Analogue Systems RS370 & RS375

Polyphonic Harmonic Generator & Expander

Published in SOS June 2005
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Reviews : Synthesizer

Having built their reputation on knob-heavy modular synths, British manufacturer Analogue Systems surprise everyone by bringing out a menu-driven additive synthesis module! But don't worry — the optional expander is covered with things to tweak and turn...

Paul Nagle

AS RS 1 Header.s
Photos: Mark Ewing
For this review, Analogue Systems supplied us with a small modular system containing a selection of modules, including the RS370 and RS375 under review (on the top row of the review system). The other modules, on the bottom row, were: (from left to right) the RS210 Fixed Filter bank, the RS360 Vocal/Phase Filter, the RS400 Phase Shifter, the RS120 Comb Filter, the RS310 Reverb/Chorus, and the RS390 Echo.
For this review, Analogue Systems supplied us with a small modular system containing a selection of modules, including the RS370 and RS375 under review (on the top row of the review system). The other modules, on the bottom row, were: (from left to right) the RS210 Fixed Filter bank, the RS360 Vocal/Phase Filter, the RS400 Phase Shifter, the RS120 Comb Filter, the RS310 Reverb/Chorus, and the RS390 Echo.

One of the greatest strengths of an analogue modular synthesizer is its scalability — it can grow not only with your budget but also according to personal taste. With over 40 modules currently available, nobody could accuse Analogue Systems of denying their users a wealth of expansion options. There's something for every synthesist, whether it's typical analogue building blocks or more exotic add-ons.

In this review, I'll be looking at two new modules that slot firmly into the 'exotic' category. They are the RS370 Polyphonic Harmonic Generator and the RS375 Harmonic Generator Expander. Together they form a six-note polyphonic DSP-based synth, a polyphonic MIDI-to-CV interface, or a monophonic synthesizer whose individual harmonics can be modulated in real time.

Socket To Me

The RS370 is 36HP wide (as with most modular synth modules, Analogue Systems modules are measured in Horizontal Pitch units) and features a single control knob, 25 neatly-arranged 3.5mm sockets, MIDI In and Thru (of which more later) plus a small backlit screen which is navigated using Edit and Cancel controls. The screen is the focus of all RS370 operations, and it soon became apparent that, unlike most SOS modular synth reviews, writing this one would involve a lot of menu-gazing.

Of the mini-jack sockets, six provide individual outputs for each of the synth's voices, although you can opt to mix them internally and route the audio to all outputs simultaneously. CV and trigger points are present, giving individual access to the pitch and triggering of each voice. There is a single control knob (Ctrl 1) which can be freely assigned to perform a selection of roles, and three CV inputs (labelled Ctrl 2 to Ctrl 4) that can open up a wealth of external control if you attach suitable CV sequencers, envelopes, joysticks and so on.

Some of the sockets are named rather vaguely. For example, the three control inputs are labelled 'In' but the outputs (Ctrl 1 to Ctrl 4) aren't given corresponding 'Out' labels. To add to the confusion, the CV and Trigger sockets may be either inputs or outputs depending on the RS370's selected mode.


The RS370 offers four different modes of operation. They are: 'polyphonic MIDI', 'polyphonic analogue', 'real-time MIDI' and 'real-time analogue'. The two real-time modes transform the RS370 into a monophonic sound source but by way of payback, you gain real-time control (via the RS375 module) of the first 16 harmonics — something unavailable in polyphonic mode. We'll look at real-time operation when we discuss the RS375 later in this review.

In polyphonic MIDI mode, the RS370 operates as a polyphonic synthesizer with up to six voices. As it uses additive synthesis techniques, there's a lot of raw sound potential even before you connect up an external filter or six. At the same time, the RS370 serves as a MIDI/CV converter, its CV and trigger sockets becoming outputs. In polyphonic analogue mode, these sockets switch to input duties, governing the pitch and triggering of all six individual voices. In this mode, the RS370 works as an oscillator bank, with common waveforms for each.

The arpeggiator included on the RS370 has the customary up, down, alternate and random modes, and may be sync'ed to incoming MIDI Clock with the same set of clock divisions as the LFOs. A latch function can be enabled via an incoming MIDI controller message or by a voltage at one of the CV inputs (you could even allocate the Ctrl In 1 knob to the role).
On The Menu

At the very top level, the RS370 features eight menu headings, as follows: Copy From Memory, Synthesizer, LFO, Arpeggiator, Memories, MIDI/CV conversion, MIDI Options, and Special Options. When presented with a list like this, pushing the Edit knob serves as an Enter key to access sub-menus. When you reach a value that requires editing, pushing the knob again selects it. Turning the same knob makes the adjustment, and once you are satisfied, a final push confirms the updated value. To annul an edit or to back out of a menu, you push the Cancel button. This method of navigation is fine for relatively simple menu-hopping, but when employed extensively, as here, I'd say the controls have been pruned below the minimum for pleasurable use.

The first of the top-level menus, Copy From Memory, is used to load a previously stored patch. A number of these are present to get you going, including organs, polyphonic synths, leads and so on. Many serve as helpful illustrations of the complex modulation routings and MIDI control assignments available. There are just 46 memory locations, selected manually or via MIDI program changes.

AS RS 2 Screen Close-up.s
With its lone Edit knob and menu-driven user interface, the RS370 is a bit of a departure for Analogue Systems products, but it works well, and the optional RS375 expander (see page 86) is available if you miss the hands-on, real-time control.
With its lone Edit knob and menu-driven user interface, the RS370 is a bit of a departure for Analogue Systems products, but it works well, and the optional RS375 expander (see page 86) is available if you miss the hands-on, real-time control.

Next in the list is the Synthesizer menu. This contains the bulk of the voice-tweaking sub-menus, including voice control, envelope settings, the mapping of control inputs and MIDI control messages, the Expander module's function (if it's fitted), the current Mode and any overall transposition value. Drill down into voice control, and you discover further menus for selecting oscillator waveform, harmonic content, tuning, output mixing, plus 'vintage drift' amount and speed. I mention all these to highlight the level of navigation required, but fortunately, many parameters can be programmed for external control. As an extra timesaver, superfluous menu items are removed automatically according to context. For example, you only see the options for pulse width when you select a square waveform, and you don't see the menu to set harmonic levels unless the 'waveform=synthesized' option is taken.

The RS370 is a novel concept: a software polysynth designed for close interaction with a hardware modular. Fortunately, much is familiar. Its oscillator waveforms include four synthesizer stalwarts: sine, triangle, sawtooth and variable square. A fifth waveform, 'synthesized' offers additive synthesis, and this is probably its most exciting aspect. Select this option and you can scroll through up to 32 harmonics and set their amplitudes in the range -127 to +127; negative amounts invert the phase. When generating waveforms in this way, the display shows the results graphically. It's rather wonderful to see the waveform changing as you adjust each harmonic level — and educational too. If you don't want to program all 32 levels whenever you make a patch, six rather conventional harmonic templates are provided, including a saw/square mixture, a low-pass sawtooth wave and a square wave. Any of these may be used as starting points, after which you perform further adjustments as required.

Each voice consists of four oscillators. Although these all produce the same waveform, they can be detuned with respect to one another for some dense, warm textures. Each oscillator's level and pitch settings can be modulated by either MIDI controllers or incoming voltages. To further muddy things up, Vintage Drift acts on the oscillators' detune values; increasing its amount increases 'thickness' dramatically (very much like the effect of lager...).

I've already mentioned that voices can be mixed internally, so that all six are present at every Voice output. By routing voices individually, you could (with sufficient external modules) pan, process or filter all of them in ways rarely possible on even the most sophisticated polyphonic synths. I should add a note of caution here: your modular could grow dangerously and expensively large if you attempt to explore the full potential of this particular module!

In order to be as self-contained as possible, the RS370 includes software envelopes and LFOs. As the envelopes have 10 stages, each appearing on a separate screen, I must admit I often wished for a simple ADSR. The parameters available include attack time and level, three separate decay slopes, and release time and level. There's also an option to compress or expand the entire envelope, or compress it dynamically at higher pitches. Fortunately, there are a couple of ways to improve access to the envelopes, one of which requires the Expander module, the RS375. The other uses the modulation matrix.

AS RS 3 RS370.s
The RS370.
The RS370.

Generally speaking, modulation is well catered for, with the RS370's four analogue voltage sources routable to 30 destinations. Each source can be mapped to just one of these and should you select a destination that's already in use, it is instantly zapped in favour of the new one (there's no Undo facility, so it's a good idea to plan your modulation connections in advance...!). Destinations include overall pitch, oscillator levels and detune, all the envelope stages, the speed of the two LFOs, oscillator pulse width and 'harmonic wave morphing', plus arpeggiator and portamento settings. Harmonic wave morphing is worth a quick mention — if modulated through its full range, it sweeps the waveform from a sine wave to a square wave via a sawtooth on the way.

In addition to the analogue modulation inputs, up to 16 MIDI controllers (plus aftertouch and pitch-bend) can be added to the equation, with the same list of destinations available. So by cunning allocation of 10 of these, you could gain complete control over the envelopes and thus reduce the time spent in menus.

Of the RS370's analogue modulation sources, one of these is in the form of a control knob (Ctrl In 1) whilst the remaining three are control inputs (Ctrl Ins 2 to 4). For maximum flexibility the voltage range may be set individually for each CV input.

Still on the topic of modulation, the two LFOs feature the expected selection of waveforms: sine, triangle, saw, square and random. They may be free-running or can sync to MIDI Clock with an impressive set of divisions — from every sixth of a beat right up to one step every five beats. Usefully, you can control LFO amplitude via a mod wheel or aftertouch, but the available routings are pretty stingy in comparison with other modulation connections. There are just four possible LFO destinations: pulse width, harmonic wave morph, fine pitch and fine pitch 2 (so each LFO can control pitch at a different rate and depth).

Once you've created a patch, programmed extensive modulations and tweaked harmonics, envelopes and LFOs, the next logical step is to secure your hard work. With no MIDI Out socket, it seems, at first glance, that there's no way to do this. However, within the Memories menu is the 'SysEx Dump Memory' option and here, a push of the Edit knob squirts the patch out in SysEx form via the MIDI Thru port, revealing this to be a 'soft Thru'. Patches are transmitted individually and when they are restored, they enter the edit buffer, so you can place them (manually) in the memory location of your choice.

The RS375 Harmonic Generator Expander
The RS375 (shown below) is a 48HP wide expander module featuring 16 knobs, each with an associated CV input. This module could be considered a luxury addition if you mainly intend to use the RS370 polyphonically. It offers a quick and direct way to tweak the onboard envelopes, but then you can achieve the same thing using MIDI. It also throws in an extra four control inputs (labelled Ctrl In 5 to Ctrl In 8) and four control outputs (Ctrl 5 to Ctrl 8), supplementing those on the RS370. These inputs are also available on the smaller, cheaper RS376 module.
However, if real-time harmonic synthesis is your bag, the RS375 is essential. In its real-time modes (MIDI or analogue), the RS370 becomes a monophonic synthesizer, and the knobs of the expander can be assigned to set the levels of the first 16 harmonics. On top of this, connections made at the CV control points (say, sequencers or LFOs) will transform the generated waveform dynamically. Varying harmonic content with knobs (rather than one at a time via the menu) was great fun, and I stumbled across some particularly dramatic changes when I connected the internal envelopes to several of the harmonic CV inputs.
Considered together as a monophonic synthesizer, the RS370 and RS375 don't come cheap. However, they offer a range of highly malleable sound sources that could transform the personality of your modular synth.

As a MIDI/CV converter, the RS370 is worthy of note, because polyphony is rarely something incorporated into such devices. It receives on a single MIDI channel and can drive up to six voices of either 1V-per-octave synthesizers or those using the Hertz-per-Volt convention (eg. certain Korg and Yamaha monosynths). Even when you use the 'polyphonic analogue' mode of operation and trigger each voice independently via CV and Gate, incoming MIDI controllers are still available as modulation sources.

Trigger outputs may be chosen from a selection including conventional gate signals, S-Trigs (the 'short trigger' used by Moog synths) and a trigger of user-specified length. Finally, 'mixed triggers' are provided — something you don't encounter every day. In order to understand these, cast your minds back to certain limited polysynths of old. Korg made quite a few, such as the the Poly 800 and the Delta, where the synth lacked separate filters/envelopes for each voice. Using a single filter saved money, and of course made for limitations, but it also gave rise to some unique playing styles. You could retrigger the filter for all held notes by playing a new note, for example. The RS370's mixed triggers recreate this effect by sending every trigger or gate to all six trigger outputs simultaneously.

AS RS 4 RS375.s
The RS375 Expander module.
The RS375 Expander module.

Another welcome addition, the Zipper Noise Filter, is a low-pass filter (often referred to as a lag processor or slew generator) whose job is to smooth the seven-bit MIDI control values and eliminate the artefacts known as zipper noise. Its settings range from 0 to 127, and progressively higher values reduce the definition and response of incoming controller curves. So you would tweak this value according to need; in most cases, I found a setting slightly below the mid point worked well.

The RS370 has four control outputs (Ctrl 1 to Ctrl 4) and each of these can generate a voltage sourced from the MIDI controller of your choice (or aftertouch, or pitch-bend). In addition, you can choose whether to send voltages proportional to incoming MIDI velocity or from the internal envelopes. Sadly, the output of the two software LFOs is not available; these may only be routed internally.

Whether used polyphonically or monophonically, you need to decide what should happen when you play more notes than your synth can handle. The RS370 offers three choices whose functions are self-explanatory: high-note or low-note priority, or 'drop oldest note'. Although you would generally use the latter option for polyphonic playing, there are interesting results to be had by using high- or low-note priority instead. It might have been handy to include 'drop lowest-velocity note' too — perhaps in a future firmware release? I mention this because, happily, Analogue Systems have provided an operating system that can be upgraded via MIDI. This function appears under the Special Options menu, along with calibration routines, an option to display the voices used, display of incoming MIDI, and display of CV inputs (from the RS370 and the RS375, or the more affordable RS376 Expander if this is fitted).


Incorporating a polyphonic DSP synth into an analogue modular is a bold step. The RS370 is innovative and occasionally mind-boggling. Even when used to create traditional fare such as strings, organs or brass, its modularity means you rarely approach it as you would a conventional polysynth. In conjunction with the RS375 Expander, you can switch to monophonic mode and gain unique real-time control of harmonics. Although representing considerable outlay for just two modules, it's undeniably powerful.

On the other hand, the RS370 is a step away from the immediacy of analogue modulars, and if you wanted to build a complete polysynth, with an analogue filter per voice, you would need to add a whole bunch of duplicate modules to your system. But having said that, the RS370 can take your modular into realms you never thought it could explore. I'll be fascinated to see what users do with it.

Analogue Systems RS370/RS375 £595/£285
Allows you to integrate a DSP polysynth into your analogue modular.
Unique real-time harmonic control via the RS375.
Patch memories and extensive external control.
RS370 serves as a versatile MIDI interface.
These aren't cheap modules, especially if you purchase enough VCFs, mixers, envelopes and LFOs to make a complete six-voice polysynth.
Lots of menu-hopping to set up patches on the RS370 initially.
Only 46 patch locations.
Incorporating a software-based synth into an analogue modular is an excellent idea. Whether used as an oscillator bank, for real-time harmonic changes or as an additive polyphonic synth, the RS370/375 modules offer something genuinely different. True, you have to be prepared to spend some time setting things up, but with the available control routings and ability to store user patches, even this becomes less of a chore after a while.
RS370, £595; RS375, £285; RS376, £130. Prices include VAT.
Analogue Systems +44 (0)1726 850103.
+44 (0)1726 850103.

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