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Author Topic: When to use a compressor  (Read 2308 times)

Mike Caldwell

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Re: When to use a compressor
« Reply #10 on: May 05, 2018, 08:14:21 am »

Thanks for all the input guys. I wasn't really getting the answers I was looking for just by Googling. I had to go somewhere and ask. Actually, I have one last question. Normally I would start another thread for this, but I think it can be answered quickly.

What is the order (Chain?) for all this equipment?

Does it go: DJ mixer - EQ - Compressor - Amplifier - Drive Rack - Speakers

Close....

DJ Mixer - EQ - Compressor - Drive Rack - Amplifier - Speakers

However.....
Some would swap positions of the EQ and Compressor.


I assume with "drive rack" your talking about one of the various DBX units.
The Drive Rack either a DBX or a model from one of many manufacturers will do everything in that one unit eliminating the need for the EQ and compressor and can be locked out to prevent everyone making adjustments.

Ivan Beaver

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Re: When to use a compressor
« Reply #11 on: May 05, 2018, 09:49:55 am »

Close....

DJ Mixer - EQ - Compressor - Drive Rack - Amplifier - Speakers

However.....
Some would swap positions of the EQ and Compressor.


I assume with "drive rack" your talking about one of the various DBX units.
The Drive Rack either a DBX or a model from one of many manufacturers will do everything in that one unit eliminating the need for the EQ and compressor and can be locked out to prevent everyone making adjustments.
Often "drive rack" is sated, but they don't actually mean DBX units-it has become standard for processor, like Kleenex for facial tissue.

There is no need for an eq (for a DJ anyway) since the tonal issues should be sorted out in the system processor (driverack).

I would argue that if you must have a EQ for the Dj to screw up, it MUST go in front of the compressor.

If it goes after, then they have control over the actual output levels.  That can be dangerous.

I believe in hard clamping of systems that don't have a qualified tech to look after it.  DJ house systems are a classic example, and HS football stadiums.

Both have amatures that don't care about the sound quality and the only thing they know if how to turn it up to make it loud.  Sad-but true
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Mike Caldwell

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Re: When to use a compressor
« Reply #12 on: May 05, 2018, 05:01:40 pm »




I believe in hard clamping of systems that don't have a qualified tech to look after it.  DJ house systems are a classic example, and HS football stadiums.

Both have amatures that don't care about the sound quality and the only thing they know if how to turn it up to make it loud.  Sad-but true

That's true for any system that the general public has access to!

Luke Geis

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Re: When to use a compressor
« Reply #13 on: May 05, 2018, 05:16:27 pm »

For the purposes of system protection, a compressor is not the tool to use. It is a tool that in conjunction with a limiter can be utilized for system protection, but alone it will not work.

When to use a compressor is more a matter of taste and desired goals. A compressor, I would say, is never truly needed. However when utilized well it can really help things out. Wrongly utilized it can create as many issues as it solves though. In general, the idea behind a compressor is to reduce and control the dynamic range of the media so that you can place it in your mix and have that media stay closer to the desired level. It reduces peaks and increases the noise floor making quieter passages appear louder, while quieting the peaks of the media. This in turn increases the RMS level of the media giving it a higher perceived listening level even though the channel may not actually be any louder than without compressor.
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Michael Lawrence

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Re: When to use a compressor
« Reply #14 on: May 05, 2018, 06:09:52 pm »

I would like to add a little clarification for you here, Steve.

We need to draw a distinction between a compressor used during the mixing process and a compressor used as part of system processing.

System optimization guru Bob McCarthy has a term - "The Art - Science Line" - to describe the transition that happens at the outputs of the mixing console. In a DJ situation, the DJ mixer's outputs play the same role. Once the signal leaves the mixer, it crosses the Art-Science line, and everything downstream of there (system processor, amps, loudspeakers...) is tasked with simply relaying the input signal to the audience as faithfully as possible. This is an objective goal, with the aim being that what goes into the system is what the listeners hear, period. Anything done artistically - adjusting tone, level, balance of instruments, the mix itself - happens before this point, and this is where the artistic decisions come into the process. By the same thinking, the USPS should not alter the content of your letters, they should just deliver them to the destination.

In the mixing stage, compressors basically will change the way the signal level varies over time. Compression sets a threshold. Anything below the threshold passes right through unaltered. Anything above the threshold is turned down ("attenuated") by an amount you set. (A ratio, measured in dB.) This might happen so quickly that percussive events like drum hits actually lose their attack, or it might happen much slower, such that a singer gets a few syllables out before the gain reduction kicks in. That's up to you, and how you set the compressor. Once the signal level drops back below the threshold, the gain reduction is reduced again. As others have said, there are many various applications for compressors in the mix. Everyone's got their own recipe here.

Once the mix leaves the mixer, the use of compression is different.
There are two main causes of loudspeaker damage: thermal and mechanical.

Mechanical damage occurs very quickly. If a mic is dropped or an open channel is "stabbed" (among other things), a loud POP will issue from the loudspeakers, which is the sound of the loudspeaker diaphragms ramming all the way forward or backward, and the voice coil can actually jump out of its 'gap' and become permanently damaged. This is called over-excursion and it happens very very quickly, so to protect against it, system techs use a VERY fast-acting limiter that simply will not allow the level to increase past a certain threshold, and will catch it (in theory) before damage occurs. These are digital devices with sub-millisecond response time. A compressor will not help you here.

Where it will help you is with thermal damage. Past a point, a loudspeaker doesn't get louder, it just gets hotter. If the loudspeaker voice coils overheat, they can melt and burn and short out or "open" or all sorts of nasty things that result in your system not working. In these cases we're more concerned with the system's heat buildup over time. A single snare drum transient isn't going to heat up the voice coils in the same way that a sustained high-level sine wave will, so we're looking at RMS levels (basically, average levels) which gives us a better indicator of how the system is faring thermally. A compressor can be used to place an RMS limit on a system's signal level. So system protection generally includes an RMS compressor - or a limiter - for longer-term stuff and a peak limiter for shorter-term stuff.

That's most likely waaaay more information than you wanted, but you can see that a reasonable answer to your question of "Where in the signal chain does this compressor go?" is "Depends on what you are trying to do: protect the system, or control the dynamic range of your mix in an artistic sense?"
« Last Edit: May 06, 2018, 01:59:13 am by Michael Lawrence »
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Steve Mick

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Re: When to use a compressor
« Reply #15 on: May 05, 2018, 10:57:42 pm »

For the purposes of system protection, a compressor is not the tool to use. It is a tool that in conjunction with a limiter can be utilized for system protection, but alone it will not work.

When to use a compressor is more a matter of taste and desired goals. A compressor, I would say, is never truly needed. However when utilized well it can really help things out. Wrongly utilized it can create as many issues as it solves though. In general, the idea behind a compressor is to reduce and control the dynamic range of the media so that you can place it in your mix and have that media stay closer to the desired level. It reduces peaks and increases the noise floor making quieter passages appear louder, while quieting the peaks of the media. This in turn increases the RMS level of the media giving it a higher perceived listening level even though the channel may not actually be any louder than without compressor.

Gotcha. So the limiter would be the last part of the chain before the amps then, right?

Also, is there anything that goes in between the amps and the speakers? Let's say your running four 18" subwoofers and 10 12" loudspeakers with horns. Would you need several different amps for this type of setup or can you just use a couple of really powerful amps and then somehow bridge that power out to all the speakers?

Sorry for all the questions, but I might as well just get this all figured out now.
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Stephen Kirby

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Re: When to use a compressor
« Reply #16 on: May 05, 2018, 11:00:04 pm »

Actually highly compressed signals are what cause voice coil overheating.  The average level is higher.  So depending on what the overall dynamic range of the signal is, limiters may need to be adjusted to control the amount of heat build up in the drivers.  Highly compressed music with very little dynamic range needs to have more aggressive limiting, at lower overall levels.  Most modern dance music would be like this.  A system where someone is using a fairly high compression ratio like 8:1 and the input signal (even if it is -16 LUFS pop tracks) is cranked up so that the compressor is at max will also heat up voice coils.  So using a compressor as a "sort of" limiter can also be dangerous.  If the input levels are kept down so that the gain reduction is just barely tripping in time to the music, then you're just squashing the dynamic range slightly off the top.
What you want is a compressor set to give some consistency between highly compressed dance tracks and recently mixed pop tracks.  And then a peak limiter to put a hard stop on the levels at a point where the drivers can survive.
I mentioned something above with regards to pop tracks that may need more clarification.  Because of the various streaming services the industry has clamped down on the loudness wars.  To get music onto YouTube or iTunes, it has to have an average level around 16dB below full scale.  The industry has termed this total average level LUFS or Loudness Units Full Scale.  Meaning if it is some super compressed track with 3-4dB of total dynamic range from 5 years ago, it gets turned down so that the peaks are 12dB below full volume.  While if it has occasional peaks, they can be right up against full volume (0.1dB for iTunes, 1dB for YouTube or Spotify) as long as the total average is 16dB under full volume.  This is wonderful for music as the music can be mastered to breathe again.  My latest version of Izotrope Ozone has a metering function as well as settings on the level maximizer plug in to set the LUFS target.
What this means to a DJ is that recent pop tracks are going to have higher dynamic range than the classic dance tracks of a few years ago or dance specific music that is still compressed down to nothing as part of the genre.  So some degree of compression in the system may be in order.
So now your signal chain goes like:  Mixer - EQ (of debatable utility) - compressor - peak limiter - system processor/driverack (which may have both compression and peak limiting functions built in) - amps - speakers.
Amp to speaker relationships are a whole different subject.  Covered in many threads here.  The use of the term "bridged" is a bit concerning as many folks have the notion that bridging gives them more power.  In reality a particular amplifier can only source so much current.  Playing games with the impedance load to get more "watts" is just a math game.  There's only so much "power" available.  How much excess power over a speakers rating is a part of system design and over what kind of dynamic range the system will be used.  And folks like Ivan have written very good recommendations on various threads.
« Last Edit: May 05, 2018, 11:05:47 pm by Stephen Kirby »
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Shane Ervin

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Re: When to use a compressor
« Reply #17 on: May 06, 2018, 11:29:55 am »

<snip> Because of the various streaming services the industry has clamped down on the loudness wars.  To get music onto YouTube or iTunes, it has to have an average level around 16dB below full scale.  The industry has termed this total average level LUFS or Loudness Units Full Scale.  <snip>

Great news about Volume Wars coming to an end, or at least an armistice.
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Luke Geis

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Re: When to use a compressor
« Reply #18 on: May 06, 2018, 11:16:39 pm »

It has been coming to an end for years thankfully. It has only in the last few years been regulated in such a way where certain formats will not accept the media unless it is within a particular crest factor format. There was an individual who's name slips my mind, who was shooting for a -12 to -14db crest factor as being the ideal range to shoot for. While commercial media has not yet gotten this memo, at the very least, some media formats and playback services are sticking to one. I feel that -16db is a little unrealistic, but is a range where just about any individual can process their media to, so it makes sense. I think -10db is an absolute max and -12 is probably the most realistic and reasonable crest factor to shoot for. Any hotter and it does begin to sound processed, or at least unreal. A crest factor of -15 to -20 is about where most un compressed live music seems to sit, so -16 would just be a little too low for any engineer who utilizes fair amounts of compression / limiting to control peaks in their production. I can get to around a -10db crest factor with my live band productions and by this point it starts to sound produced. A -12 to -14 db crest factor would really keep live and commercial media within a range that at least a live band will sound more like the record potentially.

When you start compressing vocals and acoustic instruments 10db, it is not only noticeable, it starts to become more processed than live sounding. While it is nice to be able to utilize every last bit of the PA and have a performance that sounds close to a CD, the current commercial level of media is simply making us have to live up to an unrealistic standard. I have had just as many mixes sound amazing without any compression / limiting as I have with it, and with uncompressed live sound, it certainly is easier to get a live sounding / realistic mix. I err to the more processed sound because bands seem to expect it and I can usually retain some headroom in my system doing so and still have a very high impact show.

Compression has its place in live production, but as with anything " too much of a good thing is still to much ".
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Michael Lawrence

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Re: When to use a compressor
« Reply #19 on: May 07, 2018, 12:36:04 am »

There's a ton of Loudness Wars data - charts showing average levels over the past years, etc - in mastering engineer Bob Katz's book "Mastering Audio: The Art and the Science." Definitely worth checking out.
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ProSoundWeb Community

Re: When to use a compressor
« Reply #19 on: May 07, 2018, 12:36:04 am »


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