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> Introduction To Effects And Their Use Part 2, Compression and compressors
Saoirse O'Shea
post Sep 27 2007, 12:44 AM
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Effects part 2 – Compression and compressors

Introduction
Strictly speaking a compressor is a dynamics processor rather than an effect and its role is to control the gain of a signal so that when a signal is compressed loud parts become quieter and quiet ones louder. It’s rather like using the volume fader on a mixing desk to keep a signal within a specific range but a compressor excels because it can react rapidly, in a reproducible manner and with precision. A human recording engineer, even a really good, may not react quickly enough to cope with very fast transient noises like a sudden bang or an occasional loud guitar note.

Compressors work by in effect monitoring part of the audio signal level electronically through a ‘side chain’ circuit. This circuit follows the envelop (shape) of the signal, normally at the compressor’s output, and generates a control signal from it that is then fed back into the gain control circuit. If the output starts to rise beyond a defined level a control signal is generated and the gain reduced.

A Kjaerhaus Audio Classic VSTI compressor plug-in.


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The standard controls on compressors are:

Threshold.
This sets the level at which compression begins to occur. When the input signal starts to exceed this threshold the output signal is attenuated. Below the threshold no processing occurs.

Ratio.
If the threshold is exceed then gain reduction occurs, the actual amount of which depends on a ratio setting. Ratio effectively varies the amount of gain reduction. The Ratio is usually expressed in the form, for instance, of 2:1. A 2:1 ratio means that for every 2 dB that the input level rises above the threshold level there will be 1 dB increase in the utput level (similarly 16:1 means that you would need the input to rise by 16dB to get a 1 dB increase in output). A 1:1 ratio is equivalent to no gain reduction. Ratios typically can be set from 1:1 through to infinty:1 (where the output level is never allowed to rise above the Threshold setting. This setting is also referred to as limiting).

Attack time.
This determines how quickly the expander/noise gate opens to allow the signal through once the signal has exceeded the threshold Or to put I slightly different it’s the time taken for the compressor to pull the signal down after it has exceed the threshold. So a fast attack time means that the compressor responds almost immediately, a slow attack time will allow the transient signal to pass through unaltered. Attack times of 1-20msec are often used. Setting the attack time to a few milliseconds is often done to improve the percussive characteristic of a guitar.

Release time.
This is how long it takes for the compressor’s gain to return to normal once the input signal has fallen back below threshold. A fast release can make the signal ‘pump’ so that you can hear the signal going, or pumping, up and down. IN most cases pumping is not desired but in rock music it can have its uses. If the release time is too long the gain may not have recovered (returned to below threshold) before the threshold is exceeded again. Release times are often 0.2-0.6 msec.

Auto attack/release
Some compressors can be set to adjust the attack and release according to the dynamics of the music. In complex recordings an auto mode may follow the rapidly changing dynamics better.

Hold time
This control allows you to set a defined amount of time that the signal level will be held for before compression occurs. Bass notes have a much longer cycle time then high pitched ones. A suitably long hold time will allow a bass note to complete a cycle before the release time is applied and so can help prevent signal distortion. A 50msec hold is suitable down to 20Hz.

Hard knee and Soft knee
These are not usually a control but a characteristic of the actual compressor (some compressors allow you to choose between different hard/soft knee characteristics). In a compressor nothing happens until the point where the threshold is exceeded. If the compressor now immediately comes into play according to its settings the characteristic is a hard knee – there is a very clear and sudden change in the input to output signal gain at the threshold. A soft knee by contrast brings the gain reduction in more slowly over a range of say 10dB. As the input level increases the compression ratio is relatively slowly increased up to the ratio set until the threshold level is reached. Soft knee gives a more subtle effect then hard knee and can be useful for complex mixes.

Stereo link
In processing a stereo signal it is important that both channels work in unison. Without it the stereo signal can appear to wander around the sound stage: if one channel reacts with more gain reduction then the other the signal can appear to move to the channel with the least amount of reduction because it sounds louder! A stereo link forces both channels to act together.

Using a compressor
As with eq, use sparingly is my best advice. In particular if you use too much compression whilst you record then it becomes very hard to use it during mix down – in essence you have reduced the signal range whilst recording and so have less range to play with in mixing.

The less compression you use the more natural a mix can sound – this is one area where automation is really useful. You can automate the volume on a sequencer such as Reaper rather than use compression.

I would suggest that for anyone using a software sequencer – pay attention to your levels when you record. Digital distortion/hard clipping is particular horrid and can occur easily if your recording signal goes into the red (more about this elsewhere).

It is routine to compress vocals, bass guitars, acoustics and occasionally electric guitars but overdriven/distorted guitars require little if any processing (it’s pretty much already occurred). Some useful settings to start with are given in table 1 below. Table 2 gives some specific settings for guitar.

Table 1 – Useful compressor settings

Vocal:
Attack – fast. Release – 0.5s/auto. Ratio – 2:1-8:1. Soft knee. Gain reduction – 3-8 dB.
Rock vocal
Attack – fast. Release 0.2 s. Ratio 4:1-10:1. Hard. Gain red. – 5-15 dB.
Acoustic Guitar
Attack – 5-40ms. Release – 0.5s/auto. Ratio 5-10:1. Soft/Hard. Gain red. – 5-12dB.
Electric guitar
Attack – 2-5ms. Release – 0.5s/auto. Ratio – 8:1. Hard. Gain red. – 5-20dB.
Kick/snare drum
Attack – 1-5ms. Release – 0.2s/auto. Ratio – 5-10:1. Hard. Gain red. – 5-15dB.
Mixes
Attack – fast. Release – 0.4s/auto. Ratio 2-6:1. Soft. Gain red – 2-10dB with stereo link on.

Table 2 – compression for electric guitar

1970’s-80’s Heavy rock
Attack – 11ms. Release – 0.1s. Ratio 5:1. Hard. Gain Red – 15dB.
Funk
Attack – 60ms. Release – 0.08s. Ratio – 7:1. Hard. Gain red – 21dB.
Jazz
Attack – 50ms. Release – 0.03s. Ratio – 10:1. Soft. Gain red- 27 dB.
Country
Attack - 1ms. Release – 0.025s. Ration – 6:1. Hard. Gain red – 30 dB.

Coming up – reverb.


Cheers,
Tony


Editorial note: published 1 Oct 2007


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Andrew Cockburn
post Sep 27 2007, 08:02 PM
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Coolness smile.gif

One comment from me - when you say not to record with much compression - I agree and would strengthen that to say never record with compression unless it is absolutely necessary. You can't take it off after the fact but you can always add it in the mix. With todays 24 bit low noise input cards, there is very rarely a need to squash the dynamics when recording (which you would typically do to ensure you get decent levels and no overloads) - compression in the mix is more about how it sounds with other instruments and in the overall context, and you need as wide a range of options as possible.

An exception to this is if you have a hardware compressor that you really like the sound of, or you are going for a very particular compressed effect. In general, I NEVER record anything with effects on it so I can make changes later on.

By the way T9ony, your tables of guide values are VERY useful - I am always scrabbling to find this sort of info smile.gif

This post has been edited by Andrew Cockburn: Sep 27 2007, 08:03 PM


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Saoirse O'Shea
post Sep 27 2007, 08:13 PM
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QUOTE (Andrew Cockburn @ Sep 27 2007, 01:02 PM) *
Coolness smile.gif

One comment from me - when you say not to record with much compression - I agree and would strengthen that to say never record with compression unless it is absolutely necessary. You can't take it off after the fact but you can always add it in the mix. With todays 24 bit low noise input cards, there is very rarely a need to squash the dynamics when recording (which you would typically do to ensure you get decent levels and no overloads) - compression in the mix is more about how it sounds with other instruments and in the overall context, and you need as wide a range of options as possible.

An exception to this is if you have a hardware compressor that you really like the sound of, or you are going for a very particular compressed effect. In general, I NEVER record anything with effects on it so I can make changes later on.

By the way T9ony, your tables of guide values are VERY useful - I am always scrabbling to find this sort of info smile.gif


Agree Andrew about not recording with compression - like you say add the compression to early and you've affected the dynamic range and so have so much less room to play with in the mix. Only reason why I didn't say so was both the exception you mention about a really good hardware compressor and also I've met some vocalists who really seem to need compression to help them 'get their sound'. (I've also met some who really need a heavy dose of Autotune laugh.gif )

Maybe should also add that if you have a compressor with a gate leave the gate turned off until the mixing stage biggrin.gif .

Cheers,
Tony


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Andrew Cockburn
post Sep 27 2007, 08:59 PM
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Ahh yes, vocalists - picky bunch ... "I want this exact compression. Turn down the guitar. Hey, I need some reverb. Not that much!". A genuine pleasure to engineer for some of them wink.gif


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Juan M. Valero
post Sep 28 2007, 03:51 PM
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Excelent Toni !!!! as ever wink.gif
Another thing, I saw in your profile that you are in Cadiz, isn't it ?? so, do you speak Spanich ?? well, I just want that you take a look at this post: https://www.guitarmasterclass.net/guitar_fo...?showtopic=7287
I have a problem with the noise of my rack.... (this horrible zuuuummm sad.gif)


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mattacuk
post Sep 28 2007, 04:27 PM
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Fabulous article Tony!! I never really understood what compression actually was!! smile.gif


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Saoirse O'Shea
post Sep 28 2007, 05:18 PM
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Glad you guys are enjoying it so far smile.gif .

Juan - I've posted a reply - sounds like it may be mains hum.

I can speak some Spanish - still learning - but with a local accent and I use lots of local terms so it's not correct Castellean Spanish blink.gif . Locals here understand it fine but I find I'm often corrected if I go to a big city. One of my neighbours is originally from Barcelona and he says he still has problems with the accent and getting people to understand him when he speaks Spanish blink.gif .

(Locally we don't say, for example, 'Buenas dias' but either 'buena dia' ('S' at ends of words are always dropped, or even 'Bue' rolleyes.gif.)

Cheers,
Tony


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Smurkas
post Sep 28 2007, 11:41 PM
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Great article on compression. I also love the fact that you give guidelines for compression of different things. Often when I've been to courses and such and asked for guidelines the only answer I ever get is "it depends". Of course it depends on the situation but for the new guy some starting references is pure gold, especially with compression which gave me alot of trouble in the beginning.

Everyone was talking about how important it is but few explained how it really worked or gave som really useful tips on how to use it. Nice to hear that I'm not the only one who's had trouble with vocalists biggrin.gif. Absolute nightmares. Once again, great article!


/Marcus
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Muris Varajic
post Oct 2 2007, 12:14 AM
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Awesome,just brilliant Tony!!! smile.gif

I'm messing around with compressors here and there,usually just turning knobs to see what will I get.
Now many things are much clearer!! cool.gif


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Frederik
post Aug 24 2009, 12:29 PM
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What is mains hum?
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thefireball
post Dec 12 2011, 06:58 PM
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WOW. Great post here. I didn't know anything about compression...really until now. Now I have a little more knowledge. Thanks.


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