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Author Topic: Dolby CP750 sound issue
Jay Glaus
Expert Film Handler

Posts: 113
From: Pittsburgh, PA USA
Registered: Dec 2010


 - posted 06-28-2013 01:04 AM      Profile for Jay Glaus   Author's Homepage   Email Jay Glaus   Send New Private Message       Edit/Delete Post 
Hi all,

I recently had an issue with my CP750 in one of my booths with a barco 23b / GDC set up.

First, keep in mind that this is a drive-in, so we are not going directly into amplifiers. We are going from the dolby to a mixer to an FM transmitter.
Part way through the first movie, people started complaining that they could not hear anyone talk, the voices were getting squashed by all of the other sounds. Others were saying it was too loud. When you turned your radio up to hear people talking, when anything else happened it was unbearable to listen to, you had to turn it down.

The staff member that got their first, I had him turn down the Dolby. He informed me that the Dolby was at 7.8 and not at 7.0. Why that would be baffled me. It always goes to 7 when it kicks into digital 1, that's what we run it at. But ok, so we turn it down... to 5. Still doesn't help. They say you just have to turn it up more to hear the voices, and anything else still makes it over the top. Basically, its proportional.
Ok, makes sense. So, I turn the center channel on the mixer up, while dropping the left and right a little. We're getting there. I take the surrounds way down as well. Better. I just about have the center channel on the mixer a few clicks from max and a few people came back to tell me it got better.

I thought maybe it was just the first movie (even though this never happened the whole time we played it, gives you something to hang onto). Second movie is the same. Its better, but people are still saying there is something wrong. One customer pointed out that there is a scene where a gentleman is running and talking, thats all that is going on. It's hard to hear his voice over his footsteps, they are overpowering.

The next day, our tech came out and took a look. Of course, we shut our sound rack off every night, so the dolby had a chance to reboot. He said everything just seemed way too hot when he heard it. He went in the Dolby on the laptop, brought the center up, took left and right down, and took the surrounds down. Then he boosted the vocal range while taming down the extreme highs and lows a little bit.

Sounds fine now, and we've run without complaints. My question is, what would cause it to do that? Its almost like the center channel dropped out, or partially dropped out. That's all I can think of. One thing I wish I would have done but it slipped my mind was look at the screen on the 750 and make sure the center was pumping when it should have been. And as far as why it would have been on 7.8 is beyond me. Its almost like there was one major glitch.

I certainly understand those happen with computers, but my concerns are: where did the glitch happen, why did the glitch happen, and will it happen again? I'm happy its fixed and sounds fine now, but if it decides to do this again on a saturday night with a full house, I'm screwed! I want to try to figure out what happened. I recently had this problem (and still do) that I posted about a while back. But as far as we could tell, that previous problem was purely cosmetic and was not life threatening in any way to the dolby.

Which leads me to my next question. I have my old CP500's sitting around from 35mm. I have 685 cards for them as well. What I wanted to do, was hook them up in my sound rack, so if my 750 ever decides to bite the dust, I have something I can use to get me by. I figure the stuff is sitting around, I might as well use it. How would I connect that CP500 to my digital projector? I believe I need a scaler as well? I was told a little while ago how exactly the hook up would be but darned if I can remember.

Anyhow, and light shed on that situation would be appreciated. Thanks!
Jay

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Gavin Lewarne
Master Film Handler

Posts: 278
From: Plymouth, UK
Registered: Aug 2009


 - posted 06-28-2013 02:35 AM      Profile for Gavin Lewarne   Email Gavin Lewarne   Send New Private Message       Edit/Delete Post 
My Colleague swears our 750 turned itself up once. He left the room, locked the door, went to get his dinner, came back in and the processor was a full 1.5 higher than.it should be.

Personally I have never seen it.

Is it possibler someone knocked the gain controls on your mixer without noticing?

Edit..

I also have the same problem with sporadic washed out display. I read your other thread....I will try reseating the cables as well.

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Dave Macaulay
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From: Toronto, Canada
Registered: Apr 2001


 - posted 06-28-2013 07:51 AM      Profile for Dave Macaulay   Email Dave Macaulay   Send New Private Message       Edit/Delete Post 
Why are you using a CP750 at a drive-in? This seems like an expensive overkill.
I've seen occasional oddities with CP750s as well, very rarely. What you describe sounds more like a mixer issue than the CP though. This is a common problem when using a PA mixer to get stereo from 5.1, I prefer a fixed gain mixer unit (Odyssey has one if you don't want to make your own, it's just a resistor network). The lack of adjustability means that although you can't tweak the sound on purpose... it won't happen inadvertently either. The usualy PA mixers have many possible ways to screw up the sound, and a complex array of knobs and switches that all have to be correctly set.
Your CP750 is now your film sound D-A converter (unless you are the victim of an astoundingly predatory salesman). You will need to get an outboard D-A converter to bypass the CP750 and use the CP500, it does not have a digital input. The 685 card adds a 6 channel analog input.
But it doesn't make sense to me to use your old CP500 for a backup system. But, then again, the CP750 makes no sense to me either. Maybe there's an excellent reason for needing it?
If you really want to have a backup system, get or make a 5.1-stereo downmixer and buy a suitable D-A converter (Doremi's AUD-2A is good but only handles the film audio, USL's ECI-60 has several input options that you can automate for switching between film/nonsync/alternate content sound, and the Dolby DMA8+ is somewhat more flexible). Plug the D-A output into the downmixer and plug its stereo output into your transmitter input. Adjust transmitter gain to suit. Done. Add a simple stereo mixer and you have an announcement mic as well.
If you actually do that, use it as the main (only) system and sell the CP750. Much less to go wrong and a bunch cheaper.

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Tony Bandiera Jr
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Posts: 2942
From: Moreland Idaho
Registered: Apr 2004


 - posted 06-28-2013 10:58 AM      Profile for Tony Bandiera Jr   Email Tony Bandiera Jr   Send New Private Message       Edit/Delete Post 
Jay, Iam gonna ask you now-famous question in this forum: Are the CP750 (AND server and projector electronics) on a UPS?

If not, you are going to continue to have problems galore. All of this stuff is REALLY sensitive to incoming power issues, and all it takes is one power glitch (which may or may not be noticeable to the naked eye) to create problems.

AT the bare minimum, get the CP 750 and server on a ups..anything 500-750va or higher will work. (If the GDC draws the power I think it does.)

As Dave said, to use the CP500 as a backup you need the 685 card, AND you would need a DMA8 or ECI-60. Too cumbersome IMHO.

quote: Dave Macaulay
Why are you using a CP750 at a drive-in? This seems like an expensive overkill.
Dave, are you sure about that? First off, even a DI MUST have EQ on the sound. A DMA-8 will not do that. (I am not sure if the ECI-60 does eq but I doubt it.)

Second, CP-750's are not "expensive" anymore. Price is reasonably in line with the expense of using a DMA8 plus adding the outboard EQ's you would need, and it gives you flexibility.

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Scott Norwood
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From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99


 - posted 06-28-2013 11:13 AM      Profile for Scott Norwood   Author's Homepage   Email Scott Norwood   Send New Private Message       Edit/Delete Post 
Why would a drive-in need EQ? Drive-in speakers are going to sound like crap with it or without it. For radio transmission, I could see using a compressor, but most car radios at least have "bass" and "treble" adjustments.

Would the purpose be to add emphasis to dialogue frequencies?

(I am genuinely curious here. The DI sound systems that I have seen--both speakers and radio--have all been rather low-tech, using stuff that would be found at (and probably was purchased from) the local Radio Shack. I can think of one place that has (had?) a CP55, but I think that they just took the signal off of the backplane after the preamp and NR stages.)

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Jay Glaus
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Posts: 113
From: Pittsburgh, PA USA
Registered: Dec 2010


 - posted 06-28-2013 12:48 PM      Profile for Jay Glaus   Author's Homepage   Email Jay Glaus   Send New Private Message       Edit/Delete Post 
Thanks for all the replies guys. Going down the line...

Gavin, that's basically what i think happened, it seems like it turned itself up. I've tossed around whether someone bumped it or changed it. As far as I can see, none of us would have. The location of the rack is all the way against the wall, and for normal operation you don't even really get close to it. Could a customer? Maybe. Sometimes I think yes, sometimes I think no. We built the digital booth way up on stilts above the old booth so you have to walk behind the old booth and go upstairs. I doubt a customer would have gone up and changed anything, but I guess its possible. Naturally, the one place we don't have cameras. Let us know if the reseating helps your display.

Dave, we had mixed the sound before out there without processing it at all back on 35mm, and weren't crazy about it. Things just didn't seem as clean and dynamic as when we processed it. That's why we used the processors. You can't really tell on the drive in speakers, but if someone comes in with a really good sound system in their car you can tell. As far as the 750? We had asked our tech before the installation what we could use to get the best possible sound we could have, and he said that would be top of the line. Maybe it really is overkill, who knows. I guess as long as people are happy with it that is what counts. Thanks for the info on the back up, I appreciate it!

Tony, exactly my line of thinking. The server is on a UPS. The dolby is not. I am thinking of putting it on a UPS though. The projector is too far away from the rack to reach the server's UPS, and I am not one for running extension cords across the room so I am thinking of getting a little UPS for my rack. Right now, its just in one of those little rack mount power conditioners. I think they are ok, and probably do the job, but when I saw the type of UPS they sent with the projector, I liked it better. I'd like to put it on one with a battery back up so if the power flashes my rack doesn't have to boot back up. I even was thinking of getting UPS's for my projector's power. I'd need a three phase UPS for my Barco 32b and a single phase for my 23b, probably a pretty penny to get one, but I personally think they are worth it.

Scott, in your car you can notice the difference, the speakers are just the same either way. I just crank up the dolby and that scares all the bees out of the speaker poles [Smile] . But yes, we had our sound not processed before, and it sounded better to us once we put a processor on, so I guess its just a matter of personal taste.

Anyhow, thanks for all the answers! I'll let everyone know if it decides to go nuts again.
Jay

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Randy Stankey
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From: Erie, Pennsylvania
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 - posted 06-28-2013 01:00 PM      Profile for Randy Stankey   Email Randy Stankey   Send New Private Message       Edit/Delete Post 
quote: Jay Glaus
...if someone comes in with a really good sound system in their car you can tell.
Since American car manufacturers are incapable of making better cars on substantive grounds, they are selling them on the basis of gewgaws and gadgets like voice activated navigation systems and stereos.

Since more cars have good stereos in them, these days, sound is probably going to become more important, in the future, for drive-ins.

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Harold Hallikainen
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From: Denver, CO, USA
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 - posted 06-28-2013 04:07 PM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
Speaking of drive-ins, note that the USL JSD-60 has drive-in as one of the speaker configurations. It does the required down-mixes for stereo transmission, field speakers, and snack bar. See pdf page 25 of the manual. .

Harold

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Louis Bornwasser
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Posts: 4426
From: prospect ky usa
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 - posted 06-28-2013 08:36 PM      Profile for Louis Bornwasser   Author's Homepage   Email Louis Bornwasser   Send New Private Message       Edit/Delete Post 
In my experience (35mm analog) SR really helped in drive ins as well as serious compression. What we ended up with was:
1. flat to 15k
2. compressed and limited
3. reduced to 2 channels (Lt and Rt)

Eq not needed except on A chain.

Digital Cinema is already flat to 20K; Mix to suit and then compress and limit; straight to FM. (You must process since film is really a wide dynamic range medium. FM, however, is not. More importantly the EXPECTATION of FM is that you must be somewhere near the modulation density of the average FM station or the customer will think the drive in's system is broken.) Louis

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Tim Sherman
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From: North Ridgeville, OH, USA
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 - posted 06-29-2013 12:08 AM      Profile for Tim Sherman   Author's Homepage   Email Tim Sherman   Send New Private Message       Edit/Delete Post 
I run 2 JSD-60's at my drive-in, the sound is phenomenal. Sounds like you are having two different things come into play here.

First scrap the dj mixer you have and use the Drive-In odessey unit DI-60 I think is the model number. The downmix is already preconfigured non adjustable. I have heard no complaints on the unit, and know many drive-ins running them.

The second issue I think you are having is dynamic range. This is much more prevalent now with digital than it was when running 35. Think about it like this. The movie audio is supposed to be more like real life, well a gun shot is much louder than people talking. So in all reality it should be really loud on certain scenes. Only way to fix this is either using a compressor (set up incorrectly will make the audio sound horrible) And or the CP-750 might have a headroom setting that could help, I know the JSD-60 does.

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Mark Gulbrandsen
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From: Bountiful, Utah
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 - posted 06-30-2013 10:32 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
I've done several drive in conversions in the last year all using USL processors, either JSD-100 or JSD-60. I also did a bit of experimenting with combining the channels. The JSD-60 actually offers a Drive In mode of operation. As far as I know it's the only current processor that offers it. I found it to be pretty good overall but lacking in subwoofer drive.

I also tested a passive combiner and I found that going passive as the DI-60 does leads to too much phase shift at higher frequencies creating a smear to the sound when listening on very high quality auto sound systems. Thus I have stuck with a decent 10 in 2 out mixer and it gives far superior results once set up. In short, you will know when there are vehicles that have power subwoofers on the lot!

Also, I highly recommend the Broadcast Warehouse TX-5 transmitter. Instead of compressing it all before hand the TX-5 has a very fast limiter built in that has a range of 22db. It is so fast that you won't hear it working. I run the TX-5's at the lowest power out setting of 300mw and they have worked out really well. The flexibility they offer and the digital control is great! I do recommend you go inside and set the chosen frequency to what ever the DI is wanting, otherwise dial twidlers may unknowingly change it...

Mark

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Louis Bornwasser
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From: prospect ky usa
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 - posted 07-01-2013 08:06 AM      Profile for Louis Bornwasser   Author's Homepage   Email Louis Bornwasser   Send New Private Message       Edit/Delete Post 
I also use the Broadcast Warehouse transmitter. A small hint: mount the antenna as LOW as you can! Counterintuitive, but you want a high power signal that doesn't go too far. With the antenna at the baseboard of the booth, it is about right. Louis

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Mark Gulbrandsen
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From: Bountiful, Utah
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 - posted 07-01-2013 04:56 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
I totally agree Louis. 300 mw is still "illegal" but also as low as the tranny will adjust without resorting to an evil rf attenuator. Even with the antenna just above the roof trim line we still had better then a mile radius coverage and better sound quality than most of the local FM stations in those areas. Thankfully the one's I've done are in very rural areas!

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Harold Hallikainen
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From: Denver, CO, USA
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 - posted 07-01-2013 08:27 PM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
Thanks for the comments on the JSD-60. What do you mean "lacking subwoofer drive? Maybe we can fix that.

Thanks!

Harold

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Tim Sherman
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From: North Ridgeville, OH, USA
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 - posted 07-02-2013 01:02 AM      Profile for Tim Sherman   Author's Homepage   Email Tim Sherman   Send New Private Message       Edit/Delete Post 
Mark,
I couldn't disagree with you more on the sub woofer output level. There is plenty coming off of the jsd-60. I am also a Advanced MECP (mobile electronics certified professional) certified through CES Installer. If you think you need more there is something wrong. And just for the record I am using BW tx-5's on both of my screens.

Harold I would't change the level. And I have heard no complaints from other owners about the downmix.

If you really feel that you need more from the LFE channel then go into the advanced tab and modify the downmix.

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