ryg0r
Advanced Member
Australia
2,807 posts
Joined: Aug, 2002
34 hardcore releases
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 Posted - 2005/09/21 : 05:03:05
This is blatantly stolen from the ReasonStation.net board, but its great info.
quote:
Compression is used for a few reasons but probably the main reason is to make a certain instrument or the total mix louder. How does compressing a "signal" or "sound" make it louder? It doesn't. or at least not exactly. Before you go crazy let me explain.
Below is one example:
A signal or sound may be comprised of several notes. Lets visualize for example that a particular signal has 7 notes and 6 of them are at a lower "volume" lets say -20dB level and 1 note is at a significantly higher dB level lets say the maximum before clipping 0dB. So overall our example signal is pretty "hot" in that it peaks at 0dB, but the problem is while it does peak at 0dB most of the signal does not exist at 0dB, in fact most of the signal is audible at -20dB as mentioned. We want to be able to turn up the volume to hear those lower -20dB notes, but pushing up our channel fader on the mixer even just a hair, triggers clipping because the overall signal is already at 0dB mainly due to that one note. So what do we do now? We strap on a compressor. Why? Because a compressor with the proper settings will tame that 0dB peak and "push it down" or compress the peak down towards a specified point, in our case down closer towards the -20dB level. By compressing it, now, the overall signal is quieter since that loud peak was pushed down, and NOW, we can make the overall signal louder by turning up the OUTPUT GAIN without the fear of clipping, because by compressing or pushing down that peak we freed up a lot of headroom that that peak was taking up. Now our lower notes are louder and the signal isnt clipping.
I hope that makes sense..
Ok so you are probably wondering how this all translates to the knobs on that spiffy new MClass Compressor. I'll do my best to explain. The most important setting really is the THRESHOLD. Why? Because the the THRESHOLD sets the point or zone at which the compressor focuses on compressing. Lets say you dial in -20dB please see FIG. 1 By doing this you are telling the compressor "Anything that goes over -20dB should be dealt with! Anything below leave it alone" How the compressor deals with a signal that goes over the -20dB THRESHOLD is determined by the rest of the knobs.
The RATIO dictates how much of that part of the signal that has crossed the THRESHOLD should actually be compressed. Or in simpler terms the compressor wants to know "How much do you want to turn down the volume (gain) on this specified loud stuff?" You answer it by dialing in a RATIO setting. A RATIO of 1:1 does nothing. It means for every 1dB that goes into the area passed the THRESHOLD, 1dB will come out unaffected. So, a 1:1 RATIO is like switching on the bypass button on the compressor, nothing is actually being compressed. 2:1 means that for every 2dB that go over the THRESHOLD "I (the compressor) will only let 1dB out unaffected!". So in essence and in relationship to the diagrams and a 2:1 RATIO, the compression (red rectangle) stretches down and squashes that signal to half its size see FIG.1. - FIG. 3. By dialing in a setting of 8:1, at that point you are practically limiting since you are squashing that peak down so heavily to the THRESHOLD. If you want to limit, and need to make sure that you are limiting severely, use a RATIO setting of ∞:1 which actually means that no matter how high the dB only 1dB will pass through unaffected.
Limiting = prevents or limits the sound from getting louder than a certain level.
The RATIO of volume that you have turned down or compressed in the area past the THRESHOLD, is displayed in the GAIN REDUCTION METER. So for example see FIG.1 - FIG. 3 with the THRESHOLD set at -20dB and a RATIO Set to 2:1 that 0dB peak is squashed in half, down to -10dB. So you reduced the gain by -10dB which should be displayed in the meter accordingly.
The compressor also wants to know how fast to slam that compression down to the set RATIO. This is determined by the ATTACK knob. 1ms being the fastest setting and 100ms being he slowest. The compressor also wants to know "how long should I hold the compression down for?" or in other words "how long will i compress at this RATIO before releasing the compression". This is obviously determined by the RELEASE knob which ranges from 50ms being the quickest release time to 600ms being the slowest release time.
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