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Author Topic: Side Chain Limiting  (Read 10089 times)

Mark Wilkinson

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Re: Side Chain Limiting
« Reply #20 on: November 12, 2019, 12:37:24 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? :)

Hi Langston, I've become interested in sidechain limiting and found this really cool older post of yours.
Have you done more with this?

It looks like a brilliant way of handling over-excursion. 
Programs like hornresp seem to be doing a pretty job of predicting excursion, so I'm thinking I should give this a try.
That said, i get that burst testing is the only way to know for sure...what excursion actually is, and does the sidechain limiter work.
Anyway, if you did take this further...would love to hear about it..
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Marcel de Graaf

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Re: Side Chain Limiting
« Reply #21 on: November 15, 2019, 10:59:31 am »

Hi Mark (and of course Langston),

Thnx for putting this up again, I really have missed this wonderfull post at that moment.

I have a few questions:

- The subject is in main based on the excursion of the driver. What is a good and pratical way to measure this?
- I don`t come across loudspeaker DSP with a side chain input. What is a good way to do this after an analogue mixing table?

gr. Marcel
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Frank Koenig

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Re: Side Chain Limiting
« Reply #22 on: November 15, 2019, 07:55:03 pm »

- I don`t come across loudspeaker DSP with a side chain input.

The newer Powersofts (after the k series) allow you to insert filters in the limiter side chain. I assume some of the other new DSP equipped amps do too. I think the question here is what to do with those filters to make the peak voltage limiter a better proxy for an excursion limiter. Interesting subject.  --Frank

« Last Edit: November 15, 2019, 08:02:10 pm by Frank Koenig »
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Chris Grimshaw

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Re: Side Chain Limiting
« Reply #23 on: November 16, 2019, 08:42:07 am »

I wasn't sure about how useful that'd be - Armonia says that'll onset whole-band limiting, when really I just want to stop 25Hz getting too loud.
I don't want 25Hz to pull 60Hz down with it.

I have a load of Powersoft T-series here, so I'll see about rigging something up and doing a test.

Something along the lines of:
- Apply some guesswork settings on the screen Frank has shown above
- Set limiting threshold nice and low so I don't melt the speaker
- Mic in front of speaker
- 25Hz and 60Hz tones into the mixing desk, so I've got control over relative (and overall) signal levels. Feed to amplifier.
- Feed the mic into an RTA.
- Watch the 60Hz level on the RTA as I push the 25Hz fader.

My suspicion is that, with the Powersoft limiter, 25Hz will pull 60Hz down. I suppose this test will find out.

Chris

PS - This'd be much easier with a spectrum analyser, but I don't have one that'll cope with the output voltages these amps are capable of, even with the limiters clamped right down.
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Mark Wilkinson

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Re: Side Chain Limiting
« Reply #24 on: November 16, 2019, 10:10:38 am »

Hi Mark (and of course Langston),

Thnx for putting this up again, I really have missed this wonderfull post at that moment.

I have a few questions:

- The subject is in main based on the excursion of the driver. What is a good and pratical way to measure this?
- I don`t come across loudspeaker DSP with a side chain input. What is a good way to do this after an analogue mixing table?

gr. Marcel

Hi Marcel, hi guys,

Yeah, if anybody knows an accurate cheap way to measure excursion, I'd love to hear it. 
Apart from listening to tone bursts as Langston suggests, I just make a jig that holds something in front of the cone where I can make visual guesstimates.
But I suspect nothing can replace the tone burst listening.

I guess a good DSP after an analog mixer is required...I've no clue how this could be done without dsp...barely a clue with it haha
Although I think I've made some progress using q-sys and a Core110f...

First  response below is limiting via sidechain.  No hpf, xover, eq, etc. 
It's for getting the most out of a lightweight sub I mentioned in another thread.  Xmax is 9mm.
The thing is, excursion exceeds 9mm above tuning at 41Hz, ....at tuning excursion is minimal.
I've found a regular full-band limiter completely kills 40 Hz response, which needs a lot of power, but is excursion safe. (Thermal is real issue.)
So hopefully the sidechain limiter is gonna help here.

Second set of responses are curves from top down, -24dBFS RMS, -21, and -18, using pink noise.
The idea is I wanted full sidechain  limiting at -18dBFS, and wanted to see how limiting fell off at lower signals.
I can see the crest factor of the pink giving a bit of limiting at -21, and limiting looks gone at -24dBFS, so i think I'm ok.
Oh, by limiting is gone, I mean for the response notch countering over-excursion, 50-80Hz.
There are two stages of sidechain, a second one for the replacement of the hpf, which kicks in at about -36dBFS.

Now I need to add regular full-band peak and RMS limiters and do some listening and tests.  I'm encouraged so far.
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Frank Koenig

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Re: Side Chain Limiting
« Reply #25 on: November 16, 2019, 12:28:04 pm »

My suspicion is that, with the Powersoft limiter, 25Hz will pull 60Hz down. I suppose this test will find out.

My suspicion, too. It's just a limiter, not a sub-band limiter. But please do experiment. And it still may be a step forward from a broad-band voltage limiter. If you're hitting the threshold because of 25 Hz and it's pulling down the 60 Hz, you, the human, has to remove some 25 Hz from the program with EQ or, in other words, rejigger the system high-pass.

A true excursion limiter needs either to measure excursion directly or a have a computational model of the physical system that can estimate excursion based on the electrical input. Does anyone know whether the big boys who make fully integrated speaker systems (Meyer, for example) do either of these? Or something simpler, like some sort of sub-band limiter, that's good enough? Better yet, has anyone reverse-engineered any of these systems?

--Frank
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Frank Koenig

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Re: Side Chain Limiting
« Reply #26 on: November 16, 2019, 12:46:48 pm »

First  response below is limiting via sidechain.  No hpf, xover, eq, etc. 
It's for getting the most out of a lightweight sub I mentioned in another thread.  Xmax is 9mm.
The thing is, excursion exceeds 9mm above tuning at 41Hz, ....at tuning excursion is minimal.
I've found a regular full-band limiter completely kills 40 Hz response, which needs a lot of power, but is excursion safe. (Thermal is real issue.)
So hopefully the sidechain limiter is gonna help here.

Second set of responses are curves from top down, -24dBFS RMS, -21, and -18, using pink noise.
The idea is I wanted full sidechain  limiting at -18dBFS, and wanted to see how limiting fell off at lower signals.
I can see the crest factor of the pink giving a bit of limiting at -21, and limiting looks gone at -24dBFS, so i think I'm ok.
Oh, by limiting is gone, I mean for the response notch countering over-excursion, 50-80Hz.
There are two stages of sidechain, a second one for the replacement of the hpf, which kicks in at about -36dBFS.

Now I need to add regular full-band peak and RMS limiters and do some listening and tests.  I'm encouraged so far.

Good work, Mark. Keep it coming.

On measuring excursion: If you're downtown get a laser interferometer. Short of that, I wonder if there are any geometrical optical tricks that might be good enough -- we don't need a great deal of precision. Something that involves sticking a small mirror to the speaker and measuring the width of a line on a screen, maybe with a linear photodiode array or video camera, or a ruler for that matter. For detecting whether a certain peak exclusion has been reached it seems a simple, well-damped, spring, that is not too sensitive to the air motion, could be rigged to touch the cone and make noise. Maybe a piece of soft rubber or urethane foam with the right properties? Just free associating here.

--Frank

Scratch the video camera. It's not going to work because of aliasing by the frame rate of the video, at least in its simplest form.

As for transient test signals, how samples of actual kick drum hits, at least as a guide for choosing a tone burst envelope?
« Last Edit: November 16, 2019, 07:03:56 pm by Frank Koenig »
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Chris Grimshaw

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Re: Side Chain Limiting
« Reply #27 on: November 16, 2019, 02:05:44 pm »

I saw a nice way of measuring cone excursion a while ago:
- Get a strip of cardboard, and cut one end at 45 degrees.
- Attach strip to the cone, so the 45 degree end is standing up away from the cone
- Play the test tone
- Hold a ruler to the cardboard strip, and use persistence of vision to make a measurement

Might've been Linkwitz that suggested that - can't remember.

Chris
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Mark Wilkinson

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Re: Side Chain Limiting
« Reply #28 on: November 18, 2019, 10:50:18 am »

Thx Frank, Chris,

Yeah, I also saw the taped strip to cone thingy of the Linkwitz site somewhere. I've tried that and a few other gizmos that all come down to visual guesstimates.
Wish I was rich, i'd get one of the laser devices like in the ARTA manual.
Must say though, I think listening to the sine burst tests, like CEA-2010, is the real way to judge excursion. Or better yet, just stopping when measurable distortion fails the test.
REW has a CEA-2010 routine that I've toyed with in past and plan to use dialing in the sidechain limiter attempt.

Been having good luck with the sidechain limiter, at least so far with voltage measurements and transfers.
Two really cool things have popped out.
 
If the sidechain limiter follows the normal peak limiter, where the normal peak limiter is set to allow maximum output for frequencies where excursion is no problem, it's very easy to define an almost "parametric" limiting curve at lower output levels for any excursion limited frequency range or ranges.

Second, it's looking like sidechain limiting can totally replace a HPF for subs, in a very superior fashion. 
So far, I'm getting better protection (steeper curve/higher order) with considerably less phase lag / group delay.  I've become more excited over this than the frequency dependent excursion limiting.
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Mark Wilkinson

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Re: Side Chain Limiting
« Reply #29 on: December 11, 2019, 11:18:41 am »

OK, between wind, rain, and too damn cold just to make another measurement...I haven't been able to measure the sub itself with the sidechain limiter.
But I have refined the limiter a fair amount...

Below is a set of traces that show how the limiter phases in a various drive levels.
The part of the limiter that acts as a high pass filter starts engaging below -40dBFS,
whereas the part that acts a frequency dependent excursion filter starts engaging at about -18dBFS.

It is all fully engaged at -12dBFS, and the peak voltage by frequency set up with a PL340 is shown in the side chart.


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Re: Side Chain Limiting
« Reply #29 on: December 11, 2019, 11:18:41 am »


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