I just installed a sub I built, and now I'm in the process of properly tuning my system (active 2-way and rear fills using AudioControl D-6.1200 amp/processor). I'm doing a polarity check, and I'm getting a negative phase result on the right side of both the tweeter and midbass, but, I know that it sounds in phase so I 'm not sure what's going on.

I'm using AudioControl's USB mic and Mobile Tools app - it has a polarity checker, and I'm playing the speaker pop signal and checking for a + or - result. I'm isolating each speaker when doing the test. All RTA EQ is bypassed and there's no time delay.

What I believe it is, is a problem with the wiring on the head unit front outs, not a speaker wiring problem. I can tell because on my processor I can select HU front or rear signals to send to the speakers. When I select HU rear, the polarity check results are + on all 6 speakers. When I select HU front, the left side is + and the right side is - on all speaker pairs. But I've done some very careful listening with recorded phase tests (voice/pink noise in phase then out of phase) and it is in phase acoustically. So if I reverse + and - wires on the amp input for the HU front right side, it then shows + polarity test result but sounds out of phase on all pairs. (I'm checking each pair of speakers individually when listening for phase).

It just seems so weird to have the test result show negative polarity but it sounds in phase. [emoji53]

Where is the mic positioned when doing this? You should check it right at the speaker, as close as you can get it.

 

Mic is right up on each grill when I test, practically touching. And levels on the app are in the 'good' range. But since I'm getting the same negative result on the right sides of all 3 pairs I don't see it having to do with mic placement.

 

MIc is right up on each grill when I test, practically touching. And levels on the app are in the 'good' range. But since I'm getting the same negative result on the right sides of all 3 pairs I don't see it having to do with mic placement.

Had to make sure, some people try measuring from the driver's seat. That is perplexing that it sounds in phase but measures out of phase.

 

Thanks for the reply. This is my first time doing a polarity test like this. I really don't know how it works - how does the speaker pop signal tell the app whether it's + or - polarity? Is there room for error?

I feel like I might need to verify the head unit wiring to the amp by getting to where the wiring is tapped (behind one of the kick panels), and buzzing it out with my DMM. Hopefully I can tell which wire is + and - on the HU.

 

The pop should be half of a square wave, like only the positive polarity half. The physics of a speaker will round off that wave and cause some residual oscillation that produces a negative wave also (much lower amplitude), probably a few more full waves at even lower amplitude. There is definitely room for error, but it's usually caused by distance from the speaker.

 

I just installed a sub I built, and now I'm in the process of properly tuning my system (active 2-way and rear fills using AudioControl D-6.1200 amp/processor). I'm doing a polarity check, and I'm getting a negative phase result on the right side of both the tweeter and midbass, but, I know that it sounds in phase so I 'm not sure what's going on.

I'm using AudioControl's iOS calibration mic and Mobile Tools app - it has a polarity checker, and I'm playing the speaker pop signal and checking for a + or - result. I'm isolating each speaker when doing the test. All RTA EQ is bypassed and there's no time delay.

What I believe it is, is a problem with the wiring on the head unit front outs, not a speaker wiring problem. I can tell because on my processor I can select HU front or rear signals to send to the speakers. When I select HU rear, the polarity check results are + on all 6 speakers. When I select HU front, the left side is + and the right side is - on all speaker pairs. But I've done some very careful listening with recorded phase tests (voice/pink noise in phase then out of phase) and it is in phase acoustically. So if I reverse + and - wires on the amp input for the HU front right side, it then shows + polarity test result but sounds out of phase on all pairs. (I'm checking each pair of speakers individually when listening for phase).

It just seems so weird to have the test result show negative polarity but it sounds in phase. [emoji53]

Is this a factory head unit? If so, there could be some time alignment or eq applied there.

 

Yeah it's factory HU. What do you think - just leave it be since I'm pretty sure it sounds correct? Or does this negative test mean there's a problem somewhere that I should track down?

 

Any way to send the pop sound through your system other than the Head-Unit? (by-pass the HU)

Try a different speaker if you have one laying around somewhere.....

 

Yeah it's factory HU. What do you think - just leave it be since I'm pretty sure it sounds correct? Or does this negative test mean there's a problem somewhere that I should track down?

Any way to send the pop sound through your system other than the Head-Unit? (by-pass the HU)

Try a different speaker if you have one laying around somewhere.....

That's what you should do.^^

 

So when I was chasing / checking my phase I found it very useful to play a tone at the xover point, then invert channels in the DSP to see if the relative level climbs or falls.

Also what crossover slopes are you running? 12 dB slope will throw one driver 180 degrees out from the other where it's being attenuated (well one 90 head, one 90 behind)

 

So I bypassed the HU and wow does it sound different!

Good news is all the speakers show + polarity so that's great to know the wiring is correct.
So the HU front right out is no good? Something is definitely messed up here.

I was able to A/B the same song playing on my HU and iPhone for a very good comparison. I'm using my iPhone 12 with a Lightning to 3.5" stereo adapter patched to my amp line ins. Definitely some EQ/filters going on in the HU. There is SO much more bass going direct it is nuts. The mids sound much smoother. The HU sounds tinnier for sure. The high end isn't terribly different, but brighter from the HU.

Well this opens up an unwelcome can of worms! Looks like I'm going to need to replace the HU

Also what crossover slopes are you running?

I'm using 24dB LR slopes for all the crossovers. 12dB LR for the very top and bottom.

 

Thats cool, sounds like you got it figured out. It's looking like a bad HU...

 

Thinking I should check the wiring to be sure. My amp installation was done by a shop and they tapped into the HU wiring to connect cables that run to the amp in the rear cargo area. Maybe there's a bad splice or something. Since I know the problem is on the right output it shouldn't be too difficult to try and troubleshoot it....I hope!

But clearly the HU just doesn't sound good. It only has high level speaker outs as well. So if I want to get a replacement HU (with line outs), how do you check to see if there's any internal EQ or filter that would cause a similar problem?

 

Thinking I should check the wiring to be sure. My amp installation was done by a shop and they tapped into the HU wiring to connect cables that run to the amp in the rear cargo area. Maybe there's a bad splice or something. Since I know the problem is on the right output it shouldn't be too difficult to try and troubleshoot it....I hope!

My initial thoughts after I read your original post was that it sounded like a crossed wire somewhere on your output. I deff recommend double checking the wiring from HU to Amp !

[/QUOTE] But clearly the HU just doesn't sound good. It only has high level speaker outs as well. So if I want to get a replacement HU (with line outs), how do you check to see if there's any internal EQ or filter that would cause a similar problem?
[/QUOTE]

Aftermarket HU do not have any factory EQ'ing, filters or timing that is hardcoded or preset, unless its a select-able mode to choose from kind of thing. Its usually the Factory Amps that have all that junk preset(and no way to adjust)... So no worries in regards to Aftermarket HU, and you can always do the 'Default Reset' to ensure you're starting with a clean slate.

 

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

 

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.

 

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....

 

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.

 

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.

 

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....

 

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.

 

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.

 

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

 

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.

 

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

 

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....

 

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!

 

Measuring correlation between a reference and the output of the amp is one way to view this, but you'll need to delay the source so that it aligns with the output of the amp. You could also do this by recording the source and the output of the radio in your DAW. Then, if you need to align the beginning of the waveform, you can do it easily there. Correlation will also be affected by the phase changes that attend the frequency response EQ. You can eliminate that by equalizing the response of the radio's output in the channel strip for that. Then, you'd have a pretty good representation of the difference in terms of phase.


You can also record the left and right output of the radio into the DAW and if the EQ is the same, mix them down to a single track and the areas in which the phase is not the same will result in a frequency response difference. You can do this in an RTA program that will display the sum of left and right. www.ymec.com is a good place to get this--DSSF3 also has a correlation and cross correlation function as well as a phase display.

I think that for mixing and mastering another use of an APF might be to separate a vocal or an instrument to the left and right instead of in the center--using this instead of delay might work better because you'd be able to shift the phase of everything above or below some frequency by the same amount.

 

So I just tried the Voxengo correlation meter, and it's showing phase relationships on a multiband level - very cool! I'm using it in Pro Tools. I can set it for 31 bands, although it can go up to 64 bands as seen below.
In phase audio shows meters at the very top of the scale

Out of phase shows meters at the very bottom

Here's a screenshot of uncorrelated pink noise

and here's a bit of movie of the same

 

This is cool. I routinely mock up stuff in Audiomulch with VSTs when I teach seminars or when I need a graphic to explain something. This will be really useful. Thanks for the find. Of course, explaining correlation and the way it's displayed will be another exercise.

 

Now I'm wondering why the uncorrelated pink noise has much more phase difference in the low frequencies

 

I recorded mono pink noise thru my HU, line out of my AudioControl amp into Pro Tools. Using the Voxengo phase meter you can clearly see the phasing at specific frequencies. So the results make things much easier to understand - although it's a bit dejecting seeing how messed up the sound is! I was (sort of) surprised to learn it looks like there's an all pass filter on the rear outs as well

First, here's what the front outs look like. Most of the spectrum is fully out of phase, with 200Hz and 800Hz close to in phase, but not fully in phase.

And here's the rear outs. Most of the spectrum is fully in phase, except 150 - 400Hz getting almost fully out of phase.

@GotFrogs Andy can you tell what order of filters these are? And on the fronts are there two all pass filters with the signal wired out of phase?

 

Hooooly **** man.....after about 2 weeks dicking around with polarity on my factory outputs this what I discovered too. But I couldn't see all that.
So I have been thinking that the rears pair were out phase. But I see I my thinking could be backwards. I see why they used all pass filters now. They open up a whole new meaning to the phrase " in phase".
With all pass filters polarity becomes almost fluid.
But how would you model this. I mean it seems almost ludicrous for kia to put this thought into the radio of a 12k car. Unless they have had this figured out for years or something.

 

And furthermore, these systems in vehicles that are deployed thousands of times with connectors that cannot be reversed are not subject to some polarity reversal mistake. The fact that it sounds better with the polarity the way it's currently connected is a BIG ASS hint that it was intentional and designed to work WITH this phase EQ>

 

I had some time today to get behind the front kick panels and look at the HU wiring my installer did, I wanted to see if I could identify the front right HU outs and verify polarity. I couldn't get to the actual HU harness but I found a red wire which connected to the wire feeding the amp front right negative in, and a black wire feeding the amp positive in. Which means this was a mistake on the installer's part and it was wired reverse. So I changed it so red feeds positive and black feeds negative - reversing the polarity. Now the speaker pop test reads + on the front right, as expected.

I re-recorded my mono pink noise through the HU to check correlation, and got the results below. I'm going to show the correlation between HU L+R like I did earlier in the thread, then show correlation between each HU channel and the source pink noise.

Here's HU Front L+R correlation, which now looks correct, showing what looks like two all pass filters

Here's HU Rear L+R (same as last result) which looks like one all pass filter

And here's the four channels, each correlated to the source pink noise:
HU Front L+Source:

HU Front R+Source:

HU Rear L+Source:

HU Rear R+Source:

The first thing that looks wrong is these results all seem 180 degrees out of phase. Not sure what that's about.

But aside from that - it seems the front all pass filter is on the left out, not the right.
And on the rear outs I don't understand what I'm seeing - this result doesn't seem to relate to the single all pass filter that I was expecting.

FYI to get these HU+Source results I lined up the recorded audio with the mono source in Pro Tools so they started together at the beginning of the waveform, and panned each hard L/R, sending them to the correlation meter.

Well the HU+Source results just seems to create more questions - why are they 180 out of phase....and why are the rear outs looking like this? @GotFrogs I hope you can help!

 

Looks like two 1st order all pass filters on the front--one at about 250 and the other at about 500. Looks to me like those two frequencies (or thereabouts) are about 90 degrees (if a correlation value of 1 is correlated and -1 is perfectly out of phase (180 degrees). Then, the dip in between is -180 from the two peaks.

Not sure about the rear. Have to think about that.

 

Looks like two 1st order all pass filters on the front--one at about 250 and the other at about 500. Looks to me like those two frequencies (or thereabouts) are about 90 degrees (if a correlation value of 1 is correlated and -1 is perfectly out of phase (180 degrees). Then, the dip in between is -180 from the two peaks.

Not sure about the rear. Have to think about that.

Thanks for the reply Andy! Now that I have a really good understanding of what's going on, I'm going to install the Kicker Keyloc and see if it will negate the APFs. I'm just going to use the HU front outs, and I'll send them to all the speakers. This way I don't have to worry about the rear outs, which have a steep HP filter. Once I setup the Keyloc I'll check the results and see what's going on. Keeping my fingers crossed!

 

So the Kicker Keyloc did not fully negate the APFs. I set it up on the HU Front L & R outs. It does an auto calibration off of a series of audio files they provide (pink noise, sine wave sweeps, noise floor) which I played into the Keyloc. After it ran through the setup I checked the HU out L&R correlation. The results looked to be improved - but not 100%. Of the two APFs in the factory HU, the lower frequency filter looked to be corrected, but the higher frequency filter was not removed. The sound was better (marginally) but the idea of putting the audio through an extra processor just doesn't seem like the right thing to do if it's not gonna be fully effective. So I returned it.

I don't think I will be happy unless I replace the HU, or install a separate player that bypasses the HU.