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Discussion Starter · #1 ·
so like the thread title indicates,


1. what are the benefits to a linear phase crossover?


2. what companies make linear phase crossovers?


is there an alternative device that can "fix" (linearize) the phase of a system after it is in place?


here is a dolby paper on linear phase crossovers:

http://www.dolby.com/uploadedFiles/z...0115%20NYC.pdf


this post was inspired by reports from tom danley and others that the subjective level of "punch" of a system is, in part, related to having all the various sounds arrive at the listener at the same instant (minimal group delay/minimal phase changes) as well as subjective reports that when linear phase crossovers were employed at stag theater (skywalker ranch), they "cleaned up" the sound quite a bit.
 

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Do a search on 'infinite slope'. Joseph Audio has been doing this passively for years, although I'd think it would be easier today in the digital domain. The bottom line: All the issues inherent with such extreme slopes are in such a narrow passband that they are relatively inaudible.


C

http://www.freepatentsonline.com/7085389.html


Here's another article on fast/slow bass suggests that driver integration is the main causality. I guess this was an issue 10 years ago too...


http://www.soundstage.com/maxdb/maxdb061999.htm
 

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Add that the drivers are not linear, it is a bit of a challenge. Linkwitz discusses it a bit in his Orion and Phoenix for the active domain. Gets a tad clumsy in the passive. Digital has it's own problems.
 

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Quote:
Originally Posted by cc00541 /forum/post/16949900


Do a search on 'infinite slope'. Joseph Audio has been doing this passively for years, although I’d think it would be easier today in the digital domain. The bottom line: All the issues inherent with such extreme slopes are in such a narrow passband that they are relatively inaudible.


C

http://www.freepatentsonline.com/7085389.html


Here's another article on fast/slow bass suggests that driver integration is the main causality. I guess this was an issue 10 years ago too...


http://www.soundstage.com/maxdb/maxdb061999.htm

Curt, I only skimmed the patent but my understanding of the Joseph Audio XO is it's a variation on the Cauer elliptical theme -- using a notch filter to increase the slope of a standard analog XO immediately above/below the XO frequency. Each of the filters is still minimum phase although the sum isn't. Linear-phase filters are different -- no phase shift as the magnitude changes.


Bruno Putzeys and Siegfried Linkwitz have both done pieces showing why linear-phase crossovers can be 'perfect' on axis but they ring when you move off axis. Bruno is a digital kind of guy (invented the UcD amps) but he favors low-slope analog or IIR (digital equivalent of analog) crossover filters.
 

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Discussion Starter · #5 ·
personally, i am only interested in active crossovers, but if linear phase is possible passively some folks would surely be intereted in that.
 

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Quote:
Originally Posted by catapult /forum/post/16951056


low-slope analog or FIR (digital equivalent of analog) crossover filters.

you got it backwards.

http://en.wikipedia.org/wiki/Audio_crossover

Quote:
Active crossovers can be implemented digitally using a DSP chip or other microprocessor. They either use digital approximations to traditional analog circuits, known as IIR filters (Bessel, Butterworth, Linkwitz-Riley etc.), or they use Finite impulse response (FIR) filters. IIR filters have many similarities with analog filters and are relatively undemanding of CPU resources; FIR filters on the other hand usually have a higher order and therefore require more resources for similar characteristics. They can be designed and built so that they have a linear phase response, which is thought desirable by many involved in sound reproduction. There are drawbacks though - in order to achieve linear phase response, a longer delay time is incurred than would be necessary with an IIR filter. IIR filters, which are by nature recursive have the drawback that if not carefully designed they may enter limit cycles resulting in non-linear distortion.

so its IIR that we have in our Behringers. FIR is what they have in DEQX and Dolby Lake processor.


also i recall from reading the writeup on EAW NT speakers ( digital prosound speakers ) that they had to develop a new kind of digital filter that had the benefits of FIR fitlers but didn't have the drawback of TIME DELAY


now the time delay talked about here i believe is IRRELEVANT FOR HOME AUDIO. but in LIVE PERFORMANCE you would hope that the sound comes at the same time as performer moves his lips or strikes the cymbals, so time delay is BAD there.


it seems like you can't get around time delay. you can only have it come evenly at all frequencies ( linear phase ) or have it all mixed into some sort of audio soup ( IIR and Analog ).


with Analog and IIR there is a tradeoff between amount of attenuation ( 6db/oct, 12db/oct, 24db/oct etc ) and phase error ( 90 degrees, 180 degrees, 360 degrees etc ). some designers will say attenuation is more important and go with 48db/oct ( Alesis studio monitors ) and others will say phase is more important and use only 6db/oct ( Dynaudio home speakers ).


but with FIR you just use 300db/oct and no phase error ! problem solved. you just need to shell out a couple grand on the crossover ! ! !


as most digital crossovers are developed for prosound it would explain why very few of them use FIR filters. for a live performance time apparently is more important than phase. even though you probably think its the same thing but it's not !


after all they're not called "zero phase" but LINEAR phase.
 

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Quote:
Originally Posted by LTD02 /forum/post/16947858


is there an alternative device that can "fix" (linearize) the phase of a system after it is in place?

i am certain such a device could be built. but i doubt it would be cheaper to use a regular crossover and than "fix" it then to just use a FIR crossover in the first place.


a device to linearize phase might be worth it to correct for the INHERENT phase errors that arise due to FINITE bandwidth of any physical speaker.


so if such a device existed i would use it around 20hz and around 20khz to flatten speaker's phase THERE but for a crossover i would just do it right form the beginning.
 

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Quote:
Originally Posted by vasyachkin /forum/post/16952598


you got it backwards..... its IIR that we have in our Behringers.

Oops, brain fart, I meant IIR. I edited my post.
 

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Quote:
Originally Posted by LTD02 /forum/post/16947858


so like the thread title indicates,


1. what are the benefits to a linear phase crossover?

Minimum phase is a more accurate term because system amplitude response deviations from flat imply a phase shift and all audio systems have finite bandwidth.


With added delay elsewhere they'll get you imaging between different driver configurations like a WMTW center channel and TM mains and they let marketing departments brag that a square wave going in looks like a square wave coming out on a scope.


This disregards that people can't hear the phase distortion of second order all-pass filters up through LR4. Event the paper you cite says

Quote:
3. APPLICATION OF LPBW TO LOUDSPEAKER ARRAYS


One area of considerable research interest is phase distortion in loudspeaker crossover networks. In previous work, including the references in this paper, the discussion has been focused upon the audibility of phase distortion within a single loudspeaker system. Through subjective and empirical tests, it has been determined that the phase distortion introduced by a conventional crossover network is insignificant.

Reading the paper farther says that this is good in pro-sound setups with different speaker configurations (main and auxiliary). I was thinking of imaging in a home setting; but summed amplitude response being flat would be good too.


The most common analog realization of "linear phase" is a first-order analog cross-over, which still allows excursion to double with each dropping octave, leads to output level limits and/or IM distortion, often precludes using pistonic drivers so the system is always distorting, etc. Those things are all bad.


It also sounds different due to the broader but shallower power response dip about Fc compared to high-order filters which is audible and preferred by some people. The driver choices, counts, cross-over-points and resulting response will obviously be different too.

Quote:
is there an alternative device that can "fix" (linearize) the phase of a system after it is in place?

Yes. At least one of the big room correction boxes (TaCT?) will do it.

Quote:
here is a dolby paper on linear phase crossovers:

http://www.dolby.com/uploadedFiles/z...0115%20NYC.pdf

It's a paper on a specific steep-slope realization which in turn has a limited overlap region and therefore well-behaved polar response (which is audible) and good behavior when different speaker enclosures are summed together. That's good, especially in a pro-sound environment where early reflections are less an issue.


What's missing is how bad the cross-over rings off-axis in the time domain and whether that'd be audible in a home environment.
 

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Quote:
Originally Posted by Drew Eckhardt /forum/post/16954598


What's missing is how bad the cross-over rings off-axis in the time domain and whether that'd be audible in a home environment.

I've seen this mentioned twice here, and something important to remember is that this off axis effect is entirely dependent on the spacing and directivity of the devices being integrated. At lower frequencies and tighter spacings, it won't be a concern.


There are also matters of what we are after where an ideally flat phase response vs. a significant minimization in phase rotation through crossover are very different tasks. The implementation is one part of the discussion, the value of doing so and the trade offs involved will be the other half or more of the discussion.
 

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Quote:
Originally Posted by LTD02 /forum/post/16947858


is there an alternative device that can "fix" (linearize) the phase of a system after it is in place?

I forgot the Pioneer receivers. Their 'full-band phase control' claims to do that when it's enabled. I have no idea how well it works and I haven't seen any independent measurements to show how well it unwraps the phase rotation and cleans up the impulse response. Quoting the manual:

Quote:
The Full Band Phase Control feature calibrates the

frequency-phase characteristics of the speakers

connected.


Standard speakers designed exclusively for audio use

generally reproduce sound with the divided frequency

bands output from a speaker system consisting of

multiple speakers (in case of typical 3-way speakers, for

instance, the tweeter, the squawker (midrange), and the

woofer output sound in the high-, middle-, and lowfrequency

ranges, respectively). Though these speakers

are designed to flatten the frequency-amplitude

characteristics across wide ranges, there are cases

where the group delay characteristics are not effectively

flattened. This phase distortion of the speakers

subsequently causes group delay (the delay of lowfrequency

sound against high-frequency sound) during

audio signal playback.


This receiver analyzes the frequency-phase

characteristics of the speakers by calibrating test signals

output from the speakers with the supplied microphone,

therefore flattening the analyzed frequency-phase

characteristics during audio signal playback1 - the same

correction is made for a pair of left and right speakers.

This correction minimizes group delay between the

ranges of a speaker and improves the frequency-phase

characteristics across all ranges.

Furthermore, the enhanced frequency-phase

characteristics between channels ensure better

surround sound integration for multichannel setting.
http://www.pioneer.eu/files/eur/MCAC...ontrol/top.swf
 

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Discussion Starter · #14 ·

Quote:
Originally Posted by Drew Eckhardt /forum/post/16954598


Minimum phase is a more accurate term because system amplitude response deviations from flat imply a phase shift and all audio systems have finite bandwidth.


With added delay elsewhere they'll get you imaging between different driver configurations like a WMTW center channel and TM mains and they let marketing departments brag that a square wave going in looks like a square wave coming out on a scope.


This disregards that people can't hear the phase distortion of second order all-pass filters up through LR4. Event the paper you cite says




Reading the paper farther says that this is good in pro-sound setups with different speaker configurations (main and auxiliary). I was thinking of imaging in a home setting; but summed amplitude response being flat would be good too.


The most common analog realization of "linear phase" is a first-order analog cross-over, which still allows excursion to double with each dropping octave, leads to output level limits and/or IM distortion, often precludes using pistonic drivers so the system is always distorting, etc. Those things are all bad.


It also sounds different due to the broader but shallower power response dip about Fc compared to high-order filters which is audible and preferred by some people. The driver choices, counts, cross-over-points and resulting response will obviously be different too.




Yes. At least one of the big room correction boxes (TaCT?) will do it.




It's a paper on a specific steep-slope realization which in turn has a limited overlap region and therefore well-behaved polar response (which is audible) and good behavior when different speaker enclosures are summed together. That's good, especially in a pro-sound environment where early reflections are less an issue.


What's missing is how bad the cross-over rings off-axis in the time domain and whether that'd be audible in a home environment.

lot's to chew on there drew, much thanks! nice pull on the tact box. if i find any good whitepapers by them, i'll link them up in this thread.
 

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Discussion Starter · #15 ·

Quote:
Originally Posted by Mark Seaton /forum/post/16954808


There are also matters of what we are after where an ideally flat phase response vs. a significant minimization in phase rotation through crossover are very different tasks. The implementation is one part of the discussion, the value of doing so and the trade offs involved will be the other half or more of the discussion.

thanks for weighing in ms, can you elaborate a little bit or link up a couple good places to read about this?
 

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Quote:
Originally Posted by LTD02 /forum/post/16958650


again. nice pull. looks like the folks at pioneer are asking the same questions we are around here or vice versa. ;-)

pioneer is a monster on the technology side but they don't know how to package it


for example my pioneer car head unit had a microphone and a multi band equalizer and was supposed to calibrate speaker's response.


too bad i was using a system with big amplifiers and if i let it run its test tones it would probably blow all of my speakers up. so i never used it. i just set equalizer by ear.


that head unit also had an organic LED display - which obviously i didn't need.


pioneer is all about putting state of the art technology into useless products ( how japanese of them )


this would be in contrast to say APPLE which puts middle-of-the-road technology into products people actually want.
 

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Quote:
Originally Posted by catapult /forum/post/16954033

http://www.thuneau.com/arbitrator.htm

this technology actually is important.


while crossovers introduce MOST of the phase errors the transducers themselves also introduce plenty.


in my response to LTD i mentioned that this technology could be used to correct phase response at the fringes ( 20hz and 20khz ) but that's not all it should be used for.


each driver has its own finite bandwidth ( aside from the overall finite bandwidth of the sound system ) which results in phase errors. these should be fixed ( driver by driver ) using technology such as this thuneau.


after phase response of each driver has been flattened then the signal can be sent to to a linear phase crossover and that's the only way you can ensure overall linear phase performance of the sound system.


i mean you must have linear phase crossover AND linear phase drivers.


whether driver phase can be linearized in a passive system as pioneer claims to do ? i don't know. to me it seems unlikely that some DSP can take misaligned signals from several transducers and recombine them into a single signal. and even if this can be done theoretically i dont know if in practice it would do more harm than good.
 

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It is marketing mainly. Anytime you see companies making a big deal out of linear phase you can almost smell the BS.


You have to recognize that transducers are not linear phase devices to begin with. You also have to recognize that most tweeter-midwoofers are acoustically out of alignment and that they can only be time aligned, even with a DSP, for one position in space since the drivers are acoustically separate. Even a single driver loudspeaker, without a tweeter, will have time alignment mismatch (the high freq arrives slightly time aligned differently than the midrange/bass). In most cases, this is completely inaudible.


Time alignment & linear phase may mean different things although you see a lot of mixing of the terminology. You can have a smooth phase response through the crossover even though the tweeter-midwoofer are out of time alignment. Often that is the case as the typical dome tweeter on a baffle is 100-150us out of alignment with the midwoofer because the Voice Coils are at two different locations on the Y-axis of the loudspeaker (the tweeter VC is closer to the listening posn by an inch or two). The research shows this is pretty much inaudible. Correcting it buys you very little that is audible but it does give you another marketing checklist item.


Horn speakers are another matter. You can get path length differences of many inches, and in those cases DSP correction is the only method to bring them into some reasonable time alignment. You can still design passive networks that give reasonable FR behavior but there is nothing you can do with a passive network that will delay the signal. The exception is using allpass filters with 0-180 phase shifts but flat amplitude response to compensate for some of the time-delay. These require picking the correct amount of phase shift for a given crossover point because they don't delay all of the signal equally. Also, they are only really suitable for modest amounts of delay compensation. They work for baffle mounted dome tweeters, but not long path length differences you see in horns.


The bottom-line. Go with a good speaker designer and trust their work. You cannot just throw transducers together on a baffle, hook up an external crossover, linear phase or otherwise and get good results.
 
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