What is a compressor?

Discussion in 'Amps and Cabs [BG]' started by Obsolex, Apr 6, 2003.

  1. Obsolex

    Obsolex Guest

    Nov 17, 2002
    I kind of have an idea, and if I think it is what it is, i'll probably pick one up pretty soon... And what does it do... thanks, _-soto-_
  2. PollyBass

    PollyBass ******

    Jun 25, 2001
    Shreveport, LA

    *Polly rips hair from scalp*

    Please do a search kind friend. There are many, many threads already on this very hot topic. Try searching for "compressor", and I find that if I click "In thread titles only" I find the information I want quicker! Thank you.....

    AHHHHHHHHHHHHHHHHHHHHHHHHH wasa,,,,whsa,,,,wast,,,,,,wasts,,,,,,,WHAAAAAAAAAAAA!!!!!!
  3. moley


    Sep 5, 2002
    Hampshire, UK
    A compressor evens out the volume of your playing by making the louder parts quieter. When the volume of your playing reaches a certain level (the "threshold" - and you can set what level this is) it will automatically reduce the gain. Since the loudest parts are now quieter, you can make the whole level higher without distortion. This has the effect of making the overall level more consistent.
  4. Bruce Lindfield

    Bruce Lindfield Unprofessional TalkBass Contributor Gold Supporting Member

    Compressors also make the quiet parts louder - usually bringing up any background hiss - so adding noise!!

    They can also dramatically change your tone and lose a lot of bass.....:(
  5. moley


    Sep 5, 2002
    Hampshire, UK
    Yes, especially if you go mad with it. A certain amount of compression/peak limiting used on recordings, is desirable - but to be able to control your volume by virtue of your technique, is better than using compression to make up for your inconsistent playing.
  6. Mike


    Sep 7, 2000
    If you purchase a compressor buy a high quality unit. I've always had very good luck with DBX equipment, great overall compression without any tone sucking side effects.
  7. Obsolex

    Obsolex Guest

    Nov 17, 2002
    how much are they? thanks, _-soto-_
  8. Bruce Lindfield

    Bruce Lindfield Unprofessional TalkBass Contributor Gold Supporting Member

  9. Good Compressors for under $200

    I use this one:





    A compressor/limiter is essentially an automatic volume control. Imagine an engineer with his hand on a fader and his eyes on an input level meter. As long as the meter stays below a certain point (the threshold), he leaves the fader all the way up and the gain is unchanged. But the instant the sound gets louder, the engineer pulls down the fader by a certain amount. After the sound gets soft again, the engineer will push the fader back up. That's what the compressor is doing, except much faster and more accurately than humanly possible.
    Paradoxically, by cutting the peak levels, a compressor allows you to raise the average level of a sound using the Output control to make it sound louder. By using the threshold and ratio controls, you can set a stable sound that will hold its position in the mix whether the singer is whispering or screaming.
    What the controls do
    Let's go back to the "engineer with his hand on a fader and eyes on the meter" analogy. The front panel controls simply tell the "engineer" what rules he should follow. [THRESHOLD] tells him how high the input meter can rise before he has to start pulling down the fader: if it's turned full clockwise, he won't pull down his fader until the highest red LED comes on; if it's turned counter-clockwise, he'll have his hand on the fader even before the lowest green LED lights. [RATIO] tells him how far he should "pull the fader down" when the signal is above the threshold level: should he pull it down just a little bit (compression) or pull the fader as far down as necessary to make sure the output level is never higher than the threshold (limiting)? The [Over Easy (r)] switch affects how he reacts as signal approaches and travels through the threshold: does he reduce it exactly by the ratio only after it crosses the threshold, or does he gradually ease into the full ratio as it passes through? The LEDs of the gain reduction meter tell you how much the "engineer" is pulling down the "fader" at any time. If these LEDs aren't on, his hands are in his pockets.
    The [ATTACK] and [RELEASE] controls involve the speed of the engineer's response. Short attack times order the engineer to get his hands on the fader 1/10,000th of a second after he sees a too-loud signal; long attack times tell him to let transients less than 1/5th of a second pass. [RELEASE] tells the engineer how quickly he should push the fader back up again after a loud signal has stopped; when it's turned counter-clockwise, he pushes the fader back up instantly, and when it's full clockwise, he'll take longer to push his fader back up to unity gain.
    The [OUTPUT] control is simply a gain control located after our "automatic engineer in the box". The [INPUT/OUTPUT] switch allows you to see the levels before the engineer does his job, or after.
    The most important controls are the [THRESHOLD] and [RATIO] knobs. They both interact to get the effect you want, and that requires some experimenting. For example, if your average input signal is 0 dB, a ratio of 2:1 with a threshold of -12 dB will give you 6 dB of gain reduction, as will a ratio of infinity with a threshold of -6 dB. But the latter setting will sound more "squeezed" than the former.
    Avoid common compressor mistakes
    Extreme settings will lead to extreme results. If you set an infinite ratio and turn the threshold down to -40 dB, the compressor will do what it's being told to do: turn the level way down. If you then try to compensate by cranking the [OUTPUT] control to its maximum, you'll amplify the noise of your mixer, EQ, mic preamp, and the compressor itself. The noise will fade itself in whenever the input signal stops, resulting in the classic "pumping" and "breathing" problems. Noise is present in every system, and improper use of any compressor will amplify it to an obnoxious level.
    If the ratio is set to 1:1, it doesn't matter where the [THRESHOLD] control is: the compressor is being told not to change the gain at all, even if it's above the threshold level. None of the REDUCTION LEDs will light, and you may as well have the compressor in BYPASS mode. Similarly, if the ratio is infinite and the threshold is high, or the input trim of the mixer or microphone preamp is too low, you will get no compression (and, if you raise the [OUTPUT] level control, you'll be amplifying the noise floor). For low noise operation, make sure your mixer, compressor, and amplifier settings are set properly. As a general rule, you want as much gain as possible in the front of the system (at the microphone preamp), so that a good line-level signal is travelling through the whole signal path. If you have a weak signal to start with, and then amplify it at the end of the signal path (by turning the main outputs of the mixer all the way up, for example) it will be excessively noisy.
    When using a compressor on a live P.A. system, improper settings can cause feedback. Make sure that a channel is well below the feedback point when there is no gain reduction active. If you hear feedback every time the music stops, you must lower the overall level of the system.
    About stereo compression
    - For great technical information on stereo processing, see the dbx white paper called True RMS Power Summing(r), written by Roger Johnsen, Director of dbx Engineering. Otherwise read on....

    Compressors are often difficult to grasp for beginning musicians and engineers. Where an effect like pitch shifting or delay is easy to hear, compression often performs subtle changes on a signal and can be more difficult to learn. This introduction is intended as an expanded glossary which shows what the controls on the compressor's front panel are doing to the input signal.

    The THRESHOLD knob sets the level where compression will begin. As long as the input signal level is below the Threshold level, the compressor will do nothing to the signal. Once the input signal crosses the Threshold, the compressor will begin compressing at a ratio set by the ratio control.

    The RATIO knob controls the amount of compression which will happen once the input signal crosses the Threshold level, described above. Ratio controls how much the input signal will be reduced as a ratio of the input signal level. For example, if the compression ratio is set for 6:1, the input signal will have to cross the threshold by 6 dB for the output level to increase by 1dB. The maximum setting is typically labeled :1 (Infinity to 1), and is also called Limiting. This means that the output signal won't increase at all, no matter how far above the threshold the input signal goes.

    The ATTACK knob controls the amount of time before compression starts after the threshold is exceeded by the input signal. The range of this control can go anywhere from very slow (1dB/Sec) to very fast (400dB/Sec). Long attacks are useful for percussive sounds, where shorter attacks are good for melodic parts like vocals and strings. The Attack control is also useful for keeping the transients on percussive drum or bass sounds. Experiment with different short attack times on snare drums to get more or less of the "stick" attack.

    The RELEASE knob controls the amount of time the compressor takes to stop compressing after the signal crosses below the threshold. The range of this control can be from 4000dB/Sec to 10dB/Sec. Short release times are good for percussive, punchy sounds, where longer release times can make compression less obvious on vocals. Adjusting the release time may be necessary when using extreme compression and "pumping" or "breathing" is audible, or if lower level signals after peaks are getting lost. See also the section on pumping and breathing explained below.

    The OUTPUT knob controls the level of the compressor's output. The Output control is useful for making up gain which was reduced by the compression circuit or matching the input level of a mixer or recorder. If the Gain Reduction meter shows that the input signal is being attenuated by -6dB, then the Output control generally should be set around +6dB. This control is disabled if the [BYPASS] button is pressed.
    Hard knee/OverEasy®
    The OverEasy switch is used to switch between Hard and Soft knee compression styles. When the compressor is set for Hard knee, the compression ratio applies only to signals above the threshold level. If the compressor is set for Soft knee, the compression ratio gradually increases from 1:1 to the currently selected ratio over a range through the threshold area, so that the transition from uncompressed to compressed is more gradual. The difference between Hard Knee and OverEasy is more obvious at high compression ratios. Once the input signal crosses the Threshold, the unit will compress the signal at the full ratio level.
    OverEasy compression is useful when performing high-ratio compression or limiting on a signal. When the compression gradually fades in, it doesn't sound as obtrusive as when it suddenly starts limiting the signal. If you're looking for a "brick wall" limiter, the switch should be set for Hard knee to stop any transients from slipping through without affecting lower level signals. Lower Ratio levels may require a hard knee setting so that the compression slope isn't too narrow and you loose some of the compressive "punch".
  10. Donne, that is one of the best explanations of compression that I've ever read:eek: :D :) . Thanx for taking the time to explain it thoroughly, good stuff!!!
  11. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    Yep. A compressor is like any other part of your signal path:

    1) Higher quality gear generally gives better results
    2) Improper use will yield poor results regardless of quality
  12. Obsolex

    Obsolex Guest

    Nov 17, 2002
    dude, donne that explanation is crazy, i wish everyone did it like that... thanks guys... _-soto-_
  13. Bruce Lindfield

    Bruce Lindfield Unprofessional TalkBass Contributor Gold Supporting Member

    Well, I would guess that is copied from the manual to a particular model ...?

    Not all compressors are made equal!! ;)
  14. You can download the user's manual for the $89 Behringer compressor posted above, it has a hell of a lot of good instructions in it about uses for compression. It builds on what Donne said, and there's some graphs to help you understand the functions of thresholds and ratio a bit better.


    I'm pretty much a newb when it comes to this gear but that cheap unit is doing just about everything I want it to right now and not much that I don't. The built in noise gate eliminates all that nasty hiss that was mentionned earlier too. If you sing as well, this works great for running your bass into one channel and vox into the second IME, although I'll probably start using both channels for my bass signal at some point.

    If you don't really know what you're doing with these I see no real reason not to buy a cheapie rather than a $1000 unit to start off with unless you have a lot of money lying around.

    Edits: found better versions of the manual