Blu-Ray/DVD and the Dolby Cinema Sound Processor

Hi CB,

I'm not sure how many 35mm guys come here, or really even which forum to post this in. I'm interested in using our existing Dolby Cinema Sound Processor (CP55) to play audio from computers or DVD players.

Currently when we play a DVD or a BluRay disc in our theatre, we route the L/R from the RCAs on the device into a mixer, which then feeds two speakers behind our screen.

Our 35mm setup has full surround sound support, and when we want to play DVDs we simply disconnect the XLR connectors from the front left and right inputs, and connect the 'DVD Mixer' to the amp. We only get sound from front left and right, and we're missing out on the full features of our theater including The Voice (Of The Theatre, huge center horn), side fills, rear fills and the subwoofer.

This is the dolby processor we have:


I'd like to use the 'AUX' button to select the DVD/computer feed.

Here's the back of the unit (from the instruction manual):


So it just takes a line-level RCA input on L, C, R, S? Can I use one of these then?


I would be so happy.

I believe that the L, C, R and S Aux inputs will simply be directly routed to the correpsonding L, C, R and S outputs when "AUX" is selected. You may have to externally sum the left and right surround signals to a single surround signal into the CP-55.

The Aux inputs appear to all be unbalanced line level signals on a screw terminal strip with one shared ground. If so then you are going to have to work with bare ends to spade lugs or similar for the connections rather than RCA connectors.

The CP-55 is a pretty old processor. Had to go dig up a manual, but I don't think you're going to get what you want for non-film playback from it. The CP-55 is a 4.1 processor with just mono and Dolby A support. I can think of a few ways to attempt what you want, none of them great. Since the CP-55 predates Dolby 5.1 and also predates Dolby discovering balanced interfaces, it just adds to the fun. I just helped a college convert from dual 35mm with a CP-65/DTS to a single 35mm and D cinema rig. The old 35mm processor feeds the new CP-750 which handles the digital content.

How much 35mm vs DVD/Bluray content do you run? What type of projector do you use for the digital content? Do you plan to upgrade to D-Cinema as the major distributors phase out 35mm prints?

With your existing processor, there appear to be two ways to tie in. All require getting your DVD/Bluray audio up to professional line level from consumer.

First option: If you go with the Aux input, you will need to sum surround left and right off the disc before feeding in to the mono surround input. In addition, you will want to mix the .1 channel back into L and R or it will be lost. Looking at the manual, and assuming there is a Cat 241 lurking in there, the .1 will be re-derived by the processor from the Left and Right Channels.

Second option: Continue running stereo off the discs, but insert a mixer between your normal non-sync source and the non-sync input on the processor, and then tie all non-35mm sources to this mixer. You'll have Stereo only, but you will get all the EQ and the .1 processing you currently get for 35mm.

Now, if D cinema is in the cards, it might be worth looking at a CP-750. You'll need one anyways for that, and it supports a discrete 5.1 input, 2 AES and 1 SPDIF optical input, plus a stereo non-sync source. I think they're around $3Kish retail. Connect the CP-55 to the discrete/analog input, and the optical out of the disc unit to the SPDIF input, and you'll get all the processing you want/need.

If D cinema is not in the cards, it might be worth seeing if you can swing a used CP-65, CP-500, or CP-650. The CP-65 is very similar to the -55, being all analog internally. The -500 and -650 are both all digital and the analog NR circuitry is emulated in software. With the CP-65, you can convert the discrete outputs of the DVD player and use the Dolby Digital input, or enable Pro Logic decoding on the non-sync input and get 5.1 off the stereo feed that way. For the CP-65, make sure it has a Cat 441, pair of Cat 350, and a Cat 222 fitted. This will also give you Dolby SR decoding for your 35mm, which will be a step up in audio quality off the Dolby A type the CP-55 gives you.

And talk to your service company. They might have suggestions neither of us have thought of.

Wow thanks for the great info! We don't really have any service contracts or anything for this equipment. It's a dual Christie 35mm setup that gets used sporadically. Occasionally though, there will be events where 5-7 35's will be shown back-to-back, sometimes alternating between digital projection and 35s. A friendly fellow from the local IATSE (which absorbed our projectionist union) manages the actual projectors, lamps, servicing, licensing, etc. I am in charge of the sound system. Our room also has a flown concert sound system that is entirely separate.

Now, from what I understand from that hand-drawn manual page the Dolby processor expects professional +4dBu and the type of device I linked to earlier would probably output -10dBV.

Here's my plan:

The mixer that we currently use to drive the amplifiers is a Mackie CFX12. Not great by any means, but it'll do for now. This mixer has a stereo L/R bus, and 2 pre-fader aux busses and 2 post-fader aux busses. I'd like to use the L/R to feed the CP55's AUX input L/R (using only the hot pin of the XLR connection, with the cold pin connected to shield as the signal ground). I would use AUX 3 (Post Fader) to feed the Center channel, and AUX 4 (Post Fader) to feed the surround fill. The AUX outputs on this mixer are 1/4" TRS connectors, which means that if I use a TS plug it should internally tie the cold pin to the signal ground mechanically. These 4 unbalanced connections would travel to the CP-55.

Note that I used the Post-Fader aux busses. I can unassign the decoder's Center, Rear Surround Left, Rear Surround Right, Subwoofer channel from stereo and use the faders to control the relative volume of each channel.

• I can assign the decoder's L/R directly to Stereo, panned hard left and right.
• I can unassign the decoder's C from Stereo, and use Aux 3 to route this channel to the CP55 at the proper level.
• I can assign the decoder's Subwoofer channel directly to Stereo, panned dead center.
• I can unassign the decoder's additional surround channels from Stereo, and use Aux 4 to sum them all together and route to the CP55 at the proper level.

Another reason that I used the Post-Fader AUX busses is because we occasionally do skype-type calls on the big screen from a computer, and we use the pre-fader AUX 1 and 2 to route audio from microphones to Skype, then use a normal channel to return the call to the house. This prevents almost all feedback from the digital lag, and gives us an incredibly clear call that we could now play through all of the speakers.

Also, for devices that don't support 4.0/5.1 directly such as a 3.5mm laptop/iPod connection, now I can route that audio to all of the speakers.

What do you guys think?

One more question: Since the .1 is going to be rederived from L/R again, that means that the AUX input goes through the normal processing steps too, right? Does that mean that the AUX input on the CP-55 routes the signal through all of the Cat XXX EQs and such as well?

Give its a 20ish year old processor at this point, for a format that is becoming harder and harder to obtain, I would be seriously looking into ways to not use it as the primary audio processor for the space.

Hey Tim here,

I think you have attempted to call me once or twice now in Checotah, Ok. at 918-473-0361, but we keep missing each other. I have

IAmLumenator said:

......It's a dual Christie 35mm setup....Now, from what I understand from that hand-drawn manual page the Dolby processor expects professional +4dBu and the type of device I linked to earlier would probably output -10dBV.

Here's my plan:
...

Click to expand...

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I was lost at "dBu" But this ALL sounds like info I could use to do the exact same things.

So how did this all work out for you?

This is a bit off on a tangent from the original question, but for anyone attempting a similar general-purpose set-up with a newer Dolby processor, be aware that they have SIGNIFICANT input-to-output latency. I did a theatrical production in a screening room which had a CP-750, and there was almost Elvis-levels of slap-back delay between the sound coming from the actors' mouths and the sound coming from the speakers via their wireless mics. In the end, I had to get the tech guy there to rewire the processor so we could bypass it and drive the power amps directly.

simoneves said:

This is a bit off on a tangent from the original question, but for anyone attempting a similar general-purpose set-up with a newer Dolby processor, be aware that they have SIGNIFICANT input-to-output latency. I did a theatrical production in a screening room which had a CP-750, and there was almost Elvis-levels of slap-back delay between the sound coming from the actors' mouths and the sound coming from the speakers via their wireless mics. In the end, I had to get the tech guy there to rewire the processor so we could bypass it and drive the power amps directly.

Click to expand...

What was your signal path, and which input on the processor were you tied to? Haven't noticed any major latency on the digital inputs, and haven't run enough analog content to be able to tell on the install I maintain. Though that is something I can possibly test during the summer. It is possible whoever configured it stuck an unreasonably large delay on one of the inputs for some reason.

I had to Google some pictures to remind me, but I think we were going into the NonSync RCA inputs on the back panel, replacing a simple line feed from a computer sound card. I didn't consider that the processor might have had a deliberate delay configured, although according to the manual that certainly is a feature. We ended up making some adapter cables so we could run XLR direct into the L/C/R amps (which were hard-wired with screw strip connectors). Setting the delay to zero would have been a lot easier... :\

I'd try to get my hands on the configuration software for the unit, and look at the different settings. My site was lucky in that the installer left us a copy of the CD for the thing. If you're doing multi-channel off a computer, might be worth looking at the 5.1 analog input which was originally intended for legacy film projectors.

It was a one-off situation for me, but I'll definitely report your suggestions to the tech guy at the screening room.