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FAQ: Output Compression

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RoadRanger:
I use the "No Clip" preset on all my outputs  8) .

BlendedMix:
Hi Road...

Thanks for the help in the Airplay thread.  The system I described in the elem school has one cause for concern.  Blowing speakers.  There is the ability to push more than the speakers can handle.  I'm versed in some things about mixing sound, but dealing with limiters is something new to me.  It was never a concern with my acoustic guitar duo playing for small crowds.

Is there an output compression setting that will effectively limit the output to the amp to prevent sending too much to the speakers?  Does the "no clip" setting do that, or is it actually designating that it's allowing the DL to output at full strength?  It seems it could be interpreted either way.   ??? 

WK154:
It would be helpful if you had the list of equipment you are talking about. Name and model# otherwise you'll get nothing but speculation (amp and speakers). If there is any protection for the speakers it would be in the amps. Compression and limiting has little to do with it.

BlendedMix:

--- Quote from: WK154 on May 03, 2013, 11:19:24 PM ---It would be helpful if you had the list of equipment you are talking about. Name and model# otherwise you'll get nothing but speculation (amp and speakers). If there is any protection for the speakers it would be in the amps. Compression and limiting has little to do with it.

--- End quote ---

The amp for the mains is a QSC CMX300V, 120V.  I read about it, and it does appear to have some speaker protection features included, but I didn't see anything about a limiter.  I just thought I'd check to see if there was any additional ways to prevent users from getting happy with the faders on the iPad. 

I was told that since this was being run as a 70V bridged mono system, I would want to run the amp at full power, and then set my gain structure through the pre-amp and channel faders.  I've done that, but noticed some clipping on the amp that I wasn't expecting at the volume we were playing music through it.

Admittedly, it was "Harlem Shake" and it was a terrible recording, which likely had a lot to do with it.  It just reminded me that it's EXACTLY the type of thing that is going to happen in a school cafetorium, so I thought I'd at least make every precaution necessary to preserve the speakers.

The second amp is a Boeringer BE-EPQ2000.  It just powers the single hanging monitor, which sounds and works surprisingly well, and I'm not concerned about its volume, because it is plenty loud enough on even a moderate channel volume and the amp is more power than necessary. 

The speakers are a Vector Ceiling Array speaker manufactured by Pure Resonance, and they are a 120 Watts Built-in 8 Ohm / 70 Volts.  We have four of them "daisy chained" which is a total of 12 speakers.  This system is mono bridged and running in the 70 volt configuration. 

So that's the rundown.  He also recommended, since these are children's voices, to also have an Aphex Big Bottom and Aural Exciter to "beef up" the sound.  I have to admit, I was very skeptical about that since it is typically used for recording applications, but there is a very noticeable difference with it on or off in the chain, and my guess is that it has something to do with the limited low end response of the Vector ceiling speakers.  Maybe a really good engineer could get the same out of the 31 band EQ on the DL, but I can't. 

Surprisingly, the relatively inexpensive AKG DMS70D with DHT70 mics sound pretty good.  I used my SM58 right next to one, and the difference wasn't what I expected.  We went low cost, because we acknowledge that damage will occur, and they are only $165.00 to replace.  Pleasant surprise thus far...

So that's what I'm up against, and the more I type here and think about it, I'm considering looking into a one time fee for a sound engineer to come tweak the system and get it set up rather than trying to "amateur" my way through it.  I know good sound when I hear it, and I can mix an analog Yamaha powered mixing board and make it sing for two guitarists and vocals, but this thing is getting a little above my pay grade, which is strictly volunteer.   :D

Once again, thanks for taking the time to help, and it has been invaluable to this point.       

Greg C.:
Some considerations: compression in the traditional sense can actually cause speaker voice coils to heat up more then a non-compressed signal because it increase the rms voltage value of the signal. Higher rms levels = more heat. So compression is actually not a form of protection at all and can actually promote damage if your compressing the signal and running things full tilt boogie due to the low crest factor of the signal. The only time a compressor can be a relatively protective is when is used as an "rms limiter" where very slow attack and release times are used with a hard limiter ratio to prevent excessive voice coil heating. These types of limiters requires specific non-trivial calculations to avoid waisting system headroom on one side or cooking your voice coils on the other. Standard compressors also don't make good peak/clip limiters because they often don't have a fast enough attack to stop the fast transients from getting though. Most amps have clip limiters in them for basic protection. But they can be punched through on the more basic amps.

Higher end amps with full flown DSP usually have rms limiters as well as peak limiting that work very well. There are outboard speaker processors that have this 2-stage type limiting as well. Neither is cheap. But setup correctly, you can hammer them into oblivion with overdriven signals without damaging your speakers (though it may not sound great). Well made self powered speakers (i.e. not Mackie) will have solid rms and clip limiters as well as protection from driver over-excursion so they hold up to abuse pretty well. Again, they're usually not cheap.

Lastly, keep in mind with distributed systems that you should avoid driving them with a lot of low end frequency content. They're not designed for it. The 70v transformers on the speakers will saturate and distort easily with low frequencies. This also causes problems for the amps which may be why you're clipping more than you think should. You should keep low frequency content below 100Hz to a minimum. A high pass filter around 70Hz is usually a good idea.

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