Compression how-to

[Elektronkind]

I recently learned this trick about correctly configuring a compressor, and I love it.

Think of a compressor with it's 4 main settings (Attack, Release, Ratio, and Threshold) as a safe, and you have to turn the tumblers in a certain order to open that safe.

ARRT - in that order.

Preset the knobs as such:

Attack - anywhere
Release - minimum
Ratio - maximum
Threshold - most sensative

Then fine tune the above settings in the following order:

Attack
Adjust the attack of the compressor until the sound you want is satisfactory. It may be a thick sound, or depending on the instrument, a thin one. whatever the case is, stop fiddling with the attack once you hear something you like.

Release
The trick with the release is to tune it so that the sound stays within the groove of your song. You want the sound to glide back at you, so don't set it so low that the trail off is burried. Remember, this affects your groove and is hand-in-hand with Attack as far as asthetics go.

Ratio
The trick with Ratio, which you pre-set to maximum, is to lower it without losing what you came up with in setting the Attack and Release... and too *little* Ratio is too much compression. Adsjust the Ratio so that your sound stays wide and big but still within bounds.

Threshold
For drums I keep this at 1:1. But for sounds which vary in intensity (vocals, wind instruments, etc), you don't want the compressor compressing all the time, so that's where threshold comes into play.

/dale

[Elektronkind]

I would like to add that if you have ANOTHER compressor in your chain somewhere, remember that compressors are NOT additive towards one another, they're multiplicative.

if you have your first compressor set at 5:1 and the next one at 2:1, you're not getting a total ratio of 7:1 but actually 10:1.

This is why I hate clubs who think putting a compressor into their system when playing recorded, mastered music is such a good idea... it isn't, and only serves to make things sound shittier because it's now over compressed and is at god knows what setting.

/dale

[Elektronkind]

Okay here are two samples I just made using a dry drum loop out of Stylus

With no compression:
http://elektronkind.org/audio/loop-nocompress.wav

With Compression:
http://elektronkind.org/audio/loop-compressed.wav

(661kbytes each)

Notice with the compressed version, the snares and hi hats have more "meat" on their bones and really aren't as thin as the uncompressed version is. The difference is subtle but that's the goal... otherwise the it sounds kind of naked.

/dale

[Euphoria227]

This is why I hate clubs who think putting a compressor into their system when playing recorded, mastered music is such a good idea... it isn't, and only serves to make things sound shittier because it's now over compressed and is at god knows what setting.

/dale

Using compressors in a club system is not option, it's a requirement. Compressing signals prevents distortion and clipping, which sounds like shit and blows expensive drivers. The idea is to turn the source, in a club setting the DJ, all the way up without producing a distorted signal from the mixer, then split the audio signal into appropriate band passes for the sound system with a crossover (usually low, mid, and high) and compress each band pass so the amp does not clip. DJs who push the equipment harder than it was designed to go at that end can create distortion by turning the gain up too high or turning the EQs knobs all the way up.

But in the system, the compression serves to eliminate clipping. When setting it all up, the engineer would unplug all the speakers and run a strong signal to the crossover. First the crossover points would be set, the frequencies where sound is divided into low, mid, and high, or more if the system is quad-amped. Like I said, each bandpass out of the crossover is run through a compressor, then to the amps powering those drivers. The engineer starts with a ratio of Infinty:1 with the threshold at its highest setting. By raising each band pass on the crossover until it just barely clips the amp, the engineer can then back down the threshold to produce 2-6db reduction and eliminate clipping. This is done to each bandpass to get the most headroom out of the system. Lows are the loudest sound in most club music. Running one compressor, the lows would trip the compressor and squash the mids and highs too much.

[Elektronkind]

Oh god, so you're one of THOSE people.

"Those people" being the kind that say "let's just throw a compressor in there!"

Compressors are a very specific thing, they're not something you can stick in a system permanently, on some permanent setting, and expect it to work for every situation. I've had my low end up to 3khz totally drowned out by a club's over zealous use of compression in the chain. Could it be bypassed or at least set to better levels? Noooo, the "sound guy" said it was at some best setting, as if it were some audio magic wand.

And I don't know where you get "compression prevents clipping." A compressor can easily send out too hot a signal if it's set wrong (see above.) This is the job of a limiter, which should be in the chain prior to compression and post-fader if there's a house mixing board involved. The limiter keeps the peaks under control, and if there must be a compressor, it should be chained afterwards to expand the quiet moments.

/dale

[rajdeep]

I thought the attack, release and the threshold essentially worked like a limiter. If you set a quicker attack and release with a 0db threshold you'd get the desired effect of a limiter - compressing signals that cross the 0db threshold but making sure there's no clipping by smoothing out the wave using quick attack and release times.

[Elektronkind]

The ratio is what sets a compressor a part from a limiter.

I failed to notice in my reply that Euphoria mentioned *infinity*:1 raitio... and that is in the limiter function realm of a compressor.

Compression is where you have ratios set from 1:1 to 10:1. Anything higher than 10:1 up to infinity:1 is considered a limiter.

When Euphoria was talking about compression, I thought he was talking about 1:1 - 10:1, not the ratio range considered limiting... but he did say inifinity:1 so props to him.

/dale

[empath]

that was all fucking greek to me... i need to read up on sound engineering..

[hue-e]

being new to production for the most part... this is exactly what I needed to make my shit sound decent... thanks :thumbsup:

[rajdeep]

John, get hold of an Alesis Compressor manual. They explain the terms in there with graphics.

[Elektronkind]

Here's a visual aid. A good thing to have in our aural world.

Below is a screenshot I did using input from my DJ mixer.

The top window is just a channel fader/EQ/VU. I pumped the bass up a bit to cause almost a constant overage on the signal. Look at the VU meter... man, that's a hot signal! We need to do something about that.

The bottom left window is the next item in the chain, a compressor. But it is set up as a Limiter. Note both the low Ratio and the high Threshold. Attack is set (in milliseconds) so that overages are glided into, as too low an attack will cause a "pumping" sound... which may actualy be desireable in music production but likely not on the DJ booth.

The bottom right window is the last item in the signal chain, another compressor. Only this one is set as a true compressor, not a limiter as the other one. I set the Release to give the burried kick drum a bit more staying power in the particular track I was playing. But overall everything stood out.

The Limiter, followed by Compressor setup gives you two things:

1) The limiter is the first to see the signal, and so it pulls down any threshold crossings and keeps the signal cool.

2) The compressor takes any quiet parts of your signal and expands them, giving them a wider spacial feel.





/dale

[rajdeep]

I'm a bit lost on the ratio. What are we comparing when we say the ratio is X:1?

[Elektronkind]

Here's an analogy for the ratio.

Think of the ratio as a telephoto lens. The higher the ratio (ex: 15:1), the smaller and more controlled the sound gets (hence the high ratio compressor being called a limiter.)

The smaller the ratio number, the bigger the sound gets and feels.

The Ratio comes into play when the sound crosses the level as set by Threshold. So take the bottom left window in my screenshot. The Ratio is set at 2.15:1. This means that if the signal is 2.15dB higer than the Threshold, the signal is increased by 1dB.

Once you get into high ratio, as in a Limiter situation, the opposite happens... the sound gets smaller and more contained.

/dale

[Euphoria227]

Oh god, so you're one of THOSE people.

"Those people" being the kind that say "let's just throw a compressor in there!"

Compressors are a very specific thing, they're not something you can stick in a system permanently, on some permanent setting, and expect it to work for every situation. I've had my low end up to 3khz totally drowned out by a club's over zealous use of compression in the chain. Could it be bypassed or at least set to better levels? Noooo, the "sound guy" said it was at some best setting, as if it were some audio magic wand.

And I don't know where you get "compression prevents clipping." A compressor can easily send out too hot a signal if it's set wrong (see above.) This is the job of a limiter, which should be in the chain prior to compression and post-fader if there's a house mixing board involved. The limiter keeps the peaks under control, and if there must be a compressor, it should be chained afterwards to expand the quiet moments.

/dale

Actually I'm a manager of a professional sound company that handles events all over DC, Baltimore, Southern PA, and Virginia.

I did loosely use compression when I probably should have said limiting. Our company uses two kinds of compressor/limiter/gate units. DBX 166XL used mainly in our amp racks to limit band passes pre-amp. The others are Behringer Composer Pros, used in FOH racks for gating and vocal compression. These two units have the same knobs on them, but work very differently. The important ones are the ratio, threshold, and peakstop limiter.

The DBX ones on our systems use an infinity:1 limiting ratio in the compressor section. Then the peakstop limiter is applied to barely flicker. Infinity:1 limits the signal nicely. The Peakstop on this one helps lower the highest peaks the compresor didn't reduce quite enough, giving a very clean signal to the amp.

On the more barebones Behringer unit, the peakstop is set as an infinity:1 ratio. This actually makes it a little more easy to use because you can use a softer ratio, and then have a hard limiter cut the whole signal if needed.

The Alesis 3630 is another good lower priced compressor. It has a switch that lets you apply the compressor to the RMS signal or the peak signal, which can greatly improve quality, depending on application.

[Euphoria227]

I'm a bit lost on the ratio. What are we comparing when we say the ratio is X:1?

It is the ratio of input:output above the threshold. For every X db of input above the threshold, the compressor lets out only 1 db of output.

2:1 ratio, threshold at 0. +6 signal in yields only +3 output
6:1 ratio, threshold at 0. +6 signal in yielss +1 output