DiyMobileAudio.com Car Stereo Forum banner
1 - 14 of 14 Posts

·
Registered
Joined
·
4,096 Posts
Phac is purely electrical and not taking any acoustics into account or the influence of the cabin into account, so for me it’s a bit useless unless you happen to place crossovers in a minimum phase area ie the cabin has zero influence over the phase (highly unlikely)

it also assumes speaker phase is flat… it’s not and the midrange of the audio spectrum is always lagging from a driver and the sub bass and treble always leads

for want of a better description if you have a perfectly aligned two way home audio speaker the phase will generally look like a smiley face varying by 180 degrees at either end

So again phac doesn’t take the above into account
 

·
Registered
Joined
·
4,096 Posts
Yea I agree and this was my original though that it won’t help and you should adjust everything using Smaart which there you can see the impact of the acoustic environment on the phase very precisely.
But I’ve tried it and found that it works, so you can try calculate everything into you head, but something the realty is different.
I want some one with experience to try it and then check the impact it has.
If your using smaart it’s far more accurate and you don’t need to calculate everything in your head, just adjust it live and place crossovers very accurately, the above that I mentioned are not an issue when using smaart
Jl tun allows you to do a similar thing and model all pass filters based on crossovers, and again in reality it doesn’t change the fact that it’s not reflected in the real world due to phase not being perfectly flat from a driver or combination of drivers
 

·
Registered
Joined
·
4,096 Posts
If using Smaart PhaC allows you to change phase just with a slider and in real time as the calculated allpass filters are applied in real time to the system.
When watching it in real time in Smaart this is a real good way to set the phase as desired in a very quick way. Way quicker than typing in different allpass filters with bandwiths and stuff like that.
So PhaC can work if you use it a bit outside of the workflow as intended by Frank Miketta. ;)
The phase slider in helix is designed to give x amount of phase shift at the crossover, so your not guessing anything, helix software does it for you
 

·
Registered
Joined
·
4,096 Posts
If the phase on any driver is less than the driver you want to alter it to it wont work.
In your example if subs phase is -59° and woofers phase is -49° in phase you cannot use PhaC to adjust the sub to the woofer, but only the other way around, i.e. woofer to sub.
You’d simply add delay to the midbass and everything above equally in that case or adjust the crossover slope and freq of the subwoofer a touch or the midbass, in reality 10 degrees is very little to adjust
 

·
Registered
Joined
·
4,096 Posts
Same with PhaC.
But we are talking about using smaart and other dsp softwares, why do I need phac, what does it offer me

with phac alone unless you can measure phase in the environment it’s not going to help as you will have to adjust after phac for the environment

phac can’t adjust anything automatically for the environment, so your left adjusting manually regardless
 

·
Registered
Joined
·
4,096 Posts
Same with Helix.
Look... i don't use PhaC myself most of the times. But all PhaC does in the end is set a 1st or 2nd order allpass filter with a frequency and in case of 2nd order a bandwith just like the variable phase slider in the Helix software does.
With Helix you move the slider and it automatically calculates the resulting allpass filter, frequency and bandwith, PhaC does the same, you move a slider and it automatically calculates the resulting allpass filter, frequency and bandwith for a frequency that you define. Only difference is, that Helix does this in 5.something degree steps and PhaC in 1 degree steps.
I already agreed with you that you should do that with some other software like Smaart or similar tools to minimize the amount of time needed, because you can see the result in the measurement software.

So i don't get it why you're arguing with me now.
I’m saying you can’t use phac alone as it’s pointless… the original post is about this.

phac is an automated system used for ‘correcting’ the phase response based on filters applied not a phase slider like in the helix, it’s a process that doesn’t work in a real world environment, I can’t say that I use the phase slider in helix either… occasionally I will apply an all pass filter, but that’s it as far as phase manipulation goes

the original post is about using phac with other dsps, not smaart. Most people don’t have smaart so phac is pretty much useless for me
 

·
Registered
Joined
·
4,096 Posts
Frank of Mosconi knows exactly what hes doing. That said, I just don't exactly see a point in doing this. Crossovers work properly when used as intended and implemented correctly. No need to correct phase for a tweeter down to 100hz lol
It doesn’t as far as I know, it adjusts the freqs at the top end of the cover by x amount of degrees, if the other end of the all pass filter effects the lower freqs that’s a side effect of the phase shift being tiny, the helix phase shift does the exact same in reality, uses a second order all pass filter

However it still doesn’t change the facts we’ve both mentioned in that it doesn’t take into account any phase shift induced by the vehicle so may improve things or make it worse

the only time it can be handy is if you have two crossovers on a narrow band with driver as they interact and change the phase by more than if they were spaced apart, but again this assumes the driver has perfectly flat phase in the first place

you can model this in jl tun’s newest release or see it also on the helix software if you turn the all pass filters on 👍🏼
 

·
Registered
Joined
·
4,096 Posts
One question (well, actually it's more than one...). Isn't this phase shift of the cars acoustics done to all drivers at the same time? F.e. in the area where you pass over the duty from midrange to tweeter?
If yes, the relative phase would stay the same even due to the induced phase shift by the car, right?
In this case, an allpass filter as created by either PhaC or the Helix phase control would still work as intended.
Only if both drivers were in the exact same place in the car literally, even 5” centre to centre will change the reflections which will then potentially influence phase

this is why driver positioning and crossover freq have to be chosen very carefully

Playing a midrange below beaming for example doesn’t mean you can mount it anywhere as some people would have you believe, an inch difference… (aiming will change off axis response and also move this centre to surface distance in smaller degrees so does need consideration also) up/down/back/forwards can make a decent sized difference to the response in both phase and also magnitude response 👍🏼
 

·
Registered
Joined
·
4,096 Posts
Yes, but these reflections will be the very same at all times, right? No matter if you set a crossover or driver specific filters, the reflections will be always the same and therefor relative phase will stay stable. It is not like the distance changes between drivers while listening.
Therefor this cannot be the answer. When i set the time alignment without crossovers (in theory that is, of course you need crossovers) the direct sound will all arrive in time, right?
When i set crossovers and driver specific filters arrival times will be changed, especially the lowpassed drivers will lag behind the highpassed drivers, no matter the always existing reflections. When i set an appropriate allpass filter on the highpassed speaker i can bring the arrival time back in time to the lowpassed speaker as the reflections will be stable.

Or is this way of thinking too simple?

Just trying to understand.
Each driver will have its own reflections and phase issues to deal with, they will remain constant yes. The phase shifts and rolls round because of them… but each driver will be unique to its own phase shifts, not every driver will have identical shifts because they aren’t in identical places

move a mid up a pillar and it’s interaction with the car will shift in various ways

it’s why you should always put crossovers where both drivers have no big dips etc

in theory if phase was as it left the speaker then yes phac could work… however the environment skews phase very often and it’s not possible to put crossovers in a perfect textbook place

So we sometimes have to make small adjustments for the phase being out to bring it back in

it’s the environmental phase shift, no matter how constant… that phac can’t possibly predict or correct for, how can software with no idea of if mids are in upper doors or a pillars correct for phase being off or slightly out at the listening position? It also can’t predict the drivers own phase shift, be that big or small

Too many variables to predict with not enough information to predict them

Phase leads or lags with crossovers… it doesn’t change times of arrival or impulse response would change… I had this very discussion with the jl guys because it looks like timing will change, when in fact the phase rotates… it’s a hard concept to get your head around at times
 

·
Registered
Joined
·
4,096 Posts
Again , I’m a mosconi dealer , I tune a LOT of Aeros

it’s sorta works, it’s about the time that you spend about two hours fiddling with it and you get it to sound good, and then you leave the car and come back and turn it on again to a complete disaster because your ears weren’t ready for the mess that you caused…

so yeah like I said before, it makes the passband great, stop the band is a catastrophe (using phac calculator as frank designed)

I’m not a fan of really good coherence that’s narrow banded , then a mess outside of that area

I would rather sum the crossovers properly to have good allpass behavior (as a crossover network sums to be an all pass) and that’s the only time I want to move the phase in a minim phase system

as far as everything else as long as your magnitude is right that phase will fix itself. so there’s no point in trying to fix something that’s not broken when all you need to do is a better job at tuning…..


and as far as the dips again like I said the energy is still there! just because your microphone shows a dip in one spot don’t mean it’s not there , perhaps on the other side of the car…..

If you think about it like this plus take for example a 400 Hz all pass filter, with a Q around .5 or 1

The lagging side of that would be about 4 to 6 ms behind the leading side, that as a group delay why in the world would you want to add group delay 8-12 times!!! that’s an insane premise!

I can see sometimes using one way way way out of band on a driver that’s just not cooperating, and you have nothing left to do. But that’s A single driver!!! I would never ever ever add that to every driver in the networks on a multi way!

i’m surprised it doesn’t start reverberating with all of that wrapping going on!! Jesus Christ, it’s like , “ hey guys let’s intentionally add 8 more phase wraps , because it’s cool” 🤤🤤

Why would you make the compromise and trade should have one tiny part a little bit better in exchange for a very big part to be a complete disaster? Nicht verstehe!!!
That reads like all pass adds delay? They don’t add any delay in the signal, the impulse response doesn’t change at all

nor does it change through crossovers either, the phase rotation doesn’t effect time at all above the crossover
 

·
Registered
Joined
·
4,096 Posts
So , actually it does ,

but I think your just misunderstood

there’s a way you can see it , download JL TUN,

click on the crossovers icon , and it shows what you have going and it shows how many ms that LPF is set back ,

They sum to be an all pass , (that’s what it all passes a low pass in a high pass summed together) look at an all pass , the “low side” is one cycle out of time (for 2nd order) with the “high side” which isn’t out of time

that one cycle equates to a center frequency, depending how much phase (or radians) is in that circle , will dictate the delay of the low side

And it does affect the impulse , if it moves the timing it changes the impulse , whether it does it negatively or positively two different discussions
it’s obviously changing the timing somewhere or we wouldn’t be having this discussion lol

transfer function of an allpass here for example

View attachment 348338
View attachment 348339
I was referring to you saying the all pass adds msec to the signal… it doesn’t or the arrival time would move if you applied it to freqs below where the signal is

it does change the impulse shape but not the actual arrival time of the direct sound above the all pass was my point, it literally effects the phase at the freq it’s at ‘if’ there is output at the freqs… it’s why applying a second order all pass at the subwoofer crossover point has zero effect on a tweeter which I’m sure I’ve seen you suggest before
 

·
Registered
Joined
·
4,096 Posts
Music is a lot of sine waves at once, which then, when overlayed, creates a not like sine wave looking wave, but still, technically it is all sine waves.
And these waves can be out of phase if played by two speakers, be it left+right or left midrange+left tweeter.
So getting the phase in order is really important in my opinion.
Time aligning the drivers before doing everything else makes sure, that every single frequency arrives at the listening position at the same time (plus/minus a few fractions of a milliseconds).
Adding driver filters to the aligned drivers creates phase problems as every single filter alters the phase at the listening position.
With the LR24dB filters the lowpassed driver is now one cycle behind in phase, but still in time.
Having a tool to bring them back in phase without altering the time alignment is now a handy tool, and allpass filters do exactly that.
If you have no allpass filters you can do the phase alignment also with time alignment, but time alignment then puts every frequency above or below the crossover area, depending which driver you are putting additional delay to, out of time with the frequencies of the other driver, except for the crossover region that is, because there you are back in phase and still in time.

And this is what PhaC (and the Helix phase adjuster) tries to do. It gives you an easy way to manipulate phase at the crossover region without affecting the time alignment of the different drivers.
Debate is on the technical implementation of the tool, not that allpass filters don't work.
All pass filter DO NOT correct the rolling around of phase in a 24db slope crossover, they can’t, only fir filters can do that

an all pass and a crossover roll the phase the exact same way, one doesn’t counter the other, @oabeieo is accurate on that part…

If a 24db crossover made the signal one wavelength out of phase adding an all pass can only ever make it more out of phase not bring it back onto phase, yet it doesn’t

that’s not what phac does and not how it works

also all pass and a 24db crossover doesn’t add delay above it, I thought it did, however if you watch the impulse response the timing of the signal doesn’t change, the phase does alone, it’s a big thing to get your head around

I asked the same thing of Nicolas Ames of jl and he showed me with a real signal and applying an all pass to it while measuring its impulse response
 

·
Registered
Joined
·
4,096 Posts
So you are pretending that it is impossible to have two drivers in phase and time in the crossover region and their passbands, right? Because if the lowpassed driver is lagging one cycle after applying the crossovers to both drivers they are in phase, but one cycle out of time. How do you solve that?
Just trying to understand.
I was very specific and I didn’t say that… you can’t undo phase rotation in a crossover by using an all pass which adds more phase rotation, an all pass is positive phase rotation, and a crossover is positive phase rotation, one cannot undo the other, they both induce the same phase rotation

take the crossovers off of a mid and tweeter and measure impulse response then align them, now apply a low pass and a high pass respectively and check the timing again… does it change the timing? This is how Nicolas demonstrated it to me from memory, if what your saying is true the tweeter impulse would then move to a later point in time corresponding to a full wavelength worth of time, so if it was a 2.5khz crossover the time period of the time shift would be 0.4msec, but you won’t ever see that time shift with impulse response

for example if you have a tweeter and it has a 2.5khz crossover on it that crossover does not effect the timing of 8khz leaving the driver, toggle it on and off and you will not see 8khz move if you play a tone and measure it’s impulse response and arrival time, toggle the crossover on and off and it will not change in time

You are associating phase change with timing, you get phase shift through a crossover, but it doesn’t delay everything above the crossover

timing can be related to phase, changing time alignment effects phase variably depending on frequency, but changing phase doesn’t effect timing

It why if you change the polarity of a driver and watch it’s impulse response the impulse just flips horizontally. But it’s arrival time doesn’t change if that makes sense

all pass, polarity swaps and crossovers change the phase of the first arrival at the mic of there effected band if measuring impulse response, but not it’s actual arrival time

try it… play something and measure it’s impulse response and then swap polarity and see if the impulse changes in the time domain… it won’t, but if your theory were true it would change the time of arrival at the mic by half a cycle… of all freq which would make for a very out of time freq band
 
1 - 14 of 14 Posts
Top