Compressors And Limiters

[BOD2]

[b]COMPRESSORS[/b]

Audio compression is a fairly simple, yet often misunderstood process. At its simplest a compressor reduces the difference between the loudest and quietest levels in a signal (or piece of music). It does this by reducing the gain (volume) of the louder parts of the signal without changing the quieter parts.

It is this ability to leave the quieter parts untouched that makes it different from merely adjusting the overall volume of the signal. If you simply used the volume control to reduce the louder parts of the signal, then the quieter parts would become even quieter. Similarly, if you use the volume control to raise the quiet parts in volume then the louder parts would also be raised, becoming too loud.

[b]The Threshold [/b]

The level above which the compressor operates is known as the threshold. If the signal level rises above the threshold, then the compressor will kick in and reduce the level of the signal back down towards the threshold level. If, however, the signal level remains below the threshold then the compressor will do nothing.

[attachment=130:Comp1.jpg]

The diagram above shows an audio signal on a graph. The signal level (loudness) is shown in dB (decibels) on the vertical axis and time passes along the horizontal axis. The red lines show the threshold for the compressor. Only when the signal goes beyond the red lines will these peaks will be compressed. The blue line (-7dB) highlights a peak in the signal that is below the threshold.

[b]The Ratio [/b]

The ratio is the amount of compression applied. This is expressed as two numbers (e.g. 3:1 or 4:1 etc.). The first number shows by how much the incoming signal must increase before the compressor will allow the outgoing signal to increase by the amount of the second number.

[attachment=133:Comp4.jpg]

The diagram above shows the effect of different compression ratios on the output level of a signal as it rises above the threshold. A 4:1 ratio means that the incoming signal must increase by 4 dB to get an increase of 1 dB at the output. The higher the ratio, the more that loud signals will be reduced in volume.

[attachment=131:Comp2.jpg]

The diagram above shows the result of compression. The threshold has been set to 10 dB (so that only signals above 10dB are affected) and the ratio is set to 4:1. This means an input signal of 14 dB (the threshold value + 4 dB) will be reduced to 11 dB (the threshold value + 1 dB).

Note that the compressed signal is allowed to go above the 10dB threshold and that the signal peak at –7dB is not changed.


[b]Make-up or Output Gain [/b]

This is the clever bit. After the signal has been compressed the overall gain (volume) of the signal can be increased so that the loudest part of the compressed signal is restored back to its original level. This is applied to the whole signal so the end result is that the loud sections are restored to (or near) their original volume but the quiet sections become louder.

[attachment=132:Comp3.jpg]

The diagram above shows the compressed signal after make-up gain has been applied.

The volume of the loudest parts of the signal have been restored to original levels, but note that the peak originally at 7dB is now also louder as the make-up gain has been applied to the whole signal.

The result of this is that the loudest peak of the compressed signal is no louder than the original signal but the quieter parts of the signal are louder. This signal will sound punchier and more consistent in volume, with less variation in loudness.

[b]Attack Time [/b]

The attack time defines how quickly the compressor will act and reduce the volume of any signals above the threshold level. With a fast attack the signal is compressed almost immediately whereas a slower attack will allow the initial loud pulse to pass through before the level is reduced.

[b]Release Time (Sustain) [/b]

The release time controls how quickly the compressor will stop working and release the signal back to its original level after it has dropped below the threshold. If the release is too fast then the compressor may “pump” or “breathe” up and down as the compressor comes on and off quickly in time with the music.

[b]Hard Knee/Soft Knee [/b]

Fully featured compressors may have a control to adjust the “knee” of the compressor. A “hard knee” compressor applies the full amount a gain reduction as soon as the signal level exceeds the threshold. A “soft knee” compressor progressively applies gain reduction before the threshold, as the signal approaches the threshold, producing a softer more subtle effect.

[b]Enhancer [/b]

Low frequency elements of a sound tend to be higher in levels and will often reach the threshold and trigger the compressor, but any high frequencies that occur at the same time will also be compressed. Some compressors have enhancer circuitry that boosts these higher frequencies to compensate for any loss in brightness.


Many compressors, especially stomp-box ones, will often have preset attack and release times, or will combine the operation of several parameters within one control to simplify operation. Likewise, some devices will automatically apply make-up gain to compensate for the reduction in the compressed signal so that the overall output level is always restored.

[b]LIMITERS [/b]

A Limiter is simply a compressor set up with an infinity:1 ratio and a very fast attack time. This means that no amount of input signal will allow the output signal to rise above the threshold. The output is effectively limited to the level set by the threshold control.

Limiters are often used as protection to avoid signal overload into a device. The threshold will be set to a level that the device can safely handle and the limiter will prevent the signal ever exceeding that level.

[b]COMPRESSORS AND BASS GUITAR [/b]

So what effect does a compressor have on your bass sound ?

Generally the effect is usually described as an “evening out” of the overall sound and an increase in “punch”– but note that fret noise and other unwanted background noise might also be increased.

Harmonic undertones of a note will usually be well below the sound level of the fundamental note. A compressor can increase the level of these harmonics making them louder and thickening up the sound and making it more even and “punchy”.

If you play slap bass then the slaps and pops will often be louder and leap out more than normal bass notes. A compressor can reduce the difference in volume between these so that the overall sound is more even.

[Toasted]

I'd just like to extend my previous appreciation to BOD2 for writing this article - that someone else saw fit to delete. Thanks for that

[i]A big thanks to Bod2 for preparing this for us. I'd like to see more of this in FX. If any of you have any ideas for more and informative stickies, please PM me. Thanks to all. Toasted[/i]

[Rich]

BOD, that is one of the most useful & informative FX articles I have ever read. Many thanx.
I was quite intrigued by the idea of an “evening out” of the overall sound... do you take it to the pub? The flicks? A romantic meal for two?

[BOD2]

[quote name='Rich' post='3664' date='May 21 2007, 03:13 PM']I was quite intrigued by the idea of an “evening out” of the overall sound... do you take it to the pub? The flicks? A romantic meal for two? [/quote]

lol, I was going to edit the post then I realised no one would have a clue what you were talking about, so I've left it in.

Actually "evening out the sound" is best done in conjunction with treating the neck.

[Rich]

[quote name='BOD2' post='3799' date='May 21 2007, 05:54 PM']lol, I was going to edit the post then I realised no one would have a clue what you were talking about, so I've left it in.

Actually "evening out the sound" is best done in conjunction with treating the neck. [/quote]
And presumably the tweaking, plucking and slapping comes later after the romantic meal and the fancy-coming-back-to-mine-for-coffee..?

[shaundixon666]

[quote name='Rich' post='3839' date='May 21 2007, 06:56 PM']And presumably the tweaking, plucking and slapping comes later after the romantic meal and the fancy-coming-back-to-mine-for-coffee..? [/quote]

haha awesome!!!!

Anyways thanks for this FAQ I didnt know what compressors and limiters did till i read this so thank you very much!!!

[oddjob]

Just found a link to a guy who seems to have owned a lot of compressors and writen a short review on each of them look quite usefull.

[url="http://www.ev-b.com/compressors.html"]http://www.ev-b.com/compressors.html[/url]

[BOD2]

Yes, BeLow, the rack compressors tend to have a lot more controls to allow you to adjust the various parameters....and lots more flashing LEDs.

I've always been underwhelmed by noise gates, but perhaps that stems from misunderstanding what they did when I first encountered them. For the benefit of those who've never used one before ...

A noise gate does NOT remove hiss or hum or anything at all [i][b]while you are playing[/i][/b]. When setup correctly it will remove these things when you stop playing (i.e. in between songs). Basically you set a "threshold" volume on the noise gate. When a note you are playing dies off and the volume drops below that threshold, the noise gate "closes" and you get complete silence (i.e. no noise, no hum, nothing). As soon as you start playing again the volume will instantly rise above the threshold and the noise get opens - letting everything (noise, hum, your notes) through.

If you set the threshold too high then long sustained notes will suddenly cut off as the level drops below the threshold. Set it too low and the gate will never close. So there is a bit of fine tuning to be done. It's useful if you have particularly noisy gear (some FX can create background hiss) and you need complete silence during gaps in your playing - or can be useful in a studio situation for the same reason.

Not a very exciting effect, though and may not have any flashing LEDs at all !

[danlea]

Perhaps this section could do with something about multi-band compression/limiting, the classic examples being the Trace Elliot dual band compressor (also found on Ashdown stuff obviously, and on the dreadful DigiTech XBS Bass Squeeze) and the multi-band limiting essential for good mastering.

[Neill]

Hi

My Trace Elliot SMX dual compressor has died and, unless anyone can tell me otherwise, can't be repaired (it sometimes makes horrible buzzing noizes and sometimes just goes TOTALLY quiet). I've seen loads of DBX and similar quality compressors on eBay for good prices and am considering one of them. I'e read that the amp manufacturers don't always make their loops impedance requirements right for these types of devices, sometimes requireing the effect has a similar hi-z to a guitar amp pre-amp input. This would mean I'd need a 'stomp box' style unit again or a di box to get the impedance step, wouldn't it?

Does anyone know if I could use a studio type compressor in my Marshall MB4210 fx loop of would I need a Di box, too?

Cheers, and thanks in advance for any assistance.

Neill

[bass_ferret]

You might find the Ashdown Dual Band compressor a good replacement for the Trace. It was, after all, designed by the same guy, but may lack the same quality of construction.

[Merton]

[quote name='Neill' post='307528' date='Oct 16 2008, 01:46 AM']Hi

My Trace Elliot SMX dual compressor has died and, unless anyone can tell me otherwise, can't be repaired (it sometimes makes horrible buzzing noizes and sometimes just goes TOTALLY quiet). I've seen loads of DBX and similar quality compressors on eBay for good prices and am considering one of them. I'e read that the amp manufacturers don't always make their loops impedance requirements right for these types of devices, sometimes requireing the effect has a similar hi-z to a guitar amp pre-amp input. This would mean I'd need a 'stomp box' style unit again or a di box to get the impedance step, wouldn't it?

Does anyone know if I could use a studio type compressor in my Marshall MB4210 fx loop of would I need a Di box, too?

Cheers, and thanks in advance for any assistance.

Neill[/quote]
I'm sure it could be repaired, it would just probably cost more than the unit is worth. bass_ferret's suggestion is a good one

[Neill]

[quote name='Merton' post='307875' date='Oct 16 2008, 01:55 PM']I'm sure it could be repaired, it would just probably cost more than the unit is worth. bass_ferret's suggestion is a good one [/quote]

Cheers Chaps, I'll continue to ponder my options.

I have just spoken to a friend who has an electronics degree but hasn't been able to get work in that field. He's happy to take a look just 'cos he misses the scent of solder and a knotty problem to solve

Cheers.

[M4L666]

Good guide. I want one now . My bass sound now seems very uneven. Xmas is coming up though....

[waynepunkdude]

I have just got a compressor, it has definatly improved my sound but I don't really know what I'm doing with it so I'm going to print this out and take it to practice.

Thanks a million.

Wayne

[stevie]

There's a really neat explanation by Marcus Miller on the use of compression for bass on YouTube. He's demonstrating his eponymous amp, but still, it's worth watching:

[url="http://uk.youtube.com/watch?v=ljEuObKHYFw&feature=related"]http://uk.youtube.com/watch?v=ljEuObKHYFw&...feature=related[/url]

[Happy Jack]

Finally got around to watching that YouTube link for the Marcus Miller explanation - which turns out to be quite different from the explanation by BOD!

Sorry Marcus, but BOD's explanation makes a lot more sense to me than yours ...

[TimR]

Just to clarify the Make up gain of the compressor.

The effect of the make up gain is to 'expand' the signal. Note in the picture posted above how the bottom part of the signal below the threshold appears stretched vertically. This has the effect of lengthening the sustain of the note. The gain is not 'added' to the signal it 'multiples' it. E.g. with a gain of 4 the lowest signal of 1dB would become 4dB and the loudest signal of 10db would become 40dB.

While compressors are often used to stop the loud bits being too loud, when used correctly they also allow greater control of the dynamics to the sound occurring below the threshold.

Essentially be careful how much compression ratio you apply and then how much make up gain you apply. A higher threshold, less compression and less make up gain gives better sustain and dynamics.

Edited May 29, 2009 by TimR

[sturm]

thanks for the post, amazingly useful and easy to understand

[DrVet]

Hi all,

Did any of U had or tried a Boss RCL-10 Compressor Limiter?
What are your opinion on this item? How good is it for bass guitar?

thanx,
F.

[Al Heeley]

Anyone have experience of using an MXR Dyna Comp for bass? Does it limit the lower frequencies? Is it moddable to work better for bass?

[Silent Fly]

[quote name='Al Heeley' post='516360' date='Jun 17 2009, 12:59 PM']Anyone have experience of using an MXR Dyna Comp for bass? Does it limit the lower frequencies? Is it moddable to work better for bass?[/quote]

I found this [url="http://www.ovnilab.com/reviews/dynacomp.shtml"]review[/url]. You may find it interesting.

It is part of bigger [url="http://www.ovnilab.com"]Ovnilab Compressor Review[/url] web site.

[KingPrawn]

Hey all

Just wanted to throw this wonder into the Bass chat world. I have found it difficult over the years to find a compressor that has a subtle effect on my sound. That is until i found this. I can't say enough good things about it, build quality is outstanding, easy to use. It really is amazing.

[url="http://www.telenordia.de/"]http://www.telenordia.de/[/url]

check this out.

[url="http://www.youtube.com/watch?v=adw3qO8hSSU"]http://www.youtube.com/watch?v=adw3qO8hSSU[/url] Yes its on a guitar, but just imagine what it can do on a bass

Edited August 29, 2009 by KingPrawn

[righi80]

[quote name='BOD2' post='2290' date='May 19 2007, 02:54 PM'][b]COMPRESSORS[/b]

Audio compression is a fairly simple, yet often misunderstood process. At its simplest a compressor reduces the difference between the loudest and quietest levels in a signal (or piece of music). It does this by reducing the gain (volume) of the louder parts of the signal without changing the quieter parts.

It is this ability to leave the quieter parts untouched that makes it different from merely adjusting the overall volume of the signal. If you simply used the volume control to reduce the louder parts of the signal, then the quieter parts would become even quieter. Similarly, if you use the volume control to raise the quiet parts in volume then the louder parts would also be raised, becoming too loud.

[...]


If you play slap bass then the slaps and pops will often be louder and leap out more than normal bass notes. A compressor can reduce the difference in volume between these so that the overall sound is more even.[/quote]


If i understood everything it means that you really made a good job writing this. Congrats and thank you! There were 2-3 things that i never completely understood about this matter before reading this article.

Edited January 15, 2010 by righi80

[algmusic]

I generally don't use compressors as I like the natural dymanics in my playing and I try to play consistently (well I hope I do).

That said with compression, the simpler the better. I've tried a few compressors and limiters. The ebs pedal is very nice but I found the I really didn't want extra drive as I use my sansamp for that.. so with simple in mind I think the MXR Dyna Comp is king. Simple.. two dials and you're good to go and you get a consistent sound with no noise etc and you can get a real even tone and it's great with other bass effects.. I swear by it and it's cheap, about £60..