COMPRESSION

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COMPRESSION
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• Compression is.
• - one of the most important
• elements in modern audio work
• - operates in the dynamic (volume)
• domain

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What is a compressor?

• Its a device that automatically controls the
  level of an audio signal
• Imagine an engineer using the volume fader to
  keep a singers performance at a constant level,
  making fine adjustments at the right time
• Try to think of it as an amplifier in reverse
  the signal that comes out from it is smaller than
  the signal it receives into it

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Compressor basics
25dB
15dB
Here the audio signal is being compressed
Whats happened to the volume?
-70 dB
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• Two main reasons to use compression on a single
  track, or on an a mix-
• To control the dynamics
• (2) As an effect

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Fact 1Compression controls dynamics!

• it keeps the level of the track smooth and
  even
• it does this by lifting the level of the soft
  passages and lowering the level of the loud ones

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Fact 2You can use it!

• on the bass guitar to smooth over a passage
  played unevenly
• on lead vocals where words or lines are at
  different volumes
• on drums when beats are hit with different
  intensities

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Fact 3It can be a great effect!

• It makes a track or instrument sound -
• Punchier
• Fatter
• Closer
• Exciting

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Compressor types

• 1) Hardware type
• 2) Software type

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Hardware compressor

• With a compressor we want to affect the whole
  sound and output the compressed version - the
  original, unprocessed signal must not be heard

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software compressor
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Knobs and buttons

• THRESHOLD this is the level (volume) which will
  trigger the compressor
• RATIO this is the way in which the volume will
  be treated. A compression ratio of 51 means that
  if the signal is louder than the threshold level
  by 5dB, the output signal only exceeds the
  threshold by 1dB

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• ATTACK - how long it takes for the compressor to
  act after a signal has exceeded the threshold
  level
• RELEASE - how long it takes for the compressor to
  stop acting after the input signal has fallen
  back below the threshold level
• OUTPUT GAIN allows for the signal to be boosted
  back to its original level

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Diagrammatic explanation

• Dynamic range is 5dB to 5dB
• Bulk of signal is around 1dB with a few peaks at
  5dB
• Images kindly supplied
  by www.mpc-tutor.com and www.samplecraze.com

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Compression applied

• Peaks are reduced to around 3dB
• Bulk of signal is still around 1dB

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Adjusting output level

• Increase level of signal so that peaks are at 5dB
• Bulk of signal is now around 3dB r.t 1dB

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Perceived loudness

• the signal now appears to be much louder than
  before, even though the actual output level is
  identical to the original
• TV adS always seem louder than the programmes,
  this is because advertisers apply excessive
  compression in order to attract your attention,
  without exceeding the legal output levels

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Other compressor types

• LIMITER
• extreme compression ratios (101 or more)
• Allows the signal to get to a certain level and
  then no more
• Like cruise control on a car
• Used in sound reinforcement (PA) to protect the
  speakers

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• GATE
• Also called noise gate, expander
• Keeps a signal turned off until it reaches
  threshold level, then opens the gate and lets the
  sound through
• Used to cover up buzzes, noise, coughs, low-level
  sounds EG., noisy gtr amp, leakage from drum mics

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Why you should be wary of compressing

• it can add more noise to the track/mix
• it can suck the life out of the track
• it can colour the sound
• it can cant be removed (if its been used as a
  track is being laid)

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Typical settings on a compressor these are a
GUIDE only!!

• Bass instruments Medium ratio of 41 to 81
• Attack time is slow
• Release time is slow
• Snare drum Med. ratio of 41 61
• Attack time is very fast
• Release time is fast to medium
• Vocals Low ratio of 1.51 to 21
• Attack time is fast Release time is fast

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References

• http//www.samplecraze.com/audio-tutorials/compres
  sion-techniques-1.htm
• http//www.recordingproject.com/articles/article.p
  hp?article6page1
• The Mixing Engineers Handbook. Bobby Owsinski.