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Modular synthesis rests most naturally in the arms of monophonic sound. On this occasion, I'm not referring to the single-voice sounds of monophonic melodies and basslines, but rather the monaural output of all the sound coming from the one place. Often, when we are focused on crafting sonic textures in modular, we are not too bothered about the stereo field. At a push, we might do a bit of panning if our mixer allows, or perhaps run it all through a stereo reverb. However, over the last few years, stereo signals have begun to infect our racks through increasingly common stereo filters, stereo VCAs, and stereo mixers. This can make our signals more interesting while, at the same time, doubling up on patch cables and complicating our processing.
One side-effect of bringing stereo into our signal chains is that we can start to play with Mid/Side processing. This is something that producers of more regular music and our software-based colleagues have used for some time. Mid/Side is a novel way of treating stereo signals to improve stereo imaging, enhance separation and apply more detailed EQ. It allows us to process our mix's middle (Mid) or mono centre separately from the bits to the left and right (Side). Generally speaking, boosting the Mid will bring more focus and clarity to the listener, whereas boosting the Side will increase the stereo width. There are proper ways to do this and creative ways to explore, and we'll have a look at both.
Key Modules
First, we need the right modules that can do the magic to pull the Mid and Side out for processing. You need an encoder which will combine the left and right signals into the Mid and calculate the difference to give us the Sides, and then a decoder to do the opposite once you've processed those signals. Pretty much every software plugin has a Mid/Side button or option to encode/decode the signals. It's also very common in hardware dynamics processors. In Eurorack modular, it's still pretty rare. However, we do have a few choices.
The most direct Mid/Side solution is the aptly-named LRMSMSLR Mid/Side Encoder + Decoder from WORNG Electronics. WORNG has a lot of interest in stereo in Eurorack. Almost all their modules are stereo, and they have some fascinating spatial modules that let you patch audio into a stereo space very simply. What the LRMSMSLR does is take a stereo signal, pull out the Mid and Side onto separate patch sockets for you to play with and then has return sockets to bring it back together and, finally a L/R stereo output so it can all continue on its way. Let’s call it a Mid/Side loop.
WORNG has a more advanced version of this module, called the MidSide+, which includes some built-in processing. It has a Width control to dial in some serious stereo separation and a high-pass filter to remove bass frequencies from the Sides.
The Minsk 1949 Stereo Image Processor from XAOC Devices is a similar idea. It gives you a send/return for Mid/Side and a Width control ranging from everything mono in the Mid section to pushing it all wide to the Sides. There's a high-pass filter and an extra "Dimension Effect" which will either add some subtle animation to the stereo image or simulate early reflections to produce a faux stereo effect from a mono input – that’s very useful in Eurorack.
Endorphin.es has some Mid/Side processing in the Golden Master multiband processor module. In Mid/Side mode, you can use the Low/Mid/High band knobs to adjust the stereo width for each band. However, there's no send/return loop for external Mid/Side processing.
Probably the most capable Mid/Side module is the Enhance 2 from Joranalogue. It has the send/return loop we need for our own processing, and it has a Width knob for expanding the stereo field. But it also has tilt filters on both the Mid and the Side. A Tilt filter is like a see-saw where, when it tilts one way, it reduces the bass and boosts the treble, and then does the opposite when you tilt it the other way. Being able to EQ the main, focused middle of your mix differently from the sides is one of the main advantages of Mid/Side processing.
Mid/Side-Focused Eurorack Modules
Ok, Now What?
So, you have a module with a Mid/Side loop sitting in your rack and interrupting your stereo mix; what do you do now?
At a basic level, you can get into widening or narrowing the stereo image of your mix. If this is all you do, then that's still counted as a win. Run both the Mid and the Side through a different channel on a multiple VCA module and use the gain to balance between the two. As you increase the Mid relative to the Side, the focus will come back to the middle of the mix where the bass, kick and snare usually reside, making it sound narrower and more in-your-face. Boosting the Sides will expand the focus, widen the stereo field and soften the intensity of those middle sounds. Rather than a VCA, you could use a simple attenuator and do the opposite by reducing Mid to emphasise Side and vice versa.
[Above: some M/S patches focused on the Joranalogue Enhance 2]
It's the sort of thing you can set and forget, but it's also a great way to transition from intensity to softness and back again during a performance or recording. It is also ripe for modulation.
The great thing about VCAs is that you can use an LFO to start pulsing the focus or inject a throb into the stereo width. With simple modulations, you can start generating an unusual sense of space. You can put the two at odds with each other so that you have one sound with different areas of its character being modulated differently.
Next, put the Side through a lowpass filter. You now have a bright center of the mix surrounded by a dulled or tonally shifting stereo field, or the other way around. If you consider a drum kit mix that has a kick/snare in the middle and higher frequency hi-hat and cymbal sounds around the outside, you could be filtering down on the middle while keeping the air of the high frequencies to the extremes of the stereo field.
Maybe try a pair of filters that are being modulated by envelopes. You could set up a couple of gate patterns to pull on the Mid and Side differently, creating polyrhythms and syncopation from the same sound source. Using rich pads on big synths in this way is particularly effective, but it might not be something that's easy to achieve in modular. So instead, consider using it after you've applied stereo effects to a monophonic melody line. Lean into your granular effects like Mutable Instruments Clouds or delays and reverb, which will give you lots of frequency content to play with.
So, without adding effects or using a particularly specialized module, how would you get monophonic oscillators into some of this Mid/Side action? Well, the Mid is the sum of the left and right signals, and the Side is the difference between them—so if you plug the same thing into left and right by multing the output of the oscillator, you'll get nothing from the Side output, and so there would be no stereo effect. The left and right have to be different, even if it's just a little bit.
A simple way to do this is to use a pair of slightly detuned oscillators, giving you that lovely gooey phase-modulated sound. Plug one into the left and the other into the right of your Mid/Side module, so they are essentially hard-panned. The Phase difference is enough so that as we play with the levels of the Mid and Side, we start to experience the phase shifting in stereo, making it perfect for some fun modulation, filtering, and effects processing.
[Above: some M/S examples with WORNG Acronym and MidSide+]
It's interesting to note that the Mid/Side channels are mono, so when you process them, you can use mono effects and have them contribute to the stereofication of your overall sound. Try taking the Side out from our detuned dual oscillator and running it through a chorus or phaser effect. You could pitch-shift the Side to give you some weirdly duophonic melodies or drop it through a reverb to give a strange external space with focused middles.
One side-effect of Mid/Side is the ability to scrub stereo effects from audio signals. If you're using stereo samples in your Eurorack, they may come with reverb or delays, and they tend to be most present and different in the wider stereo field. If you turn down the Mid, you'll often find that you are left with just the effect tails, so if you leave the Mid up and turn down the Sides, your stereo-effected audio will fold down to a much cleaner mono signal.
You can also use the Mid/Side encoding/decoding to perform a couple of unexpected utilities. As the Mid output is just the sum of the Left and Right inputs, you could use it simply as a unity mixer for two audio channels. You could also think of it as a sum and difference module where Mid is the sum and Side is the difference and patch in two different modulators and enjoy the different but related results. When Mid and Side are recombined, they are essentially averaging the inputs, so you could use that to create an average of two CV signals by patching them to the Mid and Side inputs and using the output.
While utilities may be useful, the fun of Mid/Side processing is all in the messing about with the stereo field and finding new ways to affect the two aspects of your single sound source. One of my favorite tricks is to put the Side through a fully wet delay module, which gives you a fabulously mono bassline with echoing stereo reflections. The key, as always, is experimentation.
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