Dynamic Processors

Hello! My name is Marcos Cirino, and I am going to be explaining the basic dynamic processors. I am from Puerto Rico and I am part of a band called 23rd street. I hope this post is helpful, and I look forward to your feedback!

A dynamic processor is something that outputs a signal, where the level of the outgoing signal is based on the level of the incoming signal. In other words, a loud signal coming in will come out differently than a quiet signal coming in. There are four basic types of dynamic processors: compressors, gates, expanders, and limiters. The most common of these three is the compressor.

First of all, we have compressors. What happens when a compressor is set? The louder a signal is coming in, the less level it provides going out. In a compressor, a target level is set — called the “threshold” — and any signal coming in that exceeds that level will be reduced. The higher the level above that threshold, the more reduction will occur.

Next, we have limiters. Limiters are like super compressors. The idea is to ensure that the level does not exceed the threshold. Because this amount of compression is extreme, a limiter relies on certain functions and design that regular compressors do not have.

Next, we have expanders. The quieter the signal is coming in, the less level an expander. provides going out. In other words — it makes quiet signals even quieter. Much like a compressor, the threshold is set at a certain level. Any signal that does NOT exceed that threshold is reduced, and the quieter the signal, the more reduction is done.

Lastly, we have gates. Gates are like super expanders. Anything that does not exceed the threshold is reduced to inaudible. Again, because gates are extreme, they often require a slightly different design than a regular expander.

Now, we have talked about a "threshold". The threshold control adjusts the level where the dynamic processor will start to work. In a compressor or limiter when the loud passages at the input exceed the threshold level set, the unit will turn down these loud passages. In an expander or gate, the unit will turn down any incoming signals that are below the threshold level.   The threshold is usually adjustable by a control marked "threshold."  Reducing the threshold level means that more peaks of the signal will trigger the compressor to turn down the gain; increasing the threshold level means that an expander or gate will turn down more low-level signals.  The attack time is how fast the dynamics processor will react to a signal crossing the threshold level, going up.  In a compressor it is the time it takes the compressor to reduce gain on a high-level passage.  On an expander, it is the time that the expander takes to restore full gain after the audio level comes up after a low level passage. The release time is how fast the dynamics processor will react to a signal crossing the threshold level, going down.  In a compressor, it is the time it takes the unit to restore gain after the high-level passage is over with.  In an expander, it is the time the expander takes to turn down a low level passage (below the threshold level). 

There also exists the attack and release times.

In conclusion, I would like to explain the ratio. The ratio control determines how much the signals that are being compressed or expanded will be turned down. If a compressor has a 2:1 ratio, the compressor will turn down the gain so that if the input signal is 2dB above the threshold level, the output increases only one dB. If the input signal is 4 dB above the threshold, the gain will be turned down so that the output only rises 2 dB above the threshold (a 2 to 1 ratio).   At a 4:1 ratio the input signal has to be 4 dB above the threshold for the output to increase 1 dB. When the ratio control is set to 10:1 or more, the compressor is called a limiter because the unit is effectively preventing the peak levels from increasing any significant degree above the threshold level.

References:

http://www.recordinginstitute.com/da154/ARP/chap3Sig/asp2.html
http://theproaudiofiles.com/dynamics-processing/