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Author Topic: DCX2496 Limiters  (Read 11132 times)

Ron Kimball

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Re: DCX2496 Limiters
« Reply #20 on: June 30, 2009, 04:11:52 PM »

Greg Cameron wrote:

After a quick glance at the manual, it's pretty simple. The limiter can be set for limiting between 0 to -24dB which should equate to +22dBu to -2dBu of actual output.
I was guessing that to be true from my own read of the manual but was hoping someone here has actually confirmed it with real measurements as the manual is more than a bit vague on the subject. Guess I'll set up some instruments and check it out myself. I'm a bit shocked that folks are using these without actually knowing FOR SURE what they do Shocked. Certainly you can feed a sine wave into it and - without speakers attached - set the limiter so your amp outputs the proper voltage sine wave to match the rms rating of your sub but if they are peak and not rms limiters you'd be severely limiting your real "music power" to well below the sub's ratings. The only loudspeaker management system that I've verified can do both peak and rms limiting is the one built into the I-Tech. I personally would be happy enough with a properly sized amp with integral clip limiter to limit the peaks and a DSP based crossover/rms-limiter to limit the temperature of the voice coils.
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Greg Cameron

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Re: DCX2496 Limiters
« Reply #21 on: June 30, 2009, 04:20:49 PM »

Aside from thermal issues, I find that when limiting for multiple band-passes, allowing any clipping on horns usually sounds much worse than half way decent limiters. So I'll always set the limiters to prevent any clipping on the horns. My particular system is 5-way. I allow mild clipping on the subs & the 15" drivers in the tops. The 10" are allowed only to clip a tiny, tiny bit. But the 2" and 1" horns are not allowed to at all. It works out pretty well even with a mediocre processor.

Greg
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Cameron Pro Audio.

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Greg Cameron

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Re: DCX2496 Limiters
« Reply #22 on: June 30, 2009, 04:38:21 PM »

FWIW, you do not want to use a sine wave signal to set peak limiters. You want a dynamic source. A quality digital recording of kick, snare and HH makes a good test signal. A speaker polarity test signal like the one The Cricket makes can work too. However, one thing I've noticed about the fast percussive attack of the cricket is that if  your peak limiters aren't capable of being really fast, it can punch through the limiters pretty easily. So a more real world source might work better in that case. But that also says something else: not all processor are created equal when it comes to limiters. And it seems only the most expensive ones do a truly good job of limiting while still sounding good. XTA comes to mind. But it's also $5000+ for a 4x8. Worth every penny if you're serious about a good sounding processor and top notch limiters.

And as I've stated before, don't over-think this. If you're amps are sized such that they're close to the program power rating of your speakers, just set the peak limiters on the processor to keep the amps from clipping or at least clipping hard on peaks. If you want the 'tits' setup with hardcore rms and peak limiting, you shouldn't be looking at Behringer gear anyway. You'll need to plunk down some cash and get I-Tech amps. Or better yet, buy self powered boxes where all this stuff is figured out and implemented already. Going a piecemeal route with getting another external device to handle rms limiting and such is adding complexity that you really don't need to for a basic setup. Sure, you can do it, get the compressor/rms limiter, calculate the max rms voltage your speakers can handle, set the compressor according, etc. But it's really not necessary. There are tons of rigs out there running simple peak limiting and not blowing up. If you're going to be pushing your particular setup that hard, you should get more rig.

Greg
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Nick Hickman

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Re: DCX2496 Limiters
« Reply #23 on: June 30, 2009, 05:12:03 PM »

Hi Ron,

Ron Kimball wrote on Fri, 26 June 2009 20:13

Does anyone know FOR SURE if the limiters on the DCX2496 are peak or RMS? If peak (= kinda useless?), can the dynamic eq be used as an RMS limiter? Does anyone know FOR SURE if the thresholds are offset off of +22dbu or ? Behringer manuals sure suck Sad!

I don't have any inside information, but I did do some tests and there appears to be a lot going on with the DCX2496 limiters.  They appear to combine both RMS and peak detectors.

The threshold you set appeared to be the RMS limit level relative to +22dBu, and the peak limit level appeared to be 3dB above the RMS limit level.  This was based on testing a full bandwidth output and hitting the limiter hard.  Things could be different based on the high-frequency limit of the band-pass or how hard it was hit.

Here's the amplitude-over-time output from a 1kHz sine wave and 1kHz square wave, both with the same peak amplitude, and both in a full-bandwidth output under heavy limiting, on the same vertical scale.  The sustained RMS level is the same in both cases, and the initial "peak" of the square wave is limited 3dB higher.

http://100dB.com/misc/dcx2496/limit.png

Limiting peaks to 3dB above RMS (if that is indeed what's happening) would seem a dubious design decision.  6dB above RMS (i.e. corresponding to the peak level entailed in AES loudspeaker power ratings) would seem more sensible.

Nick
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Ron Kimball

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Re: DCX2496 Limiters
« Reply #24 on: June 30, 2009, 07:57:30 PM »

Interesting data! It would be more useful if there was a vertical scale and even more useful if there were graphs of the inputs also. Pink and/or white noise would be good to try too Smile. If the limiters are rms I would expect the output to be only down 3db (=.707 of) from the sine - this seems to show much more?  
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Nick Hickman

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Re: DCX2496 Limiters
« Reply #25 on: July 01, 2009, 09:00:22 AM »

Hi Ron,

Ron Kimball wrote on Wed, 01 July 2009 00:57

Interesting data! It would be more useful if there was a vertical scale and even more useful if there were graphs of the inputs also. Pink and/or white noise would be good to try too Smile. If the limiters are rms I would expect the output to be only down 3db (=.707 of) from the sine - this seems to show much more?  

Exactly so.  Output measurements were as follows:

Sine wave (limiter bypassed): +1.9dBu RMS, +4.9dBu peak
Sine wave (limiter engaged): -2.2dBu RMS, +1.0dBu initial peak

Square wave (limiter bypassed): +4.8dBu RMS, +4.8dBu peak
Square wave (limiter engaged): -2.1dBu RMS, +1.4dBu initial peak

I'd encourage you to go ahead with your own measurements!

Nick
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Scott Smith

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Re: DCX2496 Limiters
« Reply #26 on: July 01, 2009, 09:51:46 AM »

Nick Hickman wrote on Tue, 30 June 2009 17:12

...I did do some tests and there appears to be a lot going on with the DCX2496 limiters.  They appear to combine both RMS and peak detectors...

GREAT TESTING!  Is that typical in limiters?  It seems to be a good solution for my needs. Very Happy
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Ron Kimball

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Re: DCX2496 Limiters
« Reply #27 on: July 01, 2009, 11:56:09 AM »

Scott Smith wrote:

GREAT TESTING!  Is that typical in limiters?  It seems to be a good solution for my needs. Very Happy
The reason for my post is that there doesn't seen to be any published data as to how these loudspeaker management limiters work. It does start to look like the Behringer does RMS limiting which is exactly what I want Smile. The DriveRack 260 also has crappy docs as to what theirs does Sad.
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Ron Kimball

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Re: DCX2496 Limiters
« Reply #28 on: July 01, 2009, 12:02:48 PM »

Nick Hickman wrote:

Output measurements were as follows:
Thanks - you Da Man!Thumbs Up
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Nick Hickman

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Re: DCX2496 Limiters
« Reply #29 on: July 01, 2009, 12:22:19 PM »

Hi Ron,

Ron Kimball wrote on Wed, 01 July 2009 16:56

The reason for my post is that there doesn't seen to be any published data as to how these loudspeaker management limiters work. It does start to look like the Behringer does RMS limiting which is exactly what I want Smile.

If my observations were correct, it effectively does RMS limiting whilst the signal crest factor is less than 3dB.  Above that, it limits based on peaks.  That margin really should be user-adjustable (or 6dB!).

I agree that loudspeaker processor limiters aren't always well-documented.  The BSS FDS-366, for one, offers an adjustable margin between its RMS limit and its peak limit.

Nick
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