How to use mid-side microphone technique

Mid-side demystified

Mid-side mic technique was created (and even patented) back in 1933 by Alan Blumlein. As opposed to using a matched pair of microphones for stereophonic sound, the mid-side technique involves using one microphone for the middle of the stereo image, and another for the sides. You don’t need an identical, matched pair of microphones for mid-side, but it is preferable.

• The mid microphone is placed directly in front of the sound source and is responsible for capturing the center information of the stereo image via a cardioid pickup pattern 
• The side mic is placed 90 degrees to the left or right of the mid microphone and is responsible for capturing left and right sides of the stereo image via its figure-8 pattern.

Using mid-side to capture a stereo image offers a number of advantages over a typical XY stereo pair, including:

• The ability to mix the mid and side information independently from one another rather than committing to a stereo image when tracking. You’ll be able to widen your stereo image by simply bringing up the Side channels in relation to the Mid channel. 
• The ability to process the mid and side information separately as well.
• Mid-Side recordings translate well to mono, as the side information will largely phase-cancel itself, resulting in a clearer reproduction on mono speakers.

How to set up mid-side technique

To configure your mics for mid-side, align them as close to each other as possible with the mid mic (cardioid) pointing at the center of your sound source, with the side mic rotated 90 degrees from that of the mid mic.

You’ll need to route the mic signals to three tracks of your DAW or mixer. Pan the side mic signals hard left and right. Don’t forget that you’ll need to invert the phase on the negative side of the side mic, as the signals from figure-8 mic’s two sides leave the mic phase-inverted from one another. 

That’s it! Now you’ve got independent and flexible control over the width of your recording. Try listening to the mid mic soloed and then the side mics soloed to hear the difference, and then blend to taste—I’d recommend grouping the two side channels in your DAW if it allows for this.

Here's a Pair of UT Twin87s in a mid-side configuration. The bottom mic is set to cardioid. The top mic is rotated 90° and set to figure-8.

Applications of Mid-Side Technique

Mid-side mic technique is ideal for recordings where the environment plays a significant role in the feel of the performance being recorded, as the side mic is largely responsible for acquiring room ambience and reverb.

A great example would be that of a choir performance in a reverberant church. The mid mic captures the core performance and the sides will capture the sound of the room. Backup vocals, and small jazz combos are also suitable, and it’s an excellent choice for recording drums if you don’t have a stocked mic locker or want a more old-school sound that avoids the surgical precision of contemporary drum production. Mid-side technique is also popular among field recordists for capturing ambient sounds of outdoor environments.

Mid-Side Compression and Processing

An oft-overlooked benefit of mid-side recording is mid-side processing. Not only do you now have control over your stereo width, but you can also process the mid and side signals independently of one another. This opens up a world of creative options otherwise unavailable.

Possible applications include:

• High-passing the side signals to keep bass elements focused to the center of the mix.
• Compressing the side signals to create a more aggressive room sound or in-your-face reverb.
• Adding a touch of modulation to the side signals for effect while keeping the center-focused element clean.
• EQing the mid and side signals differently to create a sense of unreal space.
• Using DAW automation to increase or decrease the amount of ambient room sound over time - bring it up for the choruses and down for the verses.
• Any combination of the above.

While mid-side is often referenced as some sort of mysterious or esoteric studio practice of yore, fact is it’s only a little more involved than just pointing an SM57 at a guitar cab and nowhere near as hard to understand as some would have you believe.

Try it out, and once you find you have a new level of control over your stereo image, you may well wonder how you got along without it!

Here's a look at Studio One with some creative processing of a mid-side recording. Note the independent effects applied to the Mid and Side signals, which are then bussed. Note panning and phase status of the Side channels.

Chad adds...

One of the big reasons to do M/S processing in the analog equipment world has to do with component tolerances. When you're doing mix-bus or mastering processing in analog through two-channel equipment, the relative matching of things like analog VCA's (SSL bus comps, etc), T4B opto-modules (LA2-A's, etc), discrete op-amps, vacuum tubes, etc. can create an audible discontinuity between the left and right channels. In worst case examples, the L and R channels can begin to sound like they are playing a different mix of the same song.

It's extremely hard to get these parts to match up in terms of their level, THD, noise, and response time. M/S encoding and decoding before the analog processing chain allows these differences to manifest as barely perceived brief changes in width opposed to dynamic incongruities between the L and R channels. As you might guess, in the digital domain and plug-in world, this is not so much an issue because the channels are all exactly software clones of one another (although many plug-ins still offer M/S modes because it changes the plug-in's behavior.)