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Author Topic: Side Chain Limiting  (Read 11670 times)

John Neil

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Side Chain Limiting
« on: May 13, 2012, 10:48:16 am »


X-Frame 88 Preset schematic:



Langston,

Pardon my n00b showing, but why the sidechained limiter?
« Last Edit: July 19, 2012, 03:10:17 am by Langston Holland »
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Langston Holland

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Re: Side Chain Limiting
« Reply #1 on: May 13, 2012, 09:04:38 pm »

Hi John:

I split your post off from that other thread and renamed it as seemed appropriate...

Your question is anything but newbie level (I hope) because I've put many hours into finding answers to it so far.

The reason I bought a free wire processor was to experiment with some ideas I've had for a while concerning side chain limiting. Initially, I was simply interested in playing around with a variant of feed forward limiting commonly called look ahead. After spending some time on that, a flood of other ideas came that have completely changed the way I control level dependent loudspeaker behavior. There are at least a dozen significant advantages to side chain limiting that have convinced me that the way limiting is currently employed in even the best live sound processors is a huge compromise. All of this applies to the more gentle limiters that we call compressors as well, but that's another topic.

I'm just going to point out one advantage in this post because it would take 50+ pages to explain everything I've stumbled upon to date. Charlie Hughes is responsible for striking the match on this with one of his (typical) extremely well written articles and his (typical) amazingly generous personal help.

Using Limiters to Help Protect Loudspeakers

BTW, I reject the "limiter as protection mechanism only" philosophy. I'm absolutely convinced that limiting can be a very large part of why a loudspeaker system does or does not sound good at high outputs and low outputs where the limiter is not even engaged! How can this be true? It's part of the one advantage that will be explained below. :)

I will use a Danley TH118 sub driven by a large amp with 32dB gain to demonstrate. The stimulus used in these measurements is a sine wave burst of 4 wavelengths at 1/24th octave intervals. This sub with this amp can handle a maximum of +11dBu RMS at the output of the processor in the middle of its passband before it starts to sound ugly. It can handle +2dBu at 20Hz before woofer slap (aka ugly). The following plot shows how standard limiting behaves once the threshold is reached and exceeded to the point of clipping the input of the processor. A 30Hz 4th order BW high pass is required in combination with the limiting to keep it sounding good at max output:

TH118 with Standard Limiting:



Now I'm going to do something stupid and change the high pass filter to 1st order and move it one octave lower. And experience zero woofer slap or other ugliness. And get a stunning increase in VLF output at levels prior to limiter engagement where the subs are typically run (enough rig for the gig). And get significantly better phase behavior, thus a tighter impulse, thus better impact. And get better protection of the sub. All due to the power of side chain limiting.

OK, I lied. I've mentioned more than one advantage, but there are many others for each end of each passband.

TH118 with Side Chain Limiting:




TH118 Composite Standard Limiting vs. Side Chain Limiting:

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Ivan Beaver

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Re: Side Chain Limiting
« Reply #2 on: May 13, 2012, 09:43:14 pm »

This is yet another example of why it is so hard to answer a complicated question with a "simple" answer.

That simple answer will be wrong.

The more you dig into a subject-the mroe complicated it gets.

There is A LOT mroe than just "wattage" that goes into limiter settings.  The freq is a huge part of it.  Overexcursion (as in this example) can be a real problem.

At higher power levels-the response (and output) does not stay linear.  This has to be accounted for when setting limiters.

Of course it gets a bit harder-because limiters with sidechains are not very common.  Thus complicating the whole issue and what you can do about it.

And if youa re talking about a full range product-what the woofer can take (sine wave) and what the HF/horn can take (sine wave) are very different.  Feedback really doesn't care-and can occur at most any freq.  So how do you set a limiter that can protect against feedback on either the lower or higher freq?

Again a sidechain limiter is needed.

Of course just having the proper control/tools/limiter is only the BEGINNING of the procedure of how to actually set it. 

NEXT you avhe to know what the limits are-for the lower and higher freq.  Getting this data from the manufacturers is not easy-and must be done yourself.

Of course the attack times will vary for the lower and higher freq-along with some other variables (such as loss through the crossover-due to the actual rolloff action of the crossover itself).

Why does it just seem to get more complicated the more we learn?



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Ivan Beaver
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Langston Holland

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Re: Side Chain Limiting
« Reply #3 on: May 14, 2012, 10:52:38 am »

Another advantage, this time looked at from the other end of the sub's passband:

A Beautiful Woman that can Cook

Black and blue traces use an 80Hz 4th order LR low pass filter. Blue traces use a standard limiting keyed off the post-crossover audio signal, black traces use a side chain limiter keyed from the pre-crossover audio combined with a 2nd order LR low pass filter one octave above crossover.

There's an exciting world in limiting that is largely unexplored. :)
« Last Edit: May 14, 2012, 09:04:49 pm by Langston Holland »
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Arthur Skudra

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Re: Side Chain Limiting
« Reply #4 on: May 14, 2012, 11:12:04 am »

Hey Langston!  Charlie's article on this is fantastic as always.  What are you using/doing to determine the sidechain filter curve?  Let us in on your secret sauce!  ;)
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John Roberts {JR}

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Re: Side Chain Limiting
« Reply #5 on: May 14, 2012, 11:32:33 am »



There's an exciting world in limiting that is largely unexplored. :)

Unexplored by end users but not exactly virgin territory for captive systems where the informed speaker design engineers get to twist the knobs to protect their own babies..

My world view is a future populated with "Steven Segal" (character) powered speakers that are truly "Hard to Kill".

I played around with designing a stand alone speaker protection black box and decided against it because the logistics of connecting the all the individual speaker specific data dots for end users was too much work for a small company, and such a product could be obsoleted by manufacturers getting their act together.

My preferred platform is active powered cabinets with DSP inside, but the driver protection scheme that chops the power feed (ala PWM) at each speaker, while relatively more expensive is promising too.

JR

PS: I vaguely recall a thread several years ago here that delved into the math for driver excursion protection, while that was more of a theory discussion than a how to do this with practical available gear.
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Langston Holland

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Side Chain Limiting
« Reply #6 on: May 14, 2012, 02:30:40 pm »

Hi Arthur:

If I were into secrets, I wouldn't be postin' all this stuff. :)

Plus it would be evil - everything I know is a gift from someone else.

Like you, I have a subscription to the AES E-Library and have read everything I could find on loudspeaker protection. There's really two ways to approach this, one is over my head/budget/interest that uses destructive thermal and mechanical measurement of drivers and systems to find limiting thresholds, the other can be done in a field with cows (FWC).

Folks with true mastery of limiters like NEXO probably use both methods, i.e. extensive laboratory measurement and FWC (tuning by ear).

Charlie's article takes the logical approach of keeping the driver within Xmax, which is actually pretty easy and affordable to measure these days with either CLIO or ARTA and a laser attachment. But, the entirely objective procedure of not exceeding the rule of thumb excursion threshold (assume Xmax +10%) does not fully address the issue. This will keep the system within a reasonable distortion range, i.e. probably reasonably good sound as well as protecting the driver.

Why not just cut to the chase and use Don Keele's marvelous tone bursts to find where the driver in a specific system with a specific amp REALLY starts to sound bad? In my experience, this threshold is usually achieved at levels somewhat lower than Xmax, thus driver protection comfortably exceeds the rule of thumb formula.

I've found that CLIO's longest gated tone burst of 4 wavelengths under 150Hz or so starts to sound bad at the same point as the Keele tone bursts. This makes the process much easier and quicker, though the end result is identical. CLIO v10.31 now offers Keele's tone bursts in a full CEA-2010 compliant module for max SPL testing. Incredible.

Bottom line: set the side chain curve such that the sub isn't allowed to go where it sounds bad. Result: the best possible VLF from a given sub plus fame, fortune and beautiful bovines. What else is there?

(this is a Tapatalk test post)
« Last Edit: May 14, 2012, 08:50:50 pm by Langston Holland »
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Langston Holland

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Side Chain Limiting
« Reply #7 on: May 14, 2012, 02:33:41 pm »

Good points by JR as usual - the all-in-one loudspeaker has to be the future and it surprises me that more have not followed Meyer's lead on that. Based on the access level I've been granted to date, I'm not at all convinced that the practical limits (pun) of side chain limiting have been addressed by any of the mfg's or any available research. They certainly don't appear to have left a paper trail (an ancient form of encoding that used processed trees as a substrate upon which various pigments were applied that one would physically disseminate for decoding by the recipient).

As an example; the 1st order high pass filter I used for the TH118 was simply due to the fact that the 10 year old processor I used as a learning tool could not fully implement the reciprocal response I wanted in the side chain. A newer free wire processor very probably can do this, which means NO HIGH PASS filter at all is needed. That will improve time domain response a bit more and is a far better solution than the dynamic EQ techniques (that screw up phase) currently employed by many mfg's to achieve variable LF extension.

Show me proof that someone has actually applied this and I'll quit bragging about my bovine aided research (BAR)!

Maybe I'll demonstrate how I managed to produce a look ahead limiter without inserting delay into the audio path. It was an accident that occurred at the BAR lab, rest assured that no animals were hurt in the process, though with time they will be eaten.

(2nd Tapatalk test)
« Last Edit: May 14, 2012, 07:38:23 pm by Langston Holland »
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John Roberts {JR}

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Re: Side Chain Limiting
« Reply #8 on: May 14, 2012, 03:55:42 pm »

Good points by JR as usual - the all-in-one loudspeaker has to be the future and it surprises me that more have not followed Meyer's lead on that. Based on the access level I've been granted to date, I'm not at all convinced that the practical limits (pun) of side chain limiting have been addressed by any of the mfg's or any available research. They certainly don't appear to have left a paper trail (an ancient form of encoding that used processed trees as a substrate upon which various pigments were applied that one would physically disseminate for decoding by the recipient).
paper... I remember that
Quote
As an example; the 1st order high pass filter I used for the TH118 was simply due to the fact that the 10 year old processor I used as a learning tool could not fully implement the reciprocal response I wanted in the side chain. A newer free wire processor very probably can do this, which means NO HIGH PASS filter at all is needed. That will improve time domain response a bit more and is a far better solution than the dynamic EQ techniques (that screw up phase) currently employed by many mfg's to achieve variable LF extension.
Indeed,,, more brain cells were sacrificed to making speakers sound good, than keeping them working, but then was then and now is now... I expect DSP to get so cheap it is silly not to use it in powered cabinets for intelligent protection.
Quote
Show me proof that someone has actually applied this and I'll quit bragging about my bovine aided research (BAR)!
I've been out of the trenches for 10 years, and I wasn't a speaker guy back then either...(as is probably obvious). 
Quote
Maybe I'll demonstrate how I managed to produce a look ahead limiter without inserting delay into the audio path. It was an accident that occurred at the BAR lab, rest assured that no animals were hurt in the process, though with time they will be eaten.

(2nd Tapatalk test)


God bless you and your precious family - Langston

Keep on keepin on...

JR

PS: There could be some rather short look ahead capability based on the delay inherent in typical digital filters, but using that gets a little involved since you often want to crunch data after it is filtered, but I am wandering here... never mind. 
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Langston Holland

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Re: Side Chain Limiting
« Reply #9 on: May 14, 2012, 08:48:52 pm »

Quote from: JR
PS: There could be some rather short look ahead capability based on the delay inherent in typical digital filters, but using that gets a little involved since you often want to crunch data after it is filtered, but I am wandering here... never mind.

Never mind my eye! You don't realize how much of a great idea that is...

With a typical processor, that won't help of course because everything must be converted to digital to be usable. BUT, the future is FIR and FIR involves a second processing stage with significant delay from the perspective of the shorter wavelengths involved with practical latencies in live sound. If one could feed the output limiter's side chain prior to the FIR stage, you'd have more than enough look ahead delay for the mid and high frequencies. This would be huge and I'm trying to talk my favorite processor mfg into implementing it - just been so busy with work and personal stuff lately.

With the shorter wavelength limiters able to look ahead WITHOUT actually adding delay to the audio path due to the FIR stage, how do we obtain look ahead delay for the sub passband WITHOUT adding delay to its audio path? There will be no FIR involved here because the latency penalty is prohibitive, thus the fact that all live sound FIR based processing is hybrid - FIR on top, IIR down low.

For usable look ahead, it's nice to have at least 1/4 wavelength (the first peak of the sine) of the frequency of interest for the limiter to predict the worst of what's coming. Given that the kick is the primary threat to the sub and its fundamental is about 60Hz, a 1/4 wavelength is about 4ms. I found a way to get about 5ms for free - Shazam! :)

How?

The upper orange trace in the first plot in my earlier post shows the sub's high and low pass response ballooning outward under heavy limiting. The high pass balloon endangers the drivers and the low pass balloon creates high level, high distortion mud. This ballooning occurs because the limiter is keying on the post-crossover audio signal. As the filters properly reduce the signal above and below the sub's passband, this signal also reduces the action of the limiter above and below the passband. The ballooning has to happen for the limiter to see enough level to act upon.

To fix this, you could simply feed the limiter's side chain input with the full range audio prior to the crossover, but then you'd have the upper frequencies modulating the sub output - unacceptable.

What to do? Compromise. I've found that a 2nd order LR low pass at twice the sub's actual low pass frequency works very nicely. For a sub with an 80Hz 4th order LR low pass, the low pass to the limiter's side chain would be at 160Hz. There is a great deal less group delay with the filter feeding the limiter's side chain vs. the one passing audio to the sub and this is where the free look ahead delay comes from.

Thus we are able to get usable amounts of look ahead for all passbands with no additional delay inserted into the audio.

Measurement:

« Last Edit: May 14, 2012, 08:57:04 pm by Langston Holland »
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Re: Side Chain Limiting
« Reply #9 on: May 14, 2012, 08:48:52 pm »


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