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Author Topic: Crossover Shapes  (Read 7658 times)

Timo Beckman

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Re: Crossover Shapes
« Reply #10 on: October 02, 2011, 03:42:23 PM »

Tanxs a lot for the link .
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Peter Morris

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Re: Crossover Shapes
« Reply #11 on: October 02, 2011, 09:53:37 PM »

I have been saying this for years, but it bears repeating. The real loudspeaker design engineers working at the manufacturer will have superior knowledge about the drivers being used and box design. They will also generally have superior test equipment and facilities to measure actual box with actual driver response to make corrections.

...


+1 and as I said above ...
 http://www.qscaudio.com/products/speakers/KW_Series/
QSC have been there and done that ...

http://media.qscaudio.com/pdfs/manuals/KW_Series_user_manual_EN_revC.pdf

"DSP FeaturesThe KW Series features advanced DSP (digital signal processing) circuitry that performs many functions. Some functions are set at the design/ production level and are not user accessible. These functions include crossovers, time alignment, limiting and protection, thermal management
and a number of proprietary features. QSC has designed exclusive DSP functions that greatly enhance the capabilities and performance of the KW Series systems.
Proprietary DSP FunctionsExcursion Limiting: In addition to signal limiting to protect the amplifier and transducers from overload, the KW Series utilizes a proprietary limiter
that prevents woofer over-excursion. Over-excursion occurs when a voltage presented to the woofer causes the cone to physically travel too far. This builds up excessive heat, stresses the moving parts of the woofer, produces audible artifacts and distortion and reduces the woofer’s lifespan. The proprietary algorithm contained in Excursion Limiting prevents over-excursion. Voltages that will harm the woofer through over-excursion are reduced enough to prevent over-excursion without any audible compression, limiting or loss. Taking advantage of the Excursion Limiter, the DEEP™ (Digital Extension and Excursion Processing) algorithm functions as a highly musical and non-distorting low-frequency EQ circuit. More on the DEEP function is available in the EQ section of this manual.
Intrinsic Correction™: Introduced on QSC Concert/Touring products, Intrinsic Correction is a proprietary process and set of signal processing algorithms that address correctable characteristics of transducers and waveguides. The net result is that any KW Series system will present extraordinarily even and consistent energy throughout the physical listening area of the loudspeaker, resulting in a very musical, acoustically
transparent system."
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Curtis H List (Too Tall)

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Re: Crossover Shapes
« Reply #12 on: October 04, 2011, 07:38:21 PM »

Hello Folks,

Well, excitement stirs as we are preparing for the arrival of our new QSC KW rig. With the new rig, comes new processing toys. We have chosen the BSS OmniDrive. Where before, the trusty ole' Rane crossover did a phenomenal job, the new OmniDrive opens fun new doors of adventure, specifically, in Crossover Shapes.

The Rane didn't offer any options in shapes, you just dialed in the frequency and level of each band and added delay if needed to the sub stacks. Now, I read about all the different shapes you can use in your crossover design. Although I am looking forward to the added accuracy in flattening and optimizing the system, I am a little overwhelmed at all the different shapes, Butterworth, Bessel, Linkwitz-Riley, etc.

In reading about them, I am leaning towards the Linkwitz-Riley 24dB/Octave slope, but there is also the Butterworth. So, I thought I would check in with the knowledge pool and see what ya'll use and why? What applications call for certain shapes?

Can't wait to get it out and tuned, and any advice is, as always, greatly appreciated.

Thanks,

Adam P.

Hi Adam,
Just a few points to hit on.

First I am not encouraging that you rip into the settings QSC is using.
When working on a totally different generic processer the pitfall of not agreeing on what “Q” is not withstanding.

Do not get confused with the fancy names.
It would help if you think of Hertz as CPS (Cycles per Second).
It’s nice to have something named after you, but it is also more confusing until it becomes 2nd nature.

The difference between a Linkwitz-Riley and a Butterworth is mostly the shape of the knee.
The way the knee is drawn so that when you add a Butterworth high-pass and a Butterworth low-pass you get a mild bump (about 3dB) at the xover point.

If you use Linkwitz-Riley you get a flat line at the xover point.

So why would you ever want Butterworth if it makes a bump?
Well this is where “phase” comes in.

If the phase at the xover point for both high-pass and low-pass is about 90 degrees (or 270 degrees) you may have a 3dB hole. The Butterworth “hump” takes care of this nicely.

PS- It may be a 6dB hole. I just can’t remember right now.
 Sorry.

So when you measure drivers this is something you want to keep track of. Write it down as you go.
Hope this helps.


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John Roberts {JR}

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Re: Crossover Shapes
« Reply #13 on: October 04, 2011, 09:42:14 PM »

Hi Adam,
Just a few points to hit on.

First I am not encouraging that you rip into the settings QSC is using.
When working on a totally different generic processer the pitfall of not agreeing on what “Q” is not withstanding.

Do not get confused with the fancy names.
It would help if you think of Hertz as CPS (Cycles per Second).
It’s nice to have something named after you, but it is also more confusing until it becomes 2nd nature.

The difference between a Linkwitz-Riley and a Butterworth is mostly the shape of the knee.
The way the knee is drawn so that when you add a Butterworth high-pass and a Butterworth low-pass you get a mild bump (about 3dB) at the xover point.

If you use Linkwitz-Riley you get a flat line at the xover point.

So why would you ever want Butterworth if it makes a bump?
Well this is where “phase” comes in.

If the phase at the xover point for both high-pass and low-pass is about 90 degrees (or 270 degrees) you may have a 3dB hole. The Butterworth “hump” takes care of this nicely.

PS- It may be a 6dB hole. I just can’t remember right now.
 Sorry.

So when you measure drivers this is something you want to keep track of. Write it down as you go.
Hope this helps.

I'm not changing my position that amateurs should not second guess professionals with skills and tools, but to clarify a little on your short tutorial about filter alignments, and this may be confusing too, but 4 pole L-R are simply two 2pole Butterworth alignments cascaded in series. The 3dB or 6dB characteristic at tuning frequency is related to even or odd order (number of filter poles). Since each pole is + and - 45' at tuning (90' relative to each other) we get 90" relative for 1 pole, 180' relative for 2 pole, 270' for 3 pole, 360' for 4 pole, etc.  Odd order alignments sum to unity with -3dB at tuning because of the 90' relative offset. Even order crossovers like 4 pole L-R sum to unity with both sections down -6dB at tuning. (note: old school 2 pole crossovers were 180' out so required a polarity flip to not create a notch (suck out) at the exact crossover point. Since Butterworths are normally tuned for -3dB at the crossover frequency, stacking two 2 pole Butterworths in series, auto-magically gives us the -6dB desired for L-R.

This is the cliff-notes version, and I repeat the design engineers at the speaker companies generally know what they are doing, certainly more than me.     

JR
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Re: Crossover Shapes
« Reply #13 on: October 04, 2011, 09:42:14 PM »


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