STME58
794 posts
Apr 28, 2014
5:04 PM
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I am just dipping my toe into the amplified waters and am curious about the term "compression" as applied to amplification. In some cases it seems like people use the term to refer to what I would call clipping, or saturation, where the peaks of the sound wave are chopped off because they hit the limit of the amplifier. This would give a distorted sound.
The other way I have heard it used is to bring up the volume when it is low and bring it down when it is loud. As someone who learned to play on a brass instrument, where dynamic range is something to strive for, I have a hard time understanding why you would want to do this. I worked hard to make my pianissimo as far from my fortissimo as I can, why would I want an electronic device to level it out for me?
I am hoping to get some insight into what this term means from the folks who know. Thanks!
Last Edited by STME58 on Apr 28, 2014 5:05 PM
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DukeBerryman
361 posts
Apr 28, 2014
6:26 PM
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When I use a compression pedal with my mic, it makes the low volume things louder, and the high volume things softer. It doesn't limit the dynamic range of my instrument, though. All the sound is collected, and then the volumes are slightly varied.
This presents a better signal for the amp to work with. The job of the amp is to make everything louder, and there is some more compression that happens - even clipping - during the amplification process. But that is an amplification byproduct that sounds good.
I use my compression pedal just to make my mic more usable by providing the amp with the best kind of signal to work with.
EDIT: I should add that I use a Lone Wolf Flat Cat first in the pedal chain
Last Edited by DukeBerryman on Apr 28, 2014 6:33 PM
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Goldbrick
419 posts
Apr 28, 2014
6:36 PM
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compression (or more technically Dynamic range compression) is a subtle effect primarily for electric guitar where the highest and lowest points of the sound wave are "limited". This boosts the volume of softer picked notes, while capping the louder ones, giving a more even level of volume. This is frequently used in country music, where fast clean passages can sound uneven unless artificially "squashed".
Explanation[edit]
Compression reduces the height of the signal to a preset level. This level can change over time. Known as 'attack', a greater attack level will give a more aggressive sound
Last Edited by Goldbrick on Apr 28, 2014 6:36 PM
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WinslowYerxa
573 posts
Apr 28, 2014
6:52 PM
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While pianissimo may be desirable in purely acoustic music heard live, in amplified music music those same passages will often become inaudible. A similar situation occurs with recorded music heard in your car or on a less than wonderful sound system.
Meanswhile, the fortissimo passages may overload the system, requiring that the volume be turned down, making the pianissimo even more inaudible.
Compression helps even things out so that everything gets heard at a decent volume level.
It's not just used live or in country music. Most recordings use compression, sometimes extremely heavy compression, and most decent live setups will also use it, either subtly in the case of "acoustic" music or heavily in louder music.
Harmonica players can benefit from compression as their note attacks tend to be much louder than the sustain that follows.
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Last Edited by WinslowYerxa on Apr 28, 2014 8:53 PM
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Greg Heumann
2694 posts
Apr 28, 2014
8:16 PM
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Compression, strictly defined, is indeed the softening of loud passages and amplification of quiet ones - so that the original dynamic range of the source - perhaps 50dB - is "compressed" into less dynamic range - perhaps 30dB. This is done for many, many reasons. Winslow has explained the most common - has to do with the short distance between the "floor" of audible music through a sound system and the maximum it is capable of producing.
However when you talk about clipping, you are ALSO talking about a natural form of compression. Electronic components, most notably, microphones, tubes, transistors and speakers, clip (or "chop off the tops and bottoms") of a signal when the signal is too powerful. That creates compression too - however in this case it usually comes along with considerable distortion. In the case of tube amps we tend to like the distortion created by a little clipping. In the case of transistor amps we do not - as transistors distort much more harshly when driven to clipping.
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Last Edited by Greg Heumann on Apr 28, 2014 8:21 PM
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STME58
796 posts
Apr 28, 2014
9:50 PM
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Thanks for the explanations. If i understand this correctly, compression is something that would let you listen to classical music in a noisy car without having to turn up the soft parts so you can hear them and then turn down the loud parts because it is blasting you eardrums because you turned it up for the soft part. The typical amplification system is something like the noisy car but instead of an audible noise floor it is a low response of the system at the low end that requires the soft parts to be turned up.
So if the compression is matched to the response of the amplification system, would the dynamic range the audience hears be the same range they would hear without the amplification?
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timeistight
1555 posts
Apr 28, 2014
11:25 PM
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"So if the compression is matched to the response of the amplification system, would the dynamic range the audience hears be the same range they would hear without the amplification?"
Nope. Compression reduces the the dynamic range of the signal being compressed.
Compression also allows an increase in the average level of a recording: a heavily compressed recording can sound much louder than a recording with a wider dynamic range. That's why TV commercials seem to be louder the the programs they interrupt; they're very heavily compressed to make them "pop"!
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Barley Nectar
369 posts
Apr 29, 2014
12:51 AM
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Why would I want to use a compressor live with a full blown band?
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5F6H
1774 posts
Apr 29, 2014
1:20 AM
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What "compression" means is a little relative to context.
The treatment of audio signal as described above (AKA normalisation), to even out peaks & troughs in amplitude is one, for which you would want a compressor/limiter of some sort.
Compression in a tube amp is more of about dynamic response, yes, peaks of the AC sine wave to get clipped, but this is as much relevant to "grind/distortion", or harmonics emphasised, rather than dynamic response.
Switching a fixed bias amp to hot, cathode bias (or say comparing a brown Concert to a SJ1/2) will help generate amp compression. This is because the tubes don't allow the same range of plate current swing, the tubes are biased hot, can only draw so much more current before they pull down the plate voltage. This compresses the signal, or introduces sag (sag being a compression based artefact).
A very soft rectifier (always check with the amp's manufacturer/tech before subbing rectifiers) will also introduce some compression/sag due the rectifier's internal resistance (a solid state rectifier has no internal resistance if it is just diodes - Weber Copper caps are solid state devices that do have internal resistance to emulate a tube).
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Last Edited by 5F6H on Apr 29, 2014 3:16 AM
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Martin
663 posts
Apr 29, 2014
4:51 AM
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@STME58: This is the exact same question that I asked a month or two back, and I got several replies. If you check the archives you could probably find some posts that shed added light on this elusive concept.
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rockmonkeyguitars
67 posts
Apr 29, 2014
7:27 AM
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There is another type of compression that is more to do with frequency response and less to do with dynamics. When the frequency response is squeezed into a more narrow spectrum that is also considered by many to be compression. This is something you will see a lot when people talk about speakers or guitar pickups. An example would be swapping a ceramic magnet in a guitar pickup for an alnico 2 magnet. The alnico 2 magnet will make it so the pickup isn't capable of producing overtones that are as high pitch as the ceramic magnet but rather than simply cutting them out, it squeezes them down to the lower treble and midrange. So it's different than simply using an EQ to remove tone, you are actually changing the tone by squeezing or compressing it into a narrower feild
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