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Rear fill, L-R stereo/mono question

34K views 45 replies 13 participants last post by  jwebster 
#1 ·
I'm getting to the point in my install where I'll start wiring in a few days. I have read every single thread here :), and information in other places, about doing rear fill using L-R, but I still don't have any definitive answer.

If we are trying to do the Haas effect, do the rear fill speakers (with the assumption that you have more than one speaker, in the doors or in the rear deck) need a "stereo" signal, different signal for each side, or can you use a "mono" signal for both sides?

Here's the system diagram I'm "planning" to use, including the connections for rear fill. Will this "work"?



Thanks.
 
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#3 ·
The CarPC is nothing special. An Intel D945GCLF2 motherboard with the built in dual core Atom processor and GMA 950 graphics. More than enough for my (current) needs. The video will initially be just composite video, before I start experimenting with sync-on-green and RGB signals that the BMW LCD expects. The CarPC will be connected to a SpaceNavigator (in the center console) for control, and hooked into the car's IBUS via a Resler IBUS interface.
 
#6 ·
I would think a mono signal would not work. If the sound is already mono and run into one cable, how (even if you split it into two cables) could the amp filter out the common information? It is all common to both channels in mono so there would be no sound.

What you could do is run the fronts passive also and get l-r rear fill. Or you could get an Alpine H701 and sell the 360.2, and run two-way front/l-r rear fill/sub. The last option is have a headunit with two RCA outs (not sure if you do), run the one set to your 360.2 (connected to midbass, l-r rear fill, and sub) and run the second to your tweets (the JL crossover is high enough @ 5khz on their amps and the tweets are more sesitive to amplitude to t/a isn't as needed--but you would loose eq).

I'm sure it isn't what you wanted to hear, but I hope it helps in your decision.

Looks good, but.....
You should either use the crossover, or dont use the crossover on both your component sets. My opinion only.
WHY???? His plan is to run the front stage active which gives better tuning options if done properly. The rear is just for ambiance, no need to run it active and no need to "hurt" the front stage because of this.

Not trying to sound mean at all, but I do think you should read a bit more about what the OP is trying to do if you don't understand it before giving advice.
 
#5 ·
Looks good, but.....
You should either use the crossover, or dont use the crossover on both your component sets. My opinion only.
 
#9 ·
Well...since that L-R signal is going into the 3Sixty, it can do TA/EQ etc on that. However, the $64,000 question is that if I'm running two speakers for rear fill, one on each side of the car, do they need L-R and R-L, or can L-R be fed to both of them?

Since L-R is removing all common information between L and R, L-R should "potentially" be the same as R-L, except "maybe" 180 out of phase (which I don't know yet).
 
#10 ·
I don't have the car I have this set up in in my possession right this moment, but a not from my work says that I am running L+ and R+ to each of the left and right channels. That doesn't mean that you want to run mono, because you will want to run separate EQ and TA for each side.
 
#11 ·
Now that makes sense, since the path lengths for the left and right rear speakers will be different as seen from the driver's seat. But, what's encouraging is that the same L-R signal can work for both of those speakers. Hmm....I guess, since I have only one channel of processing left on the 3Sixty, I'll live with the less than ideal scenario. Maybe attenuate the nearer rear speaker, bit more than the opposing one?
 
#12 ·
Doing l-r before the 360 would work and it would remove the center info, but when you run it from the center channel, it would still be mono meaning that the far right sound would come out the left speaker with the far left sound (so it would still be the same sound out of both speakers just minus "center" information such as vocals). I think it would be better than rear speakers, but still not true rear fill.
 
#16 ·
Oh crap... :( I just realized (late light bulb moment) that the 3Sixty can delay only up to 5ft, which would equate to roughly 5-5.5ms of delay. Crap crap, that may not be enough for rear fill..... :(

Man, I had a HUGE response typed out and my computer at work shut down. But here is a shortered version.

You can get a l-r signal (centered information cancelled out), bandpass it, eq it, and delay it. That is what is needed for true rear fill. These are all the things needed for proper l-r rear fill. The problem is that the output your using on the 360 is a single mono channel likely into a y-cable that splits the rca's so you have one for both left and right input at the amp. This means that instead of hard left information playing from the left speaker it will play from the left AND right speaker (since it is mono) and instead of hard right information playing from the right speaker it will play from the right AND left speaker (again in mono).

EX--If you have a guitar playing on the left side of the stage:
2-way: It will play from the left side front speaker
2-way w/ l-r rear fill: It will play from the left side front and rear speakers
2-way w/ l-r rear fill in mono: It will play from the left side front and the left AND right rear speakers

So you would have information on the left side "pulling" back to the right. Will it be noticeable? I don't know. Personally, I would do everything like you planned and see how it works. If it doesn't work well, just disconnect the rear fill. You could then wait until the JBL MS-8 comes out (it should actually come out in 2010) and it will actually auto eq AND is set up for l-r rear fill. You could put it right in place of the 360 and sell it later. That seems to be the cheapest way to me.

Also, I didn't take you as being a dick. As long as I feel a person has tried there hand at searching (or I feel the need to stop bad advice from being given), I have no problem helping out.
pionkej, I very much appreciate the detailed information that you keep providing. :) Thank you. I'll have to think about this a bit more I guess....
 
#15 ·
Man, I had a HUGE response typed out and my computer at work shut down. But here is a shortered version.

You can get a l-r signal (centered information cancelled out), bandpass it, eq it, and delay it. That is what is needed for true rear fill. These are all the things needed for proper l-r rear fill. The problem is that the output your using on the 360 is a single mono channel likely into a y-cable that splits the rca's so you have one for both left and right input at the amp. This means that instead of hard left information playing from the left speaker it will play from the left AND right speaker (since it is mono) and instead of hard right information playing from the right speaker it will play from the right AND left speaker (again in mono).

EX--If you have a guitar playing on the left side of the stage:
2-way: It will play from the left side front speaker
2-way w/ l-r rear fill: It will play from the left side front and rear speakers
2-way w/ l-r rear fill in mono: It will play from the left side front and the left AND right rear speakers

So you would have information on the left side "pulling" back to the right. Will it be noticeable? I don't know. Personally, I would do everything like you planned and see how it works. If it doesn't work well, just disconnect the rear fill. You could then wait until the JBL MS-8 comes out (it should actually come out in 2010) and it will actually auto eq AND is set up for l-r rear fill. You could put it right in place of the 360 and sell it later. That seems to be the cheapest way to me.

Also, I didn't take you as being a dick. As long as I feel a person has tried there hand at searching (or I feel the need to stop bad advice from being given), I have no problem helping out.
 
#17 ·
You can get a l-r signal (centered information cancelled out), bandpass it, eq it, and delay it. That is what is needed for true rear fill.
says who? true rear fill? there are many ways to acheive a rear fill that will help add to the front stage and a sense of ambience and openness. I've tried several. I don't think its fair to say a rear mono center will not work unless you have tried it.

I would say hook up the 360 and try it :)

Pionkej, can you describe your setup now? what are you using or used to achieve true rear fill?
 
#19 ·
I appreciate the info guys. Here's another thought and it gets more complex... :) Forgetting the issue of the 3Sixty having only about 5.5ms of delay for a second, let's focus on the single channel issue.

That channel can obviously have only L-R or R-L, but not both, and you can apply TA and EQ to it. I understand that the delay "ideally" should be different for the rear left and rear right "rear fill" speakers, but these are going to be attenuated a lot, and I'm not sure how much "actual" difference may be there between the two approaches.

Now, if we have only one channel for rear fill and it's L-R, we could potentially also use amp bridging to get L-R and R-L to the respective speakers, couldn't we?

Quoting WhteRabbit here from the thread "Rear Fill: do you use it" -

to my knowledge, the opposite. a bridged amp will sum the input signals. By inverting one signal, the summation will result in an L-R (as in, L + (-R)) signal. This would be run as a bridged output to a rear speaker.

By running the two rear speakers in series, they both get this L-R signal.

Finally, by inverting the right speaker, we invert the signal to get a -(L-R) signal, or an R-L signal.

Thus, by starting with only L and R and a bridgable amp and NO processing, we have rearfill where the left driver receives L-R and the right driver receives R-L
That would potentially work, right?
 
#20 ·
That only works on amps that aren't meant to be bridged or have a bridge switch (where you actually flip a switch if you want to bridge channels-for l-r you just leave it unswitched). These are old school amps we're talking about here. Like some old A/D/S power plates.

Newer amps flip the phase automatically when they "sense" the channels are briged. Which means the L + (-R) doesn't happen, it just runs the two speakers in series at an 8 ohm load and doesn't cancel the center information.

You are better off with your original idea. Since BMW's have balanced lines, you can get l-r from there.

EDIT: Try it though on the amps you have. When I tried it in my truck, no center information was cancelled out.
 
#21 · (Edited)
Thanks Pion for the explanation. you are right. werewolf's idea of rear fill is exactly what you described, however, and I want to apologize in advance if I seemed argumentative, is that you can still acheive some good results with a less than perfect setup :)

my experience with the delay is 15-20ms is ideal...5ms was not enough. you might have to attenuate it more as a tradeoff. it created an echo effect if you will and made the midrange sound strange.

I dont have an answer to the other question you asked OP
 
#22 ·
No offense taken. I like when discussion can be opened up like this as it betters everyone (or at least anyone who searches in the future before asking the same question :) ). I honestly had no idea that could work in mono, so it is very good to know that it has been tried and people have satisfactory results.
 
#25 ·
What if you just did not connect the center or subwoofer input on the 360? The 360 should create the center image ( and sub channel ) for you?

I have used the center channel output ( with no center input connected ) for rear fill with good results. (If there were more outputs available on the 360 I would do a stereo rear fill, unless the rear channel was a single speaker in the center of the rear deck though.)
 
#31 ·
I decided not to read majority of this...

Here's what I've started to put together for my application of rear fill. Your over all goal is this...

1) Recombine L and R channels as such: +L(-R) and +R(-L) You do this on your XLR cables using the product in my thread.
2) Bandpass the speakers for ~ 300hz to 3000hz. You can go as high as 5khz or 8khz but it's all up to you. I'd stay away from bass frequencies to prevent localization
3) Time Alignement. if you have the equipment time align per driver. If not, just do 20ms for your speakers and adjust as needed.

http://www.diymobileaudio.com/forum/dumb-question-forum/69956-how-create-difference-sums-l-r-r-l.html
 
#33 ·
Parts-Express.com:ART CleanBOX Stereo Balanced/Unbalanced Converter | interface adaptor line level consumer transformer

Alright, from the start you take the RCA cables from your H/U and plug them into "RCA INPUT". Now you have a balanced XLR output.

Speakers use L+, L-, R+, R-. So do balanced inputs. XLR cables also include a ground. So you recombine the XLR output cables as L+(R-) and R+(L-) and leave the ground alone. The way balanced inputs function you will cancel all common frequencies between L and R on BOTH R & L channels. This will leave you with extreme L & R channels w/o any center channel information in your XLR cables.

Now...after you're recombined you feed the XLR cables to the XLR INPUT.

Finally, feed the RCA OUTPUT/L-R signal into your amps. All done.

FYI: Since I own a 9887 and will possibly attempt a passive setup up front I will have rear channels for T/A and bandpass. Of course before I ever put together my rear fill application I need to fix the setup I have up front.
 
#34 ·
Parts-Express.com:ART CleanBOX Stereo Balanced/Unbalanced Converter | interface adaptor line level consumer transformer

Alright, from the start you take the RCA cables from your H/U and plug them into "RCA INPUT". Now you have a balanced XLR output.

Speakers use L+, L-, R+, R-. So do balanced inputs. XLR cables also include a ground. So you recombine the XLR output cables as L+(R-) and R+(L-) and leave the ground alone. The way balanced inputs function you will cancel all common frequencies between L and R on BOTH R & L channels. This will leave you with extreme L & R channels w/o any center channel information in your XLR cables.

Now...after you're recombined you feed the XLR cables to the XLR INPUT.

Finally, feed the RCA OUTPUT/L-R signal into your amps. All done.
Dude...I already HAVE balanced outs from my head unit.....:rolleyes: and I don't need to convert them to unbalanced, the 3Sixty will accept a balanced signal. (minor issues not withstanding).
 
#35 · (Edited)
Then grab an amp that accepts balanced and you're done. I believe the 3sixty will bandpass and T/A your signal. Otherwise, you need at least SOMETHING that accepts balanced to filter the signal for L-R prior to going into the amp.


EDIT: Sorry I didn't skim over your post before. Just a little tired of the rear fill questions...my bad.
 
#37 ·
You're right. But while researching, if you have the correct T/A distances some people say that even 20khz is ok when playing the rear fill. But when just using a general T/A for both speakers of say 20ms then you CAN go as high as 8k but it really depends on the vehicle and preference for sound.

personally, I don't plan on going over 3khz.

Remember...I'm just regurgitating :D I tried it once and I liked it. That's why I plan on doing it again. I'm just having a hard time finding permanent install hardware for the app. I'd prefer to have a 4-way head unit.
 
#38 ·
More good information on things you can do with rear fill that I hadn't seen before. Still not sure I would like it either, but certainly would be worth trying if you had the means to do so. Thanks for sharing.
 
#40 ·
OK, to sum it up, is this a fair description?

- A single L-R signal is "ok" to use even if you have two rear fill speakers (farthest corners, left and right), since we have removed the common information between L and R, and with that removed, the difference between L-R and R-L is negligible/wouldn't have any real world effect.
- A minimum delay for real fill is probably in the 15-20ms and above range. Anything below that will probably kill the whole rear fill effect.
- The rear signal must be bandpassed, probably between 150-200 to 2k-3k or so. This of course needs to be fine tuned.
- You must have the ability to attenuate the rear channel(s) to make them blend better (easy with amp controls).

btw, guys I REALLY appreciate all the information, this is good stuff.
 
#43 ·
I'm a little late to this thread, but thought I would set the record straight.

The rear speakers should be wired out of phase for best results with the L-R signal... so they cannot be localized.

So, you have a single (mono) signal of L-R going to the rear amp... the left speaker + goes to amp +, and negative (-) speaker terminal is wired to negative amp terminal.

Then wire the right rear speaker out of phase... + speaker terminal to negative amp terminal, and - speaker terminal to + amp terminal.

You don't need a delay, a fader works fine.

You don't need a center if you have good placement in the front (kick panels), you'll have a phantom center... the left and right speakers have the center channel information and if they are close enough (kick panels) to each other, will acoustically sum the center.

Dolby specifies a 6db per octave low pass of about 7k for the rears... this sounds best to me, but some have said that 2k works better. You can experiment with it and see what you like.

I usually high pass the rears the same as the fronts... 60 - 90 hz.

Speaker placement and installation are still king... for best results, I recommend sealed kick panel placement for the front satellites, and rear deck placement for the rears in sedans. In hatch backs you can customize a rear subwoofer box by putting the sub(s) in the middle of the box, and putting the rear satellites in either end of the box in thier own separate chambers.
 
#44 ·
Good experience based info...thanks. Instead of going 60-7khz or so, I was just planning on matching the range of speakers in my kick. I have tweeters in my kicks and "ambiance" tweeters in my pillars amd then 5.25" in my kicks and 8" in the doors. I'm using a 3" wideband for the rear-fill. I was planning on playing them around 200-5khz to match my 5.25" and kick tweeter. Does that sound like a good starting point?
 
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