rePhase, a loudspeaker phase linearization, EQ and FIR filtering tool

[Tom Danley]

Originally Posted by Pano

Thanks POS for the new version

I'm still testing with mine. So far it hasn't done any harm, but I'm not sure it's a huge improvement, either. The differences are subtle, to my ear. I've tried correcting just the box HP filter, and also the box with the crossovers. Wanting to hear what each part does.

Hope to have a report soon.

Hi Pano
If you can, try taking the measurement part of the process outdoors, at say 2 or more meters.
There are several issues which complicate the “soup” here. The magnitude and phase one measures from the loudspeaker often change slightly with distance and more so with position left to right / up and down within the radiation pattern.
Even a constant directivity horn has this effect to varying degrees. Your measurement mic only occupies one point in space and so what your electronic correction will be based on is that location.
So far as fixing the loudspeaker, ideally you would measure at a distance more like how you listen. Even this outdoors could have a built in problem is the woofer is high enough to cause a ground bounce. No amount of eq can fill in a strong cancelation notch.
Try an alignment this way with the system not including your room related issues, just loudspeaker correction.
Best,
Tom

[Pano]

Thanks Tom. I have kinda-sorta done this, without taking the speakers outside. That is a major pain, as they are so big and heavy. They barely fit thru the door.

I've done the correction from measurements taken at 5 ft, at the listening position, and from simulated filters and box response. Trying to make sense of it all.

[thierry38]

Thanks pos !

also for the "dark side of the force"

i 've just taken à look to the brickwall implementation.
as you said,complementary filter must be done manualy with the same parameters.

after generating 2 complementaries brickwall filters with 2 differents types of windows,
-export to REW and process the sum
-it does not give an all-pass.

a lot of care and minimum training ...

An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.

a deep look with rePhase next weeks.
summer starts to get down

PS:the raised cos EQ in a minimum phase option to apply shelving min.ph.

[pos]

Hi Thierry,

Optimization and energy centering are automatically disabled when a brickwall filter is used, so you just need to use the same number of taps and windowing algorithm (and same sampling frequency of course) for the LP and HP and you will get a perfectly complementary crossover.

Here is an illustration using a nuttall window:





Now here is what you get if you mismatch either the windowing algorithm or the number of taps:

[pos]

Here is another set of pictures to illustrate the effect of the number of taps, frequency, or windowing algorithm

[pos]

I implemented brickwall filters them into rePhase because many people wanted them, and because they were easy enough to add, but using "real" complementary filter slopes (LR, Horbach-Keele, or other available types) and making the result curves match the target ones as closely as possible (using optimization and the appropriate number of taps and windowing algorithm for the task) is probably the best way to go.

Brickwalls will give you the sharpest possible slope for a given number of taps and windowing algorithm (and even that is not true with optimization...), but not the lowest ripples, and no real control on the slope whatsoever.
Plus, the fact that the exact same number of taps has to be used for the LP and HP is a waste of DSP power when most hardware solutions make it possible to arbitrarily distribute taps among channels...

[OllBoll]

I sat and played around some and got some questions since I'm no FIR expert =)

Assume only phase correction:

How big an evil is it to use few taps? Say you have a 24 dB roll off @ 20 hz and use 2048 taps with a FFT length of 8192. Apart from not having a perfect LR4 phase response what is the downside of doing this? Won't the speaker itself have much bigger phase response ripples or is there something I'm not seeing?

My current plan is to phase correct a LR4 @ 100 hz, one at 200 hz and the question become if I should also correct the phase of the 20 hz LR2/LR4 low frequency roll off or should I let it be?

[OllBoll]

Or how small must the phase ripples be for it to be good enough? And lets say this phase compensation isn't broadband but rather up to 200 hz and above that another filter with less ripple is used, does that give us bigger room to play with?

[pos]

Hi OllBoll,

When doing phase-only corrections with few taps you will get alteration in the amplitude and phase curves, but the optimization steps will try to compensate any amplitude alteration. You can check this effect by choosing a very narrow amplitude scale like -1/+1dB, and you should see amplitude ripples around your corrected crossover frequency than should diminish with optimization iterations.
But these optimization will cause phase to get worse, as you noted.
The optimization focuses on amplitude as this was judged the most important thing to preserve, but there will eventually be a point of diminishing return.
Having a phase curves that does not exactly follow target is probably not a problem (and it is still better than no correction at all), but rapid variations and ripples can be problematic. Best thing would be to check what group delay those will cause (adding a group delay visualization in rephase is on the todo list...).
As to what should be corrected or not, it is up to you to try

[OllBoll]

Thanks for the answer =)

One thing I thought about when testing, would be nice to have the ripple denoted in -dB instead of having to count zeroes =)