What is compression and why is it so important to the music recording process? Before I answer that, here’s a bit of history on the technology behind the compressor. The early ones were based on electronic valves (tubes) and then as technology progressed they went to solid state (transistors). The tube-based compressors are the most highly valued because of their warm, smooth sound. Transistors don’t usually impart any color to the original signal so the sound comes out a bit dry and crisp. The latest wave of compressors use digital technology to emulate the old tube-based ones of the past but they don’t have the heft (some weighed as much as 100 pounds). Ok, enough of that—what exactly does a compressor do? To put it simply, it compresses the dynamic range of the incoming signal so that the highest volume part of the signal is lowered and the softest part is raised. The details are as following:
1) the incoming signal passes through a detector that determines the volume of
2) if the volume of the signal is more than a user defined volume (threshold), the
signal is sent to the compressor section. If it’s below the threshold, it passes
through unaffected to the gain section (more on this later)
3) the compressor section lowers the volume of the signal it receives by a user
defined amount and does it by a user defined speed (slope). Varying the time it
takes for the compressor to react is called the attack time and can be used
creatively as an effect. There is also a release time which is how quickly
(or slowly) the compressor turns off when the signal goes below the threshold.
4) the compressed signal is then sent to the gain section where the volume of the
entire signal (compressed and uncompressed) can be raised by a user defined
It doesn’t make a difference if the compressor is hardware or software, they all work in the aforementioned manner. The manufacturer may use different devices to accomplish the task but the end result is the same.
So why is compression so important? Compression makes the sound engineer’s job infinitely more enjoyable because in the past, the engineer would have to anticipate the loud sections and manually adjust the volume while recording. If you made a mistake, you’d have to start all over again and in a live situation, whatever hit the tape was what you heard. By compressing a vocal a bit during recording, you can keep the vocalist from overdriving the gain structure and introducing distortion. The same goes any instrument that has a wide dynamic range (practically all of them). Compression also has a special case called limiting where the attack time is zero and there is no slope so the signal never gets louder that the predetermined level. This is useful in mastering to make sure the overall mix doesn’t go in to the area where you might get some undesirable distortion.
Compression can also be used as an effect to increase the sustain of an instrument. This is usually used on electric guitars but it can be used on vocals and other instruments as well. One of the side benefits of increasing the sustain is that it brings out the harmonics contained in the signal so the sound is fuller and warmer.
This can be done to point where a guitar almost sounds like a violin. In fact, Jimmy Page used a bow on his guitar to complement the sound further.
Here are some sound clips so you can see what an uncompressed guitar sounds like versus a compressed guitar.
Notice how the clean compressed guitar has a rounder, punchier tone and the overdriven compressed guitar has increased sustain and a punchier tone as well. Compressing a vocal has similar results.
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