BSS Audio
DPR-402 User Manual v3.0
Page 4
1 Compressors and Limiters
1
.1 The need for Gain Control
The human ear excels in its ability to detect an extremely wide range of sound
levels. These can range from the quietest whisper to the roar of a jet aircraft. When
we attempt to reproduce this large range (dynamic range) of sounds with
amplifiers, tape recorders or radio transmitters, we run into one of the fundamental
limitations of electronic or acoustic equipment. In some cases, such as amplifiers,
the dynamic range available is quite good. However, equipment such as tape
recorders and radio transmitters have a restricted usable dynamic range.
What limits the available dynamic range of this equipment is its inherent noise
floor at the bottom end, and the maximum input signal resulting in an acceptable
amount of distortion at the upper end. The usable dynamic range sits in between
these two limits, and it is common practice to operate a piece of equipment at a
level that is somewhat below the upper distortion point, leaving a margin of safety
for the unexpected transient loudness peaks present in program material. The safety
margin is known as headroom, and is generally in the range of 10 to 20dB.
Lowering the standard operating level to increase headroom helps distortion, but
moves the average program level nearer to the noise floor, thereby compromising
the signal to noise performance.
It therefore becomes apparent that to get the most out of an audio system, the
standard operating level must be kept as high as possible without risking distortion.
One solution to this problem is for the operator of the equipment to be
continuously monitoring the program, and manually adjusting the gain to suit the
moment. When the program is quiet, the gain can be increased, and when the
program is loud the gain can be reduced. However, in most types of program there
are instantaneous short duration level peaks or transients, which would be difficult
to anticipate and impossible to respond to in the required time. Even a sound
engineer with the quickest reflexes could not bring the gain knob or fader down
quickly enough.
The need therefore arises for a fast acting automatic gain controlling device which
will track the program material constantly, and which will always adjust the gain to
maximise the signal to noise performance without incurring distortion. This device
is called a compressor or limiter, and is one part of the
DPR-402
.
