Dynamic Range Processing
From DSPWiki
Dynamic Range Processing transforms the amplitude of a signal. Some various applications of dynamic range techniques are:
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Envelope Shapers
Envelope shapers control the amplitude of a signal over time in proportion to an envelope which can be drawn by hand.
Noise Gates
Noise gates clean up audio signals by removing everything below a certain signal level. It functions as a switch, which is on when the amplitude of the input signal is above a specific threshold, and off when the amplitude is below that threshold.
Compressors
A compressor is an amplifier whose gain is controlled by its input signal. They are mainly used to keep an output signal relatively constant, or to give more presence to a particular element in a mix. They work by reducing the amplitude of transient loud spots, and then raising the overall level of the signal. One way of characterizing a compressor is by its transfer function, which shows how an input amplitude is mapped to an output amplitude.
Peak vs. Average (RMS) Detectors:
A detector circuit monitors the amplitude of the input signal, and can respond either to peak signals, or to the average amplitude of the input (typically over a span of one or two seconds).
Compression Ratio:
The compression ratio (or input/output ratio) is the ratio of change between the input signal and the output signal. A ratio of 4:1 means that a 4 dB change in the input signal causes only a 1 dB change in the output signal.
Threshold:
The compressor threshold sets the input level at which the compressor switches from a linear 1:1 ratio to a compressed ratio. This adds a bend (or knee) to the graph:
Knee
Knee adjustments make a more gradual transition between the signal that falls below the threshold and the signal above it. This is displayed graphically as a sloping transition between the 1:1 area and the compressed ratio area.
Expanders
An expander is the opposite of a compressor. It exaggerates small changes in input amplitude, and generates an output signal at the expansion ratio. A 1:4 ratio means that a 1 dB input amplitude change is translated into a 4 dB amplitude change.
Limiters
Limiting is the same as compression, only with very high compression ratios (> 10:1). The relationship between the input and output amplitude is linear up to the threshold (T). Beyond this level, the output remains constant regardless of the input level. Limiters are useful in live recording situations to ensure that digital clipping can never occur. The signal will be unaffected underneath the threshold, but limited above it.
Noise Reduction Units (Companders)
Noise reduction units usually employ a compressor on the input stage, and an expander on the output stage. This means that the signal recorded on tape has an overall higher level, and increases the signal to noise ratio caused by tape hiss. Certain noise reduction schemes, such as Dolby Noise Reduction, use a multiband compressor, which helps make the effects of compressing/expanding less noticeable.
General Dangers of Dynamic Range Processing
It is difficult to alter the amplitude of a sound in a time-varying way without introducing distortion to waveform transients (sharp edges of attacks and decays).
A dynamic range processor reacts to the amplitude of the input signal, and there is a delay between the "cause" and "effect". This delay can be reduced with processors that look ahead to determine if there are any waveforms which will trigger processing.
