A Practical MS 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.

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