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Old 12th February 2006, 10:23 PM   #401
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DEQX goodness ordered yesterday and will be with me on Tuesday Managed to find a UK dealer with stock:

http://www.tekcare.co.uk/Scripts/pro...p?idproduct=77

Not gone for the pre-amp version as I'll be doing multichannel so I'll eventually get round to building something like a 10 way DACT attenuator.

Feel like a sell out now after pimping the PC setup in such a big way. I still think that it holds way more potential than the DEQX could ever hope for but I've spent ££££'s along with 100's maybe 1000's of hours playing around and it hasn't happened.

Nevermind, I hope the DEQX really is the end to my digital crossover journey.

I'll report back with some thoughts and we'll see if it really is worth all that money. One thing is for sure; it had better fix the recent issues I've been having.
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Old 13th February 2006, 07:07 AM   #402
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Default Re: Re: The "Perfect" XO for High End Stereo Systems?!

Quote:
Originally posted by m0tion
.... but when I graph them using Excel they seem to simply oscillate back and forth between ~1 and ~-1. Is this correct?
Hi m0tion,
you investigated the rsult with Excel and observed values in the resulting file in the range +-1.

This is absolutely correct! What you see in the generated file are the real and imaginary parts of the filter response.

In the generated file the first parameter of each line is the frequency, the second the real and the third the imaginary part of the filter response. To get the amplitude of the filter you have to calculate it in your spreadsheet from the real and imaginary part with the following formula:

Amplitude = SQRT (( real*real) + (imaginary*imaginary))

This formula calculates the amplitude of the filter and could be nicely visualized in EXCEL.

I will also provide some examples so stay tuned....

Charly
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Old 13th February 2006, 07:18 PM   #403
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Quote:
Originally posted by Vil
there is pretty interesting Linear Phase EQ :
they say " Phase Linear Operation is achieved by processing your sound in both the forward-time and reverse-time directions through classic filters - all in realtime. This completely removes the phase warping caused by IIR filtering "

so thats not FIR but IIR filter with linear phase . I tested it , phase is really linear , and there is no frequency ringing up to -140dB .
Dear Vil,
in literature ringing always is described as an irregularity in the time domain and not the frequency domain.

To demonstrate his I took a very steep linear phase Linkwitz Raily filter of 40. order (240db/oct) with a XO-Frequency of 3000Hz and computed the step response for this filter. The red curve is the step response of the LP, the green one the step response if the HP and the blue line is the sum of both of them. I calculated the blue one to show that his filter is phase linear and adds up to a perfect step.

Click the image to open in full size.

The signals of the LP and the HP start oscillating at 3000Hz long before the step happens. This phenomenon is called preringing and only occurs with linear phase filters (not with minimum phase filters) and is definitely audible. Why?

The blue curve shows that summing up both signals ringing disappears because the ringing of the two signals adds to zero. This is only true adding the electric signals. Firing the ringing signals (red and green) via speakers into your listening room the addition of the wave fronts will be no more perfect so preringing will become audible.

In comparison to the linear phase 40. order LR filter the next image shows the step response of a linear phase 40. order Bessel filter (also 3000Hz) proposed in my paper.

Click the image to open in full size.

As you see this filter has no ringing at all. You could make the Bessel LP filter as steep as you like. At no circumstance ringing could be observed.
So my advice is to listen carefully before using steep linear phase filters. Caused by preringing often - especially at higher frequencies where the ear is very sensitive - linear phase filters are worse than minimum phase approaches.

By the way, using an IIR filter and calculating it from both sides not really gives any advantage. IIR has no preringing but postringing. Making it phase linear means mirroring the step response so the postringing of the IIR filter will be transformed to an audible preringing! Believe me, nobody could outflank physics ;-))

Charly
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Old 13th February 2006, 07:30 PM   #404
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What an excellent post oehlrich. I've also now managed to read through the .doc you posted earlier in this thread.

It seems you greatly understand the limits imposed by using linear phase filters. I only wish I had your knowledge and my PC filtering attempts may have been completely successful.

Keep us updated on your progress and I may just give the PC thing another go.
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Old 13th February 2006, 07:49 PM   #405
Vil is offline Vil
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good post oehlrich .

>>in literature ringing always is described as an irregularity in the time domain and not the frequency domain.

sure you are right , I learned that at my college days too , just never used english language terms to describe that , there were no possibility to access any technical books in english language ...
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Old 14th February 2006, 07:34 PM   #406
mbon is offline mbon  France
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Default Re: Re: Re: The "Perfect" XO for High End Stereo Systems?!

Quote:
This phenomenon is called preringing and only occurs with linear phase filters (not with minimum phase filters) and is definitely audible. Why?.....This is only true adding the electric signals. Firing the ringing signals (red and green) via speakers into your listening room the addition of the wave fronts...
Don't forget that what you wrote is available for any kind of filter !!!!! There's is always a big difference between theory and speakers reality !

It is well known that 1st order filter are often not so nice in real world, specially off axis !!!!!!

It is just an example !

I use linear phase filter and I offer you a champagne bottle if you can ear the pre ringing on my system !!!! Please come at home !

I've done some blind test, nobody had noticed pre ringing during test, in fact...what is pre ringing for ears in situ ????

In the real life, anyone should consider the benefits of high slopes filters specially off axis. The low speakers interraction offers more goodies than the "pre ringing".

Please notice that the new reference studio monitor made by Focal use linear phase filters (200dB/octave) !!!! and the crossover is at 2.2KHz

I'm not specially a linear phase addict but readers should read carrefully differents point of view...

I still enjoy linear phase filter....

regards
Marc
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Old 14th February 2006, 07:36 PM   #407
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Hi Marc

We all know that linear phase filters can sound great but it is not a trivial task to generate decent linear phase filters. So its more a case of not all filters are equal.
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Old 14th February 2006, 08:35 PM   #408
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BTW DEQX was waiting for me when I arrived home from work.

Just had a *very* quick play and no sign of the distortion I was talking about earlier in the thread. Even when I really crank the volume.

I have to conclude that I do hear pre-echo with DEQX on the really steep filters (300dB/oct. for example) but the problem with the PC was not this. It was more like digital clipping which is completely bizzare as all my level were backed way off when troubleshooting. Something obviously not right in the chain somewhere.

So just to recap; it wasn't pre-echo problems with the PC but something rather different.

No thoughts on just whether or not the DEQX sounds better as I've only tried a highpass on an ATC woofer so far. But I really do like the measurement and setup software for the DEQX, the best I've used yet and offers just as much accuracy and potential in the measurements as the PC did but its way, way fast. I can see it becoming a great tool for future projects.
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Old 14th February 2006, 09:25 PM   #409
Sheldon is offline Sheldon  United States
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I have followed your journey with interest. No doubt, you'll get the bug to go back to the PC at some point afte you've got everything decently sorted and have actually had a chance to enjoy music for a while. But since I have a DEQX coming in the next week or so, I'm selfishly glad that you have one in hand. Bad for you (timewise), good for us that you will be out ahead of the curve, especially with what you've already learned so far. Expect plenty of questions, if you are game.

Sheldon
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Old 15th February 2006, 07:38 AM   #410
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Default Re: Re: Re: Re: The "Perfect" XO for High End Stereo Systems?!

Quote:
Originally posted by mbon

In the real life, anyone should consider the benefits of high slopes filters specially off axis. The low speakers interraction offers more goodies than the "pre ringing".
Dear Marc,
You are right if you say steep filters have advantages. One of the advantages is (as I wrote!) that the lobbing error (this is what you call ‘off axis’) is minimal. The other big advantage is that the frequency ranges where the driver may be far away from its perfect linear operation are just switched off by the brickwall filters. These two advantages were paid by ringing and time delay.

Reading my paper you should have seen that the proposed Bessel filter has a wider overlapping region but in this very special case the softer slope did NOT create any lobing error. So here I can see no advance using a brickwall filter.
Looking at the 2. order slope of the HP I wrote that the response of the driver may pollute the response of the filter. This means applying a 2. order XO alone would not be sufficient. The driver also has to be corrected in such a way, that the convolution of driver-, correction- and XO response exactly covers the slope of the XO. Using brickwall filters and omitting driver correction creates a smaller error than using a 2. order slope without correction. Despite of this omitting driver correction always leads to errors. So I recommend to perform driver correction in any way (I plan to write about that).
Next point is ringing. I guess there is no discussion: Preringing is audible if it extends a certain degree. I admit that the point when it is audible strongly depends on the drivers used and how perfect the wavefront of both drivers add in space. The Bessel filter I proposed has no ringing at all.
Regarding delay the Bessel filter is perfect because it could be realized as minimum phase OR linear phase filer. Both of them share the same transient perfect response and also have the same perfect polar response. Just use whatever you like!

I did not state that the proposed Bessel filter is the best one could do! I just tried to show the pros and cons of the different approaches and summarized that the Bessel filer aggregates more pros than cons which makes it a good candidate for a XO.

If you have a PC based system, just give it a try and listen!

Charly
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