Hi Mark,

Wow, big subject. Now, I'm certainly not a "guru"- there are people here with great experience and insight who are more worthy of this- but hey, I love compressors!

There are so many uses for compressors- some simple and others a little more involved. But I'd say that there are two main uses for a compressor-

1. as a dynamic level control to allow a signal with a wide dynamic range to fit onto a recording format with a narrower dynamic range, and

2. as an effect or process to create a certain feel or texture to a sound.

These are not mutually exclusive- some overlap- for example it makes good sense to compress an acoustic guitar because it has very percussive transients when played with a pick, but this also gives the guitar a certain sound when recorded with compression.

It's the same with bass guitar. A bass guitar has a naturally very variable output level- try playing an open E string, and then play e.g. a bass "power chord" high up the neck- you'll notice a dramatic difference in level. A compressor will even out this dynamic to make it smoother, controlled, and more recordable.

Electric guitars are another case- it depends whether it is clean or distorted or somewhere inbetween- a clean guitar sound has a much wider dynamic range than a distorted guitar. The process of distorting a guitar signal naturally robs some of its dynamic range- but compressing any signal with a compressor can add subtle (or huge) changes to the sound depending on the type of comp.- valve(tube)/solid state, VCA/Opto etc..

This is the tip of the iceberg, I could talk all night...but I won't.... :D

I'll let other people carry on....

Mark

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"Oscillators don't, amplifiers do....."
Anon.

Mark,

> how to use certain tools like compressors, limiters, gates, filters, etc.

Good post. Following is a little blurb covering compressor basics, and you can find my more detailed article about Equalizers here:

http://www.ethanwiner.com/articles.html

Look for "The art of equalization."

--Ethan

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A compressor or limiter is an automatic volume control that reduces the volume when the input gets too loud. Originally they were used to prevent AM radio transmitters from distorting if the announcer got too close to the mike. Then some creative folks discovered that a compressor can sound cool as an effect on voices and musical instruments.

The primary controls on a compressor are:

Threshold - also called ceiling - This sets the point at which the automatic volume reduction kicks in. Below that volume the compressor does nothing. When the input gets above that level, the compressor reduces the volume automatically to keep the signal from getting much louder.

Attack time - how quickly the volume is reduced when the input exceeds the threshold. If it's too slow, then a short burst of loud music can get through and possibly cause distortion. So when using a compressor as a tool to prevent overload you generally want a very fast attack time. But when used on an electric bass to get a little more punch, 20-50 milliseconds is often good because that lets a little burst of the attack get through before the volume is reduced. So each note has a little extra "definition" but without the full length of the note being too loud.

Release time - how quickly the volume comes back up after being reduced. If it's too fast, you'll hear the volume as it goes up and down. That sound is called "pumping" or "breathing." Sometimes this sound is desirable, but often it is not. It depends on whether you're using the compressor as a tool to prevent overloading, or as an effect to create a cool sound or add more sustain to an instrument. If you don't want to hear the compressor work, set the release time fairly long (one second or more).

Compression ratio - 1:1 does nothing. 2:1 means if the input rises to 2 dB. above the threshold, the compressor will reduce the level by 1 dB. so now it's 1 dB. above. 10:1 means you have to get 10 dB. above the threshold for the output to go up by 1 dB.

Now, how do you help clear up misunderstandings about compressors? e.g., every guitar player I've tried to explain a compressor to starts off saying, "so a compressor makes things louder, right?" And then I start by saying, "well, no and yes.........

How about an explanation of how level reduction/control can actually allow for an overall increase in level, and why that is (sometimes) beneficial and desirable?

Cycle,

> How about an explanation of how level reduction/control can actually allow for an overall increase in level

Easy. I'll use a rhythm guitar track as an example, though the same principle applies to almost any instrument, and entire mixes too.

When you strum a guitar there is an initial burst of loud sound which quickly decays as the notes fade out. So the maximum volume you can make that track before distortion is the level of the initial burst. If you add compression and set the threshold and ratio to lower that burst by, say, 5 dB., you can now make the track 5 dB. louder without having it distort. Since the ear judges volume by the average level and not so much by the peaks, the track now sounds much louder.

Compression can also be used to increase sustain, so bass and other notes remain louder for a longer time. And that too increases apparent volume. To make a note sustain longer requires raising the volume of a note as it fades out. That is, making the trailing part of a note louder to counter its natural fadeout is what makes it seem to sustain more.

To do this with a compressor you'll set the threshold low enough that the volume is reduced most of the time. Then as the note fades the compressor reduces the volume less, which is the same thing as raising the volume. For example, when you play a note on an electric bass the compressor immediately reduces the volume by, say, 10 dB. because the start of the note exceeds the threshold by 10 dB. You don't hear the volume being reduced because it happens so quickly. But as the note fades over time, the compressor raises the volume which gives the effect of adding sustain.

--Ethan

wow, this is really helpful, thx guys. My knowlage has just grown, and now I won't be AS dumbfounded when I walk into the studio and my production partner starts tweaking with the compressors