Ashly DPX-100 Operating Manual page 18

Operating Manual - DPX-100 Graphic Equalizer - Compressor/Limiter
Compressor/Limiters: The Need For Gain Control
The human ear excels in its ability to detect an extremely wide range of loudness
levels, from the quietest 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 dynamic 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
distortion. 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
problem. 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 transition from clean, undistorted
operation to severe distortion 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 distortion.
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, inmost types of music there are instantaneous,
short duration volume peaks, or transients, which would be difficult 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 problems. For example, in
recording, an extra burst of enthusiasm from a lead singer might overload the capabilities
of your recording tape, causing ragged distortion and necessitating another take. In sound
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