Compression and EQ

Compression and EQ

1. What is compression?

   • Another name for a dynamic processor.
   • It evens out the audio volume.
   • The sound is processed and peaks can be removed.
   • Dynamic levels are controlled to prevent the audio from distorting or peaking.
   • Used to prevent quieter instruments from fading out and the louder instruments drowning out others and causing distortion.
   • Limiting
     • Acts as a more extreme compressor.
     • Designed to reduce the signal in specific cases to prevent it from damaging equipment.
     • Short attack.
     • Ratio close to infinity:1
     • Hard knee
     • Can be used with a compressor to clip out the loudest peaks
   • Multi-band compression
     • Targets specific frequency ranges.
     • Tries to avoid a "pumping" effect when compressing a mix.
     • E.g: used to prevent one drum triggering the entire kit's compression.
   • De-essing
     • "sss", "ch" and "t" sounds can be over-emphasised
     • Controlled via multi-band compression or a pop shield.
     • Targets 5-10kHz range.
       • Sounds are enhanced by EQ and fed into the side-chain input of the compressor , causing it to be more sensitive to these frequencies.
   • Expanders
     • Reduce signal levels that fall below threshold
     • Expands dynamic range of a piece.
     • Most commonly used to reduce background noise.
   • Noise gates
     • An extreme form of expander.
     • Uses a ratio closely set to infinity:1.
     • Can also be used creatively by feeding a drum track through the sidechain.
       • Can make a synth more rhythmic.
     • The gate is either open to let a signal through or closed to kept it from sounding at all (or significantly reducing it)
     • Can have a hold time
     • Can be used to separate instruments and for live performances where other electrical equipment may cause interference
     • Range control reduces the sound level below the threshold, rather than to cut it off completely. Creates a more natural sound.
   • Parameters
     • Threshold has to be set up. When the signal level hits the set amplitude, the peaks are reduced according to the ratio used.
     • Threshold - the amplitude level above which compression begins.
     • Ratio - the amount by which the input signal is reduced past the threshold.
       • How much the signal is attenuated by once it exceeds the threshold. Visually it affects the bend.
     • Attack time - how long the compressor takes to act after the threshold is exceeded
     • Make-up gain - the amount you boost the whole mix by afterwards to compensate for compression (lost/cut amplitudes)
     • Release - how long the compressor takes to stop acting once the signal is below the threshold
     • Knee - makes the transition less abrupt
     • Input gain - adjusts the level of input to the compressor. This is important for avoiding distortion.
     • Stereo link - this links 2 mono compressors. IT's particularly useful if the same settings are required for L and R channels otherwise the stereo image could drift around.
     • Peak/RMS - peak mode is for catching fast, transient peaks and is more accurate than the levelling mode of RMS, which is based on averaging.
   • Resultant signal is boosted
   • Squeezing the dynamic range by reducing signal level above a user-defined level by a certain amount.
   • Dynamic range - the difference between the highest and lowest amplitude in a piece

2. What is equalisation?

   • EQ
   • The process of amplifying, filtering or attenuating (opposite of amplifying) audio signal frequencies.
   • Used to enhance perhaps the treble quality of a voice or increase the bass.
   • Stereos have simple EQ controls.

3. Side chain compression

   • When you link 2 tracks so the compression peaks in one track and activates compression on another track
   • E.g: pumping sound in dance music
   • A copy of the input signal is fed to a side chain, which is monitored by the compressor.
   • It's possible to filter the tone so that the processors is more or less sensitive to different frequencies.