Signal routing


A powerful feature of a modular synthesizer is that the signal flow through a set of modules can be rerouted by using switches as an alternative for repatching patchcords or rearranging a pin-matrix. A switch can be a module by itself and be patched between other modules to create alternative routings controlled by the switch module. Clever use of switches avoids having to repatch the patch cables that connect module inputs and outputs. A choice of different selections can be made by using rotary switches. There are two types of rotary switches; one type with multiple inputs and a single output, and the other type with a single input and multiple outputs. Rotary switches can also have ‘multiple decks’, meaning that two or more similar switches are mechanically connected to allow e.g. the switching of stereo or multi-channel signals.

Some analog modules use rotary switches for selecting module options. An example is a waveform switch on an oscillator module. Often oscillators provide several different waveforms, and either several mixing knobs or instead a single waveform rotary switch could have been implemented by the synthesizer designer to route one of the oscillator waveform signals to the module output. In this example the rotary switch is a cheaper alternative to using several mixing knobs. Rotary switches built into a module can also be used to do things like switching the pitch range of an oscillator up or down by one or more octaves.


A very special type of device is the matrix switchboard or pin-matrix. This is basically a two-dimensional ‘multiple input / multiple output’ switchboard where any input can be connected to any output by either a toggle switch or a pin that must be plugged into the matrix. On an analog modular synthesizer equipped with a matrix all output to input connections can be made by using pins instead of patch cables. Matrices give a clear overview of the signal routing, it is much easier to see which output is connected to which input by just looking at the pins, instead of having to look at the noodle of cables hanging out of the front of a modular system using patchcords. In the old analog days pin matrixes used to be quite expensive and also prone to crosstalk, so they were not commonly used. On a digital system matrices can be easily programmed in code or prepatched by combining a bunch of switch and mixer modules. In the experience of the author matrix synthesizers are definitely the best balance between ease of use and flexibility.

Sources and destinations

When a prepatched synth uses rotary switches to route modulation signals, there are two systems that can be used, a system of destinations or a system of sources. The difference between the two systems is that in the destinations system a single source can be routed to one of several destinations, and that in the sources system each destination can select one of the available modulation sources. In the destinations system the rotary switch is positioned at the modulation generator module, the module would have a switch that would say; "Where do you want the modulation signal to go to". With the sources system the rotary switch is at the module to be modulated, the module would have a rotary switch saying; "Where do you want the modulation signal to come from". The system with the destinations is the cheapest to implement, but it limits a modulation source to be used for only one single possible destination. So, when a low frequency oscillator is used to add a bit of vibrato to the oscillator it can’t be used anymore to also sweep the filter, as the signal can go to only one destination. With the sources system both the oscillator and the filter can select the same low frequency oscillator as a modulation source. So, the advantage of the sources system over the destinations system is that several modules can share the same modulation source, which the destinations system does not allow. When a modular system has separate switching modules available, these will most commonly be used in a sources system, using a multiple input to one output switch to select a source for modulation or to add an effect to.

On the G2 there is the choice to use a sources or a destinations system. Instead of switches, mixer knobs with mute buttons can be used to quickly turn a modulation on or off. It is also possible to patch the equivalent of a matrix pin system, such as was used on the vintage EMS VCS3 synthesizer. By using mixer knobs with mute buttons to build a matrix the ultimate in flexibility in signal routing is achieved. A destinations system is the simplest system and cheapest to implement, but also the most limited. A matrix pin system is the most flexible, as in theory literally anything can be modulated by any available modulation source with this matrix pin system. It is also the most expensive to implement. The sources system does well in many cases and is often the best balance between flexibility and the use of computational resources. In practice many patches can use a mix of the sources system and some of the ‘add along’ mixer chains that will be explained later.

The G2 offers an abundance of different types of switches that together provide for a lot of possibilities. There are switches that have multiple inputs and a single output and switches that have one single input and multiple outputs. These two types are commonly named selectors and distributors. Both types are available as manual switches, where nameable pushbuttons select and display the source or the destination in the frontpanel displays. But there are also controllable switches that can be set into any position by a control signal. These controllable switches have no pushbuttons, but instead have a control input that defines the current position on the switch. This means that it is the level of the control signal on the control input that defines which source or destination will be connected to the output or input of the switch. Controllable switches are commonly named multiplexers or demultiplexers and the G2 has at present five of these modules.

Chaining switches into ‘multiple-deck’ switches

The nice thing about the manual switches is that all have a control output that produces a level signal with a value that denotes the position of the switch. When this control output is routed to the control input of a multiplexer it will make the multiplexer act as a slave switch, conveniently following the setting of the manual switch. The slave switch will now act as a second ‘deck’ of a mechanical multi-deck rotary switch. This allows for making stereo signal switches or complex multi-channel switches. The control outputs of the switches increase in steps of four units. The manual eight input switch will produce values of 0, 4, 8, 12, 16, 20, 24 and 28 units on its control output for the eight positions it can be in. The multiplexer modules will use these values to switch to another position. E.g. when the signal on the control input of an eight channel multiplexer is below 4 units it will be in the first position, when the control signal is 4 units or up to but not including 8 units, it will be in the second position. Exactly at 8 units it will switch to the third position, until it receives a value of 12 units, etc. At 28 units and above, the switch will rest in the eighth position. So, the control input does not need to receive an exact number, but it uses numbers within well-defined ranges.

Exceptions to the rule

The G2 ‘crossfading eight channel multiplexer module’ differs from the other controllable switch modules, as it uses steps of eight units instead of four units in its control range to select the mnext switch position. The reason is that the signal of a modulation signal generator module, which must be set to a ‘unipolar’ (=positive values only) signal output range, can be used to easily step through the whole ‘crossfading’ range of this module. Note that the first four multiplexer modules on the G2 are primarily meant to be used as slaves for the manual switches, but this special ‘crossfading multiplexer’ is specifically designed to be controlled directly from a smoothly varying modulation source signal. The eight input cross fading multiplexer can be used for a variety of special effects. Imagine that the eight outputs of an eight-tap echo delay unit are connected to the eight inputs of the multiplexer and the delay receives audio from a beat box connected to an audio input of the G2. When the delay time follows the beat of the beat box the delay will hold e.g. the last half bar or full bar of the beat box pattern. Connecting a triangle low frequency oscillator signal will dynamically switch from tap to tap. As the output signal of each tap has a different time delay the output of the multiplexer will be a signal where the audio contents of the tap delay will be warbled in time. When instead of a low frequency oscillator a sequencer module is used, the contents of the delay line can be warbled in all sorts of wacky patterns. You can let your imagination run wild on how much musical fun this ‘time warbling’ can be. It is one of the possible techniques that make the G2 unique amongst other synthesizers.

Multiplexers and sequencers

The controllable eight input and eight output multiplexer switches are very related to sequencer modules. By controlling them with an upward sloping sawtooth modulation signal the switch positions are sequenced from left to right, just like on a sequencer. A down sloping sawtooth will select the positions in reversed order. By using a triangle waveform the positions are selected back and forth. And by using a sequencer to control the position the multiplexers can be stepped in any pattern. Other possibilities are to combine two modulation signals of different rate, so the interference pattern that results makes the multiplexers step in very complex patterns back and forth. Also an envelope signal can be used to step through the positions. It is even possible to add the output of the switches to the signal that controls the position of the switch, which creates the equivalent of a ‘cellular automaton’.

A/B compare switching to turn a module function on or off

The two input switch is often used in more complex FX patches to bypass a FX modules signal chain. To do so, the first input is connected to the input of the FX chain, while the second input of the switch is connected to the output of the FX chain. On the output of the switch module there is now either the clean, unprocessed signal that is present on the input of the FX chain, or the processed output signal of the FX chain. This also allows for a chain of mutable FX modules, where each module can be conveniently switched in and out of the FX chain by using this A/B switch on every single FX module in the chain. This can also be done with the bypass buttons on the FX modules themselves, but using a switch with a nameable button allows for a clearer interfacing with the G2 frontpanel displays. And by using a two input multiplexer module (the value switches), the bypassing can be controlled with e.g. a gate signal or a clock signal, so an effect can be rhythmically turned on and off.

The two input multiplexer is a lot like a crossfader module, though it can only toggle between the two inputs and cannot fade smoothly like the crossfader can. This means that a crossfader can also be used to bypass an effect. Unlike the multiplexer switch it can also smoothly fade from dry to wet. For many effects, like echo’s, this is a nice feature. In fact such a dry/wet control is already built into e.g. the reverb module. But the ‘bare bones’ G2 echo delay line modules have no dry/wet control, nor a bypass button, and here one can easily make a bypass or dry/wet control with either a two input switch or the crossfader.