Compression

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Compression

• EL2071 Audio 2

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A Compressor

• acts like an automatic volume control, turning
  down the volume when the signal gets too loud
• it reduces the gain (amplification) when the
  input signal exceeds a preset level (the
  threshold)

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A Compressor
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Compression

• keeps level of vocals or instruments more
  constant so they are easier to hear throughout
  the mix
• it prevents loud noises that may be recorded with
  distortion on tape

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Special effects

• make drums sound fatter
• increase the sustain on a bass guitar
• usual uses vocals, bass guitar, kick drum,
  acoustic guitar

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Using a Compressor

• When? Track-laying or mixdown?
• Track-laying improves S/N ratio of tape track,
  but you must decide on settings
• Mixdown experiment with settings

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Compressor controls

• Standard
• Ratio Change in input to output level
• 21 for every 2dB change, output changes 1dB
• 21 20dB change, output changes to 10dB
• Typical ratios are 21 to 41

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Compressor controls
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Compressor controls

• Standard
• Thresholdinput level above which compression
  occurs
• Set high (-5VU) to compress only loudest notes
• Set low (-10 or -20VU) to compress broader range
• Typical setting -10 to -5
• If compressor has a fixed threshold, adjust
  amount of compression with input level control.

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Compressor controls

• Standard
• Attack time how fast the compressor reduces gain
  when its hit by an attack
• Typical times 0.25 to 10 milliseconds (msec)
• Longer time larger peaks that are passed before
  gain reduction occurs
• Shorter time reduces punch by softening attack

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Compressor controls

• Standard
• Release time how fast gain returns to normal
  after a loud passage ends
• Typical times 50 msec to several seconds
• Shorter time follows rapid volume changes keeps
  average level higher BUT
• since noise rises along with gain, can give
  pumping sound
• Longer time sounds more natural

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Compressor controls
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Compressor controls

• Standard
• Output-Level Control using a compressor often
  increases the output level. This control can
  compensate for that.

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Compressor controls

• Standard
• Side chain pair of in/out jacks for connecting
  to an equaliser
• will make compressor respond in different ways to
  different frequencies
• To compress only sibilant sounds on a vocal,
  boost side-chain EQ to around 10kHz
• To compress only breath pops on a vocal track,
  boost side-chain EQ to around 20Hz

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Connecting a Compressor

• To compress one instrument while recording
• connect compressor via Send/Return (or Channel
  Insert) in-and-out jack
• adjust controls, experiment

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Limiter

• Keeps signal peaks from exceeding a preset level.
• Prevents signal overload and clipping
• Compression ratio very high 101 or greater
• Threshold is set high 0 VU
• While compressor reduces overall dynamic range
  limiter affects only highest peaks

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Noise gate (Expander)

• Acts like an on-off switch that removes noise
  during pauses in an audio signal
• Reduces gain when input level falls below a
  preset threshold
• Uses clean up drums tracks by removing leakage
  between beats
• on a noisy guitar amp, cuts out buzz and hiss
  between phrases
• can have side chain, often used to tie in a kick
  drum signal and bass guitar

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Drawmer LX20 Expander/Compressor

• Controls
• Expand
• Compress
• Attack
• Release
• Output
• Bypass
• Stereo Link
• Side chain