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Author Topic: One thing I don't understand about compressors  (Read 22087 times)

esteso

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One thing I don't understand about compressors
« on: April 08, 2006, 03:59:05 pm »

I've been studying Bob Katz' book  'Mastering Audio' and I've run across an explanation on compression I just can't get no matter how mant times I read it. (good book, BTW.... thanks a lot Bob)

Anyway on page 119 there's a diagram of a simple tone burst. It starts loud, then drops in level and then goes back to it's original level. All of these gain changes are instantaneous so the tone burst looks like the letter "H" Next is a diagram of the same tone burst treated with compression, fast atttack, fast release and the threshold is set midway between the loud and the soft sections.

OK, so now the louder part of the burst becomes squashed. No problem. But the softer part gets brought up in volume. I just don't get this. Shouldn't the softer part return to unity gain after the signal drops below the threshold?

(hope this isn't too hard to follow, if you have the book the diagram makes it pretty clear)

Thanks for the help
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Michael Telle
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Viitalahde

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Re: One thing I don't understand about compressors
« Reply #1 on: April 08, 2006, 05:02:46 pm »

You're thinking this upside down.

The lower part is brought up - relatively. As you squash the shit out of the loud part, it'll be at a lower level, and when you bring the gain back to unity so that the loud part is at its original level, the soft passage is kinda "brought up".
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Jaakko Viitalähde
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esteso

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Re: One thing I don't understand about compressors
« Reply #2 on: April 08, 2006, 05:12:13 pm »

No, this is not what the diagram shows. The level on the lower tone burst is shown as being sinificantly louder in comparison to itself, not relatively to the loud section.

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Michael Telle
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Viitalahde

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Re: One thing I don't understand about compressors
« Reply #3 on: April 08, 2006, 05:15:57 pm »

..I'm sure Bob did it like this just to point out that the soft part IS going to pop up (which can be a pretty bad thing).
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Jaakko Viitalähde
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esteso

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Re: One thing I don't understand about compressors
« Reply #4 on: April 08, 2006, 05:39:21 pm »

No, I'm sorry to have to disagree..... but this is a scientific diagram meant to explain the technology behind compression, not a subjective analysis. Thanks for your replies but I think I need someone with access to the diagram or maybe Bob himself.

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Michael Telle
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jdg

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Re: One thing I don't understand about compressors
« Reply #5 on: April 08, 2006, 06:02:42 pm »

the diagrams on page 120 'show' it better. refer to those Smile
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john mcCaig
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esteso

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Re: One thing I don't understand about compressors
« Reply #6 on: April 08, 2006, 07:17:51 pm »

Ok, got it. I could've just turned the page but that first diagram had me totally flummoxed and I kept fixating on trying to understand (what can't be understood)

But I'm still a little cofused as to why Bob wrote, "Note that the loud passages are instantly brought down, the soft passages are instantly brought up........"

thanks
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Michael Telle
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CHANCE

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Re: One thing I don't understand about compressors
« Reply #7 on: April 08, 2006, 07:57:55 pm »

Could it be that perhaps the low passages are brought up with "make up gain"?
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Chance Pataki
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A person is a biological signal processor--EQ mag

esteso

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Re: One thing I don't understand about compressors
« Reply #8 on: April 08, 2006, 08:58:50 pm »

Good question, first thing I thought of actually. But if that were the case the level of the loud burst would match, both compressed and non-compressed, no? They do not.

Anyway, I don't need to beat this to death. I've decided that it has to be a typo (or diagrammo)

Cheers
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Michael Telle
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bblackwood

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Re: One thing I don't understand about compressors
« Reply #9 on: April 09, 2006, 09:24:39 am »

esteso wrote on Sat, 08 April 2006 18:17

But I'm still a little cofused as to why Bob wrote, "Note that the loud passages are instantly brought down, the soft passages are instantly brought up........"

Indeed. Without AGC or some follow-up gain, that's incorrect. Compression only affects the signal above threshold...
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Brad Blackwood
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zmix

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Re: One thing I don't understand about compressors
« Reply #10 on: April 09, 2006, 11:30:50 am »

esteso wrote on Sat, 08 April 2006 15:59

 Shouldn't the softer part return to unity gain after the signal drops below the threshold?


You are correct. This sounds like an editorialization of the 'horrors' of compression to me.

zmix

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Re: One thing I don't understand about compressors
« Reply #11 on: April 09, 2006, 11:32:50 am »

bblackwood wrote on Sun, 09 April 2006 09:24

Compression only affects the signal above threshold...

False. Compression operates on a point of rotation set by the threshold. Limiting affects only signals above the threshold.

esteso

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Re: One thing I don't understand about compressors
« Reply #12 on: April 09, 2006, 04:12:05 pm »

Ok, on the next page a similar quote reads, " The higher ratio clamps the high signal down farther, and with the fast release, as soon as the signal goes below the threshold, the release time aggressively brings the level up. This type of fast action can make the music strongly compressed because it brings down the loud passages and quickly brings up the soft passages."

I think Bob must have been thinking make up gain but didn't let us know. Just a theory. A great book really, I just fixate on the things I don't understand.

Thanks
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Michael Telle
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brett

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Re: One thing I don't understand about compressors
« Reply #13 on: April 09, 2006, 05:14:55 pm »

zmix wrote on Sun, 09 April 2006 16:32

bblackwood wrote on Sun, 09 April 2006 09:24

Compression only affects the signal above threshold...

False. Compression operates on a point of rotation set by the threshold. Limiting affects only signals above the threshold.



Absolutlly. The threshold triggers the compressor and if the loud transient then subsides quickly and the release is set short not to overlap the lower level material, that lower level material is braught up with the make up gain. On big 808 drum machine kicks for instance, you can use a compressor to make the boom sound louder longer.

Now, the concept is that compression is used for gain increase and loudness increase even though we logicaly use it to quiet peaks. Expansion is downward in that it makes things quieter except where the threshold is crossed and the expansion is triggered. That makes that part louder through out the release envelop. It can breath life and dynamics back into an over compresseed or flat sounding part.

Compression is upward and Expansion is downward on the overall RMS. A compounder does both compression and expansion and usually gating and limiting used on individual tracks like vocals and bass.  
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bblackwood

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Re: One thing I don't understand about compressors
« Reply #14 on: April 09, 2006, 09:51:28 pm »

zmix wrote on Sun, 09 April 2006 10:32

bblackwood wrote on Sun, 09 April 2006 09:24

Compression only affects the signal above threshold...

False. Compression operates on a point of rotation set by the threshold.

Yah, yah, I know. My point is that the way that quote is worded is incorrect without built in gain...
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Brad Blackwood
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