Film-Tech Forum ARCHIVE: No sound on Left and Right channel on Dolby CP750

This is topic No sound on Left and Right channel on Dolby CP750 in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Justin Hamaker (Member # 2165) on 03-06-2015, 09:45 PM:

This issues just started today. I don't have any sound on left and right channels in one auditorium. This appears to have just started today. I have checked all of my cabling to look for loose connections.

Discovered the issue when we had customers complain of no sound during our preshow (which I believe is just L/R). During trailers and the feature the L & R channel is completely flat when I look at the eq on the Dolby software.

I ran the Hollywood Sound Test and definitely did not have audio in the L or R channel. But did have LS, RS, C, and SW.

Using the Dolby software, I swapped the inputs for the R and C channels, and the sound followed.

Any ideas?

Posted by Stephan Shelley (Member # 8587) on 03-06-2015, 09:51 PM:

Do you have left and right with non-sync. Also check with pink noise. If you have it with those but not from the media block L/R are one AES pair. Check cable/connections between media block and the 750. It is also possible there is a problem with the media block itself or the AES connector/input on the 750.

Posted by Robert Harrison (Member # 3202) on 03-06-2015, 10:03 PM:

I have had this problem, also, and only with one unit (out of 4). In my case, I switch between discrete and pro-logic, depending on the material being 2 channels or 5.1. It happened this afternoon (again). My only recourse was to reboot the Dolby unit, which worked before. I'm going to do without the switching and leave the unit in discrete during presentations (this doesn't seem to occur in non-sync mode where I always use pro-logic). Must just be a glitch. Let's see if anyone knows of an update fix.

Posted by Justin Hamaker (Member # 2165) on 03-06-2015, 11:43 PM:

After restarting the CP750 the issue went away. What's interesting is all channels were working in non-sync before the reboot, so it would suggest the problem has something to do with the AES input, or the Digital 1 format.

Posted by Nguyen Jack (Member # 7530) on 03-07-2015, 02:00 AM:

I Think you need to check cable connect. or change another cable.

Posted by Magnus Eriksson (Member # 6085) on 03-07-2015, 04:49 AM:

There is an issue with the non-sync input. From release notes:

"Switching to the Non-sync input while configured for format 71 (Mono C), can sometimes result in distorted audio output. The CP750 must be restarted"

"Distorted" in my case meant a messed up channel mapping and/or muted channels when playing AES/EBU. A CP750 reboot fixed the sound for a day or so.
Changing the non-sync input from mono C to prologic solved the problem permanently.

Posted by Justin Hamaker (Member # 2165) on 03-07-2015, 05:09 AM:

Mangus, that does not apply to my situation. First of all, the movie was not in 7.1. Secondly, the issue was not missing channels in non-sync, it was missing channels in Digital 1.

Posted by Magnus Eriksson (Member # 6085) on 03-07-2015, 05:23 AM:

quote: Magnus Eriksson
messed up channel mapping and/or muted channels when playing AES/EBU

Sorry for my Swedish [Wink]

This was a 5.1 setup, the problem occured when the CP750 switched from non-sync to Digital 1.

Posted by Dave Macaulay (Member # 813) on 03-07-2015, 08:28 AM:

The obvious place to look is in the 4xAES cabling since L+R is on one AES stereo signal.
That also means one D-A converter in the CP750 is handling the L+R conversion so it could be the processor.
Rebooting clears up some CP750 issues but it's not prone to that kind of problem.
It could also be a server fault
In my experience "missing one AES pair" problems have been caused by the cabling between server and D-A converter.
A marginal signal can work sometimes and not other times.

Posted by Jim Cassedy (Member # 4115) on 03-07-2015, 08:52 AM:

A couple of weeks ago, I had the VI track coming out of the subwoofer.

Re-booted the 750 and the gdcdigitalcinemaserverthiny and it's
never happened again.

Remember, digital cinema is not an 'exact science'.

Sometimes stuff just happens.

Posted by Steve Guttag (Member # 268) on 03-07-2015, 10:47 AM:

The most likely culprit is the cable/connectors/adapters on pair 1.

As for the VI out of subwoofer...that is a known GDC problem...rebooting just the GDC server would have "fixed it."