Film-Tech Forum ARCHIVE: Alternative Content Audio Woes

This is topic Alternative Content Audio Woes in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Ethan Murphy (Member # 8958) on 08-05-2015, 10:44 AM:

Hi Everyone,

This is my first posting on the forum and I'm hoping you all can help me get to the bottom of an issue(s) that is plaguing a brand new DCP install/integration for us. I have lurked on the forums for quite a while and feel like a have a pretty good understanding of previous alternative content discussions and hope that this topic will elaborate on those.

Our system is a little complex with multiple sources, but I'll simplify it to the basic pipeline for the alternative content portion of our system. We start with a Oppo BDP-103 BluRay that goes into an Extron XTP matrix switcher that then feeds into the IMS on our NEC 900c projector. The image pipeline stops there, but the audio then comes out of the IMS (AES) and goes into a QSC DCP100 Cinema Processor which then feeds into a Dolby CP-650 (we have 35mm too) which then goes out to the amps and speakers.

So I've seen on these forums two different suggested approaches to alternative content and it would be great if the experts could elaborate and defend either approach below. One suggestion is to send LPCM audio out of the Oppo via HDMI and the other option is to do all audio decoding further downstream. My first question is, if nothing further downstream can decode the bitstream that leaves us with only the first option correct? The Extron switcher cannot decode all flavors of audio (EDID only offers up LPCM with stereo audio, a multi channel Dolby format and a multi channel DTS format), it is my understanding that the IMS does not decode audio at all (NECs IMS1000), The DCP-100 can decode some analog audio but that doesn't help us here and the CP-650 can decode dolby only. Would this option only work if we had an AP20 in place to do all flavors of dolby and dts audio decoding? What is the DCP-100 even doing in this pipeline? acting as a crossover monitor?

Currently when we set the oppo to output bitstream audio the room is immediately filled with horrible white noise.

So the solution is to set the BluRay to LPCM and no problem right? Wrong. When set to LPCM the center channel drops out frequently which is a show stopper (no dialogue). Not to mention what about alternative content from a computer which doesn't have the option to output LPCM? This seems to be an issue with the Extron switcher, but no firmware updates or troubleshooting steps have been offered up. The lower end DTP switcher does not have the same issue.

Right now we have stereo sound to get by, but our integrator has left us hanging for more than a month. Any advice or troubleshooting recommendations would be greatly appreciated.

-Ethan

Posted by Sam D. Chavez (Member # 1841) on 08-05-2015, 12:13 PM:

It would be good to know who your service company is but perhaps you don't wish to say.

Posted by Scott Norwood (Member # 30) on 08-05-2015, 12:28 PM:

Do you have a multichannel analog input available on either the QSC or the CP650? If so, the easiest approach would be to connect the discrete analog outputs from the Oppo to that. That way, you can let the Oppo deal with any decoding issues of Blu-Ray content.

(How does the QSC connect to the CP650? The specifications make it look as if it is designed to connect directly to QSC amplifiers and not to another processor.)

Posted by Ken Lackner (Member # 1002) on 08-05-2015, 12:55 PM:

I agree with Scott's last statement. Are you sure the CP-650 isn't fed into the DCP-100 and not the other way around?

Posted by Ethan Murphy (Member # 8958) on 08-05-2015, 12:58 PM:

I do want to allow an opportunity for the service company to deliver a functioning room.

Going direct analog out is not an option for us unless for testing purposes only because we have a total of 4 BluRay Sources and two are from the front of the theater and are being converted to Cat5/6 for the run to the matrix switcher. I tried to simplify the build to isolate the issue at hand which seems to be a common one. What is the best practice for handling alternative content audio via HDMI?

Also, yes you are both right the CP-650 is being fed into the QSC DCP-100 not vice versa like I had originally stated.

Posted by Louis Bornwasser (Member # 3063) on 08-05-2015, 01:49 PM:

Boy, how the mighty have fallen. In the film era, you would have about 36 hours to resolve this kind of problem. Guess the video babies aren't up to the task.

Posted by Marco Giustini (Member # 4544) on 08-05-2015, 02:11 PM:

that's an interesting problem.

As you say the IMS does not decode the audio and neither does the DCP. The 650 can decode AC3 only through an AES input.

If the Oppo can output LPCM though, the IMS should pass it to the DCP which in turn should happily play it for you. I am not familiar with the Oppo nor with your setup though.

Posted by Scott Norwood (Member # 30) on 08-05-2015, 03:02 PM:

It will be interesting to see the answer here and if there is a real solution. The approaches that I have seen have basically come down to the following:

- go with analog from the player and let the Oppo deal with the decoding
- decide that Blu-Ray is not worth worrying about, given that it is not a professional format, and just do L/R analog or digital into a pro-logic decoder, possibly with AC3 into the CP650s digital input as well
- install an AP20

None of those seems to be an option in this case.

Posted by Carsten Kurz (Member # 5396) on 08-05-2015, 03:46 PM:

Multichannel LPCM from all BluRay Sources is the best option, followed by multichannel discrete analog.
Now the question is, why/where does the center drop out - HDMI, or between Doremi HDMI input and AES-output?
Because that is the only real problem you seem to have with the current setup.

Your setup is complicated enough, so I would usually not recommend to add another device, but you may still want to have a look at USLs http://www.uslinc.com/index.php?option=com_virtuemart&Itemid=249&vmcchk=1&Itemid=249

Also, not ALL HDMI devices are able to output Multichannel LPCM.
So, depends which operational modes you have to support.

An AP20 is indeed very useful for all sorts of alternative sources. However, it does not support Dolbys HD codecs. Which is never really a problem, as it will always be able to decode the lowest denominator of either old or new DTS OR Dolby codecs OR LPCM. But your setup is complicated enough, I wouldn't know where to put an AP20.

Again, try to find the reason for that center dropout first.

One quick way to solve this could be to use the Oppo as the decoder for all HDMI sources and use it's multichannel analog out. But your QSCs analog input is probably already used by the CP650 out. And going into the QSC through the CP650 analog INs is not really a nice setup. But it maybe a compromise until other options have been evaluated.

- Carsten