Sound Advice – Mid-Side Stereo

Audio used to be the unwanted orphan of the video industry. Merely tolerated, and sometimes completely ignored, it was usually the last thing any videographer worried about. Now with stereo Television, VHS HI-FI and PCM audio, good audio is extremely important. In fact, in many of today’s top Television pieces, audio is the only informative thing happening. Try to imagine a late-breaking news story without audio or narration of any kind. (One of my pet peeves is field audio recorded so badly that subtitles have to be added to make it understandable).

Well done audio can help paint a visual picture as well. John Holliman, of CNN fame, appeared as a guest on a radio- theater show I engineer every month. He noted that on his return from Desert Storm, many people described in glowing terms the pictures sent back those first few days. The only problem was, they didn’t send back any pictures the first three days, only audio. The audience filled in the visual blanks with their own minds. That’s the power of audio.

With that in mind, this month’s column will describe a very useful miking technique that can be used on your next video shoot. One of my videographer buddies just came back from 3 weeks in South Africa where he used it to capture local musicians with great success. He’s leaving in a few weeks for Bolivia, and plans to use it there as well.

Mid-Side Stereo (A New Use for an Old Technique)

This technique is called Mid-Side miking (M-S for short), and while the basic setup has existed for decades, its use in video recording is relatively new. Properly implemented, split- channel Mid-Side miking allows you to record everything in both Mono and Stereo at the same time on two channels of audio. That’s right, both Mono and Stereo at the same time without compromise of either. This is NOT the same as using a pair of mikes summed to mono in post production. Summing stereo gives you extra side audio you may not want, in addition to a phase cancellation effect on sideways moving sound sources. No, split-channel M-S mixing records a Middle-channel (a cardioid or shotgun microphone element) on one audio track, and a Side-channel (a bi-directional or figure-8 microphone element) on the second track.

For those on a budget (and who isn’t), you can convert your current shotgun or cardioid pattern microphone into a Mid-Side mike by attaching a bi-directional (figure-8) mike element to the top of your existing directional microphone with the figure-8 pattern facing side-to-side. I’ve used bi-directional microphones from Countryman Associates with great results. If your field mixer can provide phantom power to run these mikes, you’re in great shape. If not, you can either buy a phantom power supply adaptor, or build one yourself. The basic plans follow.Screen Shot 2016-01-23 at 6.50.55 PM

Raw M-S audio doesn’t sound anything like stereo. It needs to be decoded into true stereo using an M+S/M-S matrix.

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As you can see from the schematic above, the signal from the “Mid” microphone is routed equally to both stereo outputs in-phase, while the signal from the “Side” microphone is phase- inverted in only one of the output channels. You can either buy a matrix decoder, or build a simple adaptor from the included plans that will allow you to simulate the circuit right in your mixing board. In post production, by using a M-S decoder matrix, you decide if you want to use the mono track, a wide stereo spread, or something in between. You can even “zoom” the audio to match the visuals.

The most important aspect of this setup is that there’s no irreversible audio decisions to be made in the field. In conventional stereo, if the room ambiance sounds bad, you’re stuck with it in post production. Why do you think so much time and money goes into fixing audio in post? In M-S miking, if the Mid/Shotgun mike sounds good, then you’ve got usable audio. The side mike can be added-in during post-production to produce extra stereo “width” when appropriate.

For instance, suppose you’re interviewing a person on the street in Puerto Rico, when a group of laughing youngsters playing soccer surround you. With standard shotgun miking, you’ve got a great visual with totally “flat” audio. If you had a “Side” mike hooked up on channel two, you could decode a full-width stereo image in post production that would wrap the viewer in the total audio-visual experience. The same goes for musical ensembles, crickets chirping, or locomotives chugging down a track. In short, any visual that involves either multiple sound sources or movement will sound better if it’s miked in stereo.

A Cheap Mid-Side Decoder Matrix

There’s nothing mysterious about decoding pure M-S audio into usable stereo. While some stereo microphones use Mid-Side elements with a decoder matrix built-in, for maximum flexiablity you really want to record raw Mid-Side audio to be decoded later during editing. If you use one of these mikes, be sure to set the pattern to straight Mid-Side rather than Stereo. Once it’s been decoded to stereo it can be summed back to mono, but widening the stereo image is a little tricky.

The simplest way to decode M-S to stereo in the editing suite is by using a technique I invented back when I didn’t have the budget for a dedicated M-S decoder. You can build one for less than 10 dollars worth of parts. All you need is three XLR input channels on your mixing board and a reverse phase “Y” cable, and you’re in business.

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First, hook up your audio source as shown in figure (5). Assign all three channels to the stereo buss. Pan the side channels to dead center first and adjust the relative gain on each channel for a null. You’ll get complete cancellation (no sound) when you are set properly. Now pan the Side-mike channels hard left and right and bring up the Mid channel to approximately the same gain as the side channels. If you use the Mid channel only, you’ll get the straight-ahead sound of the shotgun mike. By adding in signal from the Side channel you increase the width of the stereo spread.

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Another advantage is Stereo sound that’s been recorded initially with an Mid-Side mike will sum back into mono successfully without that nasty “flanging” sound you’ll get if you’re using spread stereo microphones. I use this technique for recording multiple voices that may end up on either FM or AM radio without compromising the sound for either broadcast. One thing to be careful of during mixdown, if you’re sending a signal to a digital reverb to “wet” the sound, use just the Mid channel or one of the Side channels. Since the two side channels are out of phase with each other, a send from both will tend to cancel itself out.

That’s a wrap.

Try Mid-Side recording on your next video shoot. It’s simple to implement, and won’t mess up your regular audio. Best of all,the results will astound you. Film makers have known for decades how important good sound is to their total production. Mid-Side miking can help videographers gather field sound that will “knock their socks off”.

Copyright Mike Sokol 1995

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