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Discussion Starter · #1 ·
Hey Guys

So can someone explain to me in simple language what an all pass filter is and when and how do you use it?

Can you measure the before and after effects of this filter with a RTA?

Thanks Guys


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Put simply, allpass filters are EQ for phase. They don't impact the frequency response like a regular EQ filter, they only impact the phase at the frequency and for the Q specified by the allpass filter.

The effects of an allpass filter can be measured - they can be measured directly if you measure the phase of the individual speakers - or "indirectly" by measuring the frequency response when multiple speakers are playing. Measuring a single speaker will not show the effects of an allpass filter, since phase changes only impact the frequency response when multiple speakers are playing together.

For example... Here are some old pictures of my L+R midbass response before and after I applied allpass filters.

Here are the "Before allpass filters" measurements (Blue=Left, Red=Right, Green=L+R combined) - see the big dips in the green L+R combined measurement that don't exist in the L and R individual measurements?


Now here are the same measurements after I applied a few allpass filters to make the phase match (where the dips are) between L and R - see how the big dips are completely gone in the L+R combined response?


That is what allpass filters can do for you.

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I’m trying to do some research to get my head around them as well, two spots I’ve found some helpful info was JL audios training session on their tuning software, and Helix had a basic overview on their software guides


 

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Put simply, allpass filters are EQ for phase. They don't impact the frequency response like a regular EQ filter, they only impact the phase at the frequency and for the Q specified by the allpass filter.

The effects of an allpass filter can be measured - they can be measured directly if you measure the phase of the individual speakers - or "indirectly" by measuring the frequency response when multiple speakers are playing. Measuring a single speaker will not show the effects of an allpass filter, since phase changes only impact the frequency response when multiple speakers are playing together.

For example... Here are some old pictures of my L+R midbass response before and after I applied allpass filters.

Here are the "Before allpass filters" measurements (Blue=Left, Red=Right, Green=L+R combined) - see the big dips in the green L+R combined measurement that don't exist in the L and R individual measurements?


Now here are the same measurements after I applied a few allpass filters to make the phase match (where the dips are) between L and R - see how the big dips are completely gone in the L+R combined response?


That is what allpass filters can do for you.

Sent from my ASUS Chromebook Flip C302 using Tapatalk
Nice. So, allpass filters can be configured to target certain frequencies . I did not know that.
Is there a maximum number of allpass filters that can be used at one time ?
 

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All pass filters work in the same place where boost doesn’t generally… , this often means a dip caused by a reflection or standing wave… and the eq doesn’t work so it means the reflection cancels any extra energy put in, this also effectively means there is a delayed energy that cancels the direct energy

all pass filters don’t correct this at all, what they do do is make the opposite side to the reflection phase error side have an induced phase wrap so it then matches the bad sides delay, effectively you make the good side bad but in doing that the phase then matches and sums correctly despite having a small dip

Sometimes even if you can correct a dip but it still doesn’t sum 100% you can correct the summation with an all pass filter, I have never used a first order filter to achieve this, but second order, generally higher Q all pass filters are the ones that get used
 

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Nice. So, allpass filters can be configured to target certain frequencies . I did not know that.
Is there a maximum number of allpass filters that can be used at one time ?
It really depends on the location of the drivers, I have had two phase wraps on a well placed midrange between 160 and 2.5khz, I have had 6 or 7… you don’t need to try and correct all of them, I’ve also had phase issues which were so close together that two all pass filters at the same freq cured the summation odd as that may seem

Don’t even try to fathom reflections on tweeters 😂
 

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just a heads up, you should not be throwing all pass filters at every dip you see on an RTA
Care to elaborate at all? When/why shouldn't you use allpass filters to correct multi-speaker summation issues (whether it be L+R combined or at crossovers)?

Personally, I mainly use them on my midbass and on the lower frequency areas of my widebands, if/when needed - and only if the multi-speaker response actually dips lower than the individual responses (which from my understanding, is clear evidence that there is a significant phase mismatch causing that dip). I have also used them to correct phase-related dips at crossovers as well.

I'm trying to understand why you wouldn't want to address these areas of poor summation? Why would you want to leave a significant dip there if it can easily be corrected with an allpass filter?

In my experience, where it can get tricky is when using allpass filters to address "L+R combined" phase-related dips near crossover areas. You can use an allpass filter to address the dip in the L+R combined response, only to find that it caused a new dip at the crossover. :) However, I can usually find the right combination of allpass filters to get everything summing correctly for both the L+R combined response and at crossover areas.

Personally, I haven't noticed any "cons" when fixing these kinds of dips with 2nd order allpass filters... The measured response clearly shows that I'm getting better summation - and I don't hear any "side-effects" when actually listening to the system, so I don't see why you wouldn't want to address the phase-related dips.
 

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Summation isn’t everything, if the delay that’s added on the bad side is moving the centre one way caused by a dominant reflection , and you add an all pass on the good side so it matches and sums, it’s very possible to push the centre further away, we can only add delay with all pass filters (how much is freq dependant, the higher the all pass the less delay it adds… a second order adds one wavelengths delay) so a very small amount at 1.6khz vs 160hz)

every effect has a side effect whether it’s desired or not is another matter 👍🏼
 

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I might get abuse for this, but - all-pass filters are not really for amplitude ("RTA") issues. Those dips on the graph above would not really be noticeable in themselves.

All-pass is for coherence, imaging, soundstage. Amplitude dips are a good way to see the problem, but they are not the problem that you are trying to fix. You are trying to keep phase the same L to R, and through a crossover region. This is best done by measuring phase. I struggle with OpenSoundMeter for hitting a target curve, but that is the type of tool (or a more serious/paid for alternative) that is best for setting up all-pass.
 

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Summation isn’t everything, if the delay that’s added on the bad side is moving the centre one way caused by a dominant reflection , and you add an all pass on the good side so it matches and sums, it’s very possible to push the centre further away, we can only add delay with all pass filters (how much is freq dependant, the higher the all pass the less delay it adds… a second order adds one wavelengths delay) so a very small amount at 1.6khz vs 160hz)

every effect has a side effect whether it’s desired or not is another matter
So in my midbass example above, you would simply leave the phase-related dips there? Or would it just depend on whether you hear any "bad" side-effects after using allpass filters to fix them?

Obviously, I listen to the system both before and after I make changes. If I hear anything "off" afterwards, I wouldn't leave it in place. However, that hasn't been my experience - at least with my system. I have not yet noticed any "bad" side effects from allpass filters that I've used - but I do get better summation.

In some cases, the dips that are corrected aren't necessarily easily audible via your ears - but that is kind of the reason why we use RTA's to begin with - to identify and correct issues to a point that is just not possible using your ears alone. Otherwise, we wouldn't use an RTA at all - we'd just tune everything by ear. :) However, all of those little "imperfections" add up and may be audible in certain situations. As long as they don't cause any audible (non-wanted) side-effects, I see no reason why wouldn't want to get the phase to match between L and R and get as much summation as you can.


I might get abuse for this, but - all-pass filters are not really for amplitude ("RTA") issues. Those dips on the graph above would not really be noticeable in themselves.

All-pass is for coherence, imaging, soundstage. Amplitude dips are a good way to see the problem, but they are not the problem that you are trying to fix. You are trying to keep phase the same L to R, and through a crossover region. This is best done by measuring phase. I struggle with OpenSoundMeter for hitting a target curve, but that is the type of tool (or a more serious/paid for alternative) that is best for setting up all-pass.
I completely understand that you are better off actually measuring phase directly. However, as you stated - those dips are just another way to identify phase issues. Using an RTA to identify phase issues like this may not be as accurate as actually measuring the phase, but why not make the phase match between L and R and get rid of those dips if you can (as long as there are no audible "bad" side effects)? In my case (and probably lots of others), I really don't have the correct equipment to properly measure phase and correct phase 100% - but in cases like this, it's pretty obvious that these dips are phase-induced. I am simply using the tools that I have to identify and fix what issues that I can. If you can use an allpass filter and get the L+R responses to sum better, without any audible side-effects, that seems like a "win" to me. :)

That is kind of like saying that you shouldn't fix an issue with your car because a mechanic can fix it better/faster with better tools. At the end of the day, if you can fix the summation without any bad "side effects", then why not? Sometimes, you use what you have and do what you can with what you have. :)
 

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I completely understand that you are better off actually measuring phase directly. However, as you stated - those dips are just another way to identify phase issues. Using an RTA to identify phase issues like this may not be as accurate as actually measuring the phase, but why not make the phase match between L and R and get rid of those dips if you can (as long as there are no audible "bad" side effects)? In my case (and probably lots of others), I really don't have the correct equipment to properly measure phase and correct phase 100% - but in cases like this, it's pretty obvious that these dips are phase-induced. I am simply using the tools that I have to identify and fix what issues that I can. If you can use an allpass filter and get the L+R responses to sum better, without any audible side-effects, that seems like a "win" to me. :)

That is kind of like saying that you shouldn't fix an issue with your car because a mechanic can fix it better/faster with better tools. At the end of the day, if you can fix the summation without any bad "side effects", then why not? Sometimes, you use what you have and do what you can with what you have. :)
If you see a phase related dip on the RTA, sure, try with an all-pass filter to remove it.

But, don't just then decide it's OK based on a smoother RTA. Listen, particularly for spatial issues. It may be that a couple of hundred Hz further up, you create a different phase issue, that isn't so obvious on an RTA. So yes, you may improve phase over a couple of hundred Hz window, but mess it all up everywhere else.

It is also going to be a bit of a nightmare knowing what to do without direct phase measurement. On my 3 way fronts, I have a steep phase inversion on one midbass, well into its crossover frequency. I have actually had to apply all-pass on all 5 other drivers, just to accommodate that. I don't know how you could work that out with just RTA.
 

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If you see a phase related dip on the RTA, sure, try with an all-pass filter to remove it.

But, don't just then decide it's OK based on a smoother RTA. Listen, particularly for spatial issues. It may be that a couple of hundred Hz further up, you create a different phase issue, that isn't so obvious on an RTA. So yes, you may improve phase over a couple of hundred Hz window, but mess it all up everywhere else.

It is also going to be a bit of a nightmare knowing what to do without direct phase measurement. On my 3 way fronts, I have a steep phase inversion on one midbass, well into its crossover frequency. I have actually had to apply all-pass on all 5 other drivers, just to accommodate that. I don't know how you could work that out with just RTA.
Ok, but they why wouldn't the phase issue that you created a couple of hundred Hz further up NOT show up as a phase issue on the RTA, if it really was a significant phase issue? I mean if two drivers are that far out of phase at that location, it should show as a dip on the combined L+R response, right?

To me, significant phase issues in the L+R combined response - those that actually dip lower than the individual measurements themselves - are obvious phase issues that can usually (in my experience) be corrected relatively easily with an allpass filter - without any ill side effects. I've always used high-q, 2nd-order allpass filters that only impact the phase over a very narrow range. I'm sure that there are situation where it's more complicated than that, but in my experience, the phase-induced dips have been very easy to resolve with any ill side effects. Maybe I've just been lucky - who knows. I can only go by my experiences.

Now of course you have to listen afterwards. I'm not saying to blindly throw allpass filters everywhere and leave them in place even if they do something horrible to the rest of the system - just to solve a narrow dip. :) But if I can correct a 5dB dip easily with an allpass filter, I don't see why I wouldn't.

Please don't take this as an "argument" - it's an interesting discussion and I think it's worth having. One of these days, I'll invest in the gear to properly analyze phase, but between a good XLR MIC, a decent multi-channel USB sound card and the software to use it properly, it's an expense. Although, now that we have OpenSound Meter, that makes it a lot more feasible (vs needing Smaart).
 

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So you guys are talking about measuring phase using mics/acoustics, could you also measure phase on the electronics side? For example, using a scope and measure input vs. output and comparing the phase difference there? Or is that different? I mean i know it is different, but how different, relative to this conversation?
 

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So you guys are talking about measuring phase using mics/acoustics, could you also measure phase on the electronics side? For example, using a scope and measure input vs. output and comparing the phase difference there? Or is that different? I mean i know it is different, but how different, relative to this conversation?
At least with boxes that have group delay, the accoustic phase lags the electrical phase.
And many speaker dips in amplitude have phase shifts happening there.
So it is usually best to measure the total ensemble effect.

On the crossover regions one could make a case that fixing the phase on the electrical side might make sense, before confirming it acoustically with a measurement.

Once one goes through the mic, then it sort of confirming it electrically... just electrically after the speaker ;)
 

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@jtrosky , I’ll take a stab at post #2 and post #8. Basically those small anomalies in the response you see, you probably can’t actually hear those. Many of the dips and comb filtering peaks and valleys you see on an RTA with a resolution of say more than 1/3 octave, and I would say definitely not more than 1/6 octave; you just cant hear those so don’t waste the DSP resolution and time to try and correct those.

This has been my experience anyway so often I tune using 1/3 octave and sometimes I flip back and forth from 1/6 to 1/3 for the lower octave FR response just to gain some additional insight at the larger dips and peaks. I think this is a trap that many people fall into when tuning and they fuss too much over that small stuff when it may not actually matter.
 
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Now here are the same measurements after I applied a few allpass filters to make the phase match (where the dips are) between L and R - see how the big dips are completely gone in the L+R combined response?


That is what allpass filters can do for you.

Sent from my ASUS Chromebook Flip C302 using Tapatalk
Did you apply the APF on the left Mid-base or Right Mid-base to make the above corrections? Dumdum mentioned that there is a 'bad' side and a 'good' side, so how does one determine which side to apply the APF?
 

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Discussion Starter · #19 ·
Put simply, allpass filters are EQ for phase. They don't impact the frequency response like a regular EQ filter, they only impact the phase at the frequency and for the Q specified by the allpass filter.

The effects of an allpass filter can be measured - they can be measured directly if you measure the phase of the individual speakers - or "indirectly" by measuring the frequency response when multiple speakers are playing. Measuring a single speaker will not show the effects of an allpass filter, since phase changes only impact the frequency response when multiple speakers are playing together.

For example... Here are some old pictures of my L+R midbass response before and after I applied allpass filters.

Here are the "Before allpass filters" measurements (Blue=Left, Red=Right, Green=L+R combined) - see the big dips in the green L+R combined measurement that don't exist in the L and R individual measurements?


Now here are the same measurements after I applied a few allpass filters to make the phase match (where the dips are) between L and R - see how the big dips are completely gone in the L+R combined response?


That is what allpass filters can do for you.

Sent from my ASUS Chromebook Flip C302 using Tapatalk
Great thanks - so you applied a all pass filter at your crossover points?

So for instance you crossed your mids at say 250hz do you apply the filter at 250hz?


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Discussion Starter · #20 ·
Did you apply the APF on the left Mid-base or Right Mid-base to make the above corrections? Dumdum mentioned that there is a 'bad' side and a 'good' side, so how does one determine which side to apply the APF?
Good point - I would also like to know...

So you don’t apply the filter on the combined graph? Only one side?


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