The theory and practice behind this stereo miking technique
By Phil O'Keefe
Stereo mic techniques are great for capturing and creating a natural, or even a hyper-natural sense of width and space. Probably the two most common techniques are the XY coincident arrangement, and a spaced pair. But the lesser-known Mid-Side (“M-S”) mic technique has its advantages too . . . let’s investigate.
A lot of people are hesitant to try M-S recording, maybe because they read about decoders and math formulas and feel overwhelmed. Don’t worry — we’ll keep it simple.
You’ll need two mics: One cardioid, and one bi-directional (“figure 8”). Ideally you want two similar mics, but this isn't essential; experiment with whatever mics you have that meet the polar pattern requirements.
M-S uses the center or “mid” mic in combination with the bi-directional mic to achieve stereo. As the cardioid mic points right at the sound source, it picks up the direct sound, while the off-axis bi-directional mic picks up the room ambience and reflected sound. M-S stereo “sum and difference” is just the center mic + the side mic for one channel, and center mic - the side mic for the second stereo channel, with the center mic being positive polarity and common to both sides. As the left and right sides originate from the same mic, but with the phase inverted, collapsing an M-S recording to mono cancels out the left and right sides from the bi-directional mic, leaving only the positive polarity signal from the center (cardioid) microphone. This significant advantage of M-S recordings insures perfect mono compatibility without any phase issues.
SETTING UP THE MICS
Aim the cardioid mic directly at the sound source. As with normal cardioid mic placement, adjust the “aim” to taste; but if you're a fan of close miking, try moving back a bit further from the source for M-S recordings.
Next, place the figure-8 mic so that the two lobes of the pattern are set 90° relative to the cardioid microphone. M-S is a coincident microphone technique, so you want to get the diaphragms of the two mics as close together as you can. Fig. 1 shows a Soundelux (also known as "Bock Audio") E250 (bottom) and ELUX 251 (top) set up as a M-S pair. The cardioid E250 is pointed at the sound source (in this case, the camera), while the pattern selector on the ELUX is set to bi-directional; it’s picking up to the left and right, and its side null point points directly at the sound source/camera.
Fig. 1: A Soundelux E250 (bottom) and ELUX 251 (top) set up as a M-S pair
SETTING UP THE BOARD AND RECORDING
At your DAW, simply route each mic to its own preamp, and assign the cardioid mic to a single DAW track. You can either record the bi-directional mic to two identical, separate tracks of its own and invert the polarity of one of the two tracks later, or record the bi-directional mic to only one track and use a decoder plug-in, or clone the single bi-directional mic's track later and invert the clone track’s polarity — your choice. I generally record the bi-directional mic to two tracks simultaneously in Pro Tools, labeled “SIDE+” and “SIDE-,” and insert a Trim plug-in on the “SIDE-” track to invert the phase. Fig. 2 shows a basic M-S track arrangement in Pro Tools.
Now group the two “SIDE” tracks so that any changes you make to the volume level of one will apply to the other track simultaneously, and pan the tracks hard left and right. As you raise the level of the side mic tracks, the stereo width will increase; lowering them decreases it. Being able to adjust the amount of stereo information in the recording after the fact is one of the big advantages of M-S recordings.
Fig. 2: A basic Mid-Side track and panning arrangement in Pro Tools. Note the use of the Trim plugin to reverse the polarity (red arrow) on the "Side -" cloned track
FREE DECODER RING INSIDE!
So why would you need a decoder or hardware matrix box? The downside of simply cloning the “side” mic track and inverting the polarity is that you can’t hear the final results as you position the mics — you have to record them, then flip the polarity and start playback. By encoding the sum and difference data from the two mics and recording the result, you can hear how the stereo field will sound before you track. PAiA has a hardware M-S matrix box schematic available on their website for you solder jockeys: http://www.paia.com/ProdArticles/msdecwork.htm. For other DAW users, Voxengo’s free MSED M-S decoder plug-in (Mac AU/VST, Win VST) works very well; download it from http://www.voxengo.com/downloads/.
M-S might not be used as frequently as some other stereo techniques, but the perfect mono compatibility and ability to adjust the stereo width at mixdown make it handy for broadcast production, live recordings of small ensembles, individual instruments, and small groups of background vocalists on multitrack music sessions. Give it a try!
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.