Quote from Forum Archives on October 29, 2003, 2:38 pm

Posted by: weeks <weeks@...>

Saw this on Church Sound Check and thought I'd pass it along since there
have been so many questions about compressors lately. I think this guy
did a GREAT job explaining compression. Some might benefit from it, some
might take exception but to get the basic idea across, this is a great
explaination.

>I have a few questions about compression in a live and recording
>settings. Where can I get info on compression so that I can understand
>it better and explain the need for compression. I have a novice level
>of understanding and would like to know more about it.
>

Well, it's time for a tea break so why not throw in this little
thought...

Imagine a tall straight glass with water in it. The more water you pour
into it the more the level rises at a consistent rate. That's a bit like
the audio level rising as a performer sings louder.

Now let's transfer the water into a glass which has a wider diameter
part way up. As you pour the water in the level rises at the original
rate up the straight portion. As it gets past the wider part the rate of
rise slows down. This prevents a lot of water water reaching the top too
quickly. We can think of a compressor in the same way. The audio rises
in the normal way until it reaches a 'threshold', or point of change,
where the rate of increase slows down. This prevents loud sounds
reaching the top (clipping) point too quickly. You can think of
'headroom' as the space between the top of the water and the top of the
glass. Introducing the wider section has kept the level rise under
control and prevented it from overflowing too quickly.

If we change the point on the glass at which the wider section begins
then we can adjust the point at which the 'compression' of the water
starts. If we move it down quite low then the maximum level reached by
the water is reduced noticeably, possibly lower than is necessary. To
compensate for this we can raise the glass up on a little stand. Now, in
audio terms, this is what we refer to as 'make-up gain'. If the water
was our audio then you can see that quieter (lower) moments are raised,
and louder (more volume) moments are lowered. It's a bit like
'squashing' the audio signal.

If we change the diameter of the wider section of the glass then the
rate of water rise is slowed or quickened. The difference in rate
between the normal and wider sections can be subtle or dramatic. This
difference is referred to as the 'ratio'. A ratio of 2:1 would mean that
the wider section was twice as wide, a more dramatic 4:1 ration would be
four times as wide, a 10:1 would probably result in a hardly any
noticeable rise beyond the point of change.

So, we have two things we can do with our compressor:

We could set the threshold quite high, say +10dB, and the ratio quite
high, say 8:1, and therefore slow the rate of rise of very loud moments
quite dramatically. This makes it behave a bit like a limiter, but not
quite as aggressive. It is used to control unexpected audio peaks.

On the other hand, we could set a softer ratio, say 2:1, bring the
threshold down to say -20dB, and compensate for the drop in average
level by turning up the make up gain a few dB. This would have the
effect of bringing up quieter moments and squashing louder moments. This
is known as reducing the 'dynamic range' of the signal, in other words
how soft and loud it goes. This can be most useful when dealing with a
singer or preacher who is soft one moment and shouts the next. Another
example, more dramatic control can squash the dynamic range of a bass
guitar to give it the popular solid 'compressed' sound.

So, our compressor is a bit like a glass with variable top section
diameter (ratio), variable point of diameter change (threshold),
variable table stand (make-up gain) and a few other controls such as
'knee' (a slope on the larger diameter to make the effect more gentle),
'attack' (how fast the diameter changes as the level rises) and
'release' (how fast the diameter changes when the level drops).

As for the need for compression... To each his own. Some use it to deal
with difficult sources, others as an effect. As for me, I only use
compression if absolutely necessary, for example if I can't get a singer
over the mix when he sings softly, but am pulling the fader back when he
belts out a passionate moment. It's a bit like having another, faster
finger on the fader. I like it as an effect on a bass though.

Try a test source, or get a helpful performer to spend time with you as
you experiment with the compressor settings. You might even have that
glass next to you to reduce its level from time to time!

Anyway, back to work... Hope that helped a bit.
Disclaimer - Technically, no, my glass is not an accurate alternative to
a real compressor!

- James

"I will always remember the day Rene Descartes died.  We had just
finished a wonderful meal and were sitting around plotting our next move
over coffee.  The waitress came up and asked, "More Coffee?"  Descartes
replied, "I think not."   And just disappeared right before my eyes."