threshold while quiet sounds remain unaffected. The dedicated
electronic hardware unit or audio software used to apply
compression is called a compressor.
Types
Downward compression: reduces loud sounds over a certain threshold while quiet sounds remain unaffected.
Upward compression: increases the loudness of sounds below a threshold while leaving louder passages unchanged.
Both downward and upward compression reduce the dynamic range of an audio signal.
An expander performs the opposite function, increasing the dynamic range of the audio signal.
Expanders are generally used to make
quiet sounds even quieter by reducing the level of an audio signal that falls below a set threshold level. A noise gate is a type of expander.
Controls and features
Different compression ratios
What's Threshold
A compressor reduces the level of an audio signal if its amplitude
exceeds a certain threshold. It is commonly set in decibels dB, where
a lower threshold (e.g. -60 dB) means a larger portion of the signal
will be treated (compared to a higher threshold of e.g. −5 dB).
What's Ratio
The amount of gain reduction is determined by ratio: a ratio of 4:1 means that if input level is 4 dB over the threshold, the output signal level will be 1 dB over the threshold.
The gain (level) has been reduced by 3 dB:
Threshold = −10 dB
Input = −6 dB (4 dB above the threshold)
Output = −9 dB (1 dB above the threshold)
The highest ratio of ∞:1 is often known as 'limiting'.
It is commonly achieved using a ratio of 60:1, and effectively denotes that any signal above the threshold will be brought down to the threshold level (except briefly after a sudden increase in input loudness, known as an "attack").
Attack and release
A compressor might provide a degree of control over how quickly it acts. The 'attack phase' is the period when the compressor is decreasing gain to reach the level that is determined by the ratio.
The 'release phase' is the period when the compressor is increasing gain to the level determined by the ratio, or, to zero dB, once the level has fallen below the threshold.
What's a Makeup gain
Because the compressor is reducing the gain (or level) of the signal,
the ability to add a fixed amount of make-up gain at the output is
usually provided so that an optimum level can be used.
What's look-ahead
The look-ahead function is designed to overcome the problem of
being forced to compromise between slow attack rates that produce
smooth-sounding gain changes, and fast attack rates capable of
catching transients. Look-ahead is a misnomer in that the future is
not actually observed. Instead, the input signal is split, and one side
is delayed. The non-delayed signal is used to drive the compression
of the delayed signal, which then appears at the output. This way a
smooth-sounding slower attack rate can be used to catch transients.
The cost of this solution is that the signal is delayed.