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Building an MS-Decoder
Mid-side miking technique made easy
By Jules Ryckebusch

One of the classic stereo microphone recording methods is the MS or Mid-Side microphone technique. This requires the use of two mics: one to pick up the middle (that can be any pattern but is usually omnidirectional) and one to pick up the ambience from the sides (that has to be a figure-8 mic).

We’ll explain this in more detail shortly, but you send the mid mic to the left and right channel; the side mic also goes to both channels, but one side has to have its polarity inverted—it has to be “decoded.” Most large studio consoles can perform this trick pretty easily; some smaller consoles can do it, but you end up using too much board at best and jumping through hoops to invert the polarity at worst.

So what are the options?

Well, you can break out a bunch of patch cords, perform the processing in your multitrack software program, or…build the Project r MS decoder. This article will show you how to do all three.

Y?

Why would you want to build a stand-alone MS decoder when there are other methods available to achieve the same results? Well, for one thing, the Project r MS Decoder will free up a channel on your mixing board and allow you to record MS recordings straight to a 2-channel recorder rather than requiring three channels. And it lets you hear exactly what you are recording as it is being recorded (as opposed to decoding during mixdown—although that has the advantage of allowing you to adjust the side/ambience relative to the middle later).

This is yet another of those simple but indispensable projects that’s worth an evening of your time soldering. It’s also the perfect addition to that high-end mic preamp you finally got around to buying. Because there is no signal gain associated with the circuit and high performance opamps are used, the audio specs of the Project r MS Decoder are impeccable.

MS in more detail

In the “classic” configuration for MS recording the mid mic is an omni picking up the straight sound source. After the preamp it’s sent to the stereo buss, panned center. That allows it to remain exactly the same if the recording gets played back in mono.

The side mic is a figure-8 microphone positioned so that the main sound source is actually in the microphone’s dead spot (its least receptive position). Since this microphone is capturing mainly room sound or ambience as opposed to the main sound source, obviously the better the room sounds, the better the MS stereo effect will sound.

Even in an almost acoustically dead space there will be sound that propagates to the sides differently than from the front. Close-miked acoustic guitar is a perfect example: the side mic will pick up sound from the strings and finger noise better than what’s coming directly from the sound hole.

Here’s where the audio trickery comes in. The signal from the side mic is amplified then sent to a console channel and panned hard left. At the same time a polarity-reversed side signal is sent to another console channel and panned hard right. This creates a wonderful stereo spread that sounds great over headphones or through speakers. Now here’s the cool part: if the signal is collapsed into mono, the ambience information cancels itself out and we are left with our original signal.

The polarity inversion is achieved with the ‘phase’ switch on the console channel being used. What? Your mixing board doesn’t have one? Time to fire up that soldering iron!

How it works

Check out the schematic diagram of the Project r MS Decoder. It may look complicated, but the project is actually quite simple.

Both the Mid and Side signals are initially buffered by a unity gain inverting buffer formed around IC1b and IC2b. This is necessary for two reasons: First, to ensure enough drive current for the following sections. Second, the final summing sections invert the signal; to achieve zero polarity change through the unit, one more stage of polarity inversion is required.

The Mid signal goes to level-control potentiometer R15. It is then fed equally to the left and right summing amplifiers, which are formed around the two sections of IC3. The use of R15 (and R16) is to allow adjustment of the relative levels of the mid/side levels independent of the mic pre gain setting. This is useful for feeding a recording device directly.

The Side signal has a slightly different path. After initial buffering it is fed to the right summing amplifier via one section of dual potentiometer R16. It’s also fed to a unity gain inverter formed by IC2a and its associated resistors. This inverted signal goes to the other half of the dual ganged potentiometer section.

Then it’s summed into the left channel via IC3a. One section of IC1 is not used; both its inputs are tied to ground to keep anything strange from happening. R17, R18, and C1-4 are for power supply filtering. The circuit will run on ±9–18 VDC; regulated supplies are a good idea too.

So what are R19 and R20 doing? An interesting question. They’re there to load down R16 so that when the levels of the Side signal are adjusted the potentiometer gives the same feel as the Mid level adjustment potentiometer R15.

The opamp summing sections are virtual grounds. This means that signals entering the opamp see a load equal to the input resistor. All these are 10K resistors. The Mid potentiometer feeds two of these resistors so it is presented with a 5K load. To make the load on the Side potentiometer the same, one additional 10K resistor tied to ground is added to each wiper so it also sees 5K.

Here’s the other interesting thing that all of this causes. Because all the potentiometers are linear taper, loading them down with a load much smaller than the resistance of the potentiometer causes a change in its characteristics. This in essence makes it respond more logarithmically than linearly, which is the way we hear anyway, which is a good thing!

Construction

PAiA Electronics is providing a complete parts kit with circuit board, making assembly relatively easy. See the parts list for details and PAiA’s web address. I recommend the use of shielded wire to and from the jacks and potentiometers.

For those of you who really want to roll your own, this project can be built on a Radio Shack project board 278-268. This is one of those projects that can be added to your console or built into a preamp to add this feature. It can also be powered from two 9V batteries for field recording.

PAiA is supplying either NE5532 opamps or National Semi’s LM833, both of which are excellent. Other suitable ones are OP-275s from Analog Devices or my new personal favorite, Burr Brown’s OPA2134 dual audio opamp. Burr Brown was just purchased by Texas Instruments, so hopefully these will remain in production; the OPA2134 is the best dual opamp for audio use currently made. Digi-Key Electronics carries them (www.digikey.com).

Alternative decoding methods

You can always break out the patch cords if your console has ‘phase’ (actually polarity inversion) switches. But be careful—some only have the phase switch on the channel mic preamp, not the channel itself. If so, simply patch out of the preamp on the first channel into the line input on the next channel. Depending on the patch bay, you may need to use a half-normal patch technique, which is beyond the scope of this article.

Before panning hard left and right, center both and adjust the level on the second channel until the side signal is completely cancelled. This means you have the signal levels matched. Now pan one hard right and the other hard left.

Most multitrack audio software allows you to do this too. Take the side signal pre-fader and send it to an effects buss. On the original channel, pan it hard right; on the effect buss, polarity-invert the signal and pan it to the opposite channel as the original.

Using it

Just patch the Project r MS decoder between your microphone preamps and your mixing board. You can also run the signal straight into a DAT, or to A/D converters if you’re recording into a computer.

The key to getting good results is in the ability to monitor the stereo signal while recording so you can adjust the relative levels of the front and sides. Normally the Mid level will be fully up and the Side level almost all the way up. In a relatively dead space you may want to do the opposite: back off on the Mid signal a bit.

Now here’s something really different: faux MS miking. So you don’t have a Neumann U87. In fact you don’t have a figure-8 mic at all. Well, fake it.

Close-mic your source with one mic and send that to the Mid input. Then take a second microphone and place that back from your sound source to capture room ambience and send it to the Side input. Voilà.

Okay, this isn’t the real thing, but it still sounds good. It’s completely different from just panning the second microphone to a different position than the first. To hear what this sounds like with a real setup, just switch the figure-8 microphone to cardioid or omni.

Another interesting thing is to separate the two microphones. Normally the two microphones are as close together as possible. I’ve recorded in a blues studio in Florida that uses a figure-8 mic as the room mic and mixes it in the same way as with MS miking, with the exception that there isn’t even a Mid mic. This works great as a drum overhead too.

Recording a Leslie speaker one time, we had the Mid mic in the bottom of the cabinet where the large speaker and rotating baffle are. The Side mic was placed about a foot from the upper rotating horns and about three feet from the Mid mic. Sounded great.

Experimentation is the key. Happy stereo miking.

Jules Ryckebusch may be contacted via talkback@recordingmag.com

PARTS LIST

Description / Quantity / Designation

100uF/25V Electrolytic Capacitor / 4 / C1,C2,C3,C4

NE5532 or LM833 type Dual Audio OpAmp / 3 / IC1,IC2,IC3

All resistors 5% 1/4W film, values in ohms

10k / 14 / R1, R2, R3, R6, R7, R8, R9, R10, R11, R12, R13, R14, R19, R20

47 / 2 / R4,R5

330 / 2 / R17,R18

Potentiometers

100k linear taper / 1 / R15

100k dual linear taper / 1 / R16 a&b

Misc: circuit board, knobs, hardware, power connector, jacks, wire, panel or case, etc

A complete kit of circuit board, all electronic parts, potentiometers and knobs but less jacks (order #2080K) is available (at the time of writing this) for US$33.75 + $5.00 s&h from:

PAiA Electronics

3200 Teakwood Ln

Edmond, OK 73013

phone 405-340-6300

email info@paia.com

Panels and Power supplies also available

http://paia.com/8020.htm for more information

 




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