BASIC LESSON ON COMPRESSORS?

[Mitch Glasgow]

(I also posted this at "Newbies." Can you tell?)

I've actually been recording at home for many years, for the sheer joy of it, but I have always had the nagging awareness that I don't really understand compression. I always have a hard time getting answers in English (as opposed to StarTrek-like techno-babble). I know that it is crucial to understand at least the basics, in order to avoid the most common mistakes.
Specifically, what exactly is meant by "attack," "release," "threshold," and "ratio?"
And what should I be looking for? For example, if the most wildly-fluxuating dynamics are in the guitar track, how should I compress it enough to tame it, without killing it?
I'm sure this is kindergarten stuff for a lot of you, but if you could offer any info, I would greatly appreciate it. Or, if you have better things to do than teach a total stranger (which I'm sure you do) perhaps you could suggest some understanable lessons on the subject (Compression for Dummies, perhaps?).

[Gord]

Youre not the only one who doesn't fully understand compressors, most people using them do most of it by guess work including myself. To learn how the attack and release effect sound try using it on a Bass drum. Basically the attack is the length of delay the comp waits after the sound has passed the threshold before it does anything (I hope you can follow that?). By not compressing the sound straight away the attack part of the sound will be emphasised when the compressor does kick in. As the sound falls beneath the threshold the release tells the comp how long to wait before it stops compressing. (try different settings on drum sounds and you'll hear how they effect it. The threshold setting will depend on the signal level of the sound you are compressing. A poorly recorded signal will need a more negative threshold setting before the comp takes effect. you will see it taking effect on the gain reduction LED's. If you compress a sound because you think it needs compressing and the sound suddenly doesn't seem as exciting then you've over done it (ie killed all the dynamics) alter the threshold or ratio. The compressor should make the sound punchier and 'more in you're face'. The effect of the compressor is very subtle but does (almost subconciously) make a great difference to the sound. Remember also that guitar amps (especially in overdrive) have natual compression so it depends on the type of part you are dealing with wether or not you need compression at all. There is no substitute for practice and experience. Most Mixing/mastering books explain compressors, give Amazon books a visit. Try these:-

http://www.echostarstudio.com/thecompressor.html
http://www.lis.net.au/~johnsay/Acoustics/
Best of luck

[Spider]

hi there mitch , well ive only just started using compression so im kinda still trying to get it all happening myself. Although im willing to give a few of the knobs a try ..... Might be best to get someone like sonusmen to explain , in fact about 9/10ths of the people here will give you a better explanation. What ive found is the threshhold is the point at which u want the unit to begin compressing the signal. The ratio is the amount of comprssion u want to apply to the signal, eg. a 2:1 ratio , the signal will have to raise 2db for the signal to raise 1db....make sense , please someone feel free to correct me anywhere along here ... Now the attack and release , the attack is how fast you would like the compressor to drop the signal (?????) and the release is how fast you would like the signal to return to its normal level after the input level has dropped below the threshhold..And the output level is used to make up any gain loss.. That last bit (attack and release) im really very rusty on that , so get a second oppinion.. well that maybe a basic overview of the compressor (very basic).. so if anyone could elaborate on that it would be much appreciated...
cheers
spider

[DaveO]

There was another good compression article posted here recently. It complements the one Gord cited from Sonusman. In fact, Sonusman makes a noteworthy contribution to this article :

http://www.geocities.com/shailat2000/

[Mitch Glasgow]

Wow. Thank you very much Gord and Spider for such a quick response. I think I'm starting to understand now. Maybe I can paraphrase, and you let me know if I have it right. Also, your responses raise a couple more questions.
First, if I understand correctly:
1. The threshold is a level that you set, and the job of the compressor is to compress any signal that exceeds that level.
2. Attack determines how fast the signal is compressed after it exceeds the threshold.
3. Release determines how long a signal is compressed before it is allowed to return to an uncompressed state.
4. Ratio... I'm a little hazy on this one. At a ratio of 4:1, the signal would only be allowed to increase 1dB for every 4 dB of actual increase in volume (once the threshold has been exceeded). Is that right?

Hey... I feel the power! Am I getting warmer? Let me know. Again, thank you so much.

Now for the new questions... Is the threshold measured in decibels? And if so, how do you know exactly what level you want to set as the threshold (If, for example, you want to bring up the over-all volume a bit while mastering)? Can you tell just by watching the "spikes" on the meter, or is there a more scientific way to nail it (dB meter)? Also, on a split-frequency or multi-frequency compressor, does that mean that only frequencies above or below a selected point are effected by the compressor? Gosh... that would make sense.

[Edited by Mitch Glasgow on 08-06-2000 at 20:10]

[MGS]

Mitch,

You are correct about your understanding of ratio. You are more or less correct about the release control although to be exactly correct it is the length of time that the compressor continues to compress after the signal falls below the threshold. Yes, the threshold is measured in decibels. As for setting the levels, just experiment and listen until you hear the desired effect. I don't recommend relying on the reduction meter to gauge how your product will sound. Although, I often record the same artists in the same way on a regular basis using the same equipment so, at this point, I can tell what's happening to the signal just by viewing the reduction meters on the compressor alone. Check out my response to your posting in the Newbies board and feel free to further the discussion.

Mark