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Thread: Gravity Jim.. and others.. Explain Compression to me again.

  1. #1
    ZoneFiend photoweborama's Avatar
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    Gravity Jim.. and others.. Explain Compression to me again.

    Jim explained compression to me a couple of years ago.. I should have printed it out... The explanation was not what I thought it was, but now I forgot it all.

    Can you go over it again for me?
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  2. #2
    Gravity Jim
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    A compressor does pretty much what its name implies: it "compresses" audio.

    If the gain going through a compressor gets above a certain threshold (usually set by the user, but on many guitar pedal, a pre-set limit), then the compressor reduces the output by a given ratio (again, usually set by the user).

    Let's say you set the threshold at -12db, and the radio at 2:1 (two to one). That means that any signal below -12db will be unaffected, but any audio above -12 will be reduced by a ratio of 2:1, or 50%. So, if a loud transient comes along that trips the meter at -6db, the amount of signal above the threshold (6db) will be reduced by 50%, and only 3db above -12 will be allowed to pass. So the resulting output is only -9db, instead of the -6db it would have been if left unaffected. If the ratio was set to 4:1, then only 1.5 above -12 would be allowed to pass, and so on.

    Most compressors also have an output gain control, so that if you choose to compress at low thresholds and high ratios, you can increase overall gain and make everything that passes through the unit much louder. This is why compressors get the reputation for making quiet stuff louder and increasing sustain. Those effects are by-products of compression, but that's not really what the compressor is doing.

    Compression set at higher thresholds and low ratios can be used to very naturally and transparent "smooth" out a source like a guitar or vocal. Set at lower thresholds and higher ratios, they can be used to "slam" drums or make bass guitars sound "punchier" by accentuating the attack. Studio compressors also allow the user to adjust the "attack" of the unit (how fast the compressor reacts when the gain exceeds threshold) and "release" (how fast the compressor lets go of the signal once the gain stops exceeding threshold). These controls let the engineer shape the envelope of the compression.

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    Forum Member thetallcoolone's Avatar
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    Understanding the technicality of it is one thing but applying it is another.
    Is compression the first item being applied to a track, at the initial recording, and at the end for the final mixdown?

    And where does EQing come in the chain?
    Do you EQ before, after or while compressing (using the compressor's own EQ)?
    I've also seen a "knee" knob on some compressors. Is it just another name for attack or treshold or something?
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  4. #4
    Gravity Jim
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    TC1, I agree that learing to use compression properly is one of the toughest recording challenges. Even after I understood what it was doing, it took a long time to get consistently good at it, or to know what knob to turn to get what effect. i don't think there's any substitute for practice in that regard.

    As far as what point in the chain to apply compression, there's no hard rule. Guitars and basses are often compressed as they're being recorded, and maybe again at mixdown. Vocals are usualy not compressed until mixing, but if you're going for an effect, you might plug your mic right into a guitar compressor.

    So there's no real rule, but usually it's applied during mixing, after everything else (including EQ). Most compressors don't have any equalization of their own... some multi-stage units, like the Aphex Compeller, are a mix of compressor, limiter and EQ stages.

    The "knee" knob or switch simply changes the attack curve of the unit. Think of the bend in your knee as the shape of the gain as the compressor biotes down on it. "Soft knee" has a more rounded curve, "hard knee" has a sharp, hard-wall kind of curve.

    Don't even get started on multi-band compression. I've been working with it for years now and I get reasonable results, and I'm still an amateur compared to real mastering engineers.

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    Forum Member thetallcoolone's Avatar
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    I almost threw in a question about multi-band compression. I guess I'll hold it then. :lol
    My understanding of multi-band compression is that it is used mainly at the final level of mastering, when you mix all the tracks to stereo. Because the end result of all the tracks has a more complex frequency spectrum.
    Am I right?

    And may I push the limits here and ask you to have a listen to my track Smooth Cheese and comment on the recording?
    Understand that I have a very basic home setup with only the output of my soundcard to a little home stereo system so no pro stuff here. But I feel the overall sound of the track is fairly good and I've been told by some people they were surprised at how well it sounded.

    I'd like to have the opinion of a pro.
    And don't be reserved on the comments, I'm not looking for contaplency here. If to you it sounds crap, tell me. But I'd like to know why and what would be needed to make it sound better (besides a pro studio and engineer) or what kind of mistakes I made mixing it down.

    Just so you know, all instruments, except the guitar, are MIDI softfont instruments. The guitar is recorded directly into the input of the soundcard (a Soundblaster Live!), thru a multieffect pedal in bypass mode, with amp simulation plugin in the recording software. All sound processing is done thru plugins in the recording software (Tracktion 1.6)

    If you could be so kind to take a little bit of your time to evaluate it I'd be very gratefull. There's no real other ways for me to improve and learn than to have the opinion of a pro on my work.

    Photo, I don't mean to hijack your thread but I thought you might be interested in this also, make to most out of what we've got.
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  6. #6
    Forum Member Wilko's Avatar
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    The description above is more of limiting than compression. Limiting only flattens the peaks. It limits the maximum amplitude of the wave. Compression has two main components. Limiting the max and bringing up the minimum amplitude. That second part is mentioned as a function of the "gain" control above.
    Both parts combined is what compression does. Using both is what can be explained in its use as a "Sustainer". What happens there is when a note is played you can expect that it will peak when the string is plucked (attack), then you can predict that it will fade away (decay). What the compressor does is like holding a volume knob. It holds it down when you plauck the string then turns it up automatically as the note decays making it stay the same volume instead of fading away. It's not making the note play any longer. Once it's gone it's gone. But, on the way down the compressor kept turning up the volume instead of letting it fade.

  7. #7
    Gravity Jim
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    My description above is not a description of a limiter. Limiting does not reduce gain above threshold by a given ratio... it simply stops the signal from ever exceeding the set threshold.

    What you think is the second function of a compresser is just an amplifier.Thinking of a compresser as a device that creates sustain, as guitar players usually do, is exactly why most of them can't figure out how to use one on vocals, drums and other instruments.

    Also, a compressor does not EVER automatically increase gain, and it is not turning up the volume as a guitar note decays. What is actually happening when a compressor is used that way is this:

    The unit is set to a low threshold and high ratio. This really does start to act kind of like a limiter, keeping every note from getting any louder than (let's say again) -12db. Because you have the output strongly limited, you can turn the whole thing up 12 db and not exceed 0.

    So, let's say you hit a note real hard, something that would have registered at 0 on the meters all by itself. But the output has been compressed to -12db and then amplified to 0. So as the note naturally decays, the output from the compresser stays at exactly the same level until the note naturally dies to under -12db... even though the note is decaying, you don't hear it. This creates an audio illusion similar to the sound of an amp being automatically turned up, but that's not what's happening.

  8. #8
    Gravity Jim
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    TC1 - multiband compression IS usually used in the mastering stage, and your guess about a more complex frequency profile is on the right track. Multiband gets used in mastering so that you can deal with different bands without affecting others. For example, if you compress a final mix with an ordinary stereo compressor, it will compress EVERYTHING every time the general level comes up above threshold. So if you have a kick drum that pops the track up above threshold every two beats, the compresser will compress all the guitars and vocals and everything every time it happens. But, since most of the information from the kick drum is in the down-low, if you use a multiband compressor than only the frequenciesd in that down low band (wherever you set the limits) will get compressed when the kick drum hiots, leaving the vocals and guitars and stuff alone. So you can fine tune the response of each band independently, allowing you to emphasize and de-emphasize frequency bands without creating the kind of audio havoc that EQ creates. I was instructed to think of a multiband compressor as "EQ without phase problems."

    As to your recording: the biggest problem is the build-up of frequencies. There are a lot of guitars playing at once... it sounds like there are always, like, four of them at least. Since you're recording them all in a similar fashion, you've got a lot of tracks that are all occupying the same sonic space. here are some things you can try:

    1. Clean up the arrangement. There are a bunch of rhythm parts that probably sounded real cool by themselves, or in tandem with an already existing track. But once you start burying them under two or three tracks of lead, they just turn into noise. Choose the coolest parts, play them clean and confidently, and don't feel like you need to hide them behind more parts. Give every part its own sonic space.

    2. Lose the lows. On everything but bass guitar and drums, try shelving everything under 120 hz. Then, on select tracks, try moving the shelf up closer to 300hz (this is the dreaded "mud frequency"). The goal is to get rid of all that low end information in the guitar tracks... every track is adding somethign to it, and while you're not hearing it as "guitar," it's building up to the point where you finally hear it as "mud." Once again, you're trying to give it all its own sonic space.

    Those are good places to start. You just want to get rid of that wash of low-mid freqeuncies. But remember what Roger Nichols says about this: "Don't take out too much. This is where the MUSIC lives." Meaning you can strip the guts and warmth right out of the tracks by removing too much of the low mids.

  9. #9
    ZoneFiend photoweborama's Avatar
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    Thanks Jim, I'm going to cut this thread into word this time and save it!
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    Forum Member thetallcoolone's Avatar
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    Jim, this is very cool.
    Thank you very much for taking some time to analyse it.

    I'll rework the whole track using your suggessions.
    I'll come back to you, if you don't mind, on another thread and give Photo's thread back to him.

    Thanks a lot again.
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  11. #11
    fezz parka
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    Post #8 says it all... :)

  12. #12
    Forum Member mgade's Avatar
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    Re: Gravity Jim.. and others.. Explain Compression to me again.

    Next level: Handling it all in Cool Edit :) Thanksfully most jams are simple enough that I have never done more than panning, increasing and decreasing volume levels plus a little compression here and there. That's tough/fun enough. I have never really managed the limiters! Some one could do worse than starting a class on that!

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