Specifications

11
Operating Manual - CLX-52 and CLX-51 Compressor/Limiter
9. DESIGN THEORY
THE NEED FOR GAIN CONTROL
The human ear excels in its ability to detect an
extremely wide range of loudness levels, from the quiet-
est whisper to roar of a jumbo jet. When we attempt to
reproduce this dynamic range, by means of amplifiers,
tape recorders, CD players, or radio transmitters, we run
into one of the fundamental limitations of these electronic
media: limited dynamic range. Amplifier dynamic range
is quite good, and is adequate for most musical program
material. However, some types of audio equipment, such
as cassette tape recorders, have a very narrow useful dy-
namic range.
What is it that compromises the dynamic range
of this equipment? The useful operating region of a piece
of audio equipment is squeezed in between noise and dis-
tortion. As program level decreases, it approaches what
is known as the “noise floor”, and if the volume of the
program material goes lower still, it is engulfed by the
noise. The noise floor, or minimum constant noise level,
will consist of hiss, hum, transistor noise, tape hiss, buzz
and whatever noises are inherent in the medium. When
the program level is considerably higher than the noise
floor, our hearing masks the noise, and it is not a prob-
lem. However, when listening to very quiet sections of a
program for example, a pause between movements of a
string quartet the noise can become very bothersome.
At the other end of the loudness spectrum, the
limitation on dynamic range is usually distortion, either
in the form of amplifier overload, tape saturation, or A to
D clipping. In most transistorized equipment, the transi-
tion from clean, undistorted operation to severe distor-
tion is very abrupt. Therefore, it is common practice to
operate a piece of equipment at a level that is somewhat
below the distortion point, leaving a margin of safety for
unexpected, transient volume peaks in the music. This
safety margin is known as headroom, and may range from
10 to 25 dB. Lowering our standard operating level to
leave ourselves some headroom helps prevent distortion,
but at the same time it moves our average program level
closer to the noise floor, thereby compromising signal-
to-noise performance. It becomes apparent that to get most
out of an audio system, you have to keep your standard
operating level as high as possible without risking distor-
tion.
GAIN RIDING
One solution to the noise vs. distortion trade-off
is to keep your hand on the level control and manually
adjust gain to suit the program. Indeed, there are times
when this approach is entirely satisfactory. However, in
most types of music there are instantaneous, short dura-
tion volume peaks, or transients, which would be diffi-
cult to anticipate and impossible to respond to with manual
gain riding, you simply could not bring the level down
fast enough. In many situations, this can present real prob-
lems. For example, in recording, an extra burst of enthu-
siasm from a lead singer might overload the capabilities
of your recording tape, causing ragged distortion and ne-
cessitating another take. In sound reinforcement, a sud-
den burst of energy through the system can blow fuses or
even damage loudspeakers.
In addition to the problem of response time with
manual gain riding, it also requires your constant atten-
tion, which takes you away from more important jobs.
The need for a fast-acting, reliable, automatic gain con-
trol is answered by limiters and compressors.
WHAT COMPRESSORS AND LIMITERS DO
LIMITING
In any musical program are constant changes in
loudness. It is the job of a limiter to detect when the vol-
ume has exceeded a predetermined maximum safe level,
and to then turn down the volume. When the incoming
signal returns to its original level, the limiter should re-
spond by restoring the gain to normal. Thus, when the
level is within a specified “safe” range, the limiter has no
effect. When an occasional peak occurs, the limiter re-
sponds. This situation is completely analogous to manual
gain riding, except that it occurs faster and more consis-
tently.
COMPRESSION
A very significant difference in dynamic range
is achieved simply by changing the relationship between
nominal signal level and threshold, as a result of either
increasing the GAIN and/or decreasing the THRESHOLD
control. The most interesting effect to be noted, however,
is seen by comparing the original input signal with the
output signal. The quietest portions of the original signal
will be effectively increased in volume while the loudest
portions of the original signal will be decreased. In ef-
fect, both ends of the dynamic spectrum will be pushed
toward the “middle”. This is quite different from simple
limiting, where only loud peaks are subjected to gain re-
duction. More than anything else, it is this double-ended
effect which distinguishes compression from limiting.
Compression is further differentiated from limiting by
careful selection of attack and release times. When lim-
iting is employed to protect an audio system against tran-
sient volume peaks and possible overload, attack time is