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Old 3rd September 2005, 02:42 PM   #11
jcx is offline jcx  United States
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hopefully your university library will have the Audio engineering society journal JAES - the place for articles on speaker distortion/control/equalization

http://www.klippel.de/background/introduction.asp should be a good source to start a literature search

depending on your skills/ambitions the topic could reach grad thesis level complexity
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Old 3rd September 2005, 03:39 PM   #12
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using_e:

Thank you so much for the articles.


jcx:

We do have AES journals avaliable, thanks for the suggestion, it didn't register with me to look.



So far I've only spoken with the professor and had a chance to speak about securing a DSP to use.
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Old 4th September 2005, 05:47 PM   #13
forr is offline forr  France
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Hi Using_E,

Thanks for the articles.
Another source for servo-subs is patents (Free patents on Line).
My own collection of articles on the subject has now about forty authors and is 70 Mb of hard disk space (most articles in PDF).
Sorry, I am unable to establish the full list for now.

~~~~~~ Forr

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Old 6th September 2005, 12:23 AM   #14
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Have you seen:
http://www.rythmikaudio.com/servo_product.htm

It isn't exactly what you are looking for, if you are looking for a digital solution, but has anyone tried this out? It has always seemed pretty cool to me.
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Old 6th September 2005, 05:38 AM   #15
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Nice link. I'll have to take a wonder over to the patent website and lookup the exact implementation they use (cicuitry). If anything else, it serves as good read. They make a point of explaining what some causes of the woofer distortion are.

They also seem to poo-poo on the accelerometer based approach. The "mechanical" ground issue seems to be a valid one and would dictate the sub inclosure be heavy and dead to vibrations.

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As a result, while they may produce low distortion on paper, it is not necessarily reflected into the sound qualities that audiophiles are looking for.
This statement doesn't convince me of anything other than to implant a seed of doubt, after all, they are selling something.

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Second, those subs cannot work without heavy protection circuit around them. That adds to the circuit complexity and further degrades the sound quality.
This claim seems to have more validity to its claim. After all, in some strange instance say the sub were to tip over or fall off something while on, the corrective system could damage it or the amplifier. A DSP based approach seems to lend a hand at incorporating protective circuitry without degrading signal quality though.

If anyone wants to add or disagree with something I said feel free. Discussion is good!

An accelerometer is on order and my lab partner is working on securing a DSP that we can work with. Hopefully solid progress can be reported soon.
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Old 6th September 2005, 06:47 AM   #16
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They also seem to poo-poo on the accelerometer based approach. The "mechanical" ground issue seems to be a valid one and would dictate the sub inclosure be heavy and dead to vibrations.
Unfortunately the "mechanical ground issue" is valid for ALL other approaches as well ! So claiming that their's isn't susceptible to it is pure marketing BS IMHO.


I have never heard a Rythmik SUB admittedly and they may be fine sounding devices.
But the figures they present don't look that promising at all: A reduction form 15% k2 to 10% is only an improvement form very bad to bad and doesn't make one think that the principle is very effective. I know that distortion audibility is not very high at 20 Hz. But the driver excursion was only 1/4" p-p, which is still in the linear range of many mid-woofers.

Regards

Charles
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Old 6th September 2005, 12:45 PM   #17
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Indeed, I thought the table comparing DSP with analog was either simply naive, or just plain bogus. There is nothing the DSP can't do the op-amp based design can, and the DSP has some intrinsic advantages, in particular it can include elements of time.

The other approaches rather run the range of very simplistic, to almost but not quite there.

Those designs that ignore the Theil Small behaviour of the cone are doomed to failure. But any design that seeks to correct the behaviour of the cone in a simple feedback loop should be doing some serious stability analysis, and may find some unfortunate problems. The very modest improvements seen in almost all of the works points to this.

The DSP based approach with active change to the filter coefficients is getting there, but also misses the point. The distortion mechanisms in a speaker are largely not minimum phase or model-able as something invertible with a FIR.

There does seem to be an element of hammer and nails (when all you have is a hammer, everything looks like nail.) What is needed is a model of speaker motion that includes the distortion mechanisms in their basic physical form. The issue of correcting the distortion then becomes one of parameter estimation, something there is a good body of knowledge of. But the first task is to decide on a minimum set of parameters, and finding orthogonal parameters is crucial to success. If the parameters are coupled the task of estimation, and then correction will be vastly harder. Orthogonal parameters are very likely to fall out of an analysis of the physical nature of the speaker. Applying the wrong model is likely to yield a slew of coupled parameters and an infeasable problem.

Got right such a model will find that the sensor system will spend most of its time doing nothing more than very slight fine tuning of the parameters, if anything at all. This essentially removes any issues of system stability from view.
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Old 6th September 2005, 01:08 PM   #18
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What is needed is a model of speaker motion that includes the distortion mechanisms in their basic physical form.
There was once a JAES article dealing with exactly that but I can't even remember what year but I think it's more than 10 years back.

And be aware that even a model that would be called basic would not intrinsically mean simple.

Regards

Charles
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Old 6th September 2005, 02:03 PM   #19
jcx is offline jcx  United States
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you mean like:

http://www.klippel.de/pubs/The%20Pow...r%20Models.pdf


(sorry to make it so tough on you guys, thought you could figure out to click "online pubs" in the klipple intoduction link i gave before: http://www.klippel.de/pubs/default.asp )
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Old 6th September 2005, 02:55 PM   #20
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Well that presentation pretty well gets my vote . (And I did miss checking out the publications.)

I'm not sure why their modelling is either Generic (which in general I would agree is a bad idea - although arguablely the Voltera kernel is all encompasing - and thus potentially useful) or based upon memoryless models. It strikes me that creep problems can't be modeled without memory. But I'm no mech eng.

As a guide to a very wide ranging EE project, this would have to be close to spot on.
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