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
mid-range.
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
mid-range.
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.
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.
Over Compression
No one reads the manual for a compressor, and if you did you wouldn't get any warning about the effects of over
compression. I don't mean this in the sense of too much compression, your ears will tell you that, but in the sense
of setting a lower threshold than you need to get the job done. This will always make the sound worse, with the
sole exception of percussive sounds where it might sometimes be a useful effect.
Let's assume a scenario where an instrument plays occasionally with silences in between. This is where over
compression is most likely to happen. When setting the threshold, many users have an idea of how much gain
reduction they want to hear and see on the meter. The amount of gain reduction is controlled by both the
threshold and ratio controls. Suppose these controls are set so that the desired amount of gain reduction, let's
say 12dB for example, is achieved. This should be fine shouldn't it? Look again at the gain reduction meter. While
the instrument is playing, does it ever go all the way down to zero? If it doesn't, if it only goes down to 3dB, then
you haven't applied 12dB of gain reduction, you only have 9dB of compressive gain reduction. The other 3dB could
have been achieved by simply lowering the fader. This, in itself, isn't necessarily a problem. The problem is that,
when the instrument starts to play, the compressor has to go all the way from zero gain reduction to the full
12dB. The necessity of covering that additional 3dB will audibly distort the initial transient. Try it, and you will
hear it for sure.
This leads to rule number one of gain reduction - at some point in the course of the track while the instrument is
playing, the gain reduction meter must indicate zero, otherwise the minimum reading obtained shows wasted gain
reduction and over compression, leading to the distortion of transients that follow silences
compression. I don't mean this in the sense of too much compression, your ears will tell you that, but in the sense
of setting a lower threshold than you need to get the job done. This will always make the sound worse, with the
sole exception of percussive sounds where it might sometimes be a useful effect.
Let's assume a scenario where an instrument plays occasionally with silences in between. This is where over
compression is most likely to happen. When setting the threshold, many users have an idea of how much gain
reduction they want to hear and see on the meter. The amount of gain reduction is controlled by both the
threshold and ratio controls. Suppose these controls are set so that the desired amount of gain reduction, let's
say 12dB for example, is achieved. This should be fine shouldn't it? Look again at the gain reduction meter. While
the instrument is playing, does it ever go all the way down to zero? If it doesn't, if it only goes down to 3dB, then
you haven't applied 12dB of gain reduction, you only have 9dB of compressive gain reduction. The other 3dB could
have been achieved by simply lowering the fader. This, in itself, isn't necessarily a problem. The problem is that,
when the instrument starts to play, the compressor has to go all the way from zero gain reduction to the full
12dB. The necessity of covering that additional 3dB will audibly distort the initial transient. Try it, and you will
hear it for sure.
This leads to rule number one of gain reduction - at some point in the course of the track while the instrument is
playing, the gain reduction meter must indicate zero, otherwise the minimum reading obtained shows wasted gain
reduction and over compression, leading to the distortion of transients that follow silences
Merciful Release
A long time ago, when I was a fresh-faced student of sound engineering, I went to a trade show (in the days
when you had to beg your way in, if you weren't in the business already) and alighted on the stand of a company
who had a new and wonderful compressor to show off. 'Listen to this,' said the silver-tongued salesman. I listened
as he demonstrated his amazing box. 'That's 30dB of compression. Does it sound compressed to you?' I looked at
the gain reduction meter, I listened, I looked at the gain reduction meter, I listened. Sure enough, the meter was
showing a full 30dB of gain reduction and the music I was listening to sounded as fresh as a live performance. I
knew something about compressors, and I knew that 30dB of gain reduction ought to be the sonic equivalent of
what an apple looks like after it has been through a cider press. It's a good thing I didn't have any money or I
might have bought it on the spot.
With the benefit of experience I know what happened. I am sure that it was a reasonably good compressor, but
not significantly better than any other. What the salesman had done was to turn the release control to maximum.
Release, as you know, is the time it takes for gain reduction to return to zero after the signal has passed below
the compression threshold. In this case, the signal never passed below the threshold long enough for the level to
begin to return to normal, to any significant extent. The result was indeed 30dB of gain reduction, but not 30dB of
compression. You don't need a compressor to get any amount of gain reduction - just lower the fader.
Compression implies a constantly changing amount of gain reduction, and the gain reduction meter must be visibly
dancing up and down. If it is not, you're wasting your time. How fast it dances up and down is up to you but, if
you want value-for-money compression, a short release time will give you a more audible compression effect
(Figure 1). A longer release will lessen the audibility of the compression, but you won't actually get as much real
when you had to beg your way in, if you weren't in the business already) and alighted on the stand of a company
who had a new and wonderful compressor to show off. 'Listen to this,' said the silver-tongued salesman. I listened
as he demonstrated his amazing box. 'That's 30dB of compression. Does it sound compressed to you?' I looked at
the gain reduction meter, I listened, I looked at the gain reduction meter, I listened. Sure enough, the meter was
showing a full 30dB of gain reduction and the music I was listening to sounded as fresh as a live performance. I
knew something about compressors, and I knew that 30dB of gain reduction ought to be the sonic equivalent of
what an apple looks like after it has been through a cider press. It's a good thing I didn't have any money or I
might have bought it on the spot.
With the benefit of experience I know what happened. I am sure that it was a reasonably good compressor, but
not significantly better than any other. What the salesman had done was to turn the release control to maximum.
Release, as you know, is the time it takes for gain reduction to return to zero after the signal has passed below
the compression threshold. In this case, the signal never passed below the threshold long enough for the level to
begin to return to normal, to any significant extent. The result was indeed 30dB of gain reduction, but not 30dB of
compression. You don't need a compressor to get any amount of gain reduction - just lower the fader.
Compression implies a constantly changing amount of gain reduction, and the gain reduction meter must be visibly
dancing up and down. If it is not, you're wasting your time. How fast it dances up and down is up to you but, if
you want value-for-money compression, a short release time will give you a more audible compression effect
(Figure 1). A longer release will lessen the audibility of the compression, but you won't actually get as much real
Compressors Secrets
Every studio has one, every engineer uses one, and every popular music recording - almost - dating back to the
1950s and beyond has benefited from one. Of all the many and varied types of outboard in the processing and
effects racks, the compressor is surely the one that is most often used, and one that repays its cost of ownership
countless times over during its working life. So I don't need to tell you anything about compressors then? Maybe
not - if there does happen to be anything that you don't know already then you can easily find it in textbooks and
magazine articles that are often aimed more at the beginner than the seasoned pro. However, the compressor is a
many-faceted instrument, and there are a number of tips, tricks and techniques that are not commonly covered in
print. Are these the compressor's secrets, known to the few and hidden from the many? Like the Masked
Magician, I intend to reveal these secrets to the world.
1950s and beyond has benefited from one. Of all the many and varied types of outboard in the processing and
effects racks, the compressor is surely the one that is most often used, and one that repays its cost of ownership
countless times over during its working life. So I don't need to tell you anything about compressors then? Maybe
not - if there does happen to be anything that you don't know already then you can easily find it in textbooks and
magazine articles that are often aimed more at the beginner than the seasoned pro. However, the compressor is a
many-faceted instrument, and there are a number of tips, tricks and techniques that are not commonly covered in
print. Are these the compressor's secrets, known to the few and hidden from the many? Like the Masked
Magician, I intend to reveal these secrets to the world.
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