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Author Topic: Side Chain Limiting  (Read 11841 times)

John Neil

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Re: Side Chain Limiting
« Reply #10 on: May 15, 2012, 02:04:17 am »

Langston,

Thanks for the in depth response.  I've been devoting some brain space to limiting lately due to a smoke-letting incident with one of my clients.  (How'd you know I was thinking about subs?)  The real issue there is an inadequacy in the transducer department but the most available tool is limiting.  Sigh.

I am having difficulty wrapping my head around the difference of pure tone bursts and program material...how each would uniquely impact the frequency domain for each of the three apparent tools (limiting compression, side chain limiting, and dynamic EQ). Is the additional VLF extension shown above worth driving the band pass into limit due to content that was previously a non-factor?

BSS also purports a "mid-band" method...
« Last Edit: May 15, 2012, 02:36:15 am by John Neil »
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Langston Holland

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Re: Side Chain Limiting
« Reply #11 on: May 15, 2012, 12:20:49 pm »

Quote from: John
I've been devoting some brain space to limiting lately due to a smoke-letting incident with one of my clients.  (How'd you know I was thinking about subs?)  The real issue there is an inadequacy in the transducer department but the most available tool is limiting.  Sigh.

A few thoughts:

1. Make sure your limiters are set per mfg recommendation or at the amp's max rated output into that load, whichever is less.
2. If the high pass slope on the sub is less than 4th order (24dB per octave), change it to 4th order BW.
3. Raise the high pass frequency 5Hz and listen to the difference with the type of music they play. Leave it there if it's acceptable.

Quote
I am having difficulty wrapping my head around the difference of pure tone bursts and program material...how each would uniquely impact the frequency domain for each of the three apparent tools (limiting compression, side chain limiting, and dynamic EQ). Is the additional VLF extension shown above worth driving the band pass into limit due to content that was previously a non-factor?

None of this stuff I've talked about is worth it. Monetarily. Even worse is the fact that it's just a partial introduction. I do this for personal entertainment between sessions volunteering at the DMV as a crash dummy and shooting myself with tasers.

Quote
BSS also purports a "mid-band" method...

This is simply an attempt to find more appropriate time constants for attack and release settings. It results in more conservative (shorter) times. Like Ivan would say, that little comment would require quite a bit of time to unpack and explain. In itself, it's more of a tool for the marketing department than the end user.

Traditional time constant calculation:

Attack in msec = (1000/HP Frequency of Passband)/2
Release in msec = 16x attack

Mid band time constant calculation:

Attack in msec = (1000/Midpoint of HP and LP Frequencies of Passband)/2
Release in msec = 16x attack

Package the latter calculation in your processor and send it to marketing for inclusion in the brochure. While you're at it, add an equally simple calculation for notched filters, give it a fancy name, imply complexity that can't be achieved on other processors (false), interview somebody with a professional frown that says it's good (true) and send it to marketing.
« Last Edit: May 15, 2012, 05:06:24 pm by Langston Holland »
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Brandon Romanowski

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Re: Side Chain Limiting
« Reply #12 on: May 22, 2012, 10:53:35 am »

Would anyone like to explain how to measure excursion with an accelerometer ? Does anyone use this sidechain limiting commercially ?
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Langston Holland

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Re: Side Chain Limiting
« Reply #13 on: May 22, 2012, 02:05:53 pm »

Quote from: Brandon
Would anyone like to explain how to measure excursion with an accelerometer ? Does anyone use this sidechain limiting commercially ?

Hi Brandon:

I think a laser is a more accurate way of measuring excursion and the nicest software implementation of this happens to be the least expensive as well; the STEPS module contained in ARTA.

See pp 45ff of the STEPS manual: http://www.artalabs.hr/download/STEPS-user-manual.pdf

IMO, this kind of measurement is more useful to the loudspeaker designer than to us, but the pursuit of knowledge and toys is always encouraged. :) My take on this subject is that once distortion becomes objectionable, you need to limit the system so that it doesn't go there. This may coincide with Xmax or may be a bit higher or lower depending on the quality of components and skill (and/or luck) of the designer.

The best outline for "objectionable" testing was penned by the great Tom Danley 81 moons ago:

With a number of listeners there is another test one could conduct. If one had a group of listeners, (a good distance away so ears aren’t overloaded from excess loudness), each speaker / system could be cranked up with the same program material until say of the group said “obviously musically compromised” and that peak level at one meter recorded as that boxes “maximum musical output”. Next increase the level until half said it was “unlistenable” and record that level as the “maximum usable output”. While they may go louder, being unlistenable seems like a good place to stop, no need to risk letting the magic smoke out by going higher..

There is magic in those words.

---

As to commercially available processors that employ side chain limiting, there are very few. One example that takes baby steps with the concept of look ahead limiting is available from Four Audio. It appears to be a world class loudspeaker processor.

The extensive use of side chain limiting as I’ve described is the domain of the higher-end free wire install type processors. There are other things I haven’t mentioned yet, but I’m sure I’ve thrown out enough bait to attract some interest. As mentioned earlier, I don’t see evidence of anyone else taking this seriously – but in the words of an immortal – I could be wrong. :)

---

You are engaging in dangerous thinking. This is your last chance. After this, there is no turning back. You take the blue pill - the story ends, you wake up in your bed and believe whatever you want to believe. You take the red pill - you stay in Wonderland and I show you how deep the rabbit-hole goes.
« Last Edit: May 22, 2012, 02:24:52 pm by Langston Holland »
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Doug Fowler

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Re: Side Chain Limiting
« Reply #14 on: May 22, 2012, 04:24:53 pm »

Would anyone like to explain how to measure excursion with an accelerometer ? Does anyone use this sidechain limiting commercially ?

Yes, NEXO have implemented this for quite some time. Their term is "VCEQ" or voltage controlled equalization.  The NX242 manual offers a nice explanation of what they are doing.
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Brandon Romanowski

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Re: Side Chain Limiting
« Reply #15 on: May 23, 2012, 05:12:40 pm »

Langston,
    Thanks for taking the time to reply. Have you ever used a laser to measure excursion ? What's the setup ? specifically ?
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Arthur Skudra

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Re: Side Chain Limiting
« Reply #16 on: May 23, 2012, 08:23:27 pm »

Langston,
    Thanks for taking the time to reply. Have you ever used a laser to measure excursion ? What's the setup ? specifically ?
Essentially the laser measures distance, and converts it to a voltage, which in turn is measured by software.  I know that Clio has that capability.  Essentially it's a linear scale reading.  The laser I've been looking for is made by Baumer, and it's at least $1,000!!!

Excursion measurements can be done a lot cheaper using an accelerometer, here's a link:
http://www.libinst.com/accel.htm

For a completely unrelated task, I built my own custom accelerometer preamp in a compact, watertight, indestructible Hammond project box that works off phantom power and uses the ACH-01 accelerometer, works great with Smaart for vibration analysis in the Spectrograph.  Haven't figured out a convenient method to calibrate it to translate that signal to excursion though!  You should have seen the look on the sales clerk's face at the doll store when she asked me what I was planning to do with the Quik-Stik wax!  LOL!

There is a DIY project in June 2011 Audio Xpress magazine to build a preamp for the ACH-01 specifically for speaker excursion measurements that I have been wanting to build myself for a while! I just don't have any time to design and etch the printed circuit board (I wish they would have published that in the article).  If there is anyone enterprising enough to make one, I would gladly collaborate with them for a copy of the board for me.  Hint Hint!   ;)
« Last Edit: May 23, 2012, 08:25:29 pm by Arthur Skudra »
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Langston Holland

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Re: Side Chain Limiting
« Reply #17 on: May 23, 2012, 11:41:08 pm »

Quote from: Brandon
Have you ever used a laser to measure excursion ? What's the setup ? specifically ?

Hi Brandon:

I haven't found a reason to measure excursion directly given my interest in optimization (vs. loudspeaker design). I've found that indirect measurement via distortion combined with one's ears are ideal for finding the peak usable output of a system. This will not protect the system from overheating and the sonic degradation involved with power compression, but the crest factor of music will keep you away from that problem 99% of the time in my experience. That other 1% can be expensive, thus RMS type limiting is necessary to cover all the bases.

BTW, you can get a very close approximation of excursion with a direct measurement without buying anything:

http://www.linkwitzlab.com/faq.htm#Q29

Accelerometer measurements interest me for reasons other than excursion, i.e. cabinet and other boundary resonance behavior. Small signal impedance measurement is a pretty good indirect method of spotting cabinet resonances, but an accelerometer would be ideal.
« Last Edit: May 24, 2012, 12:56:20 am by Langston Holland »
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Barry Singleton

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Re: Side Chain Limiting
« Reply #18 on: July 21, 2012, 12:44:26 am »

  Hi Langston;

  I am keenly interested in this subject. I read Charlie's paper a couple of years ago and couldn't quite wrap my head around the possibilities this opens up. Reading through this thread has more than kindled my interest.

  I don't think I have a piece of equipment capable of Side Chain Limiting, nor do I know what unit would be a good candidate. Do you have a suggestion?

  I have several measurement platforms and a fair collection of measurement gear including an acceleromoter and a lazer position system. I have never claimed to be a smart guy but maybe I could be of some value if I can find some direction on this.

  With high hopes I am looking forward to hearing from you.

  All the best,
                  Barry.

 
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Langston Holland

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Re: Side Chain Limiting
« Reply #19 on: July 22, 2012, 03:16:02 pm »

Hi Barry:

If you play with all that stuff you're more than ready to blow things up. :)

Overview:

You have a fair amount of homework to do before you need to think about finding a free-wire processor with side-chain comp/limiters. IOW, what you need to do first is learn how to measure the thresholds of where your subs say uncle with and without the current processing you have. Keep in mind that it might not be your subs that give up at full amplitude - it may be your amp. For example, with a single 4Ω TH118 on one channel of a FP14000 in stereo mode with VPL set at max volts and "hard", the peak output of the amp is about 1dB shy of what the sub can handle. With a pair of TH118's on one channel as I actually use them, the peak output of the amp is 2.5dB shy of what the subs can take. More on the procedure later.

Since it's easy to talk about, I'll start with processors. There are several out there that will do the trick. I have a decade old MediaMatrix X-Frame 88 that Ray Rayburn1 sold me that works very nicely, but I bet the current stuff would make the process easier and possibly allow slightly better results. From the point of view of electrical control of loudspeaker behavior, it appears that the NION and Q-Sys processors are the most powerful available. Interestingly, they are both based on the same code. Few people seem to appreciate that Peavey is responsible for the most powerful audio processing in the world. I do not yet own either of these fancy processors, but I'm leaning toward the Q-Sys (250i) due to the fact that it has predefined FIR crossover filters as well as direct FIR coefficient import. The latter is in my future, the former I can use immediately. The NION's only have direct FIR coefficient import, but are all about Dante and that's valuable. QSC justifies their decision to go their own way with their network protocol with some drivel about better operability with standard Ethernet networks. What idiot is going to do a high dollar install and not run a dedicated network for the audio anyway? All processors in this league can do AES3 I/O, and that's all I need - but I'm going to be buying CL5's if things go well for business in the coming elections and Dante would be convenient. Maybe QSC will see the light (a.k.a market).

Remember that several other mfg's offer side-chain comp/limiting for probably less money and fewer additional features - you'll have to pursue that (and report!) yourself, because I want a Ferrari even if I never go over 70mph. Hopefully I'll learn how to get comfortable at much higher speeds one of these days. :)

Homework:

I use CLIO, a scope and my ears to do this, but you really can do this with just your ears and a voltmeter. You will connect the amplifier to the subwoofer without a processor. You will measure the gain of your amplifier, I suggest setting it to 32dB and verifying without the sub connected with a very small sine wave and a voltmeter. You will verify, preferably with a scope, that the setup you use to drive the amplifier input will cleanly pass a 10Hz, 20Hz, 30Hz, 40Hz, 50Hz and 63Hz continuous sines at +20dBu at the same level. Do not trust anything prior to measurement. Treat audio equipment like riding a motorcycle on the street; pretend that the mfg's are trying to hit you, but do not give them the opportunity. I've also heard that you may find user error situations that you've lived with through months of shows without knowing it.

Do this outdoors so the neighbors can hear and so you can hear the onset of distortion more clearly. If possible, remove whatever's needed to be able to see the woofer(s) movement. Remove the grill and gaff the wheels or anything else that can rattle - you want to hear the drivers, not the other stuff - fix the rattlers later if needed. Connect a scope across the balanced output of the source (or the passthrough output on the amp) and monitor the stimulus peak levels. If you only have a voltmeter, that'll work - just make sure you use that same meter when you're tuning your processor's side-chain limiters. The downside to using anything other than a true peak reading meter (scope is best) is that your readings won't be comparable with anyone else's unless they have that same meter. The methods that average or RMS reading meters use to do their thing varies. I can't imagine anyone going to the trouble of this kind of tuning without owning a scope.

Now feed the 10Hz Keele tone burst to the sub at a very, Very, VERY low level. Turn it up slowly while observing your meter and listening and wondering why on earth your going to all this trouble when it won't net you a dime more in rental fees. Once you find the threshold where you hear the onset of objectionable distortion (woofer slap, etc), record the meter reading. Turn it down and back up again to verify you got it right. Rinse, repeat with the other tone bursts. It gets very easy once you do it a bit.

These are the results on a pair of TH118's. I've included 1 meter peak SPL of the pair of subs for entertainment. Not bad for single 18's.

(2) TH118 w/ FP14000 at 32dB gain:

1. 63Hz 150.57dB SPL at +14dBu
2. 50Hz 150.78dB SPL at +14dBu
3. 40Hz 149.40dB SPL at +14dBu
4. 30Hz 139.05dB SPL at +14dBu
5. 20Hz 118.63dB SPL at  +2dBu
6. 10Hz  94.79dB SPL at   0dBu


Study this: Keele Shaped Tone Burst AES Paper

Download this: Keele Shaped Tone Bursts and CBT Presentation

That download is huge and you only need a small portion of it, but Don2 bundled all this stuff for a reason and I don't want to interfere with that. The places his brain goes are well worth one's time to follow and it's just amazing to me that he puts his work into the public domain for free. He's one of the great ones.

Extra Credit:

Now add your sub processor to the chain. Put your meter across the output of the processor or input of the amp and repeat the tests. Keep your ears open. What maximum dBu levels are you now reading? Is your current processing REALLY protecting your loudspeakers? Ignorance is bliss until the brother-in-law of the lead singer shows up.

Notes:

1. Name drop! You impressed?
2. How about now? :)
« Last Edit: July 24, 2012, 09:10:39 am by Langston Holland »
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ProSoundWeb Community

Re: Side Chain Limiting
« Reply #19 on: July 22, 2012, 03:16:02 pm »


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