MS Compression

Here's an interesting curiosity. As you know, when compressing a stereo signal, a two-channel compressor must
have its sidechains linked, otherwise heavy compression in one channel will cause an image shift in the stereo
sound stage. Both channels must, at all times, be compressed equally. Of course, this assumes that you are
handling stereo as left and right channels - let's call this LR stereo. Not as popular but certainly very useful is
mid-side or MS stereo, where the M channel is the mono sum of the whole sound stage and the S channel
represents the difference between left and right. MS is a useful microphone technique and is sometimes used at
other points in the signal chain for modifying the width of the stereo image. (It's a funny thing that proponents of
MS often forget that you can do that to LR stereo signals with the pan controls.) But what about compressing a
signal in MS format? Is it possible? Does it have anything new to offer?
Yes, it is possible to compress MS signals without converting them to LR. Just pass the M signal through one
channel of the compressor and the S signal through the other. Once again, you will need to link the sidechains or
funny things will happen, but it will all work perfectly. Some might say that it works better than compressing LR
stereo since, even when sidechains are linked, it is not guaranteed that analog compressors will handle both
channels absolutely equally and some image shift may persist. But, if you compress in MS domain then any
disparity between the channels will result not in an image shift, but in a variation in the width of the stereo image,
which is arguably less obtrusive. But why not take this a stage further and do something really wacky like
compressing the S signal only. What happens now? If you compress the S signal only, then anything panned
center is unaffected and compression only affects signals panned left or right, or signals that are out of phase.
Loud signals in these modes will cause a momentary reduction in level of the S channel resulting in a narrowing of
image width. I can't say that I recognize any useful function for this myself, but in the hands of more creative

Compression Vs. Clipping

While I'm on the subject of increasing apparent loudness, I don't know whether it is as widely appreciated as it
should be that compression is only half the answer. Compression is a long-term type of gain reduction, working at
the very least over periods of tens of milliseconds. If you try to achieve very fast acting compression by using
very short attack and release times, you may well end up with distortion of low frequencies where the compressor
actually changes the shape of the waveform. There comes a point in maximizing apparent loudness where the
compressor has given all it has got to give. Clipping, on the other hand, works on a very short timescale.
Transistorized circuitry reacts within microseconds to any level that is too great for the power supply to cope
with and cuts it short, creating harsh harmonics, but at the same time extra loudness. The soft clipping of valve
and valve-emulating designs rounds rather than clips the peaks but, once again, operates on a short time scale.
The problem with soft clipping, if used alone, is that it only works on high-level signals. Clip-worthy peaks only
occur in quantity in high-level signals and low-level signals, although they may indeed have the occasional
clippable peak, are largely unaffected. The answer is to use a compressor and a soft clipper in series.
The compressor evens out the general level of the signal but, since it works over a comparatively long time scale,
the peaks are not clipped but simply brought to a more uniform level. The clipper then has more material to work
on. A useful alternative is to use a series-parallel configuration as shown in Figure 4. Here, the compressor
smooths out the levels, the valve emulation device soft clips the peaks, and the result of that whole process is
added to the uncompressed signal. The result is controllable enhancement over a wide range of levels. If you
want to go further then you might add an equalizer after the compressor so that you can choose the frequency
range that will be affected to add just the right hint of distortion without going over the top, particularly in the

Compression By Stealth

One of the best-known uses of compression is to increase the apparent loudness of a mix, or an individual voice or
instrument for that matter. Compression, as you know, works by reducing the high signal levels, bringing them
closer to the low-level passages, and then applying make-up gain. Thus the low-level signals are brought up and
the whole thing sounds louder. This is fine in theory, the trouble is that the effect of compressing the high-level
signals is very audible, necessitating great care in the set-up of the compressor and judicious compromise
between getting enough compression and not spoiling the overall sound. Ray Dolby told us this when, in the early
A-type noise reduction system, he left high levels completely alone and modified the gain only of signals below
-40dB. What we need is a compressor that only operates on low-level signals. Is there such a thing? Yes there is,
and it's in your rack already. You just have to use it in a different way. Since in this situation the object is to
bring up the lower levels of the track, what we need is a way of making the quiet sections louder without
affecting the loud sections.
The answer is to mix the uncompressed signal with a compressed version of the same (Figure 3). At levels below
the compressor's threshold the two signals will combine to produce a 6dB increase in level. Above the threshold
the compressed signal will be progressively reduced and add hardly any additional level to the mix. The result is a
form of compression where you can get more dynamic range reduction with fewer audible side-effects. I'm not
going so far as to say that it is always the best way, but it's certainly worth a try. Maybe some enterprising
company will bring out a gadget to do just this, in a convenient rack mounting package. By the way, if you try
this with a digital compressor you will get a lesson in the delay involved in digital processing. You will get comb
filtering and it will sound dreadful.

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