Attack and Release in Compression

Question

Okay, rather than a problem, another of my academic "how does it work" discussions.
On another forum, I've been debating the meaning of "attack" and "release" in compression.  My contention is that the attack time is how long it takes to achieve the preset level of gain reduction once the signal crosses the threshod level...and release is the opposite: the time it takes to return to no gain reduction, again once the levels cross the threshold in downward direction.
Another poster, who usually knows what he is talking about, contents that the attack and release times are triggered, not by levels hitting the threshold, but rather simply by the compressor sensing an upward or downward level change.
As proof he posted a test from Soundforge:  A tone that transitioned between -12 and -3dB at 300ms intervals.  He then applied compression with the threshold set to -16 (so everything should be compressed) and, indeed, the waveform shows the effect of attack and release at each transition even though the whole test should be compressed.
I tried the same in Audition and, although the artefacts aren't as noticeable as in Sound forge, you can see the pumping on the waveform.
I can't see that the attack and release can be clever enough to react to every change in level (especially on a real signal, not his test) so have to assume the cause is a bit more basic.
Here's a picture of the original file I made:
And here's the same file compressed with an attack of 2ms and a release of 5ms and a threshold of -15dB, i.e. lower than anything in the clip.
As you can see, you can see where the attack and release happens (I won't post a pic but, with attack and release set to zero, you can't see any transitions) so if anyone can explain this you'll cure my curiosity!

Answer 1

But I'd still like to know how software compression works anyway.
I do not claim credit for writing this description. It comes from another recording site I frequent but puts things in pretty plain words that we made a "sticky."
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Shotgun's Compressor Tools 1 of 2
What you have to do is understand what compressors do, and what each of the controls do IN GENERAL.  Then you apply that knowlege to what you want out of using the compressor and what your ears hear AT THE TIME OF USE so that you can adjust as necessary.  So, read below for an overview of the box as a whole and each knob you're likely to find on it.
Compression
From the name, one can surmise that a compressor is going to squish, squash, mash or pulverize something.  Given that we plug audio signals into it, we can further surmise that what is getting squished, squashed, mashed or pulverized is, indeed, our audio signal.  And one would be completely correct in assuming that.  But what does that really mean?
Well, consider an audio signal.  Let's say it's a recording of my mom yelling at me about leaving my laundry piled haphazardly in the hallway. First, mom starts out trying to reason with me, gently, "Shotgun, you know, it's just not condusive to laundry efficiency leaving that stuff piled haphazardly like that..." her voice is calm, even and even somewhat soft.  As I stare at her blankly, not understanding the finer points of sorting one's laundry and transporting it to the appropriate room in the house her voice becomes stronger and louder.  "SHOTGUN! I'M GOING TO BEAT THE LIVING **** OUT OF YOU WITH A TIRE IRON IF YOU DON'T PICK THIS **** UP IMMEDIATELY AND PUT IT WHERE IT BELONGS SO HELP ME GOD!"  Now she's yelling, screaming, in fact.  Her face is red and frankly, I've just soiled myself which makes the entire laundry issue even more complicated. 
Now, let's assume we're going to lay this recording of mom over some Nine Inch Nails-style door slamming, pipe clanging, fuzz guitar backing tracks.  It's going to be an artistic tour-de-force.  However, when mom starts out, her voice was hitting only about 65-70dB--normal conversational speech.  By the time she's done it's more like 105dB worth of banshee howling.  Unfortunately, our backing tracks are a pretty even volume the whole way through.  So, at the beginning of the track mom will be virtually inaduible whereas at the end she'll be drowning out my samples of whacking a stapler on a desk.  How do we deal with that?
WE USE A COMPRESSOR!
You see, what a compressor compresses is volume.  That is, technically, it compresses the amplitude of the signal, or its "gain".  So for every decibel that goes into the compressor, only a fraction of it will come out.  That means that (depending on our settings, see below) if mom's voice uncompressed winds up at 105dB then we can set our compressor so that it only gets as high as 52dB if we want.  How does that help you ask?  Won't it still be too low to hear over the backing music?  Yes it will, but read on and we'll cover that in the controls discussion.
Threshold
The threshold control on a compressor sets a level below which the compressor will do no work.  The control is graduated in dB (in this case dBV of signal level) and allows you to set an "on/off" point so that you can compress the LOUD parts of a signal, and leave the soft parts alone.  At times you may want to set this control low enough so that you're affecting the entire signal, at times you may not.  In the case of mom's rant-on-tape, what we may want to do is set the compressor so that it doesn't touch the signal until her voice reaches something like 85dB or so***, say, about halfway up the scale from softest to loudest.  So, we set the threshold so that we only see activity on our "gain reduction" meter when the track gets to a certain point. 
To USE the threshold control effectively, you generally need to use your ears.  Have some idea, before you start, of what you hope to accomplish by using the compressor and set the threshold to Capture the part of a signal you wish to do whatever that is to.  In our example I want to lower the louder parts of my mom's tirade so I set the threshold to activate the compression at some arbitrary point in the track.  I could have done it several other ways and the only way to learn which is best is to experiment and listen.
Ratio
This is the control that tells us how much signal comes out of the box relative to what's coming in.  It is graduated in terms of a ratio (hence the name) of output to input.  So, let's say we set the control to point at "2:1".  That means that for every 2dB of incoming signal, we're only going to get 1dB of outgoing signal.  Which means that at its very loudest, mom's voice isn't going to be nearly as loud as it was originally.  Keep in mind that this ratio only applies to signals that meet or exceed the threshold setting.  Any signal that is below the threshold just passes through as though the compressor weren't there (kinda). 
To use the ratio control effectively you, again, need some idea of what you want out of your compressor overall.  In our case I just need mom's voice to be more easily mixed in with the backing tracks so I just want it to be kind of even.  However, I still want it to start softer and get louder, just maybe not AS soft at the beginning and not AS loud at the end.  That is, still changing, just not as much.
Attack
The attack control tells us that, once a signal meets or exceeds the threshold, how quickly does the compressor put the smack down on said signal?  The control is usually graduated in intervals of time, usually marked in milliseconds.  So, let's say that I set my attack control to say 5ms.  That means that when the signal passing through reaches the threshold I've set, the compressor waits an additional 5ms before it begins to reduce the amplitude (again, gain).  This seems counter-intuitive doesn't it?  I mean, we want the level controlled WHEN it reachest threshold, right?  Not 5ms later.  Well, there are reasons for slightly delaying the attack (and for that matter release) times. 
To use the attack time effectively (and by now you should have seen this coming) you need to know what you want out of your compressor in general.  Do you want the signal clamped down on fairly quickly?  Or not?  How do you know?  This brings in one of the most important concepts of recording: attack and decay.  Each sound has an attack and a release.  Imagine hitting a drum (the easiest place to see this concept).  You hear the sharp, immediately loud sound as the stick hits the head, but you also hear the sound gently fade away, also.  That initial WHACK, that initial spike in amplitude is the sound's attack. Everything else is it's decay.  Note that I use these terms in a "Shotgun" type of way and there are more correct ways to say this, I think, but I tend to, over time, develop my own language, so you're at a disadvantage. 
So then, we can hear an attack in mom's voice, too.  It's more subtle than the attack of a drum hit with a stick, or a guitar player's pick against a string, but it's there.  And if we set our compressor's attack time too short, we will lose all the definition of the attack of the sound.  Sometimes that's desirable, but in our case it is not.  A very large percentage of how people perceive sounds comes from the attack. You must strive to preserve that unless it is your desire to purposely not.  Therefore, be very careful with the attacks under your care.  In the case of a vocal track, the attack of the voice will lend very much to the intelligibility of the track, so we do NOT want to destroy it. So, we may want a slightly longer attack time than 5ms here.  But we can only tell BY LISTENING.  LISTEN to the track, sweep the attack control back and forth and listen to what happens to the attack of the sounds. If it sucks, move the control.  Don't look at where it's pointing until you're satisfied with how it sounds.  Then only look for the sake of curiosity because that setting may never work the same way again.  if you're using a plug-in make sure you allow ample time for the movement to take effect.  Moving a plug-in's controls can sometimes not take effect for a full second or two after you move it so if you're sweeping it back and forth rapidly you'll fool yourself.  In the case of plugins, make a move and pause until it changes.  If it doesn't change within 2-3 seconds, maybe you didn't move it far enough.
Release
As you might guess the release control handles the other end of the signal from the attack.  That is, when a signal drops back below the threshold, how long does the compressor wait to actually stop compressing.  All the same counter-intuitiveness applies here as well. However, remember that the decay or "tail" of a signal isn't as important to the listener as the attack so you can get away with a little more here.  Again this control is going to be graduated in units of time, usually ms.  However, the numbers will be larger than the attack times.  Sometimes up into the 100's of ms or even full seconds. 
To set a proper release time, again, understand what you want out of your compressor.  Do you want a major thrashing to your sound, or do you just want kind of a gentle corrective measure?  What you have to look out for in the case of release times is pumping.  If your release time is set too short then the sound will drop below the threshold, the compressor will release it, but the sound will then jump UP in level because the compression is no longer making it softer, but it's below threshold.  That probably sounds confusing, but it happens.  And it will sound pretty odd.  The first time you hear it you'll understand why it's called "pumping".  It sounds almost like there's a new "attack" near the end of the signal's decay.  As I've said before, sometimes this is actually desirable.  Usually it's not though.  Your goal is to set a release time long enough to give the sound time to naturally decay to a point that when the compressor lets go it won't "pump" yet short enough so that the compressor isn't still active when the next "attack" comes along.  If you set your release time too long it will start ****ing around with the attacks because it's taking so long to let go the next loud signal is there before the last one is finished compressing.  So, if you get your attack set where you think it's right, but then you start losing your attack again, consider dropping that release time lower (faster). 
Make up gain
Here's where we answer your initial question of "Won't it still be too low to hear over the backing music?"  Remember that we noted that mom's voice started out so low that it was lost in the music.  And all we've done so far is to use our compressor to take the bite out of the louder part of the track so that it's not overpowering.  So, doesn't this leave the softer part still lost?  And, possibly, doesn't it make the WHOLE TRACK too soft now?  Yes, it absolutely does.  But that's what we have makeup gain for. 
The makeup gain is going to look very similar to any other gain control you have seen.  It will be marked off in dB, possibly starting at 0dB and moving up to some obscene amount like 20 or 40 or 60 or 100,000 or something.  (It won't really be 100,000).  The makeup gain does just what it says it does, too.  It allows you to "make up" the gain that you're losing by compressing in the first place.  Now, that doesn't mean it UNDOES what you just did, not by any means.  It means that you can now take your newly compressed signal and make the WHOLE THING louder. This is how we're going to get the parts that are too soft up where they belong. 
To set this control we're going to, of course, listen.  What we've done thus far is to compress down the loudest parts of the signal so that they're not so loud.  You can say that the loud parts are now "closer" to the soft parts so to speak.  So what you do with your makeup gain is to take the whole lot and move it back UP some smaller amount so that now the loudest parts are just still loud, but not AS loud and the softer parts are still soft, but loud enough to be heard.  Think of yourself playing basketball.  If you're short like me, there's no way you can slam dunk a basketball.  However, let's say you can lower your basketball goal by one foot.  Now it's lower, but you still can't slam dunk it, but lowering it any more would ruin the rest of the game because you'd just be dropping the thing in and not shooting.  So what you do is you make yourself magically grow a foot as well.  Now the goal is still a reasonable height, but you can slam dunk because you've grown a bit yourself.  Same sorta thing.  Your signal isn't so low it sucks now, but it isn't so high you can't get anything useful out of it as well.
Here's a shocker: in terms of makeup gain there IS a general rule you can keep in mind.  As you're setting your compressor's other settings you will notice the meter marked "gain reduction" giving you some idea of what you're doing to the signal.  It could be a schitzophrenic little peak meter or it could be a big, slow, thoughtful VU meter.  Either way it'll tell you "hey, you're getting about 5dB of gain reduction here pal!"  So, this tells you you can START your makeup gain at a setting of +5dB.  That should give you a compressed signal at the same general level as the uncompressed signal.  Kinda.  Sorta.  It's a really ROUGH starting point, but it's a starting point nonetheless.  Again, though, twist it and listen to get it where it really needs to be.  You may want more, you may want less.
The ******** you'll hear
Now, as you get replies to this thread there will be plenty of numbskulls along to give the following answers:
(1) Shotgun you're such a ****ing *******.  The guy just wanted some basic info, some basic starting points for his compressor why do you have to be such a *****? 
(2) Shotgun, you don't understand compression and you've never done any recording, HAVE you?
(3) Here are my basic settings and they'll probably work
None of that is even remotely true.  Sure, there are plenty of basic starting points anybody here could give you.  In fact, many of these folks have only been using compressors for about 6 months, but even THEY will have ONE setting group that they like for some reason and are DYING to tell you it in order to appear knowlegeable.  Do not listen to any of this ****.  Develop your own views on good starter compression settings by appying what you learn and what you hear and what you observe in your own experience.  There are so many different kinds of compressors that anybody who gives you a rough setting diatribe is just pissing in the wind.  In fact, many types of compressors don't even HAVE some of the controls I mentioned.  Some have more.  Also, there are plenty of points we haven't covered.  For example limiting, which is a special kind of compression that uses a very high ratio (often infinity:1).
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This isn't the complete post but it is the pertinent section.
Jack