Sunday, June 24, 2012

SNR 101 - Compression Part 1

Well it just didn't work out to do a podcast this week. Things piled up on everyone's schedule and before we knew it, there were only two of us and just half an hour to work with. So we bagged it. We knew starting out that doing a show every week was a pretty ambitious schedule so nobody feels too bad. The gigs come first. The plan had been to talk about compressors some more and specifically compression. We didn't cover any of the actual mechanics of dynamic processing for any newbie listeners so I thought I'd cover that in this post.

Think of a compressor as a little assistant in a box. You say to this assistant, “Watch this input and when it hits this level start to turn it down a little.” You set what level it starts to act by adjusting the threshold and it acts whenever the audio coming in exceeds that threshold. The next important control is the ratio. That tells the compressor how much to reduce the volume. A ratio of 1:1 will do nothing and a ratio of :1 is a brick wall limiter, the output never gets louder than what you set the threshold at. In practice, anything greater than about 8:1 can be considered a limiter as it will be acting pretty strongly on the signal. Ratios of 1.3:1 to about 4:1 are common so if you're just starting out with your first compressor try 2:1 and work from there.

What do all the ratios mean? If the ratio is set at 2:1, then it takes an increase of 2dB above threshold to get an increase at the output of 1dB. If the ratio is 8:1 then it must rise 8dB to see a 1dB increase. At ∞:1, there is no amount of input above threshold that will produce an increase in volume. Some compressors have a separate limiter stage that lets you just set the limit and forget it. This leaves the adjustable stage of the device free to do some more subtle processing and still have some protection in place against random pops or a dropped mic.

Most explanations of compression include graphs that show how the different ratios look. My thoughts are, I don't care how it looks, I care how it sounds. When I'm setting up a channel I put my hand on the controls and make adjustments without looking until I hear what I want to hear. Then if I look over and see settings or readings that are outrageous I just shrug and say, “Eh... if that's what it takes then so be it.”

The one last thing we'll cover about ratios is the knee. Some compressors will have an option for a hard or soft knee. A hard knee means that it will start to react right away once the threshold is exceeded. This is best for signals that are transient like drums. A soft knee will engage the reduction in gain a little more slowly and less perceptibly, ideal for vocals.

Next there are a couple controls in the middle called Attack and Release. These let you decide how quickly you want the comp to react and how long it takes to go back to normal when the level goes back below threshold again. If you're new, stop, don't panic. Many comps have a button to let you select “Auto” or “RMS” for these settings and that's great news for you. That means that the comp will look at the average level of the signal and how often it exceeds threshold and adjust the attack and release accordingly. Works great for vocals and even for drums sometimes.

When setting the attack, you need to decide if you want the comp to start right away, just a few milliseconds after the threshold is exceeded, or wait a bit to let a tasty transient through and then go to work. Drums are a great example of this. Engage too quickly and things get dull sounding. But let a little snap of the stick hit through and then compress the resonation of the shell and you've got a nice sounding drum. With the release, you just need to listen and see if you can hear the comp squeezing things too much. If it sounds like things are pumping then mess around with the release until things start to sound more natural.

The last thing to consider is the make up gain. When you compress a signal it gets fatter but it also gets a little or a lot quieter. To make it sit right in the mix you need to turn it up an appropriate amount. If you turn it up on the fader you'll just make it compress more, and depending on where it's patched in, you may or may not have somewhere on the console to turn it up after the compressor. Mindful of this, compressors big and small have a make up gain knob. You can look at the meters and see how much gain you're reducing and crank it back up that much, but again it's best to use your ears.

There's a whole lot more to the subject so check back shortly for Part Two on the subject where we'll talk about all the different ways you can patch in a compressor, and even the dark art of side chaining. Wooooooooo. Stay tuned, it's about to get interesting.


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