When mixing, I run my reverb and delays effects as aux sends rather than inserts, so I can route multiple sources to them. In a typical project, I'll have three distinct reverb tracks: one (invariably a plate type) for vocals; one for drums (usually a plate, room or booth, depending on the sound I want); and a more general-purpose one, for creating a common space in which all the various sounds can sit together. (I'll also have a few instrument-specific effects inserted directly on the track of the sound I want to process.) These stereo 'effects return' tracks provide another opportunity to play with M‑S muting and processing.
Start with the piano clip from the earlier example, routed to the Mid and Sides tracks as before. Then add a new stereo reverb track, as in the screenshot (I used a Valhalla one for the examples; they make great reverbs for a very reasonable price), and send a signal to it from the piano source track.
Now let's look at what happens according to where you place MSED in the signal chain. Place MSED, with the Mid channel muted, after the reverb. Press play, and you'll hear the original piano and a reverb effect derived from the whole piano sound. But because MSED follows the reverb, you can only hear the Sides of the output of the reverb, as indicated by the horizontal line on MSED's goniometer (the graph on the right of its GUI). Note that if you fold this mix down to mono, the reverb sound will disappear entirely — which may or may not be a good thing: it can be a useful way of adding 'fairy dust' to a stereo mix without cluttering things up in mono, but it will change the character of your mix on mono playback systems.
Now, try it the other way around: drag MSED before the reverb, again keeping MSED's Mid channel muted. Now you have a reverb that's fed only from the ambient Sides component of the piano, but as the stereo reverb spreads its output across the sound stage this will give you reverb in both mono and stereo. But in both cases, the dry sound remains nice and focused. It's worth listening to see which approach works best on any given song. And note that if you really want to go down the rabbit hole, you can go to town with multiple stages of Sides-fed stereo delay and reverb in this way...
These next two clips demonstrate the difference.
amb-piano-valhalla-sides: The reverb effect when the reverb is fed with the whole stereo piano signal, but the Mid of the reverb return is muted.
amb-piano-sides-valhalla: The reverb effect fed from the Sides-only piano signal, returning a mono-compatible reverb effect.
The next technique solves a common problem, but I need to explain the nature of the problem in some detail before we get underway. Something that seems to trouble a lot of inexperienced producers is management of their mixes' bottom end. It's not exclusive to the bass and kick drum — any sound that has elements residing in the low-frequency range needs careful consideration — but I'll focus on the kick and bass here, since it makes it easy to demonstrate the problem and the solution.
The interaction between the bass and the kick in modern EDM requires careful handling. Low frequencies have lots of energy — a higher amplitude for the same perceived level as higher frequencies. So when two or more sources with lots of low end overlap, they eat into your mix headroom and can also trigger dynamics processors (eg. a bus compressor or limiter) in an ugly way. Also worth noting is that where two sounds contain the same frequencies at the same time, the louder one will mask the quieter one, and despite the apparently high level according to your stereo bus' meters, this will leave things sounding wimpy and congested. Furthermore, where parts have only some notes/beats playing together and others in isolation, the sounds will lack consistency through the song, as the masking effect kicks in and out.
The trick is to remove from one sound frequencies that clash with those in another. But while that prevents unwanted triggering of compressors and leaves more headroom, simply EQ'ing the whole signal all of the time can compromise one sound or the other. A popular technique to address this is to 'sidechain' a compressor to duck (attenuate) one sound every time the other plays — for example, sending the kick channel to the external sidechain of a compressor placed on the bass channel, so that every time the kick rises above the compressor's threshold, gain reduction is applied to the bass. Each sound when playing on its own sounds as it normally would, but when they play together, the bass 'ducks' out of the way of the kick.
But while simple ducking can be just the ticket, sometimes there are unwanted side–effects — often, for example, it's really noticeable if you duck a bass part that has significant stereo spread (eg. chorused bass parts, or sounds treated with delays), or which is providing an important contribution to the mix higher up the frequency spectrum.
Thankfully, used in a similar way, dynamic EQ allows us to duck only the problem frequencies. And, taking it a step further, if we deploy a side-chained dynamic EQ only on the Mid component of the bass, we can often achieve the same thing even more transparently.
In this example, a drum beat and bass part combine to form a fairly busy pattern, and we'll use the kick to duck some clashing frequencies on the bass' Mid channel. These are the dry source sounds.
You can do this with any side-chainable dynamic EQ using the MSED matrix we used for the previous techniques, but it's simpler if you use a dynamic EQ which allows you to process the Mid and Sides channels separately. FabFilter Pro-Q 3 is one such tool, and I've used that here. Having inserted Pro-Q 3 on the bass track, activate its external side-chain (different DAWs handle side-chaining in different ways, so if in doubt check the manual, or Google!). Then created an EQ node, set the node's mode to Dynamic EQ, and set it to operate on the Mid channel only (you can access all these functions in Pro-Q 3 by right-clicking on the node). It's not usually difficult to find the clashing frequencies by ear, but Pro-Q 3 has a natty little feature whereby it highlights thee clashing frequencies for you — the stronger the overlap between the two signals, the redder the display. (This 'External Spectrum Visualiser' is accessed through the Analyser tab at the bottom.)
So far, so simple... but part of the joy of having a dynamic EQ is that you can treat different parts of the signal with different nodes. So, just for kicks, also try creating a second node: again, set it to dynamic, and this time set it to operate on the Sides only, and target the 'twang' frequencies in the bassline. Pro‑Q 3 has a band solo function which greatly helps in isolating the exact frequencies you want to process.
Now to tell each node how to react to the side-chain signal. We want the low-frequency Mid-only node to cut, and to do that you drag the Gain knob's outer ring anticlockwise. (Don't turn the inner part of the Gain knob or you'll apply conventional 'static' EQ.) Then set the Mid node's Q (bandwidth) so that the node ducks the shared frequencies but not more than that.
We'll use the second (Sides) node not to duck that twang but to accentuate it, just enough so it isn't obscured by the kick. All you need do differently is drag this node's dynamic EQ control clockwise. The combination of using ducking to attenuate the low frequencies and expansion to pronounce the mid-range frequencies in this way results in a controlled yet nicely dynamic response.
In the previous example, we were only concerned with managing the bass and kick in a mix, and they're generally fairly percussive sounds. This next one sheds light on how you might deploy a similar M‑S dynamic EQ tactic to manage deeper evolving sounds, whose frequencies overlap massively and together occupy a huge chunk of the spectrum. The sounds I've chosen are a cello line and a huge 'soundscape cluster' pad sound.
Insert FabFilter Pro‑Q 3 on the soundscape cluster track — it's this sound that needs to be ducked to allow the cello plucks to peep through, and also expanded so the soundscape lifts when the cello plucks hit the higher registers. Create two EQ nodes — one set to Mid, which we'll use to duck a frequency range where there's overlap, and the other for the Sides, which will expand a higher frequency range. Again, make both nodes dynamic, and this time select a pretty wide bandwidth. In the bass/kick example we were only concerned with fairly narrow ranges because both parts were short in duration, but with evolving and pattern-based sounds wider bandwidths tend to work better because they yield smoother responses. These clips demonstrate the effect.
Listen to the processed version, and notice how little congestion there is, and how the cello plucks burst through nicely thanks to the frequency conscious ducking. Once the cello plucks start to die away, the cluster takes over and a wash of lovely evolving frequencies hits you, thanks to the expansion process on the Sides — and by boosting the Sides you also get a subtle and engaging shift in the stereo width, which serves to enhance the pad sound even more.
There are countless ways to use M‑S in sound design, mixing and mastering — for instance, I've not even touched on the more conventional techniques such as M‑S mastering EQ and compression, and it would take more space than a Tolstoy novel to cover it all in detail. But I'll leave you with a few more ideas that you can look into if you want to take things further:
- In mixing and mastering, engineers often turn to M‑S equalisation to target only sounds that are panned wide (eg. a wall of electric guitars or a stereo synth pad), whilst leaving important elements like kick, snare and vocals in the middle untouched — or vice versa.
- M‑S EQ or compression can be really helpful for managing a stereo vocal group, as it allows you to process the lead vocal (usually largely in the Mid) or backing vocals (often more in the Sides) independently.
- When mixing drums, try compressing only the Mid — it usually allows you to compress the kick and snare without screwing up the cymbal or other percussion sounds that lie either side.
- A high-pass filter on the Sides combined with a low-pass filter on the Mid can help to give more focus to sounds panned centrally while tightening up the bottom end.
- Try using the Mid signal to key (side-chain) expansion on the Sides of a mix, to gently lift the air band every time the Mid rises.
- For a really different effect, use your DAW's automation to modulate a reverb between the Mid and Sides. It can create amazing motion effects. Or try auto–panning (or pan automation), place MSED post-pan, and mute the Mid. As with other techniques described here, everything in the middle remains clear and focused, but you still get that lovely panning effect.
The potential uses for M‑S are limited only by your imagination, and I implore you to get busy exploring the techniques that incorporate M‑S magic!
Eddie Bazil has been mixing and sound designing for well-known artists and synth/sampler companies since the 1980s. He is the brains behind audio-production video and eBook tutorial company Samplecraze. If this article has inspired you, check out the Samplecraze website: www.samplecraze.com
You'll sometimes see Mid and Sides called different things. For example, the Mid and Sides are often described as the 'Sum' and 'Difference', respectively, a description that's rooted in the maths used to encode L‑R channels to M‑S. Occasionally, including on the Fairchild 670 vari-mu compressor, you'll also encounter the archaic term 'Lat-Vert', which stems from vinyl mastering.
You'll find audio clips for each of the step-by-step exercises in a ZIP file of hi–res WAV audio examples in the righthand Media sidebar or use the link below. Feel free to download the source files and work through the exercises in your own DAW, or just to listen to the clips of the results.
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