BSS Audio OPAL Series DPR-944 Owner's Manual - Manual (Page 28)

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10.0 General guide to compressing
10.1 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.
General guide to compressing
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 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
Fig 10.1 Dynamic range