User`s manual

106
4.4.17: Limiter/Compressor/Expander
Application:
The LCE (Limiter/Compressor/Expander) is a three-section dynamic signal level processor,
recommended for advanced users only. Dynamic signal level processing enables the user to
manipulate the overall dynamic range of a signal, generally to correct for near-party/far-
party and/or “quiet talker” scenarios.
The three types of level processing available (three sections) are limiting, compression, and
expansion. Unlike an analog implementation of this process, this digital implementation is
substantially easier to set up and operate and is far more accurate. All matching of amplifier
gains is carried out automatically by the processor. Logarithmic level conversions are carried
out precisely in real time.
These dynamic processes modify the amplitude of the signal using a variable-gain digital
amplifier. The amplitude is a rectified and smoothed version of the signal wave form, as
measured by a real-time digital envelope detector.
The operation of the envelope detector is governed by its Attack Time and Release Time,
which are adjustable. Normally a fast attack time and slow release time are used with speech.
Setting a fast attack time (less than 10 milliseconds) causes the processor to rapidly respond
to sharp sounds. The level detector will be more peak sensitive to fast attack time and more
average-value sensitive to longer attack times. An Attack Time of 2-5 milliseconds is
recommended for Speech applications.
Short release times, less than 100 milliseconds, may make the level detector too responsive to
intra-syllabic pause creating an annoying “pumping” artifact. Conversely, long release
times, greater than 500 milliseconds for example, may fail to respond to breath group pauses
and exchanges between speakers. Therefore a Release Time of 200-400 milliseconds is
generally recommended for speech applications.
The gain algorithm dynamically adjusts the signal amplification based on input signal level
and its Gain Region (limiting, compression, or expansion). If the input signal level exceeds
the Limit Threshold, it is in the Limit region. If the input signal level exceeds the
Compression Threshold, but not the Limit Threshold, it is in the Compression region. And,
lastly, if the input signal level is below the Compression Threshold, it is in the Expansion
region. The three Gain Regions are described below:
Limit Region: In the limit region the output signal level is “damped” or kept from
exceeding the specified Limit Threshold by applying attenuation.