PreSonus AudioBox USB - Manual (Page 39)

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Tutorials 5
A Brief Tutorial on Dynamic Processing 5.2
Why do the best consoles in the world put compressors on every channel?
The answer is simply that most instruments need some form of
compression, often very subtle, to be properly heard in a mix.
Why do we need noise gates?
Consider the compressed-vocal example discussed earlier; you now have a
20 dB dynamic range for the vocal channel. Problems arise when noise or
instruments (air conditioner, loud drummer, etc.) in the background of the
vocal mic become more audible after the lower end of the dynamic range is
raised. You might attempt to mute the vocal between phrases in an attempt to
remove the unwanted sounds; however, this would probably end disastrously.
A better method is to use a noise gate. The noise-gate threshold could be
set at the bottom of the dynamic range of the vocal, say -10 dBu, such that
the gate would shut out the unwanted signals between the phrases.
If you have ever mixed live sound, you know the problems cymbals can create
by bleeding through the tom mics. As soon as you add some highs to get some
snap out of the tom, the cymbals come crashing through, placing the horn drivers
into a small orbit. Gating those tom mics so that the cymbals no longer ring
through them will give you an enormous boost in cleaning up the overall mix.
Dynamics processing is the process of altering the dynamic range of a signal, thereby
enhancing the ability of a live sound system or recording device to handle the signal
without distortion or noise and aiding in placing the signal in the overall mix.
5.2.2 Types of Dynamic Processing
Compression/Limiting
Punch, apparent loudness, presence—these are just three of the many
terms used to describe the effects of compression/limiting.
Compression and limiting are forms of dynamic-range (gain) control. Audio
signals have very wide peak-to-average signal-level ratios (sometimes
referred to as dynamic range, which is the difference between the loudest
level and the softest level). The peak signal can cause overload in the audio-
recording or sound-reinforcement chain, resulting in signal distortion.
A compressor/limiter is a type of amplifier in which gain is dependent on the
signal level passing through it. You can set the maximum level a compressor/
limiter allows to pass through, thereby causing automatic gain reduction
above some predetermined signal level, or threshold. Compression refers,
basically, to the ability to reduce, by a fixed ratio, the amount by which a
signal’s output level can increase relative to the input level. It is useful for
lowering the dynamic range of an instrument or vocal, making it easier to
record without distorting the recorder. It also assists in the mixing process by
reducing the amount of level changes needed for a particular instrument.
Take, for example, a vocalist who moves around in front of the microphone while
performing, making the output level vary up and down unnaturally. A compressor
can be applied to the signal to help correct this recording problem by reducing
the louder passages enough to be compatible with the overall performance.