A very small presentation on Dynamic Range Compression i.e. DRC generally applied on Audio...

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Dynamic Range Compression
Audio DSP Ksharing: Rajesh Sharma
www.rajeshsharma.co.in sharma.rajesh@gmail.com

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Objective
• To reduce the dynamic range of audio
signal using compressor
– Sound recording, broadcasting (loudness wars)
– Make music sound louder without changing its peak amplitude
– To deal with technical limitation of audio equipment
– To improve audibility of audio in noisy environment
• Upward Compression
• Downward compression
– Reduces the gain of an
audio signal

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Design
• Audio Signal is split:
• Feedback method: older designs
• Feed forward Methods: new designs
• Digital Audio DRC: DSP techniques are used to implement DRC.
– Digital audio editors, dedicated workstations

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Threshold/Ratio
• Threshold: The level above
which signal is reduced
– -60dB means a large portion will
be compressed
• Ratio: The input/output ratio
above threshold
– 4:1 means that a signal 4dB above
threshold will become 1dB.

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DRC Response
• Peak/RMS Sensing:
– compressor response to input signal
• Peak sensing: reacts to instantaneous changes
– reacts fast, may have distortion
– Acts more like limiter
• RMS sensing: compressor apply averaging to i/p signal
– More relaxed compression, close to loudness perception
• Soft/Hard Knees:
– Defines Response curve of DRC

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Attack/Release
• Attack/Release:
– how quickly it acts to change the signal
– Expressed as N dB/ms (e.g. 10 dB/ms)
• Attack phase (signal above threshold):
– it is the period when compressor is increasing gain reduction
• Release phase (signal below threshold):
– it is the period when compressor is decreasing gain reduction

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Stereo Linking
• Stereo Linking: compressor applies the same gain
to left/right channel
– To avoid image shifting (phantom source)
– Can be done in two ways:
• left/right channels are summed to mono
• DRC computes the gain for both channels and applies the highest
• Look Ahead: used to catch transients
– DRC is applied to Delayed Signal
– DRC gain is computed from non-delayed signal

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Side Chaining
• Side Chaining: used to apply DRC to main audio
signal, depending on external input signal level.
• Usage: DJ use it for ducking
– Ducking: to suppress audio when they speak
– Can detect strong spectral content using equalizer in SideChain
– Can be used as de-esser,
• 6-9KHz band of original signal fed to side chain

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Compressor/Limiter
• Compressor and limiter are not so much different
– A limiter is a DRC with higher ratio and fast attack

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Multi Band DRC
• MDRC: separate DRC for different bands
– Signal change in one band does not change other band signal
• e.g. low frequency kick drum
– Signal is split using band pass filters
– Each band pass signal is passed through its own DRC
– Signal are recombined at the output
• MDRC Usage:
– Audio mix engineers for audio mastering
– Radio stations use them for increasing apparent loudness
– commercials may also use them in similar fashion

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Usage
• Public Spaces: To make music sound louder
• Music Production: vocal performances in rock
concerts may use DRC to make them stand out
• Broadcasting : To boost the perceived volume of sound
• Marketing: DRC on TV commercials
• Sometimes DRC may be used for transmission purpose and
can be expanded afterward….