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January 12, 2011

11

Understanding Ableton’s Compressor

compressor

When used correctly, audio compression can really make a track sound a lot more professional. If it’s not used correctly though, it’s going to adversely effect your mix.

To do the best job you can, it helps to understand what compression is and how to use it – so here’s everything you need to know about Ableton’s compressor effect.


First, the basics.

The function of a compressor is to compress or ‘squash’ the amplitude of an audio track so that it is more consistent. Still confused? This picture demonstrates it pretty well:

As demonstrated by the above image, compressing audio also means that as a whole, your audio can be louder (without any peaking occurring). You might also notice that compression takes away some of the dynamics of the audio – so, if you over compress something it will sound very ‘flat’ dynamically, even though it may sound louder.


Threshold and Output

These form the basis of how a compressor works.

  • Threshold – anything above the threshold volume will be reduced in amplitude.
  • Output – increases the entire volume of the audio.
  • The Graph – this displays the result. The horizontal axis is input volume, and the vertical axis is output volume.

So in the below image, everything above -15.6 dB is going to be squashed a little. Use threshold to control how much of the audio is compressed, and then use ‘output’ to bring the volume up as far as you can without the audio peaking.


Attack and Decay

These control what happens when the threshold level is reached.

  • Attack – how soon compression kicks in once the threshold level is reached.
  • Decay – how long the compression continues after sound drops below the threshold level.
  • So in the image below, compression will start 1ms after the threshold level is reached. 20ms after volume drops below this level, compression will stop. Attack and decay are necessary for the compression to sound smooth – without them, compression would start and stop too quickly.


Peak/RMS/Opto

These are different ways of determining when the threshold volume is reached.

  • Peak – this measures the volume by taking the highest possible point of the audio wave.
  • RMS (or root mean square) – this measures the average volume over a short space of time. Because it’s an average, some peaks may still sneak through without much alteration from the compressor.
  • Opto (optical compression) – this is slightly different again. It emulates a form of analog compression, and in doing so can sound slightly more natural.

Ratio and Knee

These control how harsh the compression is.

  • Ratio – when the compressor kicks in, it squashes audio by a certain amount, and this amount is determined by the ratio. Let’s say the ratio is 2 – any sound over the threshold level will be reduced to half the original volume.
  • Knee – the graph shown earlier is great for seeing what this does. Instead of a single ‘kink’ in the graph at the threshold volume, a high knee value will smooth the kink out (try it and see for yourself!). Think of it as a way of applying a gradual threshold.


The Rest

The last few options don’t have as much of an impact on the sound as any of the aforementioned settings do. If you’re not sure what you’re doing, then leaving them set at the defaults is a great option!

If your’e interested though, FF1, FF2 and FB are all different types of compressor model. ‘Lookahead’ is a setting that controls how far ahead the compressor looks for threshold levels in the audio.


How To Compress

Ok – that’s great, we’ve gone through what everything actually does. But what’s the best way to actually go about compressing something? The following method works well (by no means the best method though, there’s no such thing!):

  • Turn the ratio up quite high, to say, 8.
  • Adjust the threshold to a rough value that sounds ok (don’t worry, we’ll change this again later).
  • Now, it should be easier to hear the effect that attack and release have. Adjust these to taste.
  • Now that attack/release are set, adjust the ratio so that your audio sounds punchy, while still being expressive.
  • Re-adjust the threshold to perfection.
  • Add a little knee as sounds right.
  • Bring up the gain as far as you can, without causing any peaking.

As mentioned earlier, make sure you don’t over compress anything. It will leave your track sounding flat and harsh – it’s worth compromising a little volume for some more expressiveness!

Read more from Tips + Tricks, Tutorials
11 Comments Post a comment
  1. Samuel
    Jan 12 2011

    Thanks for the great explanation!

    I was using Compressor just for the Sidechain effect you can add, which is really funny when you are a newbie, and didn’t know the “real” purpose of the effect. By the way, maybe you should have explained something about the Sidechain option inside Compressor.

    Keep on with the tutorials! :)

    Reply
    • Keith
      Jan 12 2011

      Thanks – glad you found it useful. Sidechain compression will be a separate tutorial, will add a link to it here once it’s done though :)

      Reply
  2. jasonswe
    Jan 23 2011

    Sometimes the best way to -hear- the effect Abe’s compressor is having on a piece of audio is by disabling the makeup gain option. In fact, I rarely ever use makeup gain on a single sound that maintains a semi-constant level, such as a kick drum. The output gain you can set yourself.

    Another personal tip is to switch OFF the compressor between testing different envelope follower types or compressor models so that after the adjustment is made and you switch it back ON the ears are just a few seconds fresh between different settings and certain elements of the compressor’s effect on the audio will be more present to you. For instance, sometimes Opto follower allows a bit more bass from a kick drum than Peak or RMS mode. Not to say that will be the case with every kick drum. It is worth saying that not every envelope follower mode is right for a certain piece of audio in every situation. One must use his ears carefully with a compressor while mixing and compare different settings to get the right flavour.

    Reply
    • Keith
      Jan 26 2011

      Hi Jason – awesome tips. Turning on/off the compressor is a biggie – makes hearing subtle changes just that bit more obvious. Thanks!

      Reply
  3. Jackson
    Feb 1 2011

    Hey mate,

    Great article. Compression has always been one of my weak points, as I’ve never understood how to use it. Your post is really, really helpful and I’ll definitely bookmark and come back often as reference.

    Awesome site, thanks a lot for this guide!

    Reply
  4. May 11 2012

    Hello!

    I found this guide very helpfull and i would like to ask what is the difference between FF1 + FF2 then?

    Thanks!

    Reply
  5. Bandeeto
    Aug 1 2012

    Excellent tutorial! Compression is not my strong suit so I’ve been reading through tutorials online. Yours is my favorite so far.

    Reply
  6. J
    Apr 3 2013

    But I AM interested: what are the different compressor models? How do they differ and when do I use each one?

    Reply
  7. Francesco
    Aug 21 2013

    Please what’s the best way to set this compressor for mastering a stereo full track .. ?
    I don’t have another software and i’would like to use ableton for mastering my tracks ..

    Reply
  8. Alex
    Sep 19 2013

    Thanks this is great.

    One question I have about Ableton’s compressor though: is there any way to actually see a numerical dB value for the gain reduction effected by the compressor? I’m used to compressors actually showing me a number for how much dB reduction I’m getting, and often approach compression with this in mind (i.e. i want to compress so that I’m getting between -3/-4 dB compression on average) — but in Ableton’s compressor there is a just a meter with no number. It’s nifty looking but doesn’t actually tell me what I want to know…

    Reply

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