Quadraphonic on Eurorack

If you want to get into quadraphonic sound on a eurorack modular synthesizer, here are a few modules you may want to use.

And you can see this video as a form of introduction, or listen to the Spatial Audio/Dolby Atmos playlist while you read on...

Quadraphonic sound output

My first step on quadraphonic was to use 2 stereo mixer, one for the front speakers and one for the back speakers. It is perfectly fine if you just want to place the sound in the space and not making it move. However let's look how to create a quadraphonic sound output.

Quadraphonic Mixer

Currently there is only one quadraphonic mixer:
Shakmat Aeolus Mixer

This is a quadraphonic unity mixer. It has 3 quadraphonic inputs and one quadraphonic output. It has also 2 stereo inputs. 2 outputs can be used for a 5.1 systems: the sum would go to a LFE while the center would go with a center speaker.

Outputs to Front of House (FoH) mixer

I link the Shakmat Aeolus Mixer module to a pair of intellijel stereo output with TRS balanced outputs with Intellijel headphones that can be interlinked. I use the 1U modules but the normal modules are also available.

I need a pair of each, what I have then, is 4 TRS outputs that got to the Front of the House Mixer and 2 stereo headphones outputs that go to a Zoom recorder. I can record my performance and send the output to the room speakers.

The TRS balanced outputs are very handy as they will cancel any noise that could be picked up by the cables.

Quadraphonic Modules

Shakmat Aeolus Seeds

The Aeolus Seeds allows you to place on audio input into the quadraphonic space. It works either in polar (angle and distance) or cartesian (x and y) mode. It has 2 CV inputs "time" and "depth", that can be used to control the location of the sound, but what makes this module interesting is the inclusion of a Low Frequency Oscillator (LFO) that will make the sound moves based on the mode selected. The LFO can be synced with your clock and there is time division or multiplication. The presence of the LFO and its integration with the module ensures that there is no audio bleed when the sound is moving abruptly (a problem on many mixers with CV panning when using a square signal to control the panning). If you want a module that is the close Eurorack equivalent of one of the quadraphonic voice of the Buchla 227e system interface, that would be it.

Intellijel Planar 2

The Intellijel Planar 2 can be used to pan one source into the quadraphonic space. It has a joystick, to place the sound in the space and a record and play buttons to allow you to record your movement and replay it.

One of the output can be use to send a CV to change the sound depending on the distance to the center the joystick is. For instance to control the amount of reverb of the input audio. This simulates distance in a sound.

I use this module during soundcheck to locate and balance the outputs. I input a constant sound and with the joystick ensure the sound comes out the correct speaker. I also verify that the sound coming out of each speaker is at the same volume. This saves me a great time of troubleshooting.

u-he CVilization

The u-he CVilization is a versatile module. In its quadraphonic mode it allows 4 audio inputs that will be mixed on the 4 audio outputs. Each audio input can be controlled in these different manners:

• Using CVA and CVB to locate the sound
• Making the sound rotate clockwise or counter clockwise at various speeds
• Use an enveloppe follower to place the sound on any of the 4 outputs

This last mode allows you, when used with a transient sound, to ping the speakers randomly.
The volume for each audio input can be controlled.

New System Instruments Quad Mid Side

The Quad Mid Side is an interesting utility to process a quadraphonic signal. It is the equivalent of a stereo mid side but in the quad sphere. As a matter of fact it can be used with a stereo signal.

The Quad Mid side takes the 4 input of a quad signal and convert them into Mid (The common signal between all the inputs), L-R (the left right signal), F-B (the front back signal), X (the cross over signal). This signal can then be entered back to produce the original quad signal. The idea here is to process the signal before putting it back, for instance using a reverb or a filter or a compressor on the mid signal or one of the "side" signals.

Kilpatrick Audio Sextuple Quadraphonic Spatial Director

The Kilpatrick Audio Sextuple Quadraphonic Spatial Director is a six voice quadraphonic mixer where each voice position can be controlled by a joystick or by CV panning. This gives plenty flexibility but may be you can only dynamically control manually 2 voices at most.

Koma Elektronik Poltergeist (Discontinued)

The Koma Elektronik Poltergeist is a large and deep module. It can control 4 audio inputs in a polar mode. You can either use the big knob to place the sound or a CV input to pan. If you use a sine or triangle CV, the panning will go back and forth. To get the sound go in circle you need to use a sawtooth signal and tune it correctly to get the rotation motion. It is also recommended to have a slight glide when the sawtooth drop as this sharp change may create some audio bleed in the audio output.

The module has also a stereo input which is convenient to provide other audio sources using a common mixer.

Mutable Instruments Frames (Discontinued)

The Mutable Instruments Frames allows you to program the location of one audio source in the quadraphonic space. This module allows you to do some placements that are more complex than a cartesian or polar mode. You have control on each of the individual 4 VCAs.

While this module is discontinued there are alternatives from different manufacturers