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Chapter 2: What Is A Compressor?
DN540
6 Operator Manual
The level of a vocal may vary widely. As shown in the typical example in Figure 1
“Compression - signal level graphs”, the unprocessed signal has a large dynamic range
between the highest and lowest levels. Applying compression reduces the highest
levels, thus reducing the dynamic range, while leaving the signal below threshold
relatively unaltered. Because the peak level of the signal is now lower, make up gain is
added to restore the original peak level. The result is a much more controlled and
usable sound.
Uses of compression
Compressors were originally devised to reduce the dynamic range of audio signals. To
do this they use a gain cell that makes adjustments to signal levels automatically,
dependent on the level and dynamic of the signal itself, and depending on the character
and control settings of the particular compressor unit.
Over the years many different brands of compressor have been produced using many
different types of gain cell technology, each with a distinctive sound. Users have found
applications that suit many of these, taking the use of compressors far beyond simple
dynamic range reduction and sometimes even generating the complete reverse —
dynamic enhancement. Typical applications include:
• Protecting systems from amplitude overload.
• Improving the power of signals when an artiste’s performance is inconsistent.
• Maintaining a stable signal level to help an instrument sit better within a mix.
• Changing the percussive timbre of an instrument to produce more punch.
• Compressing a mix so that it maintains a stable signal level, typically trying to make
it sound louder.
• Changing the percussive timbre of a mix to make it sound more whole and add
punch.
• Frequency-conscious compression to remove unwanted sibilance or popping.
Unwanted side effects
Very often the process of compression produces unwanted side effects, especially if the
chosen compressor does not match the application it is being put to very well. These
can include:
• Breathing — when high frequency environmental or processing noise levels jump
up and down, sounding similar to someone breathing in the background.
• Pumping — when level modulation can be heard on an instrument, making it sound
unnatural.
• Amplitude flutter — a faster oscillating modulation that makes sounds gritty.
• Source inter-modulation — when one sound source within a mix causes amplitude
changes to another, for example, the natural decay of cymbals can be modulated by
the crack of a snare drum in a drum mix.
• Transient enhancement — unwanted accentuation clicks at the start of a sound.
• Dull sound — when all the natural attack and brilliance is stripped away from the
sound source.