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SafeandSound Mastering Offline
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PostPosted: Mon May 02, 2011 6:15 am    Post subject: How to use compression Reply with quote

How to use compression - compression can be a little daunting for those who are new to audio production and with 1,000's of audio forum threads there can be a pressure to use it because everyone and their dog is. This is a basic tutorial that will help you understand why it is used and some basic advice on how to practice much needed hand to ear co-ordination.

Audio compressors can be analogue or digital and you probably already have a software compressor built into your digital audio workstation such as Logic, Cubase, Fruity loops, Reason, Ableton Live, Reaper, Adobe Audition or Pro Tools. If you wish you can purchase an analogue compressor for practicing too. A reasonable analogue compressor can be purchased for $130.00 and this will be perfect for practicing the techniques that will be explained within the body of this article. I suggest a few compressors you could try out, this is not an endorsement but just a few ideas for budget devices which will be suitable for practicing with. A Behringer Composer Pro XL MDX2600, Samson S-Com Plus, Alesis 3630 all have the controls which will be discussed.

Audio compression can be a confusing theoretical and practical audio process. The reason is that when learning to use a compressor the results of applying compression change the sound in real time. It requires a certain level of listening experience to be able to hear the changes as they are often quite subtle. As such it is best to use fairly extreme settings when learning how to use a compressor. The processing results will then be more obvious and easier to hear. We start with the basic controls on a compressor.

Threshold - control which determines at what level compression will start.
Ratio - How much compression will occur when threshold is exceeded.
Attack - How quick compression will start when threshold exceeded.
Release - How quick the compression will stop once the audio has dropped below the threshold. (dependent on detection circuit)
Make up gain - output level trim. Increases overall output level of audio signal post (after)compression.
Gain reduction - this is not a control knob, but many compressors have a meter which show how much gain reduction (compression) is being applied in dB (decibels). When learning it is a useful meter to see.

Using a compressor and learning all the different ones there are is an art unto itself. However this does not mean it is necessarily complicated to use a single device. The reason compression appears complex is mainly because there is an interplay between the controls on the units themselves and the required goal. Firstly we will ask ourselves why do we want compress sounds? A goal needs to be determined as this makes for a solid reason to employ a compressor. (as opposed to using one because it is the "done thing" or because everyone on audio forums are using compressors)

Compressors can be used for a number of reasons, compressors can even out the differences between the levels of snare hits for example. This might make mixing easier. If you want your snare drum to be more even in level you may compress it, it is a practical reason. You could not sit there pulling the fader down for every snare drum hit. Even if you automated this it would take a very long time.

Compressors can impose their envelope (time constants of attack and release) on a sound and adjust the attack and release of a sound. For example a snare drum. You can increase the attack and adjust the "body" of the snare depending on where the release time is set. So compressors can be used to tonally shape sounds.Compressors can smoothen out sounds by reducing the transient attack detail in an instrument.On a complete music mix a high end compressor can produce a sense of "glue" or "gelling" of the sounds which may previously sound a little disparate or disconnected.

Whilst compression is not particularly complicated it will take time to practice and perserverance to get to grips with. Bear this in mind when you are practicing with compressors.

Compressors control dynamic range, in short they make louder peaks in a sound source quieter, that is it the easy explanation. (like a fader or volume knob being brought down when a loud sound is detected). How they make loud sounds quieter is dependent on some of the controls. The threshold determines when a compressor starts to reduce the level of a "peak" or loud sound, usually expressed in milliseconds, the shorter the attack the quicker compression starts. Ratio is the "amount" of compression, higher ratios mean more compression is applied once the threshold is exceeded. Ratio is an expression of input level verses output level, i.e. a ratio of 4:1 means for every 4dB the threshold is exceeded the output level will rise by only 1dB. Release is how quick the compressors gain reducing action is released once the level has dropped below the threshold again. Remember, compression reduces peaks in a sound so a make up gain is there to increase the overall output level. The result of "make up gain" is that softer sounds below the threshold are lifted in level and the peaks that are reduced by the compression action are lifted to where they were before compression started. The dynamic range of the material is thus reduced. This is quite an important concept to grasp so re-read if required.

The skill involved in using a compressor is to learn the basic controls by practicing on a known sound source like a vocal performance or snare drum.We will focus on a snare drum sample, this is an easy sound source to hear the changes on. The key to success is very careful and deep listening. It takes quite a long time to learn what the compressor is doing. So adjust the controls slowly with an ear on the sound and do not expect to learn this in 1 day. If you practice a little every day for a week you may start to hear the subtle changes to sound sources when you apply compression. I suggest initially practicing with a sole sound source and stick to using the same compressor then expanding to other sounds. Set your compressor to "Peak" as opposed to "RMS" if it has these buttons.(more on this later)

cont. in part 2

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Last edited by SafeandSound Mastering on Mon May 02, 2011 6:16 am; edited 1 time in total
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PostPosted: Mon May 02, 2011 6:16 am    Post subject: Reply with quote

cont. from part 1

Firstly a few words about the threshold control. The threshold is very much dependent on the level of the signal being received at the compressors input. A low level signal will require a lower threshold for the compressor to start applying gain reduction (compression) and a high level signal at the input will require a higher threshold before compression starts. Once you have set the parameters below you can lower the threshold and watch the gain reduction meters to see how much compression is being applied.

Try these setting on a snare and listen for results, I list the result we are aiming for first:

Reducing punch in a snare:

Ratio 8:1 Attack <1ms Release 200 ms Gain reduction 8dB

Increasing snare body:

Ratio 8:1 Attack <5ms Release 50 ms Gain reduction 8dB

Increasing punch in a snare:

Ratio 8:1 Attack 20 ms Release 200ms Gain reduction 6dB

(Do bear in mind that results between different compressors will vary a little as every compressor sounds a little different due to in part to the character of the gain reduction mechanism employed such as, optical, vari-mu, diode, VCA, FET, transconductance amplifier. In addition the circuits and topology can also affect audio pass through such as Class A / Class A/B, discrete or opamp based, valves/tubes or transformer balanced ins and outs.)

In each case you can turn up the "make up gain" knob to bring the level back to normal monitoring levels. You can also try pressing the compressor "bypass" button and listen to the sound with and without compression and assess your compression settings and determine if your goals have been achieved.To further complicate, every compressor has a slightly different sound characteristic. My suggestion is to use and practice on 1 compressor so you do not confuse yourself whilst learning the basics of what compression can achieve.

There are a couple of other settings on compressors which can be found such as peak or RMS detection. These settings determine how the detection circuit senses when the threshold has been exceeded. It changes the threshold from detecting peak electrical values or average (RMS) values. RMS compression setting tends to be a little less agressive in nature than peak settings.

So if you listen with care and try various sound sources you will slowly develop better ear to hand co-ordination and be able to eventually know achieve your goal.You will dial in the required paramaters using the T, R, A, R, M knobs as outlined above.

Compression is used within recording, mixing and mastering.

Written by Barry Gardner

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PostPosted: Mon May 02, 2011 8:10 am    Post subject: Reply with quote

oh man ...... how nice!

Thanks safe and sound ..... welcome to the board ...... it's a pretty low traveled board but I hope you'll pop in from time to time.

And while I'm at it ....... if I want to use a compressor strictly as a limiter, how would I set it up?

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PostPosted: Mon May 02, 2011 10:31 am    Post subject: Reply with quote

Thanks SCB....

Well in all seriousness you may well be better off buying a limiter plug in.

Limiting when recording would be quite a bad idea, it is indeed an extreme dynamic process.

Essentially most compressors become limiters when ratios of 10:1 and above are selected. 10 dB in results in a 1dB rise, pretty extreme.

So whack up the ratio and set the attack time as low as you possibly can, release should be dependent on your source and as arule longer if it is bass heavy so the release time is not so short that it "tracks" individual waveform cycles of LF energy.

Most compressors tend to sound very extreme so a look ahead type limiter would probably be a sonically superior choice.

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PostPosted: Mon May 02, 2011 12:07 pm    Post subject: Reply with quote

Actually I only use hardware. I don't record to a 'puter and so I don't/can't use plug-ins.
In fact, I don't use compression much (at all) so I know little about it.
Typically that would mean I don't know much about recording but I actually know a LOT about it. I've played for my living for 42 years (still do) ; had a 3340 in 1969 and have always had a home studio since then plus lots of session work in real studios.

BUT, I've always also been a bit of an audiophool (vinyl especially) and so, have always been averse to comps.
I generally just watch my gain as I record.

But there are times when it really would be nice to be able to simply keep the signal from going over and I think it's time that I learned about this very basic piece of equipment.

However, you're saying a comp's not gonna be good for just keeping from peaking?

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PostPosted: Mon May 02, 2011 1:24 pm    Post subject: Reply with quote

Hi Bob, like most things in recording and mixing audio, you use what you need to get a job done and that is a personal choice. Unless you are listening to classical music there is probably more compression going on, on your average pop/rock vinyl release of the 70's/80's than you might think.

A compressor can indeed be used to avoid peaking, this relates either to input or output of a subsequent device normally so I based the reading above mainly on mixing as opposed to recording, maybe thats another text for me to start work on : )

You can adjust the threshold to just shave off 1-3dB off the top peaks and on a good quality compressor you will hardly even be aware a compressor is working.

cheers

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PostPosted: Mon May 02, 2011 11:18 pm    Post subject: Reply with quote

I think the basic skill in using a compressor is realizing when not to use it and when to opt for automation instead. Draw the volume envelopes in (or whatever you call in-the-box level changes) on the track in question, and it'll generally have more life than something that's had a compressor on it.
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PostPosted: Tue May 03, 2011 7:02 am    Post subject: Reply with quote

dobro wrote:
I think the basic skill in using a compressor is realizing when not to use it and when to opt for automation instead. Draw the volume envelopes in (or whatever you call in-the-box level changes) on the track in question, and it'll generally have more life than something that's had a compressor on it.
yeah but:
supreme commander Bob wrote:
Actually I only use hardware. I don't record to a 'puter

so that also means no envelope doohicky.

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PostPosted: Sat May 07, 2011 2:41 pm    Post subject: Reply with quote

And, unfortunately, it is somewhat difficult for the uninitiated to apply all of this rack mount talk to compressor pedals.

I agree, I can't prove it but I suspect the "pro engineers" use a lot more compression than they admit to.....but I also think the "pro performers" dont need as much compressor stuff as us amateurs might benefit from.

My impression is the overriding goal is to make the compressor do the desired job without letting it sound like there is compression going on. Compressors, imo, should be seen (the led meters show it working) but not heard. If I can hear it working then I think I used too much.

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PostPosted: Sat May 07, 2011 3:46 pm    Post subject: Reply with quote

Generally, I think you're right. I try to keep really light settings on compression because when I can hear it, I n-e-v-e-r like the sound as much as uncompressed. Part of me thinks that's because the software compressors I use aren't very good. Maybe. Another part of me thinks that as soon as you squash a sound, it doesn't sound as live as the real thing.
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