Recording - November 2015 - 58

By Paul J. Stamler

Part 11: Put A Lid On It-Compressors
Controlling dynamics in recording has always been a challenge-making sure
the loud parts don't get too loud, making sure the soft parts aren't lost in the
noise, and giving an even listening experience throughout a song. In the old
days of cutting wax masters by playing into a metal horn, producers actually
moved instruments and players toward and away from the horn during the song,
as needed. The first electrical recordings relied on "gain riding," where an engineer would manually turn levels up and down on the fly-difficult but necessary.
When radio came along, keeping maximum signal level under control
became a legal issue, as overloads could disturb neighboring channels' transmissions, and radio equipment manufacturers created the first compressors and
limiters to put a lid on levels.
These were quickly adopted for audio recordings, movies, and later TV. Audio
engineers realized that they could not only be used for keeping the overall level
of a mix under control, but for controlling the levels of individual signals before
they were combined into a mix. That's most of what I'll be talking about in this
article, but first I want to look at how the magic happens. I'll focus on hardware
compressors, but software compressors do essentially the same things.
How compressors work
Take a look at Figure 1. It's what engineers call an input/output graph or, if
they're feeling fancy, a transfer function. In this graph, the X-axis shows the compressor's input level, while the Y-axis shows the output level; both are in the standard pro-audio unit of dBu.

Figure 1: Compression

A device that doesn't mess with the signal
shows the same level at the output and the input;
it would have a straight diagonal line on this
graph, like the heavy line I've drawn.
Now imagine that inside the compressor
there's a very small engineer whose hand is on
the volume control; that engineer unerringly
reacts to the level of the input signal. Above a
certain input level, called the threshold, the little
engineer turns the volume down, and the slope
of the line on the graph decreases. In the picture,
the threshold is set at -10 dBu, and I've shown
two settings of the compressor circuit. The middle
line is labeled "2:1", and it indicates that for
every 2 dB the input signal rises above the
threshold, the output signal will rise only 1 dB.
Wrap your head around this for a moment,
because it gets squirrelly. An input signal below
-10 dBu (that is, below threshold) will come out
unchanged, but an input signal of, say, -3 dBu,
which is 7 dB higher than the threshold, will come
out 7/2 = 3.5 dB over threshold, or -6.5 dBu. An
indicator on the compressor (either a meter or a
line of LEDs) tells the user that the gain reduction
is 3.5 dB. That factor of 2:1 is called the compression ratio, or sometimes just the ratio.
Compressors can operate more drastically than
that. The lowest line in Fig. 1 is labeled 4:1; for
every 4 dB the input level increases over the threshold, the output will rise only 1 dB. So an input signal of +2 dBu (12 dB over threshold) will produce
an output rise of only 12/4, or 3 dB over threshold. That's -7 dBu, for a whopping gain reduction
of 9 dB. The little engineer is working hard.
A compressor with a very high compression
ratio, say 20:1, barely lets signals rise above
the threshold at all; compressors like that are
referred to as limiters, and they're very useful for
keeping an absolute lid on the signal level. In
effect, the little engineer draws a line at the
threshold, saying "Higher than this, you shall not
go." Limiters are useful for keeping the recording medium from overloading on momentary
peaks. They're especially useful if the recording
is digital, where overloads sound awful.
How do they do it?
Figure 2 (page 61) shows a block diagram of a
typical modern hardware compressor. The incoming signal gets split into two portions; one goes
straight ahead into the gain reduction element (the
actual volume control, of which more later), while
the other takes a sharp left turn into the sidechain,

58

RECORDING November 2015



Table of Contents for the Digital Edition of Recording - November 2015

Contents
Recording - November 2015 - Intro
Recording - November 2015 - Cover1
Recording - November 2015 - Cover2
Recording - November 2015 - 1
Recording - November 2015 - 2
Recording - November 2015 - 3
Recording - November 2015 - 4
Recording - November 2015 - 5
Recording - November 2015 - Contents
Recording - November 2015 - 7
Recording - November 2015 - 8
Recording - November 2015 - 9
Recording - November 2015 - 10
Recording - November 2015 - 11
Recording - November 2015 - 12
Recording - November 2015 - 13
Recording - November 2015 - 14
Recording - November 2015 - 15
Recording - November 2015 - 16
Recording - November 2015 - 17
Recording - November 2015 - 18
Recording - November 2015 - 19
Recording - November 2015 - 20
Recording - November 2015 - 21
Recording - November 2015 - 22
Recording - November 2015 - 23
Recording - November 2015 - 24
Recording - November 2015 - 25
Recording - November 2015 - 26
Recording - November 2015 - 27
Recording - November 2015 - 28
Recording - November 2015 - 29
Recording - November 2015 - 30
Recording - November 2015 - 31
Recording - November 2015 - 32
Recording - November 2015 - 33
Recording - November 2015 - 34
Recording - November 2015 - 35
Recording - November 2015 - 36
Recording - November 2015 - 37
Recording - November 2015 - 38
Recording - November 2015 - 39
Recording - November 2015 - 40
Recording - November 2015 - 41
Recording - November 2015 - 42
Recording - November 2015 - 43
Recording - November 2015 - 44
Recording - November 2015 - 45
Recording - November 2015 - 46
Recording - November 2015 - 47
Recording - November 2015 - 48
Recording - November 2015 - 49
Recording - November 2015 - 50
Recording - November 2015 - 51
Recording - November 2015 - 52
Recording - November 2015 - 53
Recording - November 2015 - 54
Recording - November 2015 - 55
Recording - November 2015 - 56
Recording - November 2015 - 57
Recording - November 2015 - 58
Recording - November 2015 - 59
Recording - November 2015 - 60
Recording - November 2015 - 61
Recording - November 2015 - 62
Recording - November 2015 - 63
Recording - November 2015 - 64
Recording - November 2015 - 65
Recording - November 2015 - 66
Recording - November 2015 - 67
Recording - November 2015 - 68
Recording - November 2015 - 69
Recording - November 2015 - 70
Recording - November 2015 - 71
Recording - November 2015 - 72
Recording - November 2015 - Cover3
Recording - November 2015 - Cover4
https://www.nxtbook.com/musicmaker/musicmaker/recording_202404
https://www.nxtbook.com/musicmaker/musicmaker/recording_202403
https://www.nxtbook.com/musicmaker/musicmaker/recording_202402
https://www.nxtbook.com/musicmaker/musicmaker/recording_202401
https://www.nxtbook.com/musicmaker/musicmaker/recording_202312
https://www.nxtbook.com/musicmaker/musicmaker/recording-november-2023
https://www.nxtbook.com/musicmaker/musicmaker/recording_202310
https://www.nxtbook.com/musicmaker/musicmaker/recording_202309
https://www.nxtbook.com/musicmaker/musicmaker/recording_202308
https://www.nxtbook.com/musicmaker/musicmaker/recording_202307
https://www.nxtbook.com/musicmaker/musicmaker/recording_202306
https://www.nxtbook.com/musicmaker/musicmaker/recording_202305
https://www.nxtbook.com/musicmaker/musicmaker/recording_202304
https://www.nxtbook.com/musicmaker/musicmaker/recording_202303
https://www.nxtbook.com/musicmaker/musicmaker/recording_202302
https://www.nxtbook.com/musicmaker/musicmaker/recording_202301
https://www.nxtbook.com/musicmaker/musicmaker/recording_202212
https://www.nxtbook.com/musicmaker/musicmaker/recording_202211
https://www.nxtbook.com/musicmaker/musicmaker/recording_202210
https://www.nxtbook.com/musicmaker/musicmaker/recording_202209
https://www.nxtbook.com/musicmaker/musicmaker/recording_202208
https://www.nxtbook.com/musicmaker/musicmaker/recording_202207
https://www.nxtbook.com/musicmaker/musicmaker/recording_202206
https://www.nxtbook.com/musicmaker/musicmaker/recording_202205
https://www.nxtbook.com/musicmaker/musicmaker/recording_202204
https://www.nxtbook.com/musicmaker/musicmaker/recording_202203
https://www.nxtbook.com/musicmaker/musicmaker/recording_202202
https://www.nxtbook.com/musicmaker/musicmaker/recording_202201
https://www.nxtbook.com/musicmaker/musicmaker/recording_202112
https://www.nxtbook.com/musicmaker/musicmaker/recording_202111
https://www.nxtbook.com/musicmaker/musicmaker/recording_202110
https://www.nxtbook.com/musicmaker/musicmaker/recording_202109
https://www.nxtbook.com/musicmaker/musicmaker/recording_202108
https://www.nxtbook.com/musicmaker/musicmaker/recording_202107
https://www.nxtbook.com/musicmaker/musicmaker/recording_202106
https://www.nxtbook.com/musicmaker/musicmaker/recording_202105
https://www.nxtbook.com/musicmaker/musicmaker/recording_202104
https://www.nxtbook.com/musicmaker/musicmaker/recording_202103
https://www.nxtbook.com/musicmaker/musicmaker/recording_202102
https://www.nxtbook.com/musicmaker/musicmaker/recording_202101
https://www.nxtbook.com/musicmaker/musicmaker/recording_202012
https://www.nxtbook.com/musicmaker/musicmaker/recording_202011
https://www.nxtbook.com/musicmaker/musicmaker/recording_202010
https://www.nxtbook.com/musicmaker/musicmaker/recording_202009
https://www.nxtbook.com/musicmaker/musicmaker/recording_202008
https://www.nxtbook.com/musicmaker/musicmaker/recording_202007
https://www.nxtbook.com/musicmaker/musicmaker/recording_202006
https://www.nxtbook.com/musicmaker/musicmaker/recording_20205
https://www.nxtbook.com/musicmaker/musicmaker/recording_202004
https://www.nxtbook.com/musicmaker/musicmaker/recording_202003
https://www.nxtbook.com/musicmaker/musicmaker/recording_202002
https://www.nxtbook.com/musicmaker/musicmaker/recording_202001
https://www.nxtbook.com/musicmaker/musicmaker/recording_201912
https://www.nxtbook.com/musicmaker/musicmaker/recording_201911
https://www.nxtbook.com/musicmaker/musicmaker/recording_201910
https://www.nxtbook.com/musicmaker/musicmaker/recording_201909
https://www.nxtbook.com/musicmaker/musicmaker/recording_201908
https://www.nxtbook.com/musicmaker/musicmaker/recording_201907
https://www.nxtbook.com/musicmaker/musicmaker/recording_201906
https://www.nxtbook.com/musicmaker/musicmaker/recording_201905
https://www.nxtbook.com/musicmaker/musicmaker/recording_201904
https://www.nxtbook.com/musicmaker/musicmaker/recording_201903
https://www.nxtbook.com/musicmaker/musicmaker/recording_201902
https://www.nxtbook.com/musicmaker/musicmaker/recording_201901
https://www.nxtbook.com/musicmaker/musicmaker/recording_201812
https://www.nxtbook.com/musicmaker/musicmaker/recording_201811
https://www.nxtbook.com/musicmaker/musicmaker/recording_201810
https://www.nxtbook.com/musicmaker/musicmaker/recording_201809
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201808
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201807
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201806
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201805
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201804
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201803
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201802
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201801
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201712
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201711
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201710
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201709
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201708
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201707
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201706
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201705
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201704
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201703
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201702
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201701
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201612
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201611
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201610
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201609
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201608
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201607
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201606
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201605
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201604
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201603
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201602
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201601
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201512
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201511
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201510
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201509
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201508
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201507
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201506
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201505
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201504
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201503
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201502
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201501
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201412
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201411
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201410
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201409
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201408
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201407
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201406
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201405
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201404
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201403
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201402
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201401
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201312
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201311
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201310
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201309
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201308
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201307
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201306
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201305
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201304
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201303
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201302
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201301
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201212
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201211
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201210
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201209
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201208
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201207
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201206
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201205
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201204
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201203
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201202
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201201
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201112
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201111
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201110
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201109
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201108
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201107
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201106
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201105
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201104
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201103
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201102
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201101
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201012
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201011
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201010
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201009
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201008
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201007
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201006
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201005
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201004
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201003
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201002
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201001
https://www.nxtbook.com/nxtbooks/musicmaker/recording_200912
https://www.nxtbook.com/nxtbooks/musicmaker/recording_200911
https://www.nxtbook.com/nxtbooks/musicmaker/recording_200910
https://www.nxtbook.com/nxtbooks/musicmaker/nxtd
https://www.nxtbookmedia.com