Compression Explained

Punch, apparent loudness, presence... just three of the many terms
used to describe the effects of compressing and limiting on an audio
signal.

The terms compression and limiting have been in the audio vocabulary for years,
yet there is some confusion over their definitions. The confusion arises from the
fact that both the compressor and the limiter are devices that restrict the dynamic
range of a signal, and the difference between them is one of degree, with the
limiter having the most effect. To simply define each:

Compressor: An amplifier, whose gain decreases as its input level is increased.

Limiter: A compressor, whose output level remains constant, regardless of its
input level.

Both definitions are valid only after the signal being processed reaches a certain
level. Therefore, one more definition needs to be thrown out for consideration:

Threshold: The level at or above which the compressor or limiter begins
functioning.

In a situation where input and output are idealized for a combination
compressor/limiter, as the input level increases from -10dB to 0dB, the output
level, likewise, increases from -10dB to 0dB. Here the device is functioning as a
simple unity gain amplifier, with no effect on the signal level.

Compression

Once the signal level exceeds the compression threshold of 0dB, the output level
will follow the compression curve, assuming a compression ratio of 2:1, as the
input increases 10dB; the output will yield only 5db more gain.

Limiting

In a limiting situation, with a limiting threshold of +20dB, once the input level
reaches +20dB, there is no further increase in output level. Hence, the device is
operating as a limiter,. In actual practice, compression ratios of greater than 10:1
are considered as limiting. Once the limiting threshold of +20dB has been
reached, the output level remains at +10dB, despite further increases in input
level. Therefore, it should be understood that the limiter threshold does not
necessarily indicate the maximum allowable output level of the device. Rather, it
indicates the input level at which the limiter begins working.

Variable Thresholds

It should be noted that the same compression ratio of 2:1 mentioned earlier will
have different effects on the overall dynamic range depending on the point at
which compression begins (threshold). Also, the positioning of the compression
threshold will influence the point at which limiting must begin, if a certain
maximum output level is not to be exceeded.

Variable Compression Ratios

Most state-of-the-art compressors offer audio engineers a variety of compression
ratios from which to choose. Assuming that an audio signal must be kept below
+10dB, using higher compression ratio settings will allow for a greater dynamics
range of the signal being processed.

Pumping and Breathing

It is relatively easy to determine the compression threshold and ratio needed to
prevent a wide dynamic range signal from exceeding a specified output level.
However, it should be realized that - especially at high ratios - the action of the
compressor might become audibly obtrusive. To understand why, remember that
the compressor is a variable gain device. The higher the compression ratio the
greater the change in gain. A constant high level signal, say +10dB will cause
more gain reduction. When the high level is removed, the amount of gain
reduction decreases as the compressor returns to unity gain. If the gain reduction
fluctuates rapidly, it may be quite audible as the background noise goes up and
down in time with the compressor action, i.e. attack /release times, causing a
breathing like sound. This can be used sometimes as an effect producing some
killer results.

Example: The use of extremely short attack times and longer release times may
create a backward-like sound, especially on percussive instruments. The fast
attack immediately drops the signal, and then as the signal naturally decays, the
release time setting brings up the gain, working against the normal decay. This
effect is particularly noticeable on a drum set, and particularly on cymbals.

Program Limiting

Often compression may be applied to the overall program rather than to an
individual instrument. Known as program limiting, this practice will prevent
cumulative levels of the various instruments from getting too high or falling too
low. This type of gain control must be approached with care, since the adverse
effects of compression are heard on the entire program.

Program limiting is often used to raise the apparent loudness of a record. Since
the ear averages the sound level over a period of time, a low level program with
occasional high level peaks will not seem as loud as an average level program
with no high level peaks. (Confused yet?). In the quest for louder sounding
recordings and broadcasts this type of loudness boosting is often overdone,
much to the detriment of the finished product. Meaning, what you hear coming
over the radio or television is often much different than the original producers,
engineers and mastering people had in mind.