Author Topic: Audio Compression  (Read 6570 times)

  • Wizdumb
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Audio Compression
« on: April 12, 2013, 09:04:39 PM »
Dynamic range compression, abbreviated as RDC and often referred to simply as compression is one of the most commonly used tools in the arsenal of a mixing engineer. Not only is it used efficiently by skilled audio engineers, but very much overused and misused by more unskilled or amateur producers and/or engineers. To help people understand and utilize compression in a more productive manner, I'm assembling a collection of information you need to know about audio compression in this post.

Compression controls and features on your standard compressor:
Threshold - What triggers a compressor to react and reduce the level of an audio signal is the compressors threshold. When the audio signals' level exceeds the set threshold, typically a set dB level, the compressor reacts and reduces the signal. The higher the threshold the lower amount of the signal is affected by the compressor because the signal has to at least reach or exceed the threshold for the compressor to start working. As you lower the threshold more and more of the signal is meeting or exceeding it, so more of the signal gets compressed, or at least more of the signal triggers the compressor.

Ratio: The ratio on a compressor is literally a ratio set by the engineer which determines how much gain reduction will occur when the compressor is triggered. Once the signal meets or exceeds the threshold, it is reduced by an amount determined by the ratio. A ratio of 8:1 specifies that an input of 8 dB over the set threshold will result in an output of only 1 dB greater than the threshold. This means that if the signal is to exceed the threshold by 8 dB then it will be reduced by 7 dB. Here is a simple example to make the ratio easier to understand:
If you have your threshold set to -20 dB and the input signal reaches -12 dB (8 dB over the threshold), then it will be reduced by 7 dB to an output of -19 dB (only 1 dB over the threshold rather than 8 db).

Attack:
The attack setting is used to determine how fast the compressor reacts after the signal meets or exceeds the threshold. The time it takes for the gain reduction to occur after the signal triggers the compressor by meeting or exceeding the threshold, is the attack phase. This is typically measured in ms, and the higher the attack the longer it takes to reduce the signal.

Release:
The release setting on a compressor is how long it takes for the signal to go back to an uncompressed state after being reduced. You can think of the release phase as the compressors effects wearing off in a sense. This is also measured in ms typically, and the higher the release the longer it takes for the signal to bounce back to it's natural state. If the signal hits or exceeds the threshold again before the release phase is complete, then it restarts.

Gain or "makeup gain":
The gain setting is used to increase the signal of the compressors output. Because the compressor has reduced parts of the signal that were meeting/exceeding the threshold, it's often required that you use gain to increase the entire signal back to the desired level of volume. This is not to be confused with doing the opposite of what the compressor is doing. The dynamics of the signal have been reduced by going through the compressor, meaning that the distance between the loudest and quietest parts of the signal is now smaller, so increasing the output of the gain is not just bringing those loud parts back up but increasing the gain of the whole signal.

Essentially compression is merely reducing the dynamics of the signal. It's often misunderstood as making the Volume or perceived volume of the signal louder. The reason that a sound can appear louder after compression is because after being compressed the loudest parts of the signal are closer to the levels of the more quiet parts of the signal, which means the volume of the entire signal can be increased to higher levels without clipping occurring. You should keep in mind that dynamics are a key characteristic of a sound and flattening out the dynamics of an instrument just to get higher levels does not nescessarily make the sound better, and in fact probably makes it sound worse.

If you have a sound that is too compressed you can attempt to rebuild some of those dynamics with the compressors counterpart, what's known as an "expander". This does the opposite of a compressor, reducing parts of the signal that fall below the threshold to further the distance between the loud parts and quiet parts of a signal, essentially creating more dynamics throughout the sound/signal.

Hope that was helpful, and if you have anything to add feel free to reply with additional information that should be included in the audio compression explanation...
   

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Re: Audio Compression
« Reply #1 on: April 12, 2013, 11:01:08 PM »
Wizdumb, this is a really good explanation of audio compression. I attempted to write a similar guide about audio compression on my other website a couple months ago, but it wasn't as clear or as easily understood. I'm going to go ahead and throw a sticky on this post, it'll be really helpful for beginners and people that don't quite have sound compression down yet.

  • NickJ
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Re: Audio Compression
« Reply #2 on: April 16, 2013, 09:20:31 PM »
Great and informative post. Sometimes audio compression can be the trickiest part of the overall process. One you get the hang of it, most other things should come easy. Not all, but...most. ;D

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Re: Audio Compression
« Reply #3 on: April 19, 2013, 06:39:49 AM »
Excellent post. I would like to share some of my experience in the area which is, albeit unprofessional, totally hands-on (or ears on?? :P )

Compression is an effect in the dynamics processing category. When talking about audio, the term "dynamic" appears a lot, but you have to start by "dynamic range", which is how much "space" you have to play with the level of a signal (instead of "volume", which is more related to the perceived sound pressure, which varies with frequency... and with each human as well).

http://en.wikipedia.org/wiki/Dynamic_range#Audio

When you compress a signal, you have all the parameters (threshold, ratio, release, pre and post gain, etc), and in some plugins and environments you can quantize its usage as well. And you can also side-chain it. TIP: lots of plugins and fx can be sidechained to the audio of another channel.

So, you can just compress a specific channel, to obtain the effect you desire which could be to compress the loudest signals, bring them closer to the lower/medium signals, and then boost the entire compressed signal to a new level, or even to apply an envelope and then compress, using the compressor not only as a dynamic processing effect but as one from the color processing category. It is very useful to pre and post eq / filter a compressed signal to get useful effects.

You can also compress the master channel. It is a good idea not to be so extreme in master channel compression, and use a threshold/ratio combo that does not affect the mix in its color.

When you sidechain a compressor, you usually have a BASS and a KICK, you put the compressor on the bass channel, and side chain it to the kick, then you adjust the parameters, and you will notice (remember to solo the compressed signal, mute/umute the kick, etc) how the bass obtains a RHYTHMICAL characteristic, as well as allowing the kick to be heard, as bass and kick share lower frequencies and adding up frequencies does not help in the mixing process.

USE FILTERS THE TAKE OUT UNWANTED AUDIO, to help getting a healthier mix.

To fully understand compression coloring, you can use a spectrum analyzer. If you use ableton live, that's a great combo to understand audio processing.
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Re: Audio Compression
« Reply #4 on: April 22, 2013, 06:47:08 AM »
Naomi, you will notice that there are lots of things involved. You can even filter and eq the reverb. Imagine the possibilities.

Things to master:

All types of Compression (sidechain, parallel, standard et al)
Reverb
EQ
All delay based effects and how they affect the spectrum and dynamic range occupation.
Filters and related effects and delay/filter combos.

And remember, always start at -6 / -12 db range (depending on standard note pitch and duration, or instrument type, short, sharper sounds need less db to be noted by the listener, the opposite happens with low-end sounds) , , you have time to add more volume later, at the post-mixing and mastering stages.

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  • DJ Bangher
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Re: Audio Compression
« Reply #5 on: August 28, 2013, 05:43:43 PM »
Excellent Guide/tutorial man! I really was struggling with compression until I followed this and referenced it a couple times when working on tracks. I didn't even know that compressors, limiters, and exciters were all considered "dynamic range compression" lol. Hell I didn't even know the term dynamic range compression ! Thanks a lot Wizdumb really really helped me

Re: Audio Compression
« Reply #6 on: September 02, 2013, 05:41:11 AM »
Very in depth! Thank you. I had to learn compression inside out for an exam and wish i'd found this article before!

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Re: Audio Compression
« Reply #7 on: September 13, 2013, 10:31:22 PM »
Dope post Wizdumb. Really helpful for learning audio compression.
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