Film-Tech Forum ARCHIVE: Surround EX Back Channel Power Requirements/Specs

This is topic Surround EX Back Channel Power Requirements/Specs in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Lyle Romer (Member # 1266) on 09-16-2009, 12:32 AM:

I did some searching and couldn't find the answer to this. With regular left/right surround the array should be designed so that each channel can hit 100 dBc in the center of the auditorium.

The question is, what are you designing for with the back surround channel? I guess it's a 2 part question, first what is the peak output level of the back surround channel by itself (rbs and lbs combined)? The second part of the question is where in the auditorium should this peak be hit? Is it 2/3 of the way to the front (opposite of a stage channel)?

Posted by Steve Guttag (Member # 268) on 09-16-2009, 06:11 AM:

You are close...each side should be able to hit 102dBc to half-way across the auditorium.

The rear channel should be able to hit 105dBc 1/3 of the way down the auditorium. This normally puts a significantly greater demand on the rear speakers.

When thinking about the levels remember...regardless of EX or 5.1 system, given the same signal, the SAME level should be heard in the theatre. Thus if the film has equal content in the two surround channels and you have a 5.1 system and that level measures 85dBc total (which ends up being 82dBc per side) then if you play it in EX...the level should also be 85dBc but all of the content comes from the rear.

Steve

Posted by Louis Bornwasser (Member # 3063) on 09-16-2009, 06:36 AM:

As a practical matter, large amps are cheap; make them all big amps. Then you have spares to borrow anywhere.

There is more profit in the larger amps for the factories. Therefore they can be more heavily discounted and they become a reasonable buy. Louis

Posted by Marin Zorica (Member # 1716) on 09-16-2009, 07:54 AM:

As mentioned.....there is power difference in L/R to Rear surround....also in some cinemas we have lot more surround speaker on rear wall than on back, so that comes we must put bigger speakers on back wall, or more efficient to get bigger level from it also?

Posted by Demetris Thoupis (Member # 833) on 09-16-2009, 08:06 AM:

But why would you want to invest today in the EX?

Posted by Lyle Romer (Member # 1266) on 09-16-2009, 08:34 AM:

The investment is really just an extra amplifier. I have an SA-10 adapter literally laying around.

I think pretty much all current generation processors have the EX decoding capability built in (or at least as an option).

I also assume that at some point, there will be discrete back surround mixes available with digital cinema. Especially since DCI specifies 16 audio channels. 3 could be used for the alternate discrete back channel surround mix.