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Default All Pass Filters

This is a how to use all pass filters to reinforce frequency response caused by cancellations and how to do it with maintaining staging and imaging.


Note******BEFORE EVERYONE AND THERE ďOTHER EXPERTS SAYĒ OR ANY OTHER ISSUES THIS METHOD I WILL CLEARLY STATE THIS IS JUST ONE METHOD THAT WORKS FOR ME IF IT WORKS FOR YOU THATS GREAT, IF YOUR FAVORITE EXPERT DISAGREES WITH THIS I COULDN'T FUC$&G CARE LESS SO PLEASE SHUT YOUR PIE HOLE ON ANY DISAGREEMENTS OR ANY TECHNICAL DISPUTES PLEASE *******

Letís move on!


This how to is for intermediate to advanced tuners only. If your a noob and donít understand the terminology clearly please refrain from posting questions or ask for explanations. If something dosent make sense get online and educate yourself to the degree that it does make sense. Please donít clutter this thread with a bunch of noob questions. Thanks all others are welcome.


So you have a DSP with All pass features and arenít sure what to do with all pass , you try out a few frequencies and some sound good and some donít. Hereís a way to get some good use from all pass filtering. This IS NOT THE ONLY WAY TO USE all pass filters. Thereís so many uses and I WILL go against the textbook uses so your just going to have to trust me when I say in a car with so many reflections and refractions and diffractions and comb-filters and cancellations and the listening room is a steel box and your seat is in the corner we have to deviate from textbook ďrules and normsĒ to achieve a soundstage.

So the first thing you want to do is tune your car with all allpass filters turned off. And no underlap on crossovers. DO NOT USE CROSSOVER UNDERLAP! Crossovers will shift phase so you want a ďproperĒ alignment, ( e.g. if your mids are crossed at 400 on your dsp you should also have your midbass at 400hz. I donít care what you think , your wrong, do a proper crossover) NOTE** if your crossover setup has odd order Linkwitz-Riley or Sallen key filters with a Q0 of Q0.5 like a 12db low pass on any of the speakers without a high pass turned on you will want to invert the polarity on that speaker)
Here is some sims of Linkwitz-Riley 12and 24db crossovers, you can see the 12db ones need a polarity reversal




the LR4 crossovers do not






First Set your ďsignal delaysĒ between left and right and front and rear and all the loudspeakers in your multi-way to promote a good phantom center image. The center should be placed as far back towards the windshield and as high as the top of the dash line and in the center of the car. Start with tape measure distances and adjust from there EVEN IF ITS WRONG MAKE THE AJUSTMENT ANYWAY so that the phantom center is firmly placed as high and far back as possible. I like to use all kinds of music along with the IASCA relative polarity track 3. Get the center nailed down at all costs without inverting a speaker that plays below 250hz.

Now time to do your time misalignment (just kidding I call it time misalignment because itís more of a misalignment than an alignment but thatís another story, unless your a complete stud and have equalized path lengths on speaker placemat you probably have no clue what Iím talking about with those a pillar pods causing all that chaos)

You want the frequency response as flat as you can make it preferably by only using cuts in eq (but thatís your way and I donít care. I donít boost anything and donít care about what you think or how you do it I just donít boost my preference). However the method I want you to do is the moving mic averages RTA method. DO NOT USE SEPARATE LEFT AND RIGHT EQ! You want to link left and right together in your eq on all channels so all eq work is applied to left and right together. Use the microphone and move it around your head and take averages and cut the peaks in your response first and get it flat as possible (within 3db). If you have rear speakers do the fronts first and eq the rears seperate from the fronts on all frequencies above 250hz. On frequencies below 250hz compare front and rear they would eq about the same but if the rear is massively different donít do any eq work below 250hz at all on front or rear. That is a indication of a room mode that we will be addressing with all pass.

Now that you have set your time alignment and done your eq using moving mic and no separate eq on left and right letís do some all pass filtering.

You need to use a mono signal generator. You can download one for free from your App Store or use a professional one. The signal generator should be played through the entire system and the entire signal chain is preferable. Hereís an example of one available on apple for free



When you were doing your EQ work you should have noticed what frequencies you cut on the peaks. Take a note of the center frequencies of those peaks before you eq.

Now start fresh make sure your ears are not tired for this process it may take a few tries.
With music on your system should sound good. The center should be strong. Get your ears used to the sound for about 3min at lower volumes. Now toggle between music and your signal generator. Start the signal generator at 20hz and start going up to the first big dip in the response. It should be between 65hz and 110hz. Now apply an all pass filter on the sub at the frequency that is the quietest to your ears in the first dip. Play with the Q of the all pass , you should hear what sound like a Doppler effect. You will hear the sound location move around as you move the Q of the all pass. You want to try to reinforce the dip and get reinforce the center image or if itís bass get the bass to sound like it in the front of you instead of behind you. Try a few Q adjustments and toggle back to music and listen. If itís an improvement proceed on, if it sucks try again.

Between toggling between the Q adjustment and the test tone try move the frequency up and down again and listen for the volume of the test tone. The dip should have been removed or made a lot better. Ajust the Q of the all pass to cover more or less of the dip if you have reinforcement and itís helping.

Now turn off the all pass on the sub and go to the rear speakers , try the all pass on both rear speakers so there linked and go to that first dip and repeat the process.

Now listen to music and toggle the all pass filters between the sub and the rear speakers and see which way sounds the best with music. What promotes the center and stage the most and what reinforces the center image and gets the dip in response to go away.

Once you have the first dip mostly fixed and music sounds noticeably better go back to signal generator. Go to 2nd dip. This one should be in the 125-500hz range.
This time you will use a low pass filter on the rears and place the low pass filter at the frequency where the dip is. If the dip gets better by adding a low pass to the rears take a listen, you might like not having those rears talking so much! :-p

If the low pass on rears doesn't work or donít want to low pass the rears try the all pass again on the rears. Listening to the signal generator move the frequency up and down dramatically, listen for the tone and how it reacts with your car and find the center image in the tone. If you just listened to music and had the phantom center placed, your ears should grab on to the signal generator and be able to place what frequencies are centered

Your using all pass on rears to purposely interact with the fronts to try to push the low pressure mode in where your sitting to another spot and getting the high pressure where you sit by manipulating the timing of just that frequency between front and rear or left and right. Always listen to music and verify it sounds better before moving on and making it permanent or forgetting what you did and canít get back up the rabbit hole.






Now go work on the second dip. Put the all pass filter on the rear speakers or if there are no rear speakers apply it to one of the left or right front speakers not both just one side. Choose the side that reinforces the center where the test tone sounds centered and not all the way to one side. If it is all the way to one side but the dip get louder after finding the Q of the all pass you like that makes fills the dip and the signal generator sounds like the dip is lessened when you move up and down in frequency passing the dip frequency several times. if its better go to music and listen. If the soundstage is better great, if it is worse go back and try moving the all pass up or down somewhere else in the dip and adjust the Q again and keep trying and going back to music and trying either the all pass on the left or right side. Try and audition a few different you think will be good and find the one that gives good timbre to music. this will change the timbre to the music.

If you got a reinforcement and the phantom center is better and stage has better depth than move on to next dip. If you canít get it to sound better donít put an all pass on that dip and move on. Again try a few times and a few frequencies and Q settings for the allpass within the dip.

The 3rd comb-filter or dip should now be in the 500-1.3khz range. As you move the signal generator you may notice some frequencies only sound from the left or right. You can definitely experiment with the all pass at those extremes. Just listen to music before you make any all pass filters permanent. Validate that they sound good.

Work the 3rd comb filter and work the frequencies that are also completely canceled from your position. If it makes it better great if not turn it off and move on.

I like to do these one at a time and listen for a few days to each one and make sure I like them. Some of them will be no brainers and will be obviously better especially the 1st dip one.

The frequencies above 1.6khz you certainly can use an all pass on but I donít.

Once you have gotten a couple dips fixed you can now do your left right eq work on frequencies over 1k. If the all passes work right your center should be super strong now and you wouldnít want to mess that up.

Now you can do separate left right EQ above 1.6k and only do the separate EQ ONLY UF OTS WORTH IT! Go back to music with every change you make and validate.

You can do this process with separate left and right eq as well. Just be careful and listen to every change you make with music and try lost of different tracks and validate each change. You can certainly do left and right EQ before or after this process, I strongly encourage you to try. Doing it without separate left and right EQ first helps a one that doesnít understand or fully know the behavior of there cars acoustics. If you know your car and how it behaves or already have it dialed in and want to try this method should still work.
I just wanted to put a known way of making a good sounding car first without separate left and right EQ. The linked left and right EQ with moving mic RTA is a surefire way to at least get a good sounding car without getting too technical. Separate left and right EQ and separate driver EQ should always only be done if itís worth it. Always listen with music and validate all those types of EQ.

The only thing I do without validation is the first steps of time alignment to get the center image and moving mic RTA work.

By the end of this your system would definitely sound noticeably better and at least one to two of the dips fixed.
I would not go all crazy with all pass filters. Use only what you need for the biggest of problem areas. Too many wonít sound good.
As all pass filters are added group delay.
However if properly used as described, it will actually remove group delay and make much better envelopment without going and getting some real FIR power

After everything is done and sounding better go back to rta and turn down things that are too high but listen after every change to make sure itís better.
Even if some frequencies are still missing the energy is there with all pass added it moves the energy to a diffrent part of the car. So your rta May show a change but trying to fix it may or may not make it sound worse.

Hope this helps someone.
Best of luck let us know how it goes

im so in love with my speakers I found a song I sing to them

https://youtu.be/VgU6LXY-AjI

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Default Re: All Pass Filters

Great post!

Can the Helix DSP do this?

If only running fronts and sub, how does this change the process?

Simon


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Default Re: All Pass Filters

Quote:
Originally Posted by SiW80 View Post
Great post!

Can the Helix DSP do this?

If only running fronts and sub, how does this change the process?

Simon


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Helix does it diffrent

Make everything a midrange in helix and re set crossovers
And you can phase shift every driver in increments of 11deg
So itís different, you can shift phase on seperate speakers in helix not all pass

I know minidsp
Masconi
Jl audio vxi
Can do all pass
And few others


Re read carefully I say if no rears and what to do

im so in love with my speakers I found a song I sing to them

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Default Re: All Pass Filters

You can do "parametric" all pass filters in the eq bank and set frequency, order, and q. You can do as many as you want.

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Default Re: All Pass Filters

Quote:
Originally Posted by SkizeR View Post
You can do "parametric" all pass filters in the eq bank and set frequency, order, and q. You can do as many as you want.

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Good to know thanks for clearing that up
I havenít used a helix in awhile

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Default Re: All Pass Filters

As far as if no rears just do the test tones and try the all pass on diffrent speakers or combinations of speakers to help reinforce the frequency that you can hear is missing or canceling on your listening position, again and this method will work for any way of tuning or pre tuning ,
And you can experiment with adding subtracting delays if an all pass does move your stage over after trying. Just listening to each change with music is ideal and listen to the tone of the signal generator. You want the tone to be even sounding between left and right ideally , but if the tone is louder but shifted to left or right , try it with music and at that point you could experiment with adding or subtracting delay from appropriate speaker to try to re center things with music: but if most of your frequencies were already centered it may not work, but if your car is such an acoustical nightmare that you canít get a center to appear at all this might fix that with tweaking delays after setting an allpass to reinforce magnitude or amplitude

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Default Re: All Pass Filters

Thank you. I canít wait to try this!

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Default Re: All Pass Filters

Thanks

Which tone generator app did you use?


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Default Re: All Pass Filters

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Originally Posted by SiW80 View Post
Thanks

Which tone generator app did you use?


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Signal generator iOS

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Default Re: All Pass Filters

Ok, time out. Not gunna lie, I stopped reading where he said not to match crossovers acoustically.. but are you really suggesting using sine waves to tune by ear?

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Default Re: All Pass Filters

What I got from this, and correct me if Iím wrong. Is that to get rid of the dips in response that donít go away no matter how much you gain them may be able to be fixed by adjusting the Q of an all pass filter in that range until it hits center.

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Default Re: All Pass Filters

Quote:
Originally Posted by AyOne View Post
What I got from this, and correct me if Iím wrong. Is that to get rid of the dips in response that donít go away no matter how much you gain them may be able to be fixed by adjusting the Q of an all pass filter in that range until it hits center.
Yes Aimed at reinforcement of acoustical interference and the hard dips , but with a good sober approach by listening after every adjustment to ensure soundstage is not compromised

Quote:
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Ok, time out. Not gunna lie, I stopped reading where he said not to match crossovers acoustically.. but are you really suggesting using sine waves to tune by ear?

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Skizr, no I was saying make a proper crossover, and I was suggesting if thereís no acoustic alignment process done and one was to just turn on the electronic crossover to not have underlap, but a proper electrical crossover will match the acoustic crossover, by means of using peq or whatever to make the electrical match the acoustical , not just making a set of acoustical filters. So no sir not implying that, just saying donít arbitrarily make a crossover underlap because ďit sounds betterĒ without digging into why the actual alignment is not working. Iím suggesting do a proper crossover or at a minim set in dsp to be a proper electrical crossover

And before you start picking this process apart I kindly asked to refrain from opinions , I also clearly stated that this process IS a deviation from the text book ways of doing things and to not criticize please. My approach is aimed at reinforcing dips while maintaining a soundstage. And you are one of the reasons I wrote that in there. ( see I was think about you buddy) Not everyone cars have perfect acoustics (in fact I donít know of any maybe you do)


And no not using sine waves to tune by ear, using sine waves to listen for cancellations, as you and I both know tuning or ďtuningĒ is not a process that is mathematical, it is also subjective and this process relies on your ears and common sense on what sounds better or worse.

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Default Re: All Pass Filters

Again everyone, this process has helped many cars Iíve tuned. It is in my humble opinion a excellent alternative to leaving a hard cancellation by doing nothing but eq and TA

There will be those people that think if itís not in a textbook or written by some manufacturer or some so called ďexpertĒ itís wrong.

The truth is there is no wrong. Itís what sounds better. If this process makes your car sound better , great. If it dosent , than it dosent . In no way am I saying this is scientific or the perfect way to use an all pass. But itís a way that has made many cars Iíve tuned sound better, even if I only get one all pass to work it still an improvement to my ears.
I do not care what any so called ďexpertsĒ or ďshop ownersĒ or ďmanufacturersĒ or anyone says or thinks , itís just a new thing to try. Maybe somebody will be jealous that they didnít think of it first. Maybe it sucks, I couldnít care less. But I took the time out of MY day to post this for you to try. If you want to try it great, if not , thatís your prerogative , just please do not criticize this process or find faults with it because itís not a perfect process and you wonít find it in any textbooks or online. Thanks

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Default Re: All Pass Filters

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... but are you really suggesting using sine waves to tune by ear?
FWIW, I've been using test tones for tuning midbass/sub for years and have had pretty good results. It's the only way I tune this region.

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Default Re: All Pass Filters

I appreciate the effort and time taken to present an alternative solution to a problem. I'll simply add this for consideration: The issue I've found with using the method described in this thread is that it can also detrimentally affect the areas outside of the target. So while you may fix (for example) a null at one band you create issues elsewhere. For sub and midbass where the bandpass window is smaller it may be less of an issue but with midranges where the bandpass window is over many octaves, it's been an issue in my experience.

Again, just something for others to keep in mind.

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Default Re: All Pass Filters

Quote:
Originally Posted by ErinH View Post
I appreciate the effort and time taken to present an alternative solution to a problem. I'll simply add this for consideration: The issue I've found with using the method described in this thread is that it can also detrimentally affect the areas outside of the target. So while you may fix (for example) a null at one band you create issues elsewhere. For sub and midbass where the bandpass window is smaller it may be less of an issue but with midranges where the bandpass window is over many octaves, it's been an issue in my experience.

Again, just something for others to keep in mind.

I completely agree and thank you Erin!

Yeah , and with anything the compromise and trade offs have to be worth it
And why this processes main focus is careful listening and consideration.
I usually listen one or two days and work on a single all pass and work it and ajust it until Iím happy or simply donít use it if itís not worth it .


But your totally right, it can cause issues while fixing others, thatís where youíre ears are boss and make the call ultimately.

Thanks again for chiming in :-)

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Old 3 Weeks Ago   #17
 
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Default Re: All Pass Filters

And to further what Erin was saying ,

Keep in mind an all passband is a intentional added group delay.
And that definitely can carry up the harmonic chain and mess things up

With extra emphasis on the word ďcanĒ

Watch your Q and make it the best trade off possible.

A 2nd order all pass center frequency is when the wavelength is 1/4 or 90 degrees apart on both the positive and negative sides of the filter and the filter ends at 360 degrees out of phase (which is back in phase) but added one cycle of delay. (Which can be bad or no big deal) a higher Q will make the shift in a much narrower band of the allpass but can definitely sound harsh and not flow with the signal and the speaker will exhibit it as a distortion. So mind the Q make it as high as needed to make sound better but try to keep it lower for better sonic performance.

Your acoustic behavior ďcanĒ be so bad from the get go that this process might be a total remedy to that problem area you just canít solve with eq and delay.

This process is still with all the potential negatives is still worth trying. Many times the comb filtering is so severe that just a single all pass in the right spot can make your system enjoyable so your not constantly ďtuningĒ trying to fix a problem with eq and delay that canít be fixed.

I hope some of you try it , Iím egar to hear what luck anyone has with it

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Old 3 Weeks Ago   #18
 
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Default Re: All Pass Filters

So hereís a circumstance in my car this process helped , in fact helps a great deal.

So as some of you know I use Dirac live , but I donít always use it and sometimes like to do my own tuning. So this has nothing to do with Dirac or any fir filters itís just plain old dsp work and my cars acoustic behavior.

In my car , with dash mount speakers (old install) or underdash horns and 6nd430s in the door with a 10NDL64 in sealed box under back seat firing forward. And a 15Ē sub

Out of all the combinations of pillar 6.5s or doors or horns up high or down low the car has always had a severe dip at 71hz and 145 weak as well and a huge peak at 225 and a dip at300 and a huge peak at 440 and a huge dip at 500 and a huge peak at 715 and a ginormous dip only on the left at 930 lasting to 1.2k and the right side a huge peak at 1.240khz lasting up to 1.5k ? Than things start to become even and eq starts to control things. Itís a bloody nightmare.

On the 1/3oactave scale 80 sucks and I have to cut 200, 400, 800 (approximately) by quite a bit. Super narrow Q cuts donít sound good the higher in frequency you go, so traditionally Iíve made my cuts with a Q in eq between 1.4 and 4 to get the system to sound its best in the midrange and using seperate left and right EQ kinda makes things much much worse. Because itís a comb-filter effect adding or subtracting energy with eq ruins left and right coherence.

So for me by adding an all pass on my sub at 71 (fairly high Q 13ish IIRC) makes the dip at 71 so much better and it makes my sub to midbass integrate so much better. I would date to call it a fix. Now with all the chaos in the 110-1.5k range it would be silly to add 10 or more all passes to every problem. I picked the problems that with the tone generator and with my desire for a better sounding 1khz region I picked the frequency on my tone generator where there was a complete cancellation 930hz. When I added that it made my 1k region come alive and I finally have smooth frequency response to my ears on both sides.

The problem at 140 I addressed by manipulating the crossovers delay on my back seat 10Ē midbass That are crossed at 210 using Linkwitz-Riley 48db slopes. For me I used rephase and linearized up to 24db of the rolloff delay to bring back the dip at 190 and caused the peak at 210 to cancel. But by doing that with careful listening, it made the back seat midbass sound like all the sound was coming from the front doors (using the hass principal)

It caused my vocal to become a tiny bit diffuse at 225. However when I addressed the huge dip at 500 by adding an all pass to the right door it made the 225 problem solved by adding .3ms more delay To left door. However the allpass at 500 caused a problem making 500 sound like its only coming from the right side. To me it sounded better than not at all and I used the sound coming from the right side to help reinforce my center and it works.

The overall downside to doing all this is my center is slightly more diffuse at 200-600hz but sonically the system sounds 100x better.

I was careful when I selected these all pass filters and listened to each one carefully

I wouldnít use one if you have a little dip I would only use one on the 80hz area no doubt , but the 250-1k range I would only use them if your car is a disaster of comb-filters and dips and peaks and problems and you just canít get the 630-1.2k range to sound smooth.
If your 800hz area especially just seems way too loud when eq to be flat with rest of system you might try this and see what it does.

I donít care if itís right or wrong this made my system sound better and I love a good sounding midrange that is smooth and dosent have massive dips and peaks everywhere.

My car was so bad and impossible to get to sound right and without enough power in my FIR filters to adress this kinds I felt like I was going to just sell my car and get a better sounding car. I tried this as a experiment and now my midrange and midbass finally sound good, it sounded good before but it sounds really good now. I could care less how it measures , it sounds killer and I enjoy my music.

I know if I ran some REW impulses I would find lots of issues, and that was me a year ago , now I just want it to sound really good and donít care how it measures as much as how it sounds. I definitely traded a problem for another but the trade off to me was soooo worth it .

I turned back on my Dirac room correction let it run its thing and than re engaged my all passes and some tiny tweaks to delays and levels and Iím super happy how it sounds.

And on Monday Iím building kicks for 8s (of course right when I get it sounding good just my luck)

But this whole experience prompted me to make this how to. At some point you just have to close the REW and put the mic down and just listen and make the final adjustments your ears have been wanting.

On the flip side , I done lots of customer cars that sounded so good with just EQ and TA , I did nothing else. So I would still say try it but only as a last resort to better SQ that you just canít get with the traditional tuning methods for your particular car and install

im so in love with my speakers I found a song I sing to them

https://youtu.be/VgU6LXY-AjI
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Old 2 Weeks Ago   #19
 
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Default Re: All Pass Filters

So I did some homework

If you have a dip that affects all the seats or the entire car not just one spot
You can use an all pass to correct it without worrying about making problems worse
(Thatís the short answer but basically is true)

If the dip only affects one spot than itís at your own risk and you more than likely will cause some other issue in the time domain that will affect frequency negatively

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Default Re: All Pass Filters

There will be those that say all pass is arbitrarily bad and will cause issues period

I say not so.

Itís like using seperate left and right eq , that creates a de-correlation filter more than it ďcorrectsĒ anything. You donít want to do seperate left and right eq to try to fix some issues

This method has you at least explore some of the time domain issues and if it helps , fantastic, if it makes it worse donít do it. But before going and doing a bunch of seperate left and right eq , the time domain should be looked at to see what is going on.

Proper analysis is a great way as well , this method letís you listen by ear what frequency dependent time domain issues are there by moving the frequency up and down

Maybe you donít know how to use REW or donít have all the measurement tools , with this you can at least hear what is a problem area and can do whatever is necessary to start to fix those issues by many means ......just to be clear .

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Default Re: All Pass Filters

Nice write up!

I did use an allpass filter in my old car between left and right to correct 1khz from pulling to the left mid while the rest off the frequenties where in the center, it did make the center much more solid and overall balance was better at least to my ears!
But as with all tuning options we have now days it's just one big balancing act to get it as good as possible...
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