What is compression

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Pat McGuire

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I've read recommendations for certain compression ratios for certain things i.e. eliminating the sound of fingers sliding on an acoustic guitar. I have a sense that a compressor is something I should have but since I don't really understand exactly what it does, I wouldn't know what to get, or if I even need one. What does it do?
 
Use the search function for compression and you will learn all you need to know.

Basically it does 2 things:

1. colors a sound in a pleasing manner or sometimes non pleasing manner (depends on what you're after)
2. Controls transient and unwanted volume increases or decreases, effectively smoothing the sound into a consistant level.

They can be applied to the whole track, mix or even specific frequencies on a track or a mix.

Now go research.
 
Compression is an often overused tool for controlling the dynamic range of a signal. It has 4 parameters- threshold, ratio, attack, and release. Let's say the threshold is set at 100db and the ratio is 3:1. The attack is 5 milliseconds, and the release is .5 seconds. Until the signal exceeds 100db for at least 5 milliseconds, the compressor does nothing. When the signal exceeds the threshold, for every 3 db above 100, the compressor will allow 1. When the signal drops below 100db, .5 seconds later, the compressor will go into standby mode.
As suggested above, certain compressors act as a filter, modifying the sound even when they are not acting as a compressor. This can be good or bad. The optical compressors in the Joemeek line are well known for this, and you either love them or hate them. A problem with cheap compressors is artifact, when you can actually hear it when it kicks in (or out). The FMR audio RNC is pretty much the benchmark for a cheap, clean, compressor that works, a $179 no-brainer.
When the ratio is very high, say 20:1, it is called a limiter, sort of a brick wall instead of a soft ceiling. A note on tracking- many engineers prefer to add compression after tracking, because once employed, you really can't undo it later. Lastly, to the best of my knowledge, it won't do diddly about finger noise on a guitar, although judicious EQ may help there.-Richie
 
Richard Monroe said:
A note on tracking- many engineers prefer to add compression after tracking, because once employed, you really can't undo it later.

-Richie
Click to expand...


Not to disagree too strongly with Richard, who is a knowledgeable guy, but most professional engineers DO compress during tracking. It is an issue of what they are compressing for. The type of compression which gets over used all too frequently is what I would call "color" compression, and it is usually best to do this in the mix. However, this is a relatively small segment of the professional uses of compression. The primary use of compression, indeed the very purpose for which it was invented, is to control the dynamic level going to your recording medium (or, in radio, to your transmitter), thus keeping the signal within the dynamic range of the format. While some would say that with digital formats, the dynamic range is wide enough that this is not an issue, when your dynamic range is too wide, you lose a great deal of resolution. Careful and judicious compression while tracking is also used to improve the signal to noise ratio of the signal going to tape, though this is more of an issue in an analog studio than in a digital studio.

The trick to compression is knowing when it is appropriate, and while tracking it is rarely a good idea to commit to heavy "color" compression (though sometimes that sound inspires a better performance, in some styles of music), light compression can dramatically improve the sound quality of your recordings by using the dynamic range of your format more effectively.

(When I was in school, I attended a master class by Eddie Kramer, and though he talked at length about not using EQ or compression much during tracking, every single channel had EQ and compression while he was tracking. He was just subtle about it. This type of compression is about sound quality, not "color". He did his "color" compression during the mix, which is a very different thing.)


Light

"Cowards can never be moral."
M.K. Gandhi
 
I think Light and I don't disagree much on this one. A little compression on very dynamic sources during tracking can be OK, and sometimes it's necessary. I just think you have to be very careful. I use compression in tracking when I can't avoid it, but I record mostly acoustic sources. There are some vocalists and certain songs, though, which cause me to reach for the Joemeek, or the RNC.-Richie
 
I love it when people agree. The world seems a better place, the sky is blue, the sun is shining, I will sleep better tonight...... :p
 
A compressor won't eliminate the sound of fingers sliding against guitar strings - I've heard that using a de-esser, which can be rigged up using a compressor / equaliser combo may help with this, but don't just buy a compressor if you think that's what it's going to do.

Best way to deal with that sound is not to make it, watch your EQing and reverbing.

Luck!
 
THE ONLY THINGS THAT WILL HELP SQUEAKY FINGERS ARE:
a product called "finger ease"
lift your fingers when you slide up the neck
move the mic farther away
a de esser won't help either
 
Actually, one of the best tricks I ever learned is to soak your fingers for about 10 minutes in the hottest water you can stand before tracking. I picked that up from Jon Chappell, and it works.-Richie
 
I believe he's refering to multi-band compression, and yes, that can help tame finger squeeks, as can side-chain compressing . . . as can frequency-dependent noise gates . . . or just highlighting the squeeks in your editing software and decreasing the volume.
 
The late Robbie Basho opened each performance by rubbing his nose elaborately with his fingertips to oil them up. This may be a little much. Some squeaks can be subdued with a parametric EQ but as a rule when they are on a track they are never gone.

Compression is excellent when tracking spoken-word projects and vocalists who can't work a mic. I have worked with quite a few who otherwise disappear in the softer sections and distort in the louder.
 
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