NewTek TriCaster 855 - Manual (Page 239)

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Being able to do this independently for each output too is icing on the cake, especially for
internet streaming, as it ensures correct levels at any time.
THRESHOLD
Sound above the set Threshold level will be compressed; the amount of compression and the
manner in which it is applied are both dictated by the other settings.
RATIO
A Ratio of 4:1 means that if input level is 4 dB over the threshold, the output signal level after
compression will be just 1 dB over the threshold. The gain (level) is reduced by 3dB.
Very high ratio settings are the reason for the word “limiter” is part of the title for this feature.
The highest ratio setting will effectively reduce any signal that would rise above the threshold all
the way down to the threshold level (with the exception of a brief period during a sudden
increase in source loudness, as dictated by the Attack setting).
ATTACK
Attack (like Release) is labeled in milliseconds. The setting represents the amount of time it takes
for the gain to change by a specified amount. It would not be grossly incorrect to think of this
setting as changing the slope of a graph depicting how aggressively the compressor pursues the
target value (defined by applying the Ratio setting to the amount the signal surpasses the
Threshold). Shorter values are more aggressive, while longer values are more subtle (and tend to
be less noticeable to the audience).
Compressor or Limiter – what’s the difference?
Compression and limiting are not really different processes, but really rather a matter of
degree and perceived effect. Compression, ideally, takes the form of a subtle, almost
imperceptible modulation of the sound level to bring it into a more pleasing and
convenient range. A limiter is applied more for the purpose of managing, even
crushing, unwanted spikes and transients.
That distinction aside, a limiter is essentially just a compressor set to a high ratio and,
generally, a fast attack time. Audio engineers generally consider compression with ratio
of 10:1 or more as limiting.