What's a compressor?

[Basster]

What does a compressor do? How does if effect your sound? How do you use one?

[Bob Gollihur]

http://www.harmony-central.com/Effects/Articles/Compression/ gives a pretty good description, and the contents page leading to that info has some additional resources at the bottom of the page at http://www.harmony-central.com/Effects/effects-explained.html

 

A compressor can be a necessity for some slap play, to control and limit the dynamics, but compression can be set in many different ways for a wide range of effects for all sorts of styles.

[rumpelstiltskin.]

a compressor is an effect that can sometimes be very hard to hear, in that it's not obvious like chorus or distortion.

 

the way that it works is that it reduces the difference in volume between the loudest and quietest parts of your bass signal. it is often said to give the bass a smoother, more even sound, and makes it easier to sit in a mix and not get drowned out guitars and drums.

 

compressors can be used in many different ways and have a lot to do with how basses sound recorded. again, by smoothing out the sound, it makes it easier to fit into a mix and make the bass easier to hear. there are lots of ways to do that.

 

robb.

[Dan South]

The best way to visualize what a compressor does is to think about recording a singer. A person can sing very softly or they can really belt out a scream. Yet, when you listen to a recorded song, the loud vocal parts don't seem much louder than the soft ones. Only the tone of the voice (energy, grit, etc.) changes.

 

So how did the engineers accomplish this?

 

They ran the vocal through a compressor. They set the compressor to leave the soft parts alone but to REDUCE THE VOLUME of the LOUD PARTS by a PREDETERMINED PERCENTAGE.

 

Let's translate that last sentence into compressor language. They used a control on the compressor called a THRESHOLD to set the VOLUME LEVEL where the compressor starts to work. Anything LOUDER than the threshold setting will be compressed (i.e. the volume lowered). Anything SOFTER than the threshold will be left alone. Typically, the threshold setting is done by ear, meaning until it sounds right. But it's important to understand what's happening.

 

In order to compress the louder notes by a PREDETERMINED PERCENTAGE, the engineer adjusts the RATIO setting. If the RATIO is 2:1, that means that for every increase in volume at the input, the output will only go up half as much. If the RATIO is 10:1, increases in output volume will be only one tenth as much as input volume. 2:1 is a gentle setting.

 

A lot of the singer's (or player's) dynamics will come though. 10:1 means that you'd have to sing or play a LOT louder to increase the output. As the compression ratio increases, it eventually gets to a point where NO MATTER HOW MUCH LOUDER YOU PLAY, THE OUTPUT DOESN'T GET ANY LOUDER AT ALL. This is called LIMITING. Limiting is used at the mastering phase - just before the CD is cut - to give the music an in your face volume level but without peaks that can damage speakers.

 

There are a couple of other controls on some compressors, namely ATTACK and RELEASE. These adjust how fast or slow the compression effect turns on and off. These settings will be different for different types of instruments.

 

For instance, when you slap a bass string, there's a very loud peak at the beginning of the note, then the volume dies down to the sustain level. If the compressor attacks too quickly at too high a ratio, the peak will be smashed and the sound will lose punch. However, the peaks may cause distortion when they're being recorded, so the engineer may try to compress the peaks lightly to prevent clipping.

 

Using these settings together to get a good sound takes some practice. It's very easy to create a dull, lifeless bass sound with a compressor. When in doubt keep ratios low (2:1, 3:1, or 4:1) and don't set the attack to response too quickly. You can always adjust these settings to taste.

 

One final tip. If you turn knobs and it doesn't sound like anything is happening, first check to make sure that the Bypass switch is not on (if there is one). If the compressor is not in Bypass mode and you STILL don't hear any effect, that means that your threshold level is too high. Remember that notes lower than the threshold level will NOT be compressed.

 

Hope this helps.

[Connie Z]

Just want to comment that those were VERY informative descriptions! I myself was also wondering what a compressor did.

 

Thanks!

[cornbread_medhotmail.com]

Thanks for the info. I always knew what a compressor did, but I didn't know the different settings. Now I can break out the old Autocomp and chisel the dust off of it and put it to use. I just thought it sounded bad, but I didn't know HOW to use it and never took the time. I get a little impatient at times with my gear. If it doesn't work right (the way I want it to) soon, I ditch it.

[Basster]

Thanx. The info is just what i needed.

[TomD]

Thanks, Dan! Great explanation.

[davich]

Thanks Dan for the great simple explanation.

 

However I found many in appliances, not in professional stuff, there is only one compressor function without explanation about what it is. For example in my Korg Px3B, they have compressor function and the levels. Sometimes I also use Marshall Basstate 65 combo for my church gig. There it has also only one compressor dial button.

 

My question is what it might be? Compressor gain? Compression ratio? Couldn't be threshold nor attack/release though..

[_Sweet Willie_]

Originally posted by davich:

Thanks Dan for the great simple explanation.

Ditto.

 

Originally posted by davich:

However I found many in appliances, not in professional stuff, there is only one compressor function without explanation about what it is. For example in my Korg Px3B, they have compressor function and the levels. Sometimes I also use Marshall Basstate 65 combo for my church gig. There it has also only one compressor dial button.

 

My question is what it might be? Compressor gain? Compression ratio? Couldn't be threshold nor attack/release though..

My guess would be that if there is only a single knob, most likely it is controlling the ratio. If there are two, they're probably threshold and ratio. Dan rightly pointed out that there are a number of aspects of compression that can be controlled, but depending on the compressor you use there will be limits (ha!!! -- no pun intended!) on how many of those aspects the product allows you to change. Some products designed particularly for a specific instrument, for example, may have preset attack and release settings that the manufacturer believes are optimal for that instrument. Thus leaving you room to control the threshold and ratio.

 

Also, there are compressors (e.g., multiband or dual band) that allow you to compress different frequency ranges differently. I'm still learning about the benefits of doing this, so there are probably others on the board who can speak to this kind of differentiation, its uses, and the accompanying rationale better than I!

 

I have a compression pedal that is great, but I also don't use it lots. But when I do, I really like the results. Sometimes using compression will give you a little more space to drive your amp and speakers harder, since the compression will help control sudden, unwanted peaks in your output to the amp.

 

I also advocate practicing without compression (unless you're experimenting to learn better how a particular compressor works and how it affects your sound) -- this helps me develop and maintain control over my output level with my fingers/touch/playing. With that kind of control, adding compression at a band practice or gig makes it that much more effective (or at a recording session, I presume, but I haven't done one of those in a long, long time!).

 

Peace, love, and compression. (And maybe a little chorus. )

--s-dub

[Dan South]

Originally posted by davich:

Thanks Dan for the great simple explanation.

 

However I found many in appliances, not in professional stuff, there is only one compressor function without explanation about what it is. For example in my Korg Px3B, they have compressor function and the levels. Sometimes I also use Marshall Basstate 65 combo for my church gig. There it has also only one compressor dial button.

 

My question is what it might be? Compressor gain? Compression ratio? Couldn't be threshold nor attack/release though..

Hmm.

 

Well, these circuits are probably designed differently on different units, but my guess would be that they're designed to be as cheap to implement as possible or you'd be given more knobs. My guess is that attack, decay, and ratio are fixed at some value that the engineers thought would work in the widest variety of circumstances. The single knob probably controls two parameters. As you turn the knob in the direction that causes more effect, it probably lowers the threshold to give you more compression "effect" and at the same time, it probably boosts output gain, because as more compression is applied, the output level goes down (because, of course, you're lowering the amplitude, or volume, of any signal over the threshold).

 

An alternate design might increase the ratio while simultaneously lowering the threshold and increasing makeup gain, but this would be more typical of a two-knob compressor, one knob for ratio, the other for the combination of threshold and makeup gain discussed above.

 

A compressor isn't necessarily bad if it has fewer knobs. Some of the most expensive compressors ever built, units that are very popular in big studios, don't have dedicated attack and decay controls. Simple compressors can sound good if they're designed well, particularly if you use them with the instrument that they were designed for.

 

How do they sound?

 

Hope this make sense.

[davebrownbass]

Great job Dan.

 

Looking in my user manual for the Korg Px3B, I find that the compression circuit is in the same category as Overdrive, distortion, octave divider and octave distortion.

 

The single adjustable parameter is "sensitivity." My guess, from where this is located in the signal chain, is that the circuit is a soft knee compressor with a low ratio and slow attack...primarily designed to increase sustain without "bloom." Sensitivity probably adjusts the input threshold.

 

Not really a complex or useful compressor, I think.

[davich]

Thanks Sweet Willie, Dan and Dave!

At least it gives more explanation to me. My Krog Px3B was coming from Japan with Japanese manuals. I tried to use it without manuals. So far I have done it successfully, but off course I missed all the necessary explanation. I think that single knobs should be input gain threshold.

[Bartolini]

DS,Thank you for detailed explanation.

Pls. see if my understanding is correct.

2 : 1 compression means 2 units of out put level of the sound signal is reduced to 1 unit - Correct?

 

When I play live on stage, how do I do this ratio setting if I have

1)a bass effects processor with compressor with sensitivity range from 0 to 30.

2)A compressor turn knob on my amp (I find that when turn knob is on 12O clock setting and play, the green light begins to turn red (I suppose the THRESHOLD level is at 12o clock position?)

 

When I do recording in a studio

 

1)do I do compression settings with my hardware or

2)the recording engineer does the setting when mixing?

3)should I give him the ratio I prefer or how does 2:1, 3:1, or 4:1 determined on the compressors? Are there knobs for each ratios? Sorry if I asked silly questions. I am quite ignorant about this.

 

Thank you in advance.

[Shivall]

I'll just tack this on here since I'm a thread hijacking assjackal..I mean because it's pertinent to the thread!

 

Can anyone give me the name of a good Compressor in pedal/rackmount(Don't care which really) for $100-$300? Thanks

[Dan South]

Originally posted by Bartolini:

DS,Thank you for detailed explanation.

Pls. see if my understanding is correct.

2 : 1 compression means 2 units of out put level of the sound signal is reduced to 1 unit - Correct?

 

When I play live on stage, how do I do this ratio setting if I have

1)a bass effects processor with compressor with sensitivity range from 0 to 30.

2)A compressor turn knob on my amp (I find that when turn knob is on 12O clock setting and play, the green light begins to turn red (I suppose the THRESHOLD level is at 12o clock position?)

 

When I do recording in a studio

 

1)do I do compression settings with my hardware or

2)the recording engineer does the setting when mixing?

3)should I give him the ratio I prefer or how does 2:1, 3:1, or 4:1 determined on the compressors? Are there knobs for each ratios? Sorry if I asked silly questions. I am quite ignorant about this.

 

Thank you in advance.

Bartolini,

 

First of all, in the studio do exactly what the engineer tells you to do unless he's some weenie pretender who doesn't know his job (most often this will NOT be the case). You can suggest that he A/B your sound with and without your amp's compressor, but after that go with whatever he decides. He's probably got a better idea of how your bass sound needs to fit into the mix, and you can help things out tremendously by cooperating with him.

 

Live, I think you should use one compressor or the other, not both. If one sounds better than the other, use that one. If neither sounds better relative to the other, use the amp's compressor if you want about the same amount of compression from song to song, or use the one in the effects pedal if you'd like to be able to change your sound radically from song to song. If you have a sound man, ask him (or her) which settings they prefer. Otherwise, get a friend to offer his or her opinion. Then have your FRIEND sit in with the band for a song during the sound check, and YOU go out front and listen. Have your friend play with a similar style and touch so you'll have a good comparison.

 

Note, when your compressor's light comes on, that means that it's compressing the signal. When the light's not on, the threshold is at a setting that's louder than the notes you're playing, i.e. you'd have to play louder to see the compressor work. Try slapping or picking and see if the light comes on then. The light should not be on all the time, nor should it only kick in occasionally. Strive for a happy medium.

 

Yes, 2:1 compression means that for notes that go OVER THE THRESHOLD LEVEL, the amount that they go over the threshold level will be cut in half.

 

Let's say you play three notes. The first not is very soft and it's under the threshold. When you play this note no compression occurs. The second note is twice as loud as the first note and it's exactly at the threshold level. No compression occurs here, either. The third note is twice as loud as the second note. This is definitely above the threshold, so it will be compressed. But how much will it be compressed? If the third note is twice as loud as note two, and if the ratio is 2:1, does that mean that these two notes will sound the same? The answer is no, because compression only occurs in the range ABOVE the threshold. So the third note, after compression, would appear to be one and one half times louder than the second note. The DIFFERENCE between the threshold and the note's amplitude is compressed, not the note's full amplitude (unless the compressor's threshold is set extremely low).

 

Let's say your threshold is -20dB from the maximum level that your amp can handle without distortion. You play a note at -8dB, louder than the threshold. The difference is 12dB. If the threshold is set to 2:1, the compressor will compress the difference from 12dB to 6dB. The output amplitude of the note is now

 

output = -20dB + 6dB = -14dB.

 

If the ratio had been 4:1, the difference of 12dB over the threshold would have been reduced to 3dB, so

 

output = -20dB + 3dB = -17dB.

[Bartolini]

Dan South I owe you a big THANK YOU.

So whats the difference between the LIMITER and COMPRESSION?

 

Say if I take your example Let's say you play three notes. The first not is very soft and it's under the threshold. When you play this note no compression occurs. The second note is twice as loud as the first note and it's exactly at the threshold level. No compression occurs here, either. The third note is twice as loud as the second note. This is definitely above the threshold, so it will be compressed.

 

Would the limiter increase the Threshold difference of the 1st note to Threshold level?

The second note has no fifference and the 3rd note is reduced to the Threshold level? Meaning all notes'd have the same level? If the answer is yes my question is why one want to do that since it will be without high/low level sound dynamics?

[Audioboticatoccata]

Bassikly, a kompressor squishes your soun so the kwiet parts aint so kwiet an the loud parts aint so loud so tha hole thing souns more in yer face.

 

Do it too much an it sounz like shit. Do it too little an ditto. Lern too do tasty kompreshen an you in like Flinn!

 

Anyhoo, start at -6db about 2.5 to 1 an crank it frum there to get how aggro yuu wan it!

[davich]

Bartolini, I think Dan has already explained LIMITER before. Limiter is a compressor with unlimited ratio = unlimited:1. It means how hard you play over the treshold, it stucked on threshold value.

[Bartolini]

Davich, you are right. Dan South has explained it and I apologize for over reading it. I'm sorry and thank you.

[Wally Malone]

Originally posted by Shivall:

I'll just tack this on here since I'm a thread hijacking assjackal..I mean because it's pertinent to the thread!

 

Can anyone give me the name of a good Compressor in pedal/rackmount(Don't care which really) for $100-$300? Thanks

I haven't tried one but have read from others who like the EBS MultiComp pedal. I have three other EBS pedals and can say they make a quality product. Check it out and see if it works for you.

 

Wally

[Dan South]

Originally posted by Bartolini:

Dan South I owe you a big THANK YOU.

So whats the difference between the LIMITER and COMPRESSION?

 

Say if I take your example Let's say you play three notes. The first not is very soft and it's under the threshold. When you play this note no compression occurs. The second note is twice as loud as the first note and it's exactly at the threshold level. No compression occurs here, either. The third note is twice as loud as the second note. This is definitely above the threshold, so it will be compressed.

 

Would the limiter increase the Threshold difference of the 1st note to Threshold level?

The second note has no fifference and the 3rd note is reduced to the Threshold level? Meaning all notes'd have the same level? If the answer is yes my question is why one want to do that since it will be without high/low level sound dynamics?

Bart, just glad to help!

 

In the example that I cited above, you'll recall that Note 2 was right at the threshold level. Note 1 was half as loud as Note 2, so it's under the threshold level - remember that the threshold is adjustable by YOU - and Note 3 was twice as loud as Note 2, so it's OVER the threshold.

 

In this case, a limiter would have NO EFFECT on Note1. Remember, if a note is under the threshold, it's not processed regardless of how strong the ratio is. Note 2 wouldn't be processed either, because it's right on the threshold line. Note 3 is above the threshold, so it WOULD be processed, and as someone stated correctly above, the limiter would force this note down to the threshold level. So, the output of Note 2 and Note 3 would both be at the same level, while Note 1 was still half as loud as either of the other two.

 

There are subtleties to discuss about compressors and limiters, though. One is the effect on attack transients. An attack transient is the quick, loud portion of a note just after you pluck, pick, or slap it. If you think about a note played on a bass, it's loud for an instant, and then it quickly (after less than a tenth of a second) decreases down to a sustain level. This sustain level then gradually decreases over several seconds.

 

What happens if the attack transient (the loud pluck at the beginning of the note) is OVER the compressor's threshold, but the sustain portion is UNDER the threshold? The result is that the pluck is compressed, but the rest of the note isn't. This gives the illusion of better sustain, because the sustain portion of your note is louder relative to the initial pluck.

 

However, it's that initial pluck that gives your sound "punchiness". If you compress the pluck completely, your notes will gain sustain, but lose punch. There are a couple of ways to retain some of the punchiness. One is to use a compressor with an Attack control. The attack control tells the compressor how quickly to "turn on" after it senses a note over the threshold. Using the attack parameter, you can slow down the compressor's reaction long enough to let some or all of your pluck through. In fact, you can even ADD punch to sounds that don't have much punch in the first place by setting a slow attack and a really low threshold. The low threshold lowers the volume of the sustain portion of the note, but the slow attack lets some of the note through before the compression kicks in. This gives the illusion of a pluck on a note from, say, an organ or a synthesizer that didn't have much punch in the first place.

 

Compressors that work well on bass don't always have attack controls, but they use something called Soft Knee compression. That means that when the compression effect starts, it's not an abrupt, instant effect. It kicks in gradually. This allows some of the punchy attack of each note to sneak through. Compressors in bass amps and effects are typically Soft Knee compressors. That's good, because it keeps us from having to worry about extra controls like Attack and Release.