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Syntrillium M.D. · Location: USA

Here's an excerpt from several of my threads, along with responses from some forum regulars...

A simple explanation of the Compression Ratio:

Given a 3:1 ratio, for every 3dB the signal exceeds the threshold level you have set, only 1dB of signal will pass (over and above the threshold level). ---jonrose

In other words, using 3:1, where 3=x and 1=y. What this means is that, for every X dB above threshold, the signal is Y dB at the output. So what's the difference between compression and limiting? Simply, compressing occurs with ratios below 10:1...limiting occurs with ratios above 10:1 (ie, 20:1, 30:1, infinity:1, etc)

Now, how about setting these parameters? Are there defaults that should be used? Here's some answers that might help...

THRESHOLD. This is totally dependent on the recorded level of the instrument in question. Going above the threshold is the point where compression begins. Having a default of -10 is fine, IF each and every time you record said instrument with the same levels (peak, what have you, etc)...Thresholds often need to be altered to accomodate the source recording's level

RATIO: Well, this is a bit easier to make a default out of. Again, it's about hearing what's happening to the signal when it goes above threshold, and what you're trying to achieve. Punch and Tightening can be achieved with as little as 1.75:1, 2:1, even 3:1...Beyond that, punch is not really the effect, as much as dynamic control.

ATTACK/RELEASE: Again, here's an area where listening is key. For instance, many people set a default A/R for snare drums, roughly 20ms attack and 100ms release. Fine. But, again, depending on threshold level and HOW the drums were played (and the style of the tune) a 20ms attack might be too fast, causing audible 'clicks' when the snare is hit. Backing off a few MS (ie, 25ms) will allow you to retain the transient and beef it up, without the 'clipped' sound of the attack. The release is much more dependent on the speed of the playing as well. For a fast tune, you no doubt want a fast release. For a warm-ballad, a slower release will give you a thicker, phatter snare sound.

OUTPUT COMPENSATION GAIN: This field can be your friend. If you've set your compressor properly, it's likely that the output of your file has been somewhat attenuated. Use this field to 'make up' for gain lost in the process (on some hardware devices, this control is actually titled 'Make-Up Gain')

---Syntrillium, M.D.

Syntrillium M.D. · Location: USA

From our resident forum-goer, SteveG, a bit on compression in mixing and mastering...

Basic 'String Line' mix primer

Those of us who have been fortunate enough to have been able to dissect multi-track recordings that sound as good as some of Paul Simon's do will tell you that the qualities that you are looking for have got more than a little to do with compression and limiting. It's a BIG subject, but I can certainly give you a few starting hints here.

For a start, you need to get INDIVIDUAL TRACKS sounding good and smooth on their own. Start with tracks as 'dry' as you can get them and limit any really bad peaks. How? To begin with, you probably need to get the top 5-6dB of each track squashed down to about 1-2dB. This is only a starting figure - it varies widely with individual tracks, especially vocals, which always seem to need treatment. Compression is a little harder to get right, as you have to be careful not to take all the life (dynamic range) out of the track and bring up the noise level. You have to experiment a lot with settings, but after a while you will get an ear for what's going on. Sometimes, cassette recordings 'accidentally' sound good. That's because all audio tape has a built-in mechanism which has a very similar effect to a compresser/limiter when you record it at moderately high levels. Especially cassettes. (It's all to do with magnetic saturation of the tape - any decent audio textbook will explain it.)

By now, you should be getting a good idea of the relative levels and position of each track, and hopefully the mix is starting to come together. This is often the point where you will need to apply a little overall reverb, and quite possibly a multi-band compressor to your mix, especially for radio. This will get the overall levels higher, hopefully without the bass and drums punching the rest of the track's levels up and down. You don't usually need too much of this, either - you should have got the sound pretty much right earlier in the mix. Quite often this is the 'magic fairy dust' element that gets added at the mastering stage, often with a judicious bit of final eq.
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Syntrillium M.D. · Location: USA

Also from SteveG, a little bit on Peak and RMS detection settings...

Peak or RMS? The difference is all to do with how the compressor responds to the 'loudness envelope' of your track. Peak says 'okay, there's a very high, but short transient there, so I'll use that as the reference point for the amount of compression I'm going to apply at this instant'. RMS (stands for Root of the Mean (value) Squared) effectively looks at the 'perceived loudness' values of the track and uses those to set the compression value. By and large, RMS is the one to use, as it does more or less what your ears do. Synt says that the Peak setting was only there to ensure backwards compatibility. Peak can be difficult, as it appears to do about twice as much as you think it should!

Syntrillium M.D. · Location: USA