# Time alignment on a sub? I don't get it.



## goodstuff (Jan 9, 2008)

Why would you want to time align a mono sub woofer? It's already (usually) the farthest away and there is no left or right info reaching your ears at different times.


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## bamelanc (Sep 13, 2009)

It's not left or right, it's distance...that said, rear is also a position. Someone with more technical knowledge will correct me I'm sure.


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## ErinH (Feb 14, 2007)

*Don't get wrapped up in the physical placement of a driver and trying to delay for that, solely.*

It's not really about trying to "move" the drivers. It's about trying to align the phase. Incremental delay is incremental phase adjustments most prominent at the crossover.


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## ErinH (Feb 14, 2007)

See if this helps explain the above a bit better:
Subwoofers and Time Alignment in the Car | Medley's Musings


essentially, what you're trying to do is align phase of the subwoofer/mids at the crossover via delay (time alignment). 

when talking about delay, consider the wavelength as well. That's why you may notice that even minute changes in delay (ie: 0.02ms; one click resolution on most current DSPs) will affect what you hear. In every iteration of my system in the past couple years, three 0.02ms adjustments is a way I come circle in the phase on my tweeters. One 0.02ms adjustment may be out of phase, the next is a bit more out of phase and the final (3rd) is *snap*... back in phase. I've talked to others who have experienced this. It's interesting to note that ILD is often purported as what drives our hearing of higher frequencies (2khz+) and many will say that time alignment is not necessary here because our ears are sensitive to differences in frequencies only below approximately 1khz. However, _time alignment_ will net you the ability to incrementally align phase in higher frequency(s) and result in a more coherent soundstage. 

- Erin


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## minbari (Mar 3, 2011)

if your sub is the farthest away, then that becomes your reference point for all other speakers to be aligned to.

if it is NOT the farthest away, then it will need to be aligned as well.


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## Mic10is (Aug 20, 2007)

minbari said:


> if your sub is the farthest away, then that becomes your reference point for all other speakers to be aligned to.
> 
> if it is NOT the farthest away, then it will need to be aligned as well.


Negative. As Erin said it has little to do with distance away.its about aliging the phase response of all the speakers.
Every single system I have ever heard or judged who applied this method using TA was very obvious as to the location of the subwoofer(s).

All speakers need delayed to align them in acoustic phase response


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## minbari (Mar 3, 2011)

and what is phase? a phase shift is just a time shift in relation to a reference.

I wont argue that aligning a sub might give you better bass response.


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## Shinju (Jul 11, 2008)

I have TA on my single 8" I kid you not it sounds like the bass is coming from the front with all the force being felt in the seat, and it meshes well with my 6" midbass.

TA on a sub is very useful in the right install.


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## mosconiac (Nov 12, 2009)

I think its important to remember that each driver has a specific phase response, so time-alignment is not driven solely by distance...


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## Hanatsu (Nov 9, 2010)

bikinpunk said:


> See if this helps explain the above a bit better:
> Subwoofers and Time Alignment in the Car | Medley's Musings
> 
> 
> ...


Indeed true, I always time align tweeters. Affect both height and width, when tweets are in phase the stage come "alive". Can be a bit tricky to get right sometimes. Imo it's easiest to level match/set EQ before phase match them.


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## ZAKOH (Nov 26, 2010)

I did some experiments with time aligning the subwoofer, and the benefits seemed to be extremely elusive. Most of the time, it makes no difference. This continued until I finally found a music track where delaying the front stage by a foot definitely helped to produce tighter, more realistic bass. The most important thing though, is to get the electrical phase relationship right before you mess with time alignment.


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## eddieg (Dec 20, 2009)

If you will take a wave length calculator and input frequency of 50Hz you will see that a full cycle is 6.8 meters long and it takes up to 20ms to complete. and your sub is playing probably much lower and much longer frequencies as well. 

now, if in your car you have a distance of 7 meters between your head and the sub you are either very reach or a bus driver! 

even if you take a quarter of the wave length you are still at 1.7 meters from when the sound wave would hit. 

So - if you will time correct the sub you will make that wave hit exactly at the desired location which is your front window, the windshield, as well you align the phase between the sub and the woofers and you also avoid wave cancellation issues. 

so this is why you should apply TC to a sub - since it is mono, you apply the same distance on both sub channels.


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## ErinH (Feb 14, 2007)

well, something I started to touch on but didn't regarding wavelength. With a subwoofer, given the wavelength, it's easy to find that you are phase aligned at a given frequency but not another. For example, I may have 50hz lined up but 30hz may be off. How much it matters seems to be subjective because some are better at picking this out than others. But, the fact is that it does occur.


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## chad (Jun 30, 2005)

don't forget group delay


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## minbari (Mar 3, 2011)

bikinpunk said:


> well, something I started to touch on but didn't regarding wavelength. With a subwoofer, given the wavelength, it's easy to find that you are phase aligned at a given frequency but not another. For example, I may have 50hz lined up but 30hz may be off. How much it matters seems to be subjective because some are better at picking this out than others. But, the fact is that it does occur.


yup and this is why I find T/A on the sub channel to be hoo-hum at best. if you want it aligned for a specific freq, and that makes it sound better to you, then by all means go for it. 

Unless it is naturally aligned for 70hz and just sounds likes ass, it wont help much.


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## eddieg (Dec 20, 2009)

There are several phenomenons taking place when you tune sub and woofers towards each other and not just one at a time. 

Do not forget that you do not only hear the wave created by the subwoofer alone, you also hear panel rattles and as well some vibration of the sub enclosure it self that could also give away its location. 

As you know audio frequencies below the 150Hz are non directional, this is also the main reason why a subwoofer can run mono or stereo and you could not tell the difference. 

So in order to draw the base line towards the frontal set you are required to: 

1. Make sure your woofers/midbasses produce enough bass on their own and that they can go as low as possible to get near the sub higher end of the range.

2. Cross the sub as low as you possibly can - the reason for that is that you "extend" its reach or distance by playing longer waves, the higher it will play the shorter the waves would be audible and it will give away its location more easily. 

3. Make sure the woofers and the sub are not aligned exactly at the same time as the subwoofer would cancel the woofers waves because it is stronger than them. 

4. adjust the sub gain and phase so that it would merge with the woofer and not over take them. 


If you will keep the rules above your sub would sound like it is an extension of your woofers and would become "invisible".


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## asawendo (Nov 22, 2009)

May I ask question? 

What about if we have two or more subwoofers in different location let say one on the left corner and one the right corner? So we must have them time aligned independently?


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## minbari (Mar 3, 2011)

Some may disagree, but I would align them together. This way they will sum together.

Sent from my phone using digital farts


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## 14642 (May 19, 2008)

OMG.

First, phase only matters when there is more than one speaker playing the same frequencies. If your sub is 180 degrees out of phase with the rest of your system at 30 Hz and it's the only speaker playing that frequency (within 12dB or so), then it doesn't matter. 

When more than one speaker is playing the same thing, then phase matters. If you have a speaker on the right and a speaker on the left, then it's helpful to have them in phase to hear center images and to hear bass. If you have two speakers playing the same frequencies at a crossover point, it's helpful to have them in phase so the acoustic sum is correct. 

The phase relationship only matters at the listening position. The phase of your electrical filters matters only because it contributes to the phase YOU HEAR. 

Time alignment is effective in minimizing phase differences that are the result of differing distances between your ears and the speakers. If your speakers are connected in the proper polarity (right and left should be the same and the polarity of bandwidth limited speakers should be determined by the acoustic sum measured at the listening position--while maintaining the same polarity for right and left), then time alignment FIXES phase issues caused by distance.

Now, if the DIFFERENCE IN DISTANCE between your ears and the various speakers in a bandwidth limited system corresponds to a difference in phase at the crossover point between the drivers, then time alignment will probably have a positive effect on the acoustic sum and you'll probably hear it. This is the case below 1K, where we determine the location of sounds using ITD (difference in arrival time between our left and right ears). 

OK, so a 60Hz wave is about 19 feet long. If your sub and your midbass are crossed over at 60 Hz with a 24dB/octave slope, then phase reltionship between them doesn't matter below about 45Hz and above about 80Hz. So, a difference in distance between each speaker and your ears of 9.5 feet will create a 180 degree phase shift at the crossover. If we reverse the polarity of the sub, it's fixed. No time alignment required because the sub and the midbass will be back in phase at the crossover. We don't hear phase differences of less than 90 degrees very well, so we only have to be concerned about phase differences that are greater than that and close to 180 degrees. In the example above, the band of frequencies we have to be concerned about is really narrow, so there's no chance we'll be above 90 degrees in any region where the two speakers play the same frequencies. 

So what does this mean? If the wavelenghts are long compared to the DIFFERENCE between the distance from our ears to the speakers playing the same thing, then time alignment makes little difference. If you use gradual crossover slopes then you increase the width of the band of frequencies that both speakers play. This extends the problem into higher frequencies where it may matter.

Now, time alignment for tweeters is another thing altogether. It may be helpful to align them with the midrange speakers for the same reason as the example above. We don't hear phase at high frequencies very well because the wavelengths are smaller than the distance between our ears. We use the level differences between the speakers to determine the apparent source of the sound. However, phase errors between tweeters caused by different pathlengths will cause comb filtering, which IF WE HEAR IT, will manifest as frequency response errors. There is plenty of arguement about the audibility of comb filtering when listening to music and when your head is stationary. With steady state signals (pink noise) it's easy to hear it if you move your head. 

All of this is made even more complicated by the fact that what you hear is a combination of the magnitude and phase of the direct sound from the speaker AND all of the reflections. Measuring the distance from the speaker to your head will get you close at low frequencies, but may be completely wrong for high frequencies that emanate from small speakers mounted near the glass. For tweeters anywhere near the windshield, what you measure with your tape measure will bear little resemblance to what you hear as pathlength or what you perceive as the "direct" sound.

So...the answer to whether time alignment is necessary or even helpful for woofers and tweeters is...you guessed it: MAYBE--it depends on many other factors. It should not be your first assumption when something doesn't sound right. Nor should it be assigned "cure all" status if screwing around with time alignment happens to improve a situation you don't fully understand.


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## minbari (Mar 3, 2011)

Ossum explanation! Hope you're fingers are not tired, Andy 

Sent from my phone using digital farts


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## eddieg (Dec 20, 2009)

Hi Andy, 

Wonderful reply and I liked your explanation a lot. 

Well the sub maybe the only speaker playing at 30 or 40 hz but is it the only speaker playing at 50 to 80hz? in most cases when you have a proper installed woofers - it will not be the only one, this is where the phase differences becomes audible. 

As at most cases woofers would be crossed either at first or second order while a sub is usually crossed at fourth order which is by it self a 180 degree phase shift. 

In addition to the distance between the sub to the woofers and ears and there you get the area in which this phenomenon occurs. 

As you said, shifting the phase can fix the difference of lengths - actually, in most cases it does! In a very little number of cars did I have to use TA in order to fix sub location, but in some systems the crossover points or the speaker locations do not get the sub location fixed by changing the phases, in this case, using TA (or TC) is the easiest way to correct it. 

I prefer to use TA for the subs as sometimes changing the phase also hurts the sound system clarity/detail level so with minor adjustments to the sub by using TA gets better results. 

In my case the installation is a kickpanel install of a 3-way component set. 

Eddie


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## james2266 (Sep 24, 2009)

Interesting. After reading some of these comments and Andy's rather extended explanation, I might have to revisit my time alignment and not only on my subwoofer (which has always been zero as I always used group delay).

I am curious to try using the rta for setting time alignment on a sub. Is there something in particular to be looking for? I currently have a serious problem at about 65 Hz no matter what I do. I have tried pretty much everything (crossovers, slopes, phase reversal on every combo) and can't get rid of the dip at 63 without boosting that frequency pretty big. I have looked at both the midbass and sub singly and they both seem to have issues playing at that frequency. Maybe it is just a function of the vehicle. Is there a rule of thumb which driver should be responsible for this frequency band? Personally, I want to have my midbass play this band. I have been looking at replacing my midbass soon as it sometimes has issues when it gets really cold and well, it gets really cold here quite often during winter. The fs of my midbass is 59 Hz which is right where the big dip in the response is. Could a lower fs eliminate this problem? The response of these drivers has a big hump at around 100-250, drops like a rock at around 60 Hz and then comes back up considerably at 50 Hz (about a 5-7 db dip).

Another thing I noticed and would like some clarification on is regarding comb filtering. Does it always affect both the midrange and the tweeter in a 3 way front end (if tc is off slightly) or can it manifest in the reponse of only one of the drivers? I noticed what looks like possible comb filtering on my midranges but nothing resembling this on my tweeters? Can comb filtering happen between a midbass and midrange too? I just actually had that last thought

Anyways, interesting topic and one that I have struggled with from the first day I put in a processor with time alignment and all the goodies.


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## subwoofery (Nov 9, 2008)

james2266 said:


> Interesting. After reading some of these comments and Andy's rather extended explanation, I might have to revisit my time alignment and not only on my subwoofer (which has always been zero as I always used group delay).
> 
> I am curious to try using the rta for setting time alignment on a sub. Is there something in particular to be looking for? I currently have a serious problem at about 65 Hz no matter what I do. I have tried pretty much everything (crossovers, slopes, phase reversal on every combo) and can't get rid of the dip at 63 without boosting that frequency pretty big. I have looked at both the midbass and sub singly and they both seem to have issues playing at that frequency. Maybe it is just a function of the vehicle. Is there a rule of thumb which driver should be responsible for this frequency band? Personally, I want to have my midbass play this band. I have been looking at replacing my midbass soon as it sometimes has issues when it gets really cold and well, it gets really cold here quite often during winter. The fs of my midbass is 59 Hz which is right where the big dip in the response is. Could a lower fs eliminate this problem? The response of these drivers has a big hump at around 100-250, drops like a rock at around 60 Hz and then comes back up considerably at 50 Hz (about a 5-7 db dip).
> 
> ...


65Hz dip is a vehicle acoustic problem. Play a pink noise through your midbass only (mute everything else) and watch your RTA curve - is your dip there? 
Then do the same with the subwoofer only - is your dip there? 
If the dip is there for the sub, then you'll need to play your midbass lower in order to fill that dip. Vice versa. 
FS has nothing to do with this  Well it can if you're trying to play it down to 40Hz with an FS of 80Hz  

Hump around 100Hz-250Hz is also a vehicle acoustic issue - just do some cuts there. 125Hz and 160Hz are usually the most problematic. 

Regarding comb filtering, do a search on the web, you can find files that lets you hear what it sounds like. 
Also, good info there for you http://www.diymobileaudio.com/forum/diy-mobile-audio-sq-forum/11579-flat-response-3.html - read page 4 too 

Kelvin


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## james2266 (Sep 24, 2009)

subwoofery said:


> 65Hz dip is a vehicle acoustic problem. Play a pink noise through your midbass only (mute everything else) and watch your RTA curve - is your dip there?
> Then do the same with the subwoofer only - is your dip there?
> If the dip is there for the sub, then you'll need to play your midbass lower in order to fill that dip. Vice versa.
> FS has nothing to do with this  Well it can if you're trying to play it down to 40Hz with an FS of 80Hz
> ...


Thanks Kelvin. I had read that thread when it was first done but just re-read it again to refresh my memory. I did find a youtube demo of a piece of music (drum solo) being played with and without comb filtering. It was very informative. If I assume it is cumulative with volume, I now have a feeling the problems I was having with my ml700's might just have been comb filtering induced. Have you found that some drivers are more affected by it than others? I don't notice it near as badly with my Scanspeak 10fs. 

I have measured the response of my midbasses and my sub separately with no other drivers playing and the dip at 65 or so Hertz is noticeble on both drivers. I tend to think it is a vehicle anomaly as you said as another guy on here with my same vehicle has a dip in the same spot actually. Is there anything that can be done tho other than boosting the crap out of 60 or so Hertz - maybe some kind of vehicle treatment or something?

As for the eq suggestions on the midbass, I have done that before but as 125 Hz was a huge problem 160 was down if anything and 250 Hz is stupid (in a word). Nothing that some serious eqing didn't fix tho

I will play with time alignment again and pay attention to small increments on my midranges and see if the possibly comb filtering mess goes away and I get a smoother response through that passband. Gonna be a few more days tho before I have time for that tho. Relatives


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## avanti1960 (Sep 24, 2011)

wow- interesting thread that I first thought about skipping. 

my pioneer autotune set the time alignment for my rear cargo area sub AND reversed the phase. I do have the dip @ 70Hz or so as James mentioned but the bass location and tight SQ has left me with not much to complain about except for the dip on the RTA plot. 

Question- 
as a 4th order crossover can reverse the phase- would this also apply to a DSP crossover or because of the electronic circuit layout within a passive crossover? 

I still plan on trying to fix the "dip" to see if I can- I am now leaning to possible leaking back-wave of the door panels- and adding a more rigorous application of door sealing / deadening etc. for all (4) doors. I measured one of my midbass drivers and it was shown to be out of phase in the bass response even though the driver really is in phase. Something is leaking the backwave through. 

thanks Andy for your contribution.


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## thehatedguy (May 4, 2007)

4th order is back in phase at the xo point.

And it does apply to DSP systems as well as passive.


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## tru tech99 (Jan 3, 2011)

im so lost but i good to know is not about distance


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## 14642 (May 19, 2008)

thehatedguy said:


> 4th order is back in phase at the xo point.
> 
> And it does apply to DSP systems as well as passive.


Right, but that's the electrical sum. Once that's applied to a pair of speakers, there's little chance that what you hear will match the electrical sum. That's what EQ and time alignment are for.


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## thehatedguy (May 4, 2007)

Baby steps Andy, baby steps sometimes.


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## eddieg (Dec 20, 2009)

Try to lower your sub LPF to 60 hz or even 50Hz and see it it is more "steady" at the windsheild location. 

The reason for that is the lower the frequency range your sub is playing the less directional are its sound waves and their overhaul length is longer. 

The dip you see in your RTA does not necessary mean you have to straighten it as in some cases you would get audible distortion by doing so. 

Do you sense by hearing a lack of bass? if not, then why fix it if it is not broken 

Usually when people see dips they try to fix it by raising the level instead of lowering (attenuation) everything around it, while attenuation does not add distortion it can cause lack of dynamics or boost up the noise level as you attenuate the signal, however, when you amplify or gain a specific Q for a frequency you might bring it to clipping and distortion will occur - so, do not be to rough with the EQ and your RTA -> I would suggest to also add a scope and see on that frequency range if you can see that the sound wave is clipped/distorted. 

As for fourth order crossover (24db) it does not reverse the phase, it actually set it back straight again as it shifts the phase in 360 degrees. 

But it also create a time delay of a full cycle at crossover point. 

It does not matter if it is done via DSP or physical passive crossover components, the only difference is that is a passive crossover does not have a zobel circuit or l-pad added to then the crossover point is not fixed to single frequency but it changes according to the speaker impedance at the given frequency. 

This is a part of the "magic" that many people refer to when claiming that a passive crossover would sound better than an active one, while if you plan them correctly they will do the very same thing.


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## subwoofery (Nov 9, 2008)

james2266 said:


> Thanks Kelvin. I had read that thread when it was first done but just re-read it again to refresh my memory. I did find a youtube demo of a piece of music (drum solo) being played with and without comb filtering. It was very informative. If I assume it is cumulative with volume, I now have a feeling the problems I was having with my ml700's might just have been comb filtering induced. Have you found that some drivers are more affected by it than others? I don't notice it near as badly with my Scanspeak 10fs.
> Don't think comb filtering is that much of a problem for midranges unless you let it play really high (5kHz+). If I had to guess, either the ML700 had an aiming problem or it was a phase problem (T/A).
> 
> I have measured the response of my midbasses and my sub separately with no other drivers playing and the dip at 65 or so Hertz is noticeble on both drivers. I tend to think it is a vehicle anomaly as you said as another guy on here with my same vehicle has a dip in the same spot actually. Is there anything that can be done tho other than boosting the crap out of 60 or so Hertz - maybe some kind of vehicle treatment or something? Try to move the subwoofer around front, side, (aim it to the back, to the front) and see if you have a change in response. Both the sub and the midbass have a dip? Strange, you must have a really crappy car for SQ  lol
> ...


Kelvin


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## goodstuff (Jan 9, 2008)

Wow. Over my head a bit. I normally like to swim with the big sharks but this is a little much. For me T/A has always been about making it sound like I am in the middle of the car as opposed to one side. Hence why t/a on a sub seems very useless especially since it's further away than the other drivers. YES I AM wrapped up in using it based on driver distances.( I don't measure though, just use my ears) So is t/a just adjusting phase? If so why don't they call it a phase shift? I I delay the left side so that the sound from the right reaches my ears at the same time as the left. Lol I though it was one of the few things I have down cold now you guys are blowing my mind and making me second guess myself.


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## ErinH (Feb 14, 2007)

Put your reading glasses on and pour a cup of coffee:
http://sound.westhost.com/ptd.htm


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## goodstuff (Jan 9, 2008)

bikinpunk said:


> Put your reading glasses on and pour a cup of coffee:
> Phase, Time and Distortion in Loudspeakers


I quit the bean a month ago but thanks for the link reading now.


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## james2266 (Sep 24, 2009)

bikinpunk said:


> Put your reading glasses on and pour a cup of coffee:
> Phase, Time and Distortion in Loudspeakers


Wow, what a read this is. I have been reading for over an hour trying to take it all in. I will have to finish it off tomorrow tho as I still have relatives to take care of


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## eddieg (Dec 20, 2009)

It is important to understand the difference between shift phase, polarity reverse, time correction and group delay and as well time correction methods. 

Time correction can be done via DSP buffering technology in which the whole signal is delayed without shifting phase or reversing any polarity. 

Time correction can be done by phase shifting and polarity reversing and it can be done on a certain frequency range (group delay).


The differences are that a group delay is very sensitive to audible differences so for a certain frequency you can only delay up to a small margin before it would become audible noticed. 

The stated above is why I prefer to use only buffering delay and as well do only the minimal changes required.


I also prefer to keep my subwoofers phase normal and not reversed and instead add time correction as phase reverse or polarity reverse may fix the shift phase but it can also cause other audible results, while a small time correction does not cause this. 

So - should we always use time correction? no, actually it is best to avoid if possible. 

But when using shift phasing techniques just does not cut it - TC would be very handy to have.


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