Film-Tech Forum ARCHIVE: CP650 surround levels

This is topic CP650 surround levels in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=16;t=002341

Posted by Ken Lackner (Member # 1002) on 06-23-2015, 08:25 PM:

What are the proper levels to set each surround channel at on a CP650 under the following scenarios?

1.) Cat 790 card installed, thereby enabling BSL and BSR outputs, but separate amplifier NOT used; left half of rear surrounds is wired as part of LS group and right half is part of RS group. 82dB each, right?

2.) Cat 790 card installed, separate amp for rear surrounds is used and connected to BSL and BSR outputs of the 650, but software is not updated to enable the 7.1DS format. So basically the house is a traditional 5.1 house with an extra amplifier for the rear. Since the Cat790 card was also used to enable Dolby Digital EX, the rear surrounds should be connected to the rear surround outputs, and those channels will simply duplicated the LS and RS channels when in a 5.1 format, correct? If my understanding of the above is correct, then I would think that LS and BSL combined are 82dB when in 5.1, so how are the levels set when tuning?

3.) Same as above but software IS updated to support 7.1.

Posted by Steve Guttag (Member # 268) on 06-24-2015, 06:51 AM:

In situations 2 and 3, all surrounds are set to 82dBc. The processor will adjust levels on a format by format basis to ensure the proper level...including 7.1 versus 5.1. So when you are in format 88 (6-channel PCM) it will play 5.1 equally out of the side/rear surrounds such that -20dBFS will be 82dBc in the theatre.

In 1), you'll never be correct. When you sum voltages of identical level you gain 6dB. When you some SPL of identical levels, you gain 3dB. So if you set to 82dB/quadrant. But in effect, you'll have set to 88dBc when both outputs are playing equally...the processor when playing 5.1 will drop 3dB back to 85dBc in 5.1 modes. So you will be 3dB hot/side. So you'd have to set to 79dB/quadrant to get it to be 82dB/side in 5.1 mode.

If your goal was to be able to handle 7.1 tracks on all screens and use this scheme to mix down in 5.1 houses, when in 7.1 mode, when in 7.1 mode you'll be 3dB low/quadrant.

Now if you meant that you'd just ignore the BS outputs entirely and just want to know if you need to compensate on the Ls/Rs outputs then yes...you need to set Ls/Rs to 85dB so in a 5.1 mode,it drops it back to 82dB presuming that BSl and BSr are doing their part. But this scheme only works if you only run 5.1 stuff. To convince yourself that the levels are proper...run a DCP with a 5.1 pink noise reference and measure it as it goes from channel to channel.

Posted by Ken Lackner (Member # 1002) on 06-24-2015, 09:46 AM:

Thanks for the info, Steve. I had a feeling you'd be the first to respond! [beer]

Not interested in handling 7.1 tracks on 5.1 only screens. Wondering if a 790 card is installed for onboard AES decoding, whether it matters if an additional amplifier is added to drive the rear surrounds. From your explanation, it seems like that is the best way to do it. (Then you basically have a 7.1 house and may as well upgrade the software. "3" was just for clarification; I was mainly wondering about scenarios 1 and 2.")

Posted by Terry Monohan (Member # 8285) on 06-24-2015, 10:08 AM:

Many theatres I go to these days have the surround speakers turned way down. Almost no sound coming out at all. When I complain to the candy/projection managers they will say they had to many complaints about them and they turned the volume down. It is wonderful that I have a good Dolby®5.1 set up in my home to catch all the great surround sound on DVD's that I missed on the surround tracks in some cinemas. A few theatre mangers I know turn them up past the setting level and It turns the whole cinema into a great wrap-a-round sound experience. Only once in all my years of going to stereo movies I asked the manager to turn them down. The surrounds need not be as you know as loud as the main speakers. In this theatre more sound was coming out of the side/back surround speakers then the stage horns. Every movie is mixed different for surround sound today. If you own a small single screen theatre try to adjust the surround level for each movie you run and make It a nice sound day at the movies for your paying guests. The old people can wear ear plugs If the sound is to loud for them. You don't need to blast out everyone, a movie theatre is not a disco!

Posted by Mike Blakesley (Member # 26) on 06-24-2015, 12:19 PM:

The surrounds are supposed to blend with the front speakers, not call attention to themselves all the time.

Posted by Marco Giustini (Member # 4544) on 06-24-2015, 02:20 PM:

Terry,

That's exactly why I love QSC DCA amps and sound processors with passwords.

Mike,
Correct, if I may add: they're supposed to be at the CORRECT level and not tampered with.

Posted by Sam D. Chavez (Member # 1841) on 06-24-2015, 03:14 PM:

Terry,

I'm not so sure the managers are not just humoring you and pretending to turn up the surrounds to your liking. I happen to know it's happened as I did it my self to more than one director and they went away happy none the wiser.

But, that being said, I did take you seriously regarding the Castro and rechecked the surround levels in 5.1 and 7.1.

You'll be happy to know the was a problem under certain scenarios where the back wall surrounds were not working at all. This has since been dealt with and my apologies for doubting you.

Posted by Terry Monohan (Member # 8285) on 06-24-2015, 08:14 PM:

Thanks Sam for checking the old Castro's surround speakers. I new there was a problem and now you fixed them. Next time I go I'll be glad to hear surround again in the great Castro Theatre SF. Thanks again!