A friend of mine just bought a new Alpine HU, it has 4ch Digital Time Correction... what is this used for? why would you use it?

It also has Subwoofer Phase Control ... what is that? how does it work?

 

The digital time correction is intended to synchronize the arrival times of the various sources (speakers) by delaying the farther sources so that the signal sums at a particular location (your head) coincidently. This theoretically avoids "smearing", "slapback" and combing effects that can make things sound bad.

Normally this kind of thing is used in large stadiums and even home theaters where you have multiple sources and significant arrival time differences.

The only use that would make sense in a car would be to align a tweeter 6" from your ear and a woofer in the trunk.

The sub phase control--well think about a pure, clean bass note. It is basically a sinusoidal + and - pressure variation above and below ambient air pressure. At each point in the cycle there is an amplitude of wave and an associated phase, relative to the initial zero point. One complete + and - cycle sweeps through 360 electrical degrees (like a complete circle is 360 mechanical degrees.)

The idea is to get the sub output "in phase" with the woofers at the overlapping frequencies so the acoustic output adds for strong bass. Since the manufacturer doesnt know the relative polarities of the various woofers and subs being connected (determined by how a particular driver is charged (magnetized) and connected, usually when a positive voltage is applied to a + terminal, the woofer cone will move "out" towards you, you can check with a 9V battery.) or the location of the sub relative to the woofers, they need a way to make sure you can make the output of the sub and the woofers add in such a way as to reinforce each other--if they dont add in phase you will notch the bass output in the overlap region.

If you add additional reactive network components (caps, inductors, etc.)they add phase shift and it might turn out that you need to intentionally invert the sub electrical phase relative tp the woofer so they will sum acouistically.

You can also use the phase control to reduce huge peaks in the sub-woofer overlap area by intentionally ofsetting the relative phase of the sub output wave relative to the woofer.

This does not even touch on bass response and modal behavior in a small pressure box (a car interior), etc.

 

Error--first line should read "...by delaying the nearer sources to match the farther sources..."

 

Thanks ... do you have any idea how this would be setup in a car with 4 speakers and 2 seats, I understand how to do it for 1 person in the car, but for 2 i am stuck?

 

I would say start with your focus point right in the middle of the dashboard, right under the rear view mirror, and see if you like the imaging and staging.

It's just a trial and error deal unless you have access to an acoustic measurement system, and ultimately if it sounds good to you then you're dialed!

 

Thanks

 

I was reading an article yesterday about this subject. It was about this vehicle.

http://teamgates.net/bronco.html

A link to the article
http://www.wired.com/wired/archive/8...reocar_pr.html

I would imagine that most people on here have heard about this truck but it is something when you look at the alternator arrangement. This stereo puts out roughly at 175db. They have a computer if I am not mistaken that does the staging and synchronizing. The sub hits and it draws something like 10k amps for a second to a second and a half. They targeted a spot in the vehicle where a judge would put the microphone in an SPL competition. This is quite outrageous I must say.

Just my opinion though. Thanks,


[Modified by MY2K2C5, 10:37 AM 8/1/2002]

 

I use time correction in all of my vehicles. As JackAttack said, it delays the sound so it arrives from all drivers at the same time. I think it really helps in autos because there can be such a huge difference between right and left distances for a specific listener.

There are two basic ways to set it up. If you want it to be optimal for everyone, then find a spot on the center of your dahs and dela the drives for that. If you want it optimal for the driver, which is what i do, then you measure the differences to all the speakers from the drivers head. This will create an unreal soundstage.

Proper time alignment really works best when all the drivers have been x'dover active, or very close together if passive.

hope that helps.

Scott

 

WHen you say x'overed closely what exactly do you mean. We installed MB Quart Q Series Componenets in all 4 locations and are using the crossovers that they came with. THe amps are JL Audio, which have a crossover built in, we set those to match the speakers, they are set to high-pass at the same level as the speakers ... is there any thing I am doing wrong ...

 

If you are going to use passive xovers, then each driver of the set needs to be roughly the same distance from the "center point" in order for the time alignment to work correctly. I've seen it cause problems when someone puts the woofer in the lower door and the tweeter in the dash. In other words, with a passive xover, the drivers need to roughly be in the same place. If you are putting the tweeter and woofer in the same place, you are a.o.k.!

The above example would require a active xover with a channels of amplication for each driver in order to sound correctly.

btw, have you tried alpine's ipersonalize? it does all the work and lets you create a cdr that will autoload all the settings into your headunit. It can be found on their website.

Make sense? replay again if it doesn't. I don't do the best job explaining at times.


Scott

 

This is only relevant when a person has a truly digital HU correct? I do not see how a person could do it with a normal HU. Like AT&T saying that broadband cable is digital... :U

 

No, there are external processors out there that will do time alignment. It will go through another stage of a/d>d/a conversion. Some companies like Alpine and Pioneer have hu/processor combos where the hu feeds the processor digitally.

Scott

 

This years top of the line Alpine units have it built in ....!!! thats one of the good things about them :yesnod:

 

The unit that he has is 1 model below the "i-personalize" its the CDA7997? you can log online and get a printout of numbers to automatically program it ... but not a CD ... only the 7998 does that this year...

As far as the time correction is concerned ... I get your point ... if the speakers have a distance between them the timing will be off... but they are very close ... so I should be okay ...

 

Wow that is pretty cool. I haven't studied up much on the car audio stuff much. I have been focusing on my home theatre system and now devoting time to the car audio in the vette until my wife has had enough... :eek:

Last couple of years I have been to sick to really go out and do things so I worked on setting up a nice home theatre setup. Now that I feel better thanks to the marvels of modern science, I am getting into car audio.

 

NewVetteLover:
Exactly! btw, I didn't realize only the 7998 did the ipersonalize. hmmm.....

MY2K2C5:
same concept applies to ht. It comes in VERY handy if ht is not setup symmetrical in the room. I've used this feature on several peoples ht who didn't even realize they had it. :) It made amazing differences.

Scott

 

yeah, my home theatre isn't the best but it is decent I think. I have a Pioneer Elite A/V receiver and the Pioneer Elite DVD player. The receiver is pretty amazing. I had to set all of the items that you are all talking about here. I do have 3 - 15" power subs in my home though... they sound pretty awesome. I like having all THX Ultra certified components and speakers. Listening in 6.1 is pretty nice I must admit. I should look into doing it in my car also. hmmmmm....now I am thinking again... :yesnod:

 

Alpine is introducing an new processor, the PXA-H700, that will have Dolby Digital, DTS, and Dolby Prologic II. I hear it works great on making surround on a 2 channel, four speaker system.

Scott

 

Sounds pretty intense! :cool: Would it then be possible to hear, say, XM radio in true digital format? Do the seperate components use digital interconnects?

 

Probably, but don't know since I haven't actually seen all the components to know for sure.

Scott