The document discusses dynamic range compression (DRC) in audio signals. DRC aims to reduce the volume of loud sounds or increase soft sounds to reduce the difference between loud and soft. This makes the audio sound louder without increasing its peak amplitude. DRC works by applying gain reduction to audio levels above a set threshold, according to a ratio. Parameters like attack, release, and knee control how quickly DRC responds. DRC is used in music production, broadcasting, public spaces, and marketing to improve loudness.

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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….