# lets talk about allpass filters?



## Lycancatt (Apr 11, 2010)

I know how they work..basicly..and I know how they can help with the crossover points and improve dispursion in pro audio, but where do you guys see there effectiveness in a car? now that we have micro adjustable delays, are they redundant/just a different approach? I want to learn how to better use them as many new processors have them and I'd like to expand what I know how to do.


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## Lycancatt (Apr 11, 2010)

reading a bit via google, an allpass filter lets you correct phase issues introduced by the crossover, I knew that. it also can be used as a form of analog delay, I knew that too because I've used it on a crossover to aline subs and top speakers in pro audio. what I'd like to see from yall is how it helps in car audio? could I for example, introduce a lot of phase error by cutting an eq band very deeply, then pull it back into line with an allpass filter? in analog, eq phase shift is a big deal, but is it in digital?


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## oabeieo (Feb 22, 2015)

So , a biquad APF depending on its order will shift 
To either 90degrees or 180degrees

A 1st order APF will shift to 90deg 
2nd order 180deg

The Q of it will determine how fast it goes from 0 to its order (90-180)
So a Q of .5 will slowly move phase to its order where a Q of 15 would be an immediate brick wall type phase shift.

Cascading APF can get to 270deg and using delay on the other channel that does not have APF active can change the way it behaves against the two stereo channels.

The corner frequency is the frequency where the phase shift starts. Unlike a crossover that would be 3db down , the corner is approximated by lining up the two streight legs of the filter till they join. Hence "corner frequency " 

Moving phase on one driver can sometimes be of benefit, but usually causes some other unwanted issue. You can cascade APF together to move phase in one area and than move it again to -360 however especially in low frequency-360 is not the same a zero so a complicated delay on the other channel would be needed to augment that. In low frequency if delay ability is not tight enough it could indeed not be possible. Meaning most dsp uses delay in too widely spaced increments. Or not enough delay. Also cascaded APF will add ringing to the impulse as too many IIR ring backs on the same area would start to have an effect electrically . So a fir option is preferred, but in a pinch an APF can be extremely useful


The way the helix uses APF is brilliant by shifting in 11.5 degree increments , I am unsure how they do it , I would guess that they use a first order all pass filter , with a corner frequency at crossover and then use a complicated delay structure on the opposing channels to achieve the shift . Or it's not a biquad APF at all and some other phase control, don't know haven't studied it but it works fantastic!


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## oabeieo (Feb 22, 2015)

Lycancatt said:


> reading a bit via google, an allpass filter lets you correct phase issues introduced by the crossover, I knew that. it also can be used as a form of analog delay, I knew that too because I've used it on a crossover to aline subs and top speakers in pro audio. what I'd like to see from yall is how it helps in car audio? could I for example, introduce a lot of phase error by cutting an eq band very deeply, then pull it back into line with an allpass filter? in analog, eq phase shift is a big deal, but is it in digital?




Sort of......

An APF generally shifts in one direction and can not shift backwards (like a LPF-HPF are backwards of each other) it usually shifts in one direction. 

As far as corrections to a crossover slope , it would actually just make it worce on a LPF and could help an HPF but would also shift the entire passband as well, if both channels used one and a super complicated polarity flip and delay structure implementation on other drivers could help do just that. But god.....that's a lot of work and not sure how well it would sound when it's done. 


Yeah if in an arena where subs are quite some distance apart an APF could definitely be of some great help. In car.....would be problems. 

If anyone downloaded minidsp biquad maker you can play with the APF tab and 
Make APFs and see on the graph how it will affect phase on a frequency scale graph. That is especially helpful to know what it's going to do to the signal. 

I've had success using APF to move midbass at 250hz on with a 1st or 2nd order APF with a Q between .7-1.2 and get midbass to work on both sides of car. 

Or use an APF against front and rear midbass to help reinforce a cabin null at 70 he with some success .


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## Lycancatt (Apr 11, 2010)

cool, those were kind of the same uses I've been thinking of coming from my pro audio background.


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## Justin Zazzi (May 28, 2012)

A high pass and low pass crossover pair will sum together perfectly if they follow one of the textbook alignments (such as a Linkwitz-Riley 4th-order) and if they are part of a minimum phase system. This is a mathematical certainty.

However, there are some non-minimum-phase phenomenon inside a car (and in lots of other installations) where this rule does not apply. In those cases, and this is very frequency-dependent, you can use all-pass filters to make phase changes independent of frequency response changes to adjust areas that are behaving in a non-minimum-phase way.

You can also use APF to minimize the extreme group delay of certain higher-order speaker enclosures. For example the group delay of some ported enclosures and some bandpass enclosures are really high, but you can use an APF to balance this and still get all the benefits of the strange enclosure designs. It's a way to have your cake and eat it too, which is the dumbest saying ever, and I hate using it.


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## oabeieo (Feb 22, 2015)

Jazzi said:


> A high pass and low pass crossover pair will sum together perfectly if they follow one of the textbook alignments (such as a Linkwitz-Riley 4th-order) and if they are part of a minimum phase system. This is a mathematical certainty.
> 
> However, there are some non-minimum-phase phenomenon inside a car (and in lots of other installations) where this rule does not apply. In those cases, and this is very frequency-dependent, you can use all-pass filters to make phase changes independent of frequency response changes to adjust areas that are behaving in a non-minimum-phase way.
> 
> You can also use APF to minimize the extreme group delay of certain higher-order speaker enclosures. For example the group delay of some ported enclosures and some bandpass enclosures are really high, but you can use an APF to balance this and still get all the benefits of the strange enclosure designs. It's a way to have your cake and eat it too, which is the dumbest saying ever, and I hate using it.


Couldn't have said it better, 
THats exactly right. In a car those alignments often go into the window (pun on out the window  )


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## SkizeR (Apr 19, 2011)

sub'd and interested since i just got my dsp.2 up and running which features all pass filters


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## subterFUSE (Sep 21, 2009)

oabeieo said:


> The way the helix uses APF is brilliant by shifting in 11.5 degree increments , I am unsure how they do it , I would guess that they use a first order all pass filter , with a corner frequency at crossover and then use a complicated delay structure on the opposing channels to achieve the shift . Or it's not a biquad APF at all and some other phase control, don't know haven't studied it but it works fantastic!



Helix uses 2nd order all pass filters with variable corner frequency to do their phase adjustment by 11.5 degree increments.

The corner frequency is not user-definable. It is automatically selected by the software based on the high pass filter frequency and the amount of phase rotation desired.

Except for the subwoofer, where the frequency is based on the low pass frequency of the sub channel.


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## Babs (Jul 6, 2007)

I've pondered this myself with a weird phase dropout in midbass when timed up, at about 159hz. And I think they're out of phase at the bottom even though they may be timed up at the top. Seen it with two different sets of drivers. Have never been able to figure out how to combat it. Subterfuse saw this in my car as well as Erin. It causes havoc trying to phase up my subs as well. The APF was mentioned as a possible cure but since I'm only using REW and don't have Smaart or Systune running to see real-time phase, diagnosing what's going on has been beyond my skills. 


Sent from my iPhone using Tapatalk Pro


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## Lycancatt (Apr 11, 2010)

to add some randomness to this thread, I read somewhere that alppass filters can be used to create a center/mono output to send to a center channel speaker without doing actual 5.1 processing, this has me really curious! anyone have a clue what I'm talking about?


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## oabeieo (Feb 22, 2015)

subterFUSE said:


> Helix uses 2nd order all pass filters with variable corner frequency to do their phase adjustment by 11.5 degree increments.
> 
> The corner frequency is not user-definable. It is automatically selected by the software based on the high pass filter frequency and the amount of phase rotation desired.
> 
> Except for the subwoofer, where the frequency is based on the low pass frequency of the sub channel.



Where do you get this info from? (<-----not doubting you buddy) 

2nd order would shift to -180 at a rate depending on the Q
So it would have to change delay on other side to compensate 

Unless there's something I don't know.... I've been curious how they do that 11.5 shift through the entire passband ......

Any docs on this or its methodology?


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## oabeieo (Feb 22, 2015)

So maybe if it's HPF enabled only and they inserted the APF on the frequency where the crossover is down 168.5deg and flip the polarity it would sum to 11.5. 

That's a guess tho, would be very interested in exactly how.
If that were the case it would only work if HPF was enabled.

And if that is also the case throw crossover alignments out the window. 
But with a shift on passband the alignment would be screwed up regardless.
............


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## Orion525iT (Mar 6, 2011)

Jazzi said:


> You can also use APF to minimize the extreme group delay of certain higher-order speaker enclosures. For example the group delay of some ported enclosures and some bandpass enclosures are really high, but you can use an APF to balance this and still get all the benefits of the strange enclosure designs. It's a way to have your cake and eat it too, which is the dumbest saying ever, and I hate using it.


This what I am going to be dealing with in the future. I got my bandpass midbass installed and have given the system a quick test. The staging is interesting and holds promise, but I have a feeling the tuning and crossover selections are going to be a bit tricky. I did what I could to keep the phase shift of the boxes to a minimum, but I really have no idea how its going to work out in practice. I plan to use FIR, but something tells me I am going to need a heck of a lot of seat time and I will need to clear another level of the learning curve.


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## Justin Zazzi (May 28, 2012)

Orion525iT said:


> This what I am going to be dealing with in the future. I got my bandpass midbass installed and have given the system a quick test. The staging is interesting and holds promise, but I have a feeling the tuning and crossover selections are going to be a bit tricky. I did what I could to keep the phase shift of the boxes to a minimum, but I really have no idea how its going to work out in practice. I plan to use FIR, but something tells me I am going to need a heck of a lot of seat time and I will need to clear another level of the learning curve.


Heh, yeah, making the leap up to understanding how to use FIR/all-pass filters is a whole 'nother plateau. Have fun!


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## oabeieo (Feb 22, 2015)

Fir all pass works exactly the same as an IIR 

Fir is non recursive so it will sound better.


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## Rrrrolla (Nov 13, 2008)

A couple of years ago I used Re-Phase to undo crossover phase shifts. I recall it made an audible difference for the better. Snappier, would be the best description. I made no other changes, just re-recorded a few songs with the phase changes applied.


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## Patrick Bateman (Sep 11, 2006)

Lycancatt said:


> I know how they work..basicly..and I know how they can help with the crossover points and improve dispursion in pro audio, but where do you guys see there effectiveness in a car? now that we have micro adjustable delays, are they redundant/just a different approach? I want to learn how to better use them as many new processors have them and I'd like to expand what I know how to do.


As I understand it:

1) processors like miniDSP can insert a arbitrary delay to a single. For instance, a one millisecond delay.

2) processors like MiniDSP HD have FIR filters; they can adjust the delay to different levels depending on frequency. For instance, you could have a 1ms delay at 1000Hz and a 10ms delay at 100Hz.

3) processors like MiniDSP offer all pass filters. These give you the opportunity to have a filter with delay *but no affect on the frequency.* For instance, a first order filter will have a delay that gets larger and larger as frequencies get lower and lower. You can create an all-pass filter that has the same delay as a first order filter, *but with no effect on the frequency response.*


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## Patrick Bateman (Sep 11, 2006)

Rrrrolla said:


> A couple of years ago I used Re-Phase to undo crossover phase shifts. I recall it made an audible difference for the better. Snappier, would be the best description. I made no other changes, just re-recorded a few songs with the phase changes applied.


In my experience, fixing phase does three things:

1) intelligibility is improved a great deal. For instance, I don't listen to a lot of music, I mostly listen to podcast. (I'm a work-at-home software developer, I have a lot of time to listen to talk shows.) Conventional loudspeakers have a big phase shift at the crossover point, in the midrange. That phase shift screws up the intelligibility. Basically you have a scenario where the midrange is a full wavelength ahead of the tweeter. With a crossover point of 2000Hz, that's like having the midrange a full seven inches ahead of the tweeter. As you can imagine, that will make things unintelligible.

One of the things that I like about fixing the phase of a loudspeaker is that it improves the performance no matter how loud you listen. IE, people obsess about power handling and distortion, but that's mostly a factor if you're listening at high SPLs. Phase is an issue at all volume levels.

2) Percussion sounds better. This is because percussion has fundamentals that are played by the midrange, but harmonics that are played by the tweeter. As noted in point number one, high order filters create a scenario where the midrange can be a full wavelength out-of-phase with the tweeter. That creates issues with percussion.

3) Imaging is improved.


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## subterFUSE (Sep 21, 2009)

oabeieo said:


> Where do you get this info from? (<-----not doubting you buddy)


That's good, because doubting me is unwise. 

The info I gave on how the Helix phase adjust works is directly from the tuning manual on their website.

2nd order all pass with automatic adjusting corner frequency based on the HPF freq setting.

Consequently, they say to use the APF you must start with low drivers and work your way up to the high drivers, or else you'll go insane trying to get them to work.






Sent from my iPhone using Tapatalk


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## oabeieo (Feb 22, 2015)

subterFUSE said:


> That's good, because doubting me is unwise.
> 
> The info I gave on how the Helix phase adjust works is directly from the tuning manual on their website.
> 
> ...



Oh okay ,
I'm just curious as the nitty gritty as I want to make an fir with that type as part of it.


I've installed two helix pros , used the function on both installs with tremendous results, compared to if it didn't have that feature.

It's works much different than the minidsp version, very curious as to excactly how.

I emailed helix got a reply with no answer , and than they never answered me again.


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## Hammer1 (Jan 30, 2011)

I need to learn more about All Pass Filters since my Mosconi 6to8 Aerospace has them. Any links to read up on using them.


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## bnae38 (Oct 3, 2015)

Subd


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## phil r (Sep 25, 2014)

sub'd
yes please, a guide for an APF virgin with a Helix would be good

can't find anything about APF in the Helix tuning manual.....
thanks


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## Hammer1 (Jan 30, 2011)

https://timobeckmangeluid.wordpress.com/2016/10/24/once-upon-a-time/

Found some info on all pass filters worth reading


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## subterFUSE (Sep 21, 2009)

phil r said:


> sub'd
> yes please, a guide for an APF virgin with a Helix would be good
> 
> can't find anything about APF in the Helix tuning manual.....
> thanks


You need an advanced measurement software like Smaart or SysTune to be able to get APF right. You need live phase measurements.


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## bnae38 (Oct 3, 2015)

I started a similar thread called helix and apf, some info in there.

Can get a 1month trial for systune. I did, then pretty promptly got burned out on tuning lol.

Got my phase aligned though with midrange phase adjustment and a 2nd order apf at 160 on the rmb.

Doing some trial and error on overall tonality atm..


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## mojozoom (Feb 11, 2012)

Babs said:


> I've pondered this myself with a weird phase dropout in midbass when timed up, at about 159hz. And I think they're out of phase at the bottom even though they may be timed up at the top. Seen it with two different sets of drivers. Have never been able to figure out how to combat it. Subterfuse saw this in my car as well as Erin. It causes havoc trying to phase up my subs as well. The APF was mentioned as a possible cure but since I'm only using REW and don't have Smaart or Systune running to see real-time phase, diagnosing what's going on has been beyond my skills.


I'd expect most cars with a console and 6.5" door mounted mids would need a 2nd order all-pass filter at about 160 Hz center freq with a Q of about 5 applied to the right mid to get it near phase alignment with the left mid. It's sad that you have to mess up the right side phase response as it's actually the better of the two, but you can't go back in time on the left side, so all you can do is add delay to the phase of the corresponding frequencies on the right side.


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## opekone (Mar 24, 2020)

Hammer1 said:


> Once upon a time
> 
> Found some info on all pass filters worth reading


He has a nice youtube channel where he doesn't talk but simply makes maniupulations in smaart. I find many of his videos quite elucidating.



Is it clear to anyone how many mics and where they are placed in the second to last video, "once upon a time 3"


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## opekone (Mar 24, 2020)

subterFUSE said:


> You need an advanced measurement software like Smaart or SysTune to be able to get APF right. You need live phase measurements.


How do you determine when the phase response your mic is reading is garbage and when it's not? I assume you're talking about using a single mic at listening position?


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