A compressor reduces the dynamic content of a signal (that is, the level difference between weak and strong signal levels). If the input level is weak the output level is increased, while if the input level is strong the output level is decreased. Only decreasing strong signals without increasing the weak ones it called limiting.
By compressing a signal it becomes possible to use stronger average signal levels without overloading, since the signal peaks (that otherwise cause the overload) have become limited. This feature is very popular when the goal is to be as intrusive on the listener as possible. In mastering studios the expression "radio-friendly" has been used to describe this process: the more you can compress the sound, the higher you can set the average level and hopefully be heard louder than the competition. Unfortunately too much compression also makes the sound dull and lifeless, and the listener will soon experience listening fatigue.
There are certainly situations when compression can enhance a sound:
You can also reduce dynamics without using a compressor, for example with overdrive effects or by using very light guitar strings or soft picks. You can also make your own compressor/limiter from any effect level parameter, using the internal modifiers of the G-Force.
Note that even if a compressed signal is equally loud on average as a dry signal, the level peaks are lower due to the reduced dynamics. This can be seen on level meters able to display peak values. Some LED meters let you view either peak or average levels, while analog meters can only manage average levels due to their physical limits.
When a strong signal enters the G-Force Compressor the Gain Reduction meter on the front panel lights up, indicating how much the signal level is attenuated. The behavior of the Compressor is defined by the Ratio and Threshold parameters, and is indicated by the Response Curve. The Ratio's lowest value (1:1) will not do anything to the signal, while its highest value (Infinity:1) sets an absolute top limit on the signal level.
The attack of a compressor determines how fast the gain reduction starts working. In the G-Force this is done automatically. Sometimes it's possible to hear clicking sounds when you first pluck a string, which might be the attack failing to react fast enough, thus allowing uncompressed sound to slip through. The Release parameter adjusts how fast the gain reduction stops working. If you hear pumping effects from a compressor this may be cured with the release parameter.
Since the strongest signal peaks are attenuated in a compressor, its average output level can be increased without resulting in internal overload. This is is done automatically in the G-Force compressor and is called auto-makeup gain in the manual. If you bypass the Compressor block you'll notice that the compressed signal appears to be much stronger than the dry signal. This level increase can make you believe the sound has changed more than is actually the case, but by reducing the compressor level a bit you'll find that the sound difference is more subtle.
There's a software bug affecting the Compressor: nothing happens when you link a modifier to the CPR input level in the Modifier Matrix.
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