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I keep hearing about allpass filters but, what are Allpass filters, what are they used for & etc?
 

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They delay a certain frequency/frequencies, usually used at crossovers to get them back in phase. Most of them are in orders (1st order, 2nd and so on [1st order being 90 degrees, 2nd 180 and so on]). You’ll have to research phase to get a decent understanding of them
 

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Iirc this is a good thread that might help.


Sent from my SM-G960U1 using Tapatalk
 

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They are the root of evil for Toyota/Lexus owners. And I'm sure other manufactures as well. From my minimal understanding, they add these filters to help promote a two seat tune environment. So both driver and passenger can enjoy the stock crappyness. Additionally, the actual filter variations vary depending on the vehicle and it's size.

Look to remedy this prior to any gear searching or buying. How will you capture, isolate, and resolve in a dsp. Or the best route if possible just do away with the stock head unit.
 

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how can one tell if their system has these employed? do you need to test & graph phase v.s. frequency plot for each speaker?
 

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how can one tell if their system has these employed? do you need to test & graph phase v.s. frequency plot for each speaker?
Correct. We used a mini DSP mic and ran pink noise through the system. And REW showed a giant dip where the factory applied the all pass. That is generally a good starting point to begin correcting it.

I wonder how many folks are out there who have this issue and might not even know it. After it’s corrected is generally when all the gear really shines.
 

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how can one tell if their system has these employed? do you need to test & graph phase v.s. frequency plot for each speaker?
If you have a helix dsp with input analyser you can see these by playing left/right channels alone and then summing them and seeing if they sum with 6db or if you have areas with less summation… where there is less summation that’s where your all pass is 👍🏼
 

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neat! i personally do not have any acoustic measuring equipment yet.
i meant electrical measurements of phase, not acoustic measurements.

https ://en.wikipedia.org/wiki/Bode_plot

ok, thanks!
 

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i bet the acoustic measurement method is a lot faster :)
would you play pink noise through one speaker at a time then?

Correct. We used a mini DSP mic and ran pink noise through the system. And REW showed a giant dip where the factory applied the all pass. That is generally a good starting point to begin correcting it.

I wonder how many folks are out there who have this issue and might not even know it. After it’s corrected is generally when all the gear really shines.
 

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i bet the acoustic measurement method is a lot faster :)
would you play pink noise through one speaker at a time then?
I think he did a few sweeps. Firs the whole system, then speaker by speaker to compare results.

And @dumdum is absolutely correct, the helix analyzer is awesome to figure this out.
 

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interesting, thx! so if i played pink noise one speaker at a time, i would see "unnatural" dips at frequencies where there was all pass filtering.
 

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interesting, thx! so if i played pink noise one speaker at a time, i would see "unnatural" dips at frequencies where there was all pass filtering.
No, you won’t see any dips where all pass filters are, they effect phase and not the amplitude
 

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you would see dips in magnitude if you play multiple speakers with pink noise?
 

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When speakers play in phase, (like singing in harmony) it will be a smooth response. When speakers play out of phase together it will present itself as response anomalies.

1 singer by himself would sound normal, but 2 together you notice if they aren't singing in harmony

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ok, i'm going to stop asking questions here, i'll try to read up on those other threads & get more knowledgeable.
i was trying to ask how to check if all-pass filters are used in your oem vehicle sound system. 🤷‍♂️
i don't follow people comments on the measuring for all pass filters acoustically, some comments seems contradictory. but again, i am not knowledgeable , so what do i know.

if they are just phase adjustments, you should be able to measure phase electrically, not acoustically and plot phase out for each speaker.
this would take some doing, but should work i would imagine.
 

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They are the root of evil for Toyota/Lexus owners. And I'm sure other manufactures as well. From my minimal understanding, they add these filters to help promote a two seat tune environment. So both driver and passenger can enjoy the stock crappyness. Additionally, the actual filter variations vary depending on the vehicle and it's size.

Look to remedy this prior to any gear searching or buying. How will you capture, isolate, and resolve in a dsp. Or the best route if possible just do away with the stock head unit.
they are easy to deal with in toyota/lexus. Bose systems is where theyre a real *****
 

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ok, i'm going to stop asking questions here, i'll try to read up on those other threads & get more knowledgeable.
i was trying to ask how to check if all-pass filters are used in your oem vehicle sound system. 🤷‍♂️
i don't follow people comments on the measuring for all pass filters acoustically, some comments seems contradictory. but again, i am not knowledgeable , so what do i know.

if they are just phase adjustments, you should be able to measure phase electrically, not acoustically and plot phase out for each speaker.
this would take some doing, but should work i would imagine.
if you like, feel free to pm me we can discuss in depth. Right or wrong I can only share my experience and what I saw after the REW sweep. And I know for a fact we saw a massive dip at certain frequencies. Hence adding the all pass to the DSP to present a flatter output.
 
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