OPERATION
13 | PreSonus 2007
Why do you need noise gates?
Consider the compressed vocal example above and you now have a 20dB dynamic range for the vocal
channel. Problems arise when there is noise or instruments in the background of the vocal mic that became
more audible after the lower end of the dynamic range was raised (air conditioner, loud drummer, etc). You
might attempt to mute the vocal between phrases in an attempt to remove the unwanted signals; 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 -10dBu, such that the gate would ‘close’ out the
unwanted signals between the phrases.
If you have ever mixed live you know well the problem cymbals can add to your job by bleeding through your
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 toms so that the cymbals no longer ring
through the tom mics will give you an enormous boost in cleaning up the overall mix.
3.2.2 Types of Dynamics Processing
Dynamics processing is the process of altering the dynamic range or levels 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.
Compression / Limiting
Punch, apparent loudness, presence…just three of many terms used to describe the effects of
compression/limiting.
Compression and limiting are forms of dynamic range (volume) 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 reproduction
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 the output level of an audio signal by a fixed ratio relative to the input. 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.
How severely the compressor reduces the signal is determined by the compression ratio and compression
threshold. A ratio of 2:1 or less is considered mild compression, reducing the output by a factor of two for
signals greater than the compression threshold. Ratios above 10:1 are considered hard limiting.
Limiting
refers to the point at which the signal is restrained from going any louder at the output. The level of
input signal at which the output is reduced is determined by the compression threshold. As the compression
threshold is lowered, more and more of the input signal is compressed (assuming a nominal input signal
level). Care must be taken not to ‘over compress’ a signal. Too much compression destroys the acoustic
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