Extensive compressor reviews and FAQ

What is an expander, a gate, a noise suppressor, or an upward compressor?
Expanders and gates are two versions of one type of processor--basically the opposite of a compressor. Most (but not all) noise suppressors are also the same thing. Their sensors respond to incoming signal levels just like a compressor does, but they affect the levels in the opposite manner, increasing the dynamic range of your signal rather than decreasing it. The controls, and the meaning/function of those controls, are otherwise the same as the ones on a compressor. The threshold controls when your signal will trigger the effect, and the ratio controls how much the signal is cut or boosted upon crossing the threshold.
Upward compression has a few different meanings depending on who you ask. Most people use "upward" to mean parallel compression; this is a lazy and inaccurate use of the term, but it is the most common mainstream usage. The more correct interpretation is to reduce the dynamic range of the signal below the threshold, which can be achieved with various types of compression circuit. Parallel compression doesn't actually do that, but some people claim the results sound the same. Also some DSP plugins (like Waves) have processes they call "upward compression" that may not fit any definition but their own.
There are upward and downward expanders. Upward expansion boosts your signal by a dynamically increasing amount when the signal goes over the threshold. Downward expansion, which is what you usually find in typical rack processors, cuts your signal dynamically when the signal is below the threshold. Expanders are often used to improve the signal-to-noise ratio of a recording, but they can also be used for special dynamic effects like making a bass line more percussive, or helping vocals pop out in the mix.
A gate (sometimes called "noise gate") is a downward expander with an extremely high ratio. When your signal level reaches the threshold, a gate goes from "closed" (no signal passes through) to "open" (all of your signal passes through). It's all or nothing, on/off. Some gates offer more advanced controls such as attack or decay time, but basic ones have only a threshold knob. This is a common way of controlling noise and hum from high-gain distortions, single-coil pickups, and other noisy circumstances; when you pluck your note, the gate opens, and when the note dies down the gate closes, keeping quiet until the next note. Of course this means that while you are playing, any noise you had will still be there, at full volume. I really don't recommend this unless you are making intentionally choppy-sounding music.
In the cases where a noise suppressor is not just a gate, it will be a filter that cuts out the high frequency range where noise is usually found (up near 20 KHz)--or sometimes both a downward expander and a filter. In either case, they work by removing part of your signal, cutting out highs or cutting off the beginning and end of your notes. A few of them do this cutting with less brutality, more elegance, but there is no getting around the fact that they will remove more of your signal than just the noise. Because of this I do not recommend ANY noise suppressor, at any price, unless you have no other option.
Many guitarists will find that with a high-gain amp setup and high-gain pedals there is just no way to prevent getting a wash of noise and feedback; so for them a noise gate may be a necessary and normal part of working with that big distortion sound onstage.
For anyone else who isn't cascading high-gain stages, there is usually a better way to reduce or eliminate noise. Try shielding the cavities of your instrument, or making sure it is grounded properly. Identify inherently noisy pedals, and replace them with non-noisy ones. Try using isolated power supplies for your pedals. Search for ground loops. Check out your gain staging to be sure you have each stage set as clean as possible, often by setting the input gain low and the output level high. This is because input level (gain) is usually an active boost and will add or amplify noise; while output ("master") volume is typically a passive cut from the maximum internal gain, and does not add extra noise. One way or another, it is almost always better to solve the source of noise problems rather than spending money on another piece of equipment just to do a mediocre job of hiding the noise!

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