO1 waveforms, and the upper right 3 knobs do the same thing for the 3 VCO2 waveforms. That leaves us with 3 knobs in the upper left which are LFO speed, VCO1 tuning and VCO2 tuning, and 2 knobs in the lower right which are filter cutoff and resonance, just where they are on the Cat.

Interesting modifications
A lot of the Cat's nasty sound comes from all those rough edges in the waveforms. You can smooth the Cat a good deal by changing the suboscillators to sine waves. This will lose a bunch of high frequencies from the sound; it can sound very good but it won't sound like a Cat. Many of the weirder Cat sounds are had when you do bi-directional FM, where VCO1 modulates VCO2 and vice-versa. You can route a signal for FM into either an oscillators pitch input or its FM input. These inputs have very different ranges and somewhat different responses, so I let you pick whichever sound works better for you; pick the ones you like in each of the two "+FM type"modules.

I sold my original white Odyssey to finance the purchase of bigger synths, and then bought another early black face Odyssey several years later during a sudden attack of gear nostalgia. I've used it from time to time, but maintenance is an issue, as it is with all vintage analog synths. Thus, it seemed like a good idea to replicate the Odyssey in the Nord Modular.

The Odyssey used an ingenious system of patch switches to quickly select a variety of patching configurations. This was a good compromise between versatility and speed, without the hassles of patch cords or the limitations of a completely hard-wired synth. My goal was to replicate the Odyssey as completely and accurately as possible, including the method used to set up a patch. This meant devising a basic patch architecture that included patch switches instead of requiring the user to draw patch cords.

On the Virtual Odyssey, the patch switches are represented by the switches in the 4-to-1 switch modules, and the names for these modules show the available switch selections. For example, the module labeled "NG/RING MOD" selects between the noise generator and the ring modulator as the input to the first knob in the audio mixer.

Thus, by simply selecting switch positions and turning virtual knobs, you can replicate any sound that was possible on the real Arp Odyssey, without drawing any patch cords.

In fact, you can make sounds that were not possible on the real Odyssey, because some parameters on the NM have a greater range than the equivalent parameter in the real Odyssey. For example, the LFO can run slower and the envelope release times can be longer than the same parameters on the real Odyssey.

There are some modules which are used to process the Nord's signals to more closely resemble the signals on the Odyssey. The waveforms in the Odyssey's audio oscillators and LFO were positive-going only, unlike the bipolar waveforms in the Nord Modular. Thus, a collection of Level Shifter/Inverter modules were used to alter the Nord's waveforms. Although an audio oscillator sounds the same whether it is bipolar or unipolar, it makes a difference when you use these waveforms as modulation sources.

Since the Odyssey had more sliders than the Nord has knobs (I think the Odyssey has 32 sliders), I opted not to assign any parameters to the Nord's front panel. I'll leave it up to the user to assign them as needed when setting up a patch.

The real Odyssey had one major feature that I was not able to replicate on the Nord. The Odyssey had a unique "duophonic" keyboard. When you pressed two keys, OSC1 played the low note and OSC2 played the high note. This was useful for producing distortion effects when using the ring modulator, among other things. Note that this is not the same as simply setting the Nord to have two voices. If you get ambitious, please feel free to try to figure out how to achieve the duophonic effect.

One minor difference between the Nord and the Odyssey is the way the filter responds when resonance is set to maximum. On the Nord, you must send some amount of audio into the filter to trigger filter self-oscillation. This was not necessary on the real Odyssey.

The one feature I added, which is not true to the original Odyssey voice architecture, is the additional Vibrato LFO, which is controlled by the mod wheel of a midi controller keyboard (via the red morph group). The Odyssey had really lame performance controls, and I just had to do something about it. I hope the purists are not too offended :-)

The first try-out we did was during a gig and the sound was so terrible, the organization of the party threw us out! After that failure we build the Hallsey Mark 2. (The name Hallsey was chosen 'cause our bassman made MARSHALL on two plates, MARS did fell off the fake amplifier and we added 'sey') The Mark 2 was a real modular synth, only its oscillators wouldn't tune in line... (It had sample&hold, mixers, LFO's and all that stuff) The Mark 2 was stolen during a gig and some days later our leadsinger found the poor thing at the side of a road, 'cause the thieves couldn't make any sense out of it, me think.

Mark 3, of course also from the famous 'Hallsey factory', was, so I believe, one of the first Synths which used AD and DA converters. Two of our gang worked at the university, where such electronic equipment was developed for Philips. (No, they didn't steal them: it was just an university, you know, where the professors also carry uranium 238 in their pockets) I think only the Mark 1 is interesting to emulate here ;-) Played in the lower regions, it was a wonderful beatbox, but we didn't knew then... (Press the low C and hold it a while) Those Shapers were really there. The LFO's in a way too. And the distortion remains!