A Practical Mid-Side Decoder Circuit

by Jules Ryckebusch

The schematic diagram of the MS Decoder may look complicated but is actually quite simple. Both the Mid and Side signals are initially buffered by unity gain inverting buffers formed around IC1b and IC2 b. 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 phase shift through the unit, one more stage of phase 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 directly feeding a recording device.

The Side signal has a little different path. After initial buffering, it is fed to the right summing amplifier via one section of dual potentiometer R16. It is 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 is summed into the left channel via IC3a. One section of IC1 is not used. Both of its inputs are tied to ground to keep any thing strange from happening.

R17, R18, and C1-4 are for power supply filtering. The circuit will run on +/-9 to +/-18 VDC.

OK, so what are R19 and R20 doing? That is an interesting question. These are 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 op-amp summing sections are virtual grounds. This means that signals entering the op-amp see a load equal to the input resistor. All of these are 10K resistors. The Mid potentiometer feeds two of these 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 ”sees” 5K also. Here is 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 the characteristics of the potentiometer. This in essence, makes them respond more logarithmically than linearly, which is the way we hear anyway which is a good thing!

Alternatives:

Well, you can always break out the patch cords if your console supports phase switches. Be careful, some only have the phase switch on the channel microphone pre-amp, not the channel itself. Simply patch out of the preamp on the first channel and into the line input on the next channel. Depending on the patch bay, you may need to use a half 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 that you have the signal levels matched. Now pan one hard right and the other hard left. Most multi-track audio programs allow you to do this too. I use Studio Vision Pro and Pro Tools but I know most others have similar features. 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, phase invert the signal and pan it to the opposite channel as the original. Voila! You are done. There are usually many ways to skin a cat.

So why build or use an outboard MS Decoder? The real benefit from having a dedicated MS Decoder is that you can listen to the effect in real time while you position the microphones and make adjustments before recording. I have in the past, recorded the Mid and Side signals directly only to find out they really didn’t work together while decoding during playback and mix down. We were recording a Leslie and fortunately after taking my lumps, I was able to re-record it. I had to buy a round of beers over that one.

Use:

Just patch the Project r MS decoder between your microphone pre-amps and your mixing board. You can also run the signal straight into a DAT or you’re A/D converters if recording into your computer. The key to good results is the ability to monitor the stereo signal while recording. Then you can hear exactly how the final MS recording will sound. This allows adjustments of the relative levels of mid/side and microphone positions. Normally the Mid level will be fully up and the Side level almost all the way up. Depending on the amount of ambient information, you may want to back off a bit. In a relatively dead space, you may want to do the opposite and back off on the Mid signal a bit.

Now here is something that is really different: Faux MS miking. So you don’t have Neumann U87. In fact you don’t have a figure 8 microphone at all. Well, fake it. Close mike your source with one microphone and send that to the Mid input. Then take a second microphone and place that back form your sound source to capture room ambience and send that to the Side input. Viola! Fake MS miking. It isn’t the real thing but it sounds good. It is completely different than just panning the second microphone to a different position than the first. To hear what this sounds like just switch the Figure 8 microphone to cardioid or omni. Another interesting thing to do is to separate the two microphones used. Normally the two microphones are as close together as possible. I have recorded in a blues studio in Florida that uses a figure 8 mic as the room mike and mixes that in the same way as MS miking with the exception that there isn’t even a Mid mic. This works great as a drum overhead too. Experimentation is the key. With that Leslie I mentioned earlier, we had the mid microphone 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. Happy Soldering. Even if you don’t build the Project r MS Decoder, hopefully you picked up a couple new tips for your audio arsenal.