The expander is a type of dynamic processor. As the name implies, it increases the dynamic range of a signal such that low level signals are attenuated while the louder portions are neither attenuated or amplified. This behavior is opposite to that of the compressor. The noise gate is expansion is taken to the extreme, where it will heavily attenuate the input or eliminate it entirely, leaving only silence.
How it Works
The expander is essentially an amplifier with a variable gain control. The gain is never greater than one, and it is controlled by the level of the input signal. When the signal level is high, the expander has a unity gain, and when the signal level drops, the gain decreases, making the signal even lower. Figure 1 shows the basic structure of an expander.
We often represent the input/output relationship of the expander with a simple graph, like the one in Figure 2. The level of the input signal is given by the horizontal axis, and the output level is given by the vertical axis. When the slope of the line is unity (angled at 45 degrees), the gain of the expander is one - the output level is identical to the input level. A change in the line's slope means a change in the expander's gain. For the expander, part of the line will have larger slope (a steeper line). The point where the slope of the line changes is called the threshold, which is adjustable in many expanders. When the input signal level is above the threshold nothing happens, but when it drops below the threshold, the gain reduction kicks in. The gain reduction lowers the input level, increasing or expanding the dynamic range. These signal levels generally are NOT the actual values of the signal at a single point in time, but rather an average over some short time interval, often a root-mean-square (RMS) calculation. For example, a pure sine wave may be zero at certain times, but that does not mean there is no signal. The expander's gain control will see a signal with a smooth, non-zero level.
The amount of expansion that is applied is usually expressed as a ratio, such as 2:1, 4:1, etc. This is telling you that while the input is below the threshold, a change in the input level produces a change in the output that is two times, four times, etc, as large. So with a 4:1 expansion ratio (with the input level below the threshold), a dip of 3 dB in the input will produce a drop of 12 dB in the output.
When an expander is used with extreme settings where the input/output characteristic becomes almost vertical below the threshold (say, and expansion ratio larger than 10:1), it is often called a noise gate. In this case, the input signal may be very heavily attenuated or removed entirely. This is kind of an on/off switch for an audio signal. When the signal is high enough, the switch is on and input appears at the output, but when it drops below the threshold, the switch is off and there is no output. The key parameter in this case is the threshold.
Since the level sensing function is a short time average, it takes some time for a change in the input level to be detected, which then triggers the change in the gain. Like the compressor, we characterize the expander by its attack and release times. The attack time is the time required for the expander to restore the gain to one once the input level rises above the threshold. Likewise, the time taken for the expander to reduce its gain after the input drops below the threshold is the release time. The attack and release times give the expander a smoother change in the gain, rather than abrupt changes that may produce pops and other noise. Figure 3 shows how the attack and release times affect an example input signal.
Figure 3: The effect of an expander on a signal. Only the middle portion of the input is above the expander's threshold value. It takes a bit of time for the expander to increase the gain when the input level rises above the threshold. When the input level drops below the threshold, the expander gradually reduces its gain.
Why use Expansion/Noise Gating?
Expanders do have some applications in consumer electronics. For example, people may use expanders to produce more extremes in recordings on cassettes or vinyl records, which have a limited dynamic range. An expander will make the dynamics much more dramatic during playback.
The bigger application for expanders is probably in noise reduction. They will help reduce feedback and unwanted audio - ambiance sounds and bleeding from other instruments. Noise gates are most often used to eliminate noise or hiss, which may otherwise be amplified and heard when an instrument is not being played. The threshold needs to be high enough such that the ambient noise falls below it, but not so high that the instrument's sound and sustained notes are cut off prematurely.
Expanders can also be used along with compressors to reduce the effects of noise when transmitting a signal, audio or otherwise. The transmission channel has some limited dynamic range capacity. Compressing the signal you want to transmit allows you to increase the average level of the signal with respect to the noise in the system, reducing the effect of the noise. An expander is then used on the receiving end to return the transmitted signal back to its original dynamic range. This process of compressing and then expanding a signal is called companding. For a more tangible example, the transmission channel above could be a cassette tape that is being recorded on. Compressing the signal when recording and then expanding it on playback reduces the overall noise level. This is the basic idea behind the Dolby A noise reduction technique.
Placement in the Effects Chain
It is universally agreed that the noise gate should be near the end of the effects chain. Since other effects can produce noise, you want to have the noise gate after them so that noise is not heard and amplified when you are not playing. However if you are using delay and reverb effects, you may want to place them after the noise gate so that the sound trails off naturally rather than an abrupt silence.
Eargle, John. Handbook of Recording Engineering. New York: Van Nostrand Reinhold, 1992. (ISBN 0442222904) Keene, Sherman. Practical Techniques for the Recording Engineer. Sedona: S. K. E. Publishing, 1981. (ISBN 0942080122) Orfanidis, Sophocles. Introduction to Signal Processing. New Jersey, Prentice Hall. 1996. (ISBN 0132091720)
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