[citysoundman]

All pass filter in the factory Subaru head unit. Leave the polarity alone and don't apply time alignment to the front speakers.

So I really don't know how an all pass filter works - never had any experience with it. Would you be so kind as to explain why you say to leave the polarity alone and don't apply TA to the fronts?

And is this filter on the front only, not the rears? Is this why the front right side is giving me negative polarity result?

sorry to bomb you with all the q's but I really want to understand what's up with my system

 

[citysoundman]

I found a related thread on all pass filters, with this comment that seems to describe just what I'm experiencing. @GotFrogs is this what's going on in my factory HU? I really want to know if the right side negative polarity I'm seeing is a problem with my installation (like a bad tap) or not. Thanks for your input!

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.

 

[tjk_bail]

I found a related thread on all pass filters, with this comment that seems to describe just what I'm experiencing. @GotFrogs is this what's going on in my factory HU? I really want to know if the right side negative polarity I'm seeing is a problem with my installation (like a bad tap) or not. Thanks for your input!

The above info most likely would be used on a Aftermarket HU that does not have any factory APF applied. I dont know how to resolve your issue, but there is a user that used to frequent this board often, but no longer is here much anymore. His is known as Skizer here. I'm pretty sure he can be found on CarAudioJunkies. He made a video awhile back showing how to find the AllPassFilter comming from a factory Amp, and how to fix it, you may find that video on the other forum... but not sure if its still there....

 

[sqshoestring]

Factory stuff. I worked on this old pontiac with the monsoon system factory, 2002 or something. They want bucks for the rear 6x9 so I put something else in, they are biamped. I rewire and tear the working tweets out of the old speaker and mount them too. It worked fine but what I found is only the tweeters had the left/right reversed in the rear. It was factory that way and its still that way lol.

 

[citysoundman]

So I have to figure out what to do...replacing the head unit is an expensive proposition. And my other options?

Perhaps there's some way to 'fix' the all pass filter issue...but I don't actually know what the filter is doing to the sound. This seems extremely complicated.

My audio is playing from my iPhone, I use Car Play and love the integration. Is there another way to get music to my amp, bypassing the HU, while Car Play is active? Like a DAC connected to the iPhone using a splitter or something like that? Then I can send the DAC outs to my amp line ins. This sounds a bit nutty huh.

 

[tjk_bail]

It is complicated for sure. If you do not have a DSP that has APF's capabilities, then you cannot fix it. Skiezer's video shows how to counter the effects of factory incoded APF's using his Helix DSP AllPass Filter options. So prolly will need to replace with an Aftermarket HU, or purchase a DSP.

You could just run your whole system off the rear channels from the HU..........Could run the entire system with or without a DSP, but without a DSP you would not have fade/volume control between front and rear speakers. Or dont use rear speakers at all..... ya see, all kinds of options....

My audio is playing from my iPhone, I use Car Play and love the integration. Is there another way to get music to my amp, bypassing the HU, while Car Play is active? Like a DAC connected to the iPhone using a splitter or something like that? Then I can send the DAC outs to my amp line ins. This sounds a bit nutty huh.

Iphone to DAC to Amp.... not that nutty... i've seen peeps on this forum do that....

 

[Stycker]

It is complicated for sure. If you do not have a DSP that has APF's capabilities, then you cannot fix it. Skiezer's video shows how to counter the effects of factory incoded APF's using his Helix DSP AllPass Filter options. So prolly will need to replace with an Aftermarket HU, or purchase a DSP.

You could just run your whole system off the rear channels from the HU..........Could run the entire system with or without a DSP, but without a DSP you would not have fade/volume control between front and rear speakers. Or dont use rear speakers at all..... ya see, all kinds of options....




Iphone to DAC to Amp.... not that nutty... i've seen peeps on this forum do that....

Phone>DAC>DSP works great. Then just select that phone on the Headunit under bluetooth to control the phone. You will need to control the volume through the DSP but all other functions can be performed through the headunit. This only works if your phone is connected to headunit thru bluetooth and digitally output to the DAC. This setup will not allow you to connect to the headunit by way of car play or android auto.

 

[citysoundman]

Iphone to DAC to Amp.... not that nutty... i've seen peeps on this forum do that....

Phone>DAC>DSP works great. Then just select that phone on the Headunit under bluetooth to control the phone. You will need to control the volume through the DSP but all other functions can be performed through the headunit. This only works if your phone is connected to headunit thru bluetooth and digitally output to the DAC. This setup will not allow you to connect to the headunit by way of car play or android auto.

Thanks, definitely something I'll look into.

Hey, does anyone know why the APF is there on the HU - is it to help correct phase problems for the front seat listening? And how would it effect the audio sound quality/frequency response? I assume it is separate from the HP filtering. Just trying to understand what to listen for if I try and RTA the system as-is.

 

[156546]

An all pass filter is a filter that affects phase and not amplitude. It can be a first order filter which is 90 degrees out of phase at the filter frequency, 0 degrees above that and 180 below. Or it can be a second order, which is 180 at the filter frequency, 0 degrees above and 350 degrees below.


There are many ways that these are used in factory systems, but most often they are used to provide some kind of imaging in both seats at the same time. Delay only works in one seat.

So, there's no inverting an all pass filter. If you can figure out what it is and where, you can put one on the other channel so the phase matches. But adding delay to an all pass filter isn't a good idea because that makes the all pass filter work poorly. If you reverse the polarity of the speakers, then you just reverse the in phase and out of phase areas of the response.

So, either you live with it or, in this car, you replace the head unit.

 

[dumdum]

The above info most likely would be used on a Aftermarket HU that does not have any factory APF applied. I dont know how to resolve your issue, but there is a user that used to frequent this board often, but no longer is here much anymore. His is known as Skizer here. I'm pretty sure he can be found on CarAudioJunkies. He made a video awhile back showing how to find the AllPassFilter comming from a factory Amp, and how to fix it, you may find that video on the other forum... but not sure if its still there....

You can’t fix it, you add a similar all pass to the opposite channel so they match… not ideal, but you can’t undo an all pass filter unless you can flatten the factory signal and dispose of it being made in the first place

 

[ckirocz28]

Thanks, definitely something I'll look into.

Hey, does anyone know why the APF is there on the HU - is it to help correct phase problems for the front seat listening? And how would it effect the audio sound quality/frequency response? I assume it is separate from the HP filtering. Just trying to understand what to listen for if I try and RTA the system as-is.

@GotFrogs is Andy Wehmeyer, the owner of AudioFrog, just so you know, formerly worked for JBL, he is an expert. Link to the Audiofrog blog where you can find lots of info about tuning.

Audiofrog Tech Blog - Audiofrog

Audiofrog tech blog

www.audiofrog.com

So, you have, built-in to your factory head unit, eq, all-pass eq, and probably some time alignment. There's your problem, the factory head unit, not the install.

 

[Thomasluke7899]

I just went through the same ****...I ended up with this

At first i tried running the factory outputs into a line out. From the line out into the kenwood with a rca to 3.5mm. But the more i tried to dick around with it the worst it got.
So i use a old s9 for the android auto with the factory head unit. Rear speakers still hooked up to it. I just added the kenwood and use my s20 to send an atpx? Bluetooth signal to the kenwood. From my understanding after changing the audio quality on the s20 to all the highest resolution its better than a cd. Sounds great and still get to use google ass for everything. So i dont have to touch the phones.

 

[citysoundman]

OK thanks for the explanations! Now I feel like I have a good understanding of what's going on.

So being that the all pass filter is only on the right side of the HU, is this why I'm getting a negative polarity there? (still hoping to get an answer to this mystery). FYI another 2019 Forester owner tells me his speakers are all positive polarity, although he uses a 'very old Realistic tester' for what that's worth.

He also mentioned something very interesting: this Kicker Keyloc as a possible solution for me. It supposedly analyzes the signal and corrects for all pass filters, up to 3 of them. I'm not a Kicker fan, and I don't want to add a device into the chain unnecessarily, but if it does what it says then it could be the answer. It would also give me a line out to send to my DSP.
From their website:
KEYLOC’s patent-pending KEY™ algorithm automatically detects and analyzes crossovers, EQ, time delay, and all-pass filters present, and then it will level-match the left and right audio channels. Once analyzed, it corrects these various filters to give you the smoothest frequency response possible, giving you a clean, uncolored audio signal for your aftermarket gear.

ALL-Pass Filter Defeat: The KEYLOC can correct up to three all-pass filters on one channel. The All-pass filters can have a Q ranging from 0.5–3.5, as long as they do not interact with the other All-Pass filter’s phase.

@GotFrogs is Andy Wehmeyer, the owner of AudioFrog, just so you know, formerly worked for JBL, he is an expert. Link to the Audiofrog blog where you can find lots of info about tuning.

Thanks for that! I've been on the AudioFrog site quite a bit lately Andy your tuning guide and blog are incredibly clear and informative. You've really laid out a great process, and it helps me to understand how to think about delay, crossovers and tuning both individually and as a whole. I'm especially appreciative of understanding how LR 4th order crossovers work to provide a flat response. I immediately set my crossovers this way!

 

[tjk_bail]

I have no experience with KIcker KeyLoc.... if it works that would be great! The downside would be that it does exactly what it says it will do, that is to flatten your entire EQ curve setup from factory, and leave you with a total flat response in your car... A total flat/response curve in your car would sound horrible, a LOT worse than it sounds now.

You may want to look into the JL FIX86, I believe it does the same stuff the KeyLoc does, but it has a 10 band EQ with it, the KeyLoc does not have EQ.... check it out.

JL Audio FiX™ 86 Digital Signal Processor

DSP for adding amps and speakers to a factory system — 6 outputs

www.crutchfield.com

 

[ckirocz28]

OK thanks for the explanations! Now I feel like I have a good understanding of what's going on.

So being that the all pass filter is only on the right side of the HU, is this why I'm getting a negative polarity there? (still hoping to get an answer to this mystery). FYI another 2019 Forester owner tells me his speakers are all positive polarity, although he uses a 'very old Realistic tester' for what that's worth.

He also mentioned something very interesting: this Kicker Keyloc as a possible solution for me. It supposedly analyzes the signal and corrects for all pass filters, up to 3 of them. I'm not a Kicker fan, and I don't want to add a device into the chain unnecessarily, but if it does what it says then it could be the answer. It would also give me a line out to send to my DSP.
From their website:
KEYLOC’s patent-pending KEY[emoji769] algorithm automatically detects and analyzes crossovers, EQ, time delay, and all-pass filters present, and then it will level-match the left and right audio channels. Once analyzed, it corrects these various filters to give you the smoothest frequency response possible, giving you a clean, uncolored audio signal for your aftermarket gear.

ALL-Pass Filter Defeat: The KEYLOC can correct up to three all-pass filters on one channel. The All-pass filters can have a Q ranging from 0.5–3.5, as long as they do not interact with the other All-Pass filter’s phase.


Thanks for that! I've been on the AudioFrog site quite a bit lately Andy your tuning guide and blog are incredibly clear and informative. You've really laid out a great process, and it helps me to understand how to think about delay, crossovers and tuning both individually and as a whole. I'm especially appreciative of understanding how LR 4th order crossovers work to provide a flat response. I immediately set my crossovers this way!

I didn't even think about the Kicker Keyloc, that may be your solution.
Yes, the all pass filter would be why one side measures reverse polarity. All pass filters change phase at a particular frequency, so it doesn't affect all frequencies the same.

 

[citysoundman]

The downside would be that it does exactly what it says it will do, that is to flatten your entire EQ curve setup from factory, and leave you with a total flat response in your car... A total flat/response curve in your car would sound horrible, a LOT worse than it sounds now.

I'd be going HU > Keyloc > AudioControl DSP so having a flat signal from Keyloc will be fine, it would be a starting point for me to RTA from. I could probably bypass the Keyloc EQ and just use the APF correction. To me, the downside is ending up with 2 stages of DSP which isn't great. And I'm a bit skeptical that it will work as a magic bullet, but it may be worth trying. On the plus side there are some excellent reviews on Crutchfield.

Yes, the all pass filter would be why one side measures reverse polarity. All pass filters change phase at a particular frequency, so it doesn't affect all frequencies the same.

OK thanks, so I guess the amount of phase reversal is enough to cause the negative result.

 

[citysoundman]

Just ordered the Keyloc. Crossing my fingers that it works!

 

[156546]

An all pass filter will not register as out of polarity on a polarity checker that uses the standard polarity pulse (looks like a right triangle). The phase of a first or second order all pass filter looks like the phase of a first order low pass filter (or high pass filter, for that matter. Same with second order. So, if your polarity checker works on channels with high or low pass filters, it works on channels with all pass filters, too. This is precisely why the polarity pulse is what it is.

So, the ALL PASS FILTER is not changing the polarity of the channel. The likely configuration here is that one channel is reversed and a first order all pass filter is used to put the bass back in phase between the two channels. The rest of the response is out of phase, which centers the vocal at and around 250 Hz or so and then the high frequencies are out of phase which adds a sense of space or width. This is pretty common these days. The other implementation is a second order all pass on the left channel c. 250Hz. Bose systems often use a series of allpass filters on both channels in the front and sometimes another series on the rear channels.

 

[citysoundman]

Thanks for the info Andy. It's a shame there's so much processing going on. Do you have any idea if the Kicker Keyloc will be able to negate the APF?

I'm an audio engineer and have mixed lots of music, TV and film projects. I've never come across an APF, so it's interesting to encounter it first in car audio. Looks like there's a few music production tools that use these, I saw an example of someone using it when combining two mics recording a snare drum, top and bottom. Using the filter on the bottom mic resulted in a better sounding sum. Which makes sense because multiple mics picking up the same source will have phase problems when summed if they're spaced less than 3 to 1 (second mic 3 times the distance of the first mic from the source).

There's a free plug-in Voxengo Correlometer that looks like it displays stereo multiband phase. I'm going to try viewing pink noise from the HU on this, and I'll be able to compare the front outs vs the rear outs to see how the phase compares. Actually, if this works I should be able to tell how well the Keyloc does what it's supposed to do. As a matter of fact, there are probably other phase meters I can use as well. Will look into this!

 

[Thomasluke7899]

An all pass filter will not register as out of polarity on a polarity checker that uses the standard polarity pulse (looks like a right triangle). The phase of a first or second order all pass filter looks like the phase of a first order low pass filter (or high pass filter, for that matter. Same with second order. So, if your polarity checker works on channels with high or low pass filters, it works on channels with all pass filters, too. This is precisely why the polarity pulse is what it is.

So, the ALL PASS FILTER is not changing the polarity of the channel. The likely configuration here is that one channel is reversed and a first order all pass filter is used to put the bass back in phase between the two channels. The rest of the response is out of phase, which centers the vocal at and around 250 Hz or so and then the high frequencies are out of phase which adds a sense of space or width. This is pretty common these days. The other implementation is a second order all pass on the left channel c. 250Hz. Bose systems often use a series of allpass filters on both channels in the front and sometimes another series on the rear channels.

How would one go about measuring this?
What...i dont even know...i just have more questions now.
You would need to look at phase and frequency response. Over time?
I dont even know the terminology i need right now....