Expansion and Gating
Expansion and Gating
Expansion is the process by which the dynamic range of a signal is increased. The expander operates by decreasing the gain of a signal as its level falls. As the signal level falls below a predetermined level (the expansion threshold), the gain is proportionally decreased, according to the expansion ratio, in such a way that the low level signals are reduced (see Figure 1). An expander therefore, is a device whose output level decreases at a faster rate than its input level decreases. By further attenuating the lower signal levels, you are, in fact increasing (or expanding) the program's overall dynamic range. These devices can be used as noise-reduction systems by adjusting them so that noise to be removed is below the threshold level, while the desired signal is above the threshold. One of the most common uses of expander is to control leakage of an unwanted sound into another signal path (recorded track or console input).
Expansion Ratio (or Slope)
The amount a signal is attenuated is an amount relative to the input signal. The expansion ratio establishes the proportion of change between the input and output levels. Some common ratios are 1:1.5, 1:2.5, 1:4, 1:5, and 1:10. A 1:5 ratio means that for every 1 dB of input level decrease, the expander output will decrease 5 dB. A 1:10 ratio means that a 1 dB input decrease will result in a 10 dB output decrease.
Attenuation Range (or Limit)
The expansion ratio determines the rate at which the signal will be attenuated, but it does not control how much the signal will drop. The attenuation range or expansion limit controls the amount of decrease in signal level. The attenuation range control allows you to set the finite amount of attenuation the slope will achieve. This control is usually variable ranging from 0 dB (no reduction) to infinity (but more commonly, 60 dB).
The expansion threshold is the level at which the expansion ratio will take effect. It is the level at which the expander will start attenuating the input signal. The threshold control is usually a continuously variable circuit allowing the user to make adjustments based on the nature of the material being expanded (see Figure 2).
The attack time is the length of time it takes the expander to come out of its expansion mode. It determines how quickly the signal returns to unity gain as the expander comes out of attenuation. Attack times, which range in values from .25 ms to 10 ms, can have a major effect on the quality of the expanded signal. Attack times are best set by ear on the material you are expanding.
The release time is the opposite of the attack time, governing the audio release as the expander goes into the downward expansion (attenuation) mode, making the signal die away. It controls how quickly (or slowly) the expander attenuates. Release times vary from a few milliseconds up to several seconds. This control is also best done by ear while listening to the material being expanded.
One other type of expansion device is the noise gate. This device acts as an infinite expander and allows a signal above the selected threshold to be passed through to the output at unity gain. However, once the input falls below the threshold level, the device effectively shuts down the signal by applying full (infinite) attenuation to the output. In this way, the desired signal is allowed to pass, while background sounds and noise that would otherwise be heard in the pauses in the music are not. This device also is effective in reducing leakage. Expanders can be set to act as noise gates (with an attenuation range of infinity).
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