Oberheim Matrix-6

FlowerP · Last edited by FlowerP on Thu Jul 03, 2008 6:16 am; edited 1 time in total

Oberheim Matrix-6

The Matrix-6 (M6) is a 6-voice polyphonic synthesizer using hybrid digital-analogue technology. Its specifications and history are readily available on many Internet sites and will not be repeated in detail here.

The basic audio signal path of M6 is: Osc (*2) -> Mixer -> VCF -> VCA.

Looking at the hardware, it is more accurate to describe the structure as: DCO (*2) -> Waveshaper (*2) -> Mixer -> VCF -> VCA (*2).

Except for the DCOs, the audio signal path is entirely analogue. It is implemented using the CEM 3396 Microprocessor Controllable Dual Waveform Converter / Processor chip, that contains waveshapers, mixer, VCF and dual VCAs for a single synth voice. Please read the CEM 3396 data sheet for more information. (Alternate link to data sheet)

The DCOs consist of 8254 Programmable Interval Timer chips, each containing three independent 16-bit counters capable of free-running operation, i.e. acting as square wave oscillators. By programming the 8254, the microprocessor controls the oscillator frequency, hence the term DCO. In M6 the counters are clocked from three free-running analogue high frequency oscillators (HFOs). The HFOs are calibrated during the M6 tuning operation. Over a longer period of time the HFO frequencies drift slightly. Since the clock signals for DCO1 and DCO2 always are derived from different HFOs, the DCOs within a voice are never frequency locked to each other. However, several voices share the same HFO, so between these voices there is no drift at all.

The waveshaper generates a sawtooth synchronized to the positive edge of the DCO square wave. A comparator generates a variable width pulse/square wave from the sawtooth wave. The sawtooth is also changed into a sloped wave that is variable between sawtooth and triangle (and even beyond triangle, but that isn’t used in M6). Sawtooth/triangle and pulse/square wave are simultaneously available from both Osc1 and Osc2.

Oscillator sync is available in three modes. Mode 3 resets DCO1 and waveshaper 1 sawtooth on the negative edge of DCO2 square wave. This results in ‘hard sync’ of Osc1 to Osc2. In mode 1 and 2 either DCO1 or waveshaper 1 is reset, resulting in timbres not commonly available in other synths.

The mixer is simply a cross-fader for the amplitude balance between Osc1 and Osc2.

The VCF is a standard 24 dB/octave lowpass filter that self-oscillates at maximum resonance. Osc1 saw-tri wave is fed through a VCA to the filter frequency control voltage summing circuit, thus implementing audio-rate filter fm. Note that this signal is independent of Osc1 waveform selection, pulse width and mix balance. Osc1 saw/tri waveshape control does, however, affect the filter fm signal.

VCA1 and VCA2 are connected in series. According to the CEM 3396 data sheet, VCA1 has linear gain control, whereas VCA2 has exponential (“audio taper”) gain control.

The voltage controlled functions of CEM 3396 are: saw/tri shape *2, pulse width *2, osc balance, VCF frequency, VCF resonance, VCF fm, VCA1 gain and VCA2 gain. Oscillator sync mode 3 is a hybrid of analogue and digital control. All other functions available in M6 are digital.

The software generated modulation signals per voice are Env1-3, LFO1-2 and Ramp1-2. There is also a global Vibrato LFO. Received MIDI messages or locally generated keyboard/controller data (Note Number, Note Velocity, Note Release Velocity, Channel Pressure, Pitch Bend, Control Change) are also available as modulation signals. The LFOs and MIDI Note Number have optional lag processors for temporal signal smoothing (mainly portamento). A Tracking Generator is the only general purpose modulation processor. All modulation signals are available in the modulation matrix, where 10 modulation routes can be freely defined (source, destination, amount). Many modulation routes are predefined and thus don’t use the mod matrix. Modulation amounts can be positive or negative.

The envelope generators are basically standard DADSR (Delay-Attack-Decay-Sustain-Release) type. Minimum attack time is around 6-7 ms. At very short attack time settings the attack slope has a logarithmic shape. At longer attack time settings the slope is almost linear.

When envelope amplitude modulation by velocity is active the attack phase has a somewhat peculiar shape. Instead of a smooth attack slope, it is divided into 2 or 3 segments. First there is a fast attack segment ~4 ms. This is followed by a hold period ~8 ms. The hold level depends on the attack time setting. For attack time 0-2 (=shortest) the hold level is equal to the maximum envelope level. For longer attack times the hold level is increasingly lower. After the hold period follows the remaining attack period as determined by the attack time setting.

M6 (and its close relative Matrix-1000) has a large number of ready-made patches available. From those and from a few I made myself, I have selected around 40 favourites that I want to keep and be able to play in the future.

Nord Modular G2 patches for Matrix-6 emulation (M6G2 and Ob6G2)

Although being a great synth in general, the M6 has a few shortcomings, such as having only 6 voices (unless expanded with another M6) and under certain conditions showing audible artefacts due to limited modulation update frequency. I wanted to emulate M6 on NMG2 to avoid these limitations. It soon became obvious that a single patch could not do the job optimally. In order to emulate M6, a patch load well above 50 % was required, resulting in only 8 voices on an expanded G2. Therefore I created a reduced, basic version of the full emulation patch aiming for patch load <=50 % while still retaining as many vital elements as possible. Oscillator sync, filter fm and tracking generator are omitted in the basic patch. Both patches implement an oscillator (*2) -> mixer -> filter -> amplifier structure with lots of hardwired modulations.

M6 has selectable key-to-voice assignment modes: rotate and reassign (and some others that I’ve never used). NMG2 only supports rotate mode. Thus it is architecturally impossible (or at least impractical) to emulate reassign mode on NMG2. This is unfortunate, since reassign is the natural assignment mode when creating sounds that imitate musical instruments. When you repeatedly strike the same piano key or pluck the same guitar string, you expect the previous sound to be replaced by the sound from the next strike/pluck instead of each strike/pluck adding yet another sound to the one already playing. (Actually, I’m a bit disappointed that reassign isn’t the standard assignment mode in all synths!)

A random number generated at key-down adds a slight frequency variation to the oscillators and the filter to get an ‘analogue feel’.

In early patch versions I had a sawtooth-to-triangle wave converter that gave correct phase relationship between the saw/tri and pulse/square waves. Since very few of my M6 favourite sounds actually used anything else than saw and/or pulse waves, and since the converter caused audible artefacts when used with osc sync, the converter was finally dropped.

Obtaining simultaneous sawtooth and pulse waves from NMG2 to faithfully emulate M6 presents a problem. The simplest approach would be to use sawtooth and pulse oscillators both synced to keyboard gate. The disadvantage of this approach is that all voices would start with 0 degree phase shift between the oscillators and any beating from osc detune would sound exactly the same every time the same note is played. Another approach would be to have the sawtooth oscillator free running and the pulse oscillator constantly synced to the saw osc. This would lead to undesirable aliasing artefacts from the pulse osc when playing high notes. In M6G2/Ob6G2 the sawtooth oscillator is free running and the pulse oscillator is single-shot synced to the first positive direction zero-crossing of the saw osc after keyboard gate has gone positive. This ensures correct phase relationship between saw and pulse waves from a single emulated M6 oscillator. Unfortunately, without the saw-tri-converter, this does not apply when triangle wave is selected instead of sawtooth.

The modulation routes are less general than in M6. LFO1 is hardwired to osc frequency. LFO2 is hardwired to osc frequency, pulse width, mix balance, filter frequency and output amplitude. The ramps only control LFO amplitude. The amplitude envelope only controls volume. The filter envelope is hardwired to filter frequency, resonance and fm. The modulation envelope is hardwired to osc frequency, pulse width, mix balance, filter frequency, resonance and fm. The tracking generator input is limited to those signals available as morphs, e.g. keyboard. The tracking generator output is hardwired to osc frequency and filter frequency.

The filter is a standard DIY modification of the NMG2 Filter Classic allowing resonance to be modulated. The modification changes filter tuning slightly, so the frequency shown in the filter module is not quite the self-resonating frequency. Filter fm does not sound exactly as in M6, especially when filter resonance is increased. The VCF in M6 has a small leak of unfiltered sound, and this is emulated in M6G2/Ob6G2.

Patch development was an iterative process consisting trying to recreate favourite M6 sounds on NMG2, noting how the patch could be changed to facilitate emulation more efficiently, creating a revised emulation patch and then restarting by porting/recreating all sounds from previous patch versions. All in all the emulation patch went through 18 versions before the present one!

Sound recreation on NMG2 was done by dissecting the M6 patches on my Matrix-6 Rack. An oscilloscope was used for visual comparison in addition to audible comparison. With the filter fully open, organ type amplitude envelope, all modulations disabled and Osc2 disabled, I could make Osc1 on NMG2 sound as on M6. Relevant modulations were added one at a time. The same procedure was used for Osc2. Then osc balance, filter settings and filter envelope were studied in the same way. Finally the amplitude envelope was recreated on NMG2.

Velocity sensitivity on M6 is often more complex than a linear response. Due to its simple but ingenious envelope amplitude controls, various forms of curved velocity response are easily achieved. By applying velocity morphing on both the envelope modulation depth control and the linear envelope levels in NMG2, non-linear response curves can be simulated. Depending on the desired response curve, positive and negative morphs and linear/logarithmic controls can be used. This kind of intricate dual morphing isn’t something I would spontaneously do if creating a sound from scratch, but when emulating the non-linear behaviour of M6 velocity response it certainly is helpful.

Knob assignments for most modulation routes are a bit clumsy. Ideally a single knob would select both positive and negative modulation amounts. Unfortunately the patch load cost of this is too high, so instead most mod routes are controlled by Mixer 2-1B. One knob selects the mod amount (‘Gain’) and another knob/switch selects noninverted/inverted signal. Since each Mixer 2-1B has 2 inputs, it is hard to label the 4 assigned knobs/switches in a clear way.

The FX area contains a simple delay/reverb section.

The basic emulation patch is called M6G2. You get 7 voices (8 if FX area is deleted) on an unexpanded NMG2 and 15 on an expanded. The full emulation patch is called Ob6G2 (Obese?) and offers half the number of voices (4 and 8 respectively) compared to M6G2.

Here is a short recording of some M6G2/Ob6G2 sounds. Sorry for the sloppy playing. Embarassed

OberheimNMG2_600.gif

Description:

A graphic paraphrase of the Oberheim Matrix-6 logotype.

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M6G2_layout.jpg

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The basic emulation patch layout.

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Ob6G2_layout.jpg

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The full emulation patch layout. Please note that this still emulates only a subset of Matrix-6.

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M6G2-POLYPHON.pch2

Description:

A Matrix-6 factory preset.