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Rear Center Questions? Arny, Amirn, Ethan, Wayne

post #1 of 10
3/6/13 at 11:16am
Thread Starter 
Back in 2002 I made a DIY rear center setup using an old Dolby pro Logic decoder fed with the main processors rear L&R inputs. This was a somewhat popular kludge of the day.

Question now, is this doing more harm than good in my room? I am concerned about phase cancellation / combing.

Thechnical notes:
My main processor is a Lexicon MC8. The rear center processor is a Lexicon CP1. The MC8 is actually modified to provide a pre-volume 8 channel out. I use the rear L&R pre-volume signals to feed the CP1. I also have a VCA box (with a copy of the MC8 VCA circuits) that is connected to the MC8 VCA control buss so the CP1 output tracks the MC8 volume control. This goes between the CP1 center output and the rear center amp. A bit overkill but I thought the CP1 decoder may have a better S/N with full level input signals versus taking the feed from the rear amp inputs. Note that the 8 channel pre-volume out is also for my metering displays.

I would appreciate your expert opinions as well an anyone else on this matter.
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post #2 of 10
3/6/13 at 11:28am
The only time I'd use a rear center speaker is with 6.1 or 7.1 etc content that is encoded specifically for that placement. As for avoiding comb filtering, this is why you should measure the room using proper software.

--Ethan
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post #3 of 10
3/6/13 at 11:40am
Thread Starter 
Well I have and use REW but I never ventured into the rear center issue. I guess that's a project for some weekend.

With the "properly" encoded material, is the rear center still just matrixed from the rear L&R? I am not aware of any dedicated rear center channel in the hi-rez BluRay formats but I'm not up on all those either.
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post #4 of 10
3/6/13 at 12:24pm
Quote:
Originally Posted by Glimmie View Post

My main processor is a Lexicon MC8. The rear center processor is a Lexicon CP1. The MC8 is actually modified to provide a pre-volume 8 channel out. I use the rear L&R pre-volume signals to feed the CP1.
Why aren't you using the rear channel extraction in your MC-8?
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post #5 of 10
3/6/13 at 12:27pm
Thread Starter 
Quote:
Originally Posted by sdurani View Post

Why aren't you using the rear channel extraction in your MC-8?

Not familiar with that? Where does the physical rear center output come from? The MC8 only has 8 outputs, L,C,R,Sw,RL,RR,SL,SR.
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post #6 of 10
3/6/13 at 2:33pm
Quote:
Originally Posted by Glimmie View Post

Not familiar with that? Where does the physical rear center output come from? The MC8 only has 8 outputs, L,C,R,Sw,RL,RR,SL,SR.
When you say "rear centre", do you mean a 6th channel (as Ethan was referencing with his mention of 6.1 and 7.1 material), or are you talking about adding an 8th channel to a 7-speaker layout?
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post #7 of 10
3/6/13 at 4:37pm
Thread Starter 
Quote:
Originally Posted by sdurani View Post

When you say "rear centre", do you mean a 6th channel (as Ethan was referencing with his mention of 6.1 and 7.1 material), or are you talking about adding an 8th channel to a 7-speaker layout?

Yes, adding an 8th channel (or 9th if you count the sub). The proposed setup from around 2002 era was to add an old Dolby Por Logic decoder fed by the rear channels and use it's center out for the rear center.
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post #8 of 10
3/6/13 at 8:28pm
Quote:
Originally Posted by Glimmie View Post

Yes, adding an 8th channel (or 9th if you count the sub). The proposed setup from around 2002 era was to add an old Dolby Por Logic decoder fed by the rear channels and use it's center out for the rear center.
Got it. I was misunderstanding what you meant by rear centre. Back to your original question (if you're willing to accept an answer from someone who isn't Arny, Amirn, Ethan, or Wayne).
Quote:
Originally Posted by Glimmie View Post

Question now, is this doing more harm than good in my room? I am concerned about phase cancellation / combing.
Cancellations and combing were never an issue, for a couple of reasons: 1) the rear centre was never playing the same content as the left rear or right rear (centre signal is cancelled from those speakers), so you were never hearing dual-mono or triple-mono playback; and 2) even if there were moments where two speakers were producing the same sound, you wouldn't have noticed the comb filtering (our human hearing ignores it, otherwise 2-speaker playback wouldn't have worked all these decades).

As for doing more harm than good, unfortunately that has always been the case. A single speaker along the listener's centre line means that those sounds are heard equally in both ears. By reflex, our human hearing usually interprets that to mean that those sounds are directly in front of us. The problem is often referred to as back-to-front reversals or imaging reversals. This is why your MC-8 doesn't allow for a single rear speaker (6.1 set-up). This is also why Dolby, DTS and THX all recommended 2 rear speakers for playing back the mono surround-back channel of EX/ES soundtracks. Note that it was the only channel where 2 speakers were recommended for playback.
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post #9 of 10
3/7/13 at 10:51am
Thread Starter 
Quote:
Originally Posted by sdurani View Post

Got it. I was misunderstanding what you meant by rear centre. Back to your original question (if you're willing to accept an answer from someone who isn't Arny, Amirn, Ethan, or Wayne).

Sdurani, you input is highly valued too. You are one of the surround experts here IMO - and a fellow Logic 7 fan wink.gif.

I guess i will just leave it in. I can turn the rear center on and off at will via Irule (easy to do, just use an IR command to the CP1). This way I Ican have it both ways.
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post #10 of 10
3/7/13 at 11:41am
FWIWFM, I had a single rear (6.x) for a short while before getting my present 7.x system set up. I found the rear was OK for some material, but since a lot of surround material goes to the rears as well as sides I found much of the time the matrixed/summed/whatever center rear collapsed the strong "side" effects to the center, collapsing the image. Example would be a jet flying off to one side would get sort of "smeared" to the middle as it tracked back. I got rid of the center rear.

YMMV - Don
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