compressor

How Does Audio Compression Work? #1

Today we’re looking at dynamic processor – compressor.

Compressors are being used by audio recording studios throughout the world as a tool to shape the dynamics of an audio-recording. Therefore, it could be categorized as a dynamic processor or dynamic effect. Normally compressors are being used as an insert-effect, and that means it affects just one channel that’s being inserted on. For example, a compressor could be inserted into a vocal-track. Whereas the vocalist would get too loud – the compressor would automatically reduce the volume of that vocal track.

Lets take a look at a compressor’s basic overview.

A compressor reduces the volume of an audio track, just like you would do with the fader on a mixing console, but the compressor does this is a lot faster than you would be able to do manually.

The basic idea of a compressor is that you set the “threshold”. When the level of your audio signal reaches that threshold and gets over it, the compressor actually starts compressing. The amount of compression that takes place is what you set with the “ratio”, and that determines the amount of compression above the threshold.

threshold

The easiest way to explain what a compressor does is to say that it makes loud sounds softer. That’s what the traditional compressor does.

These are the first functions and settings of a compressor:

You start up with the THRESHOLD, and make sure that the level peaks over the threshold or else the compressor won’t work.

Next step is to set the RATIO, and that determines how hard the compressor works.
Ratio 2:1for every two decibels above the threshold only one will be outputted by the compressor.
Ratio 20:1for every twenty decibels above the threshold only one dB will output. So that’s is called ‘hard compression’. If you compare with ratio 2:1, that would be soft compression.
Ratio 1:1for every 1 decibel above the threshold, also 1 decibel will output. That’s a neutral position, and you will have no compression at all.

Let’s listen to a dynamic vocal track without any compression on it and then compare that with a track where I actually apply compression on so can hear the difference.

Vocal not compressed:

Vocal compressed:

compression

 

Here, I’m playing an original drumtrack, with no compression on it:

Now compressed with ratio 2:1

Ratio 4:1

Ratio 10:1

As you could hear the sound got a lot softer when I applied the ratio 10:1

Especially when I lowered the threshold. That is because I’ve just used the “threshold” and the “ratio” setting of this compressor. And I didn’t look at any of the other settings yet.
So, what I haven’t looked at yet is that we can compensate the amount of compression or gain-reduction that occurred by applying makeup-gain or compensational gain.

Here is the original drum-signal:

And here you can listen to compressed drums but with make-up gain.

I actually compress it with a ratio 10:1 and than I bring up the level to match the original input level.

What has happened here is that I have reduced the amount of dynamic range.

I made that loud sounds softer, a lot softer, so I brought them closer to the soft sounds, like the high-hat, and then I put up the volume of the overall drums back. Basically what I did is that I brought the soft sounds and the loud sounds closer together. And you can really hear it sounds more aggressive this way.

We’ve overlooked the most basic controls that a compressor has:
threshold – level at which the compressor starts working.
ratio – how hard the compressor works on signals that need to be compressed.
gain – compensates for the amount of compression.

There’s two other things that are really important of setting up a compressor.

And that is attack and release time. These are settings which work on an amount of time set in milliseconds. Some compressors actually offer you an option ‘fast-attack’ or slow-attack or auto-release and these things. But it’s always similar to that.

The attack-time of the compressor determines how fast the compressor applies the compression, when the signal gets over the threshold. Whereas the release actually tells how long it takes for the compressor to stop compressing when the level jumps below the threshold again.

That’s basically the attack and the release-time. Those are really important in setting up how fast will the compressor actually work. If you got a snare drum, and you set a compressor on it with a really slow attack time, the compressor is going to work barely or even not at all, because the snare drum was too slow.

To sum up. The attack-time determines how fast the compressor starts to work. I’m gonna show you an example, so you can hear and see it at the same time.

That concludes part one of the first tutorial about compression techniques.

In part two I’m really going to focus on how to set up a compressor, and how they’re being used. I’m going to look at different compression-techniques like New York drum-compression, side-chain compression and all these other techniques.

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Till next time!

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