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Old 26th December 2012, 02:58 PM   #71
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If the input signal has any frequency content below 200 HZ then that may introduce a pre-response. Look at a linear phase high pass filter response to a square wave. As long as the fundamental is above the cut off frequency (well in the flat band region actually) the square wave is perfectly reproduced. If the square wave fundamental is below the cut off the wave form is distorted. Here is a picture.

Click the image to open in full size.

Notice that for the 20 Hz signal the minimum phase response is causal but the linear phase is not. It has a pre-response. At 125 Hz, above the HP cut off, the minimum phase is still distorted but the linear phase is not. Neither is very good below the cut off but above the cut off the linear phase system gets it right. ( The slight droop in the linear phase 125 Hz result is because the filter isn't flat at 125 Hz so the fundamental of the square wave is slightly attenuated. )
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Old 26th December 2012, 03:26 PM   #72
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Ok, but this is true for any linear phase filter, not only for a phase-corrected BR.
If I EQ and filter a BR to obtain a acoustical LR 48khz (for example), then (as you confirmed it) it will behave exactly as a normal LR48 would have. Whether or not I subsequently correct its phase or not will have the same impact as any other LR48, right? The steeper the slope, the more preringing will be seen on the impulse, especially if there is not symmetrical LP filtered driver (which is not the case here as I want to use a BR for the midbass as part of the midbass/woofer crossover).
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Old 26th December 2012, 03:47 PM   #73
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Quote:
Originally Posted by pos View Post
Ok, but this is true for any linear phase filter, not only for a phase-corrected BR.
If I EQ and filter a BR to obtain a acoustical LR 48khz (for example), then (as you confirmed it) it will behave exactly as a normal LR48 would have. Whether or not I subsequently correct its phase or not will have the same impact as any other LR48, right? The steeper the slope, the more preringing will be seen on the impulse, especially if there is not symmetrical LP filtered driver (which is not the case here as I want to use a BR for the midbass as part of the midbass/woofer crossover).
Yes, absolutly true. It doesn't matter if it electrical, acoustical or a combination of both. If the final output is linear phase it behaves as a linear phase filter of the same amplitude respponse because that is what it is.
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Old 26th December 2012, 05:01 PM   #74
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Originally Posted by Davey View Post
Great job on this pos.
I set up my Foobar configuration (foo convolver) a few weeks back to implement a simple three-way with the "Filters Linearization" portion and it really does work well.

Dave.
why not trying FIR crossover with foobar !
VST 2.4 adapter
VStconvolver
asio driver

a way to try phase linear steep slopes.
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Old 26th December 2012, 08:18 PM   #75
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I am not convinced steep slopes are the way to go: they are more prone to preringing as soon as the high-passed et low-passed drivers are not perfectly summed in phase (which can happen quite quickly off axis, even if the system is perfectly in phase on axis).
What slopes have you tired in your system Thierry?

The main advantage I see with a pure FIR crossover, in addition to a linear phase, is the opportunity of using any complementary shape you want for your crossover, such as Horbach-Keele slopes (special MTM) for example.
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Old 26th December 2012, 09:47 PM   #76
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we can strongly lowering the tweeter cutoff frequency.
and join the mid section in a constant directivity region,avoiding too much electrical power in the tweeter.

it's interesting with 2 ways 15"+2" compression and coaxial driver.

i've tried 48 and 96 dB/oct at several frequency.
last choice is 96 dB at 140 Hz for low/mid-low cutoff.
always with passive at 2400 Hz.

i'm looking toward for a 8"+1" coaxial driver for mid/high,with a 1500 Hz 96dB/oct crossover point.

also FIR EQ seems to be more efficient.
so much possibilities...it needs time,measurement and standing back to aim better choices.
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Old 26th December 2012, 10:04 PM   #77
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Quote:
Originally Posted by pos View Post
I am not convinced steep slopes are the way to go: they are more prone to preringing as soon as the high-passed et low-passed drivers are not perfectly summed in phase (which can happen quite quickly off axis, even if the system is perfectly in phase on axis).
What slopes have you tired in your system Thierry?

The main advantage I see with a pure FIR crossover, in addition to a linear phase, is the opportunity of using any complementary shape you want for your crossover, such as Horbach-Keele slopes (special MTM) for example.
Well all those theoretical crossovers are really nothing more that acoustic targets as it is the acoustic response of the driver that must match them. That is what is so nice about the Bodzio UE. It allows you to specify the acoustic target and it then generates the filter transfer function required to achieve it based on the measure driver SPL data.
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Old 26th December 2012, 10:10 PM   #78
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Agree. That is a very nice feature.
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Old 26th December 2012, 11:16 PM   #79
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Of course, the natural frequency response (amplitude and phase) needs to be taken into account, or corrected.
I don't like the "automated" approaches: I prefer manual corrections based on different measurements, to avoid correcting things that should really be left alone (measurement dependant).

For now with rephase the user needs to import the generated impulse in HOLM (or REW) and do a convolution with the measurement(s) (C=A*B in the "manipulation" menu in HOLM).

So by going back and forth between rephase and HOLM (adjust correction, generate, impot in HOLM, do the convolution, adjust correction, ...), the user is able to first "linearize" both the amplitude and the phase of a driver within and around the passband (1 or 2 octave, depending of the slope that will subsequently be applied), and then apply any filter slope he wants.
So this is a two-steps thing, and always manual.

I will include the possibility to import measurements in rephase to directly see the effect of a correction in realtime, but it will remain manual (no automated inverse correction). I will try to make it easy to load multiple measurements and either average them or make it easy to switch between them during corrections.

Last edited by pos; 26th December 2012 at 11:19 PM.
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Old 26th December 2012, 11:45 PM   #80
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Your manual approach is pretty much the way the UE works deep down inside. You start with a measurement. That measurement can be a lot of things: the on axis response, the smoothed axial response, a spatially averaged response,..... Then, the user specifies a frequency range over which minimum phase equalization is applied to flatten the response. Then a text book type filter is applied where in the net acoustic output will exactly match the text book response over the eq'ed range. Over the frequency ranges where the eq is not applied the response will be the text book filter timed the raw SPL data. Then the final response is made linear phase (on option).

So if you want to make a tweeter have a linear (or minimum) phase LR4 type response at 3k Hz but only want to equalize the response in the actually crossover region and leave the response above say 3.5k un equalized, you can do that.

[edit] Then the response can be measured and if you are not happy you can change it. Since the UE has has the convolution engine built in you never have to dick with stuff. You know exactly what you will have because you measure the result and can compare the measured result with the predicted result.
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Last edited by john k...; 26th December 2012 at 11:53 PM.
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