BSS Audio DPR-402 Owner's Manual - Manual (Page 6)

6
Compressors and Limiters
Fig 1.2 Operating level
and Headroom
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.
Compressors and limiters have closely related effects, and in general a limiter
will reduce gain very strongly once a certain level has been reached, whereas
a compressor will act gently, but over a much wider range of volume levels.
A limiter will continuously monitor program levels, but only commence to
reduce gain once the level has exceeded a preset amount. This point is called
the threshold level. Any program level in excess of the threshold will
immediately be reduced to this threshold level.
A compressor will also continuously monitor the program and has a threshold
level. However, program signals in excess of this threshold will be
progressively reduced by an amount (ratio) depending on the degree to which
it initially exceeds the threshold. Generally, threshold levels for compressors
are set below the normal operating level to allow them to reduce the dynamic
range of the signal gradually, so that they are acceptable to following
equipment. For limiters, the threshold point will be set above the operating
level in order to provide a maximum level for signals to following equipment.
1.2 Compressors
and Limiters