Film-Tech Forum ARCHIVE: CP750 channel issue

This is topic CP750 channel issue in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Richard May (Member # 2627) on 05-30-2018, 01:23 PM:

Has anyone had this issue? Center channel is being sent to left surrounds only. Bar graph on front says center channel info is present but audio is coming out of left surrounds. A reboot fixes it but after a while, it goes back. Non Sync also comes out of left surrounds only when I put decode mode to Center only.

Posted by Steve Guttag (Member # 268) on 05-30-2018, 03:11 PM:

I have a LOT of CP750s out there. Never had that problem. My biggest problem with the CP750 (and it hasn't been too huge) has been power supplies.

I hear that circa 2010 CP750s can have a problem where the level steadily decreases on the output though (still haven't had that one either).

Posted by Leo Enticknap (Member # 534) on 05-30-2018, 06:05 PM:

I've had it, but it was left and right (stage channels) going to LS and RS. Again, a hard reboot fixed it. The unit in question just started to do this one day - no settings were changed, or anything like that.

Posted by Tony Bandiera Jr (Member # 2365) on 05-30-2018, 07:58 PM:

Richard and Leo, one question for you both: Are your CP750's on a UPS (or at minimum, a high quality surge protector?)

Reason I ask is, of all the screening rooms I and my former colleague have done, with CP650's and later 750's, we ALWAYS put the processors on a UPS with the projector electronics, and never had any such issues with either processor. (Not even the dreaded CP650 "robosound")

I firmly believe that since most projection rooms and cinemas in general have very noisy power, all it takes is for a rogue surge/spike/undervolt to make microprocessor based hardware do strange things.

A UPS, even a basic one, IMHO is cheap insurance against such problems.

Your actual mileage may vary, some assembly required, but at least batteries ARE included. [Big Grin]

Posted by Dave Macaulay (Member # 813) on 05-30-2018, 08:19 PM:

If I remember correctly, there is some weird thing that happens when you use nonsync to centre only. Avoid that.

Posted by Steve Guttag (Member # 268) on 05-31-2018, 03:29 AM:

I do use surge protection on sound electronics but rarely UPS.

Posted by Leo Enticknap (Member # 534) on 05-31-2018, 07:46 AM:

I can't remember if the unit I experienced this glitch with is powered through a UPS or not. It is normally left on 24/7, though.

The venue in question was in Hawaii, and I was helping them by phone and Teamviewer to hook a laptop to their system for a one-off screening of video files from it.

They connected the Macbook to one of the cat745's HDMI inputs, and I talked them through setting the video frame rate and audio outputs correctly. The audio in their files was a L/R TV mix, and when they started to play, they reported that it "sounded funny, like it was coming from the sides of the room." I opened the CP750 utility on the remote access PC through Teamviewer, and sure enough, a signal was registering on LS and RS, but not L and R.

To rule out a problem with their files or laptop configuration, I had them play an audio CD through the BD player that was hooked to the other HDMI input of the cat745, and got the same result. It played through LS and RS. Then I asked them to make an analog connection from the laptop's headphone jack to the nonsync input, but they reported that they didn't have the necessary cable. So, basically, because I was out of all other ideas, I asked them to hard reboot the CP750. After doing that, it worked as it should: both the CD and the laptop played through L and R.

I was going to set it to L/R ProLogic and ask them if they preferred that or the plain L/R, but after that glitch I decided that it would be better not to tempt fate, and just play the L/R. Thankfully, the show went off without any problems, and as far as I'm aware, that CP750 has behaved itself ever since.

The next time one of us goes out there for a planned maintenance visit, it's on the list of things to do to play around with it and see if we can reproduce the fault.

Posted by Magnus Eriksson (Member # 6085) on 05-31-2018, 04:04 PM:

quote: Dave Macaulay
If I remember correctly, there is some weird thing that happens when you use nonsync to centre only.

That's exactly what happened to me when I tried to play mono muzak in nonsync five years ago. Random channel routing could occur on any input frome time to time.

quote: Dave Macaulay
Avoid that.

That's what Dolby support told me. It was a bug of some sort.
I configured nonsync to prologic and connected the mono signal to both left & right input. I've had no problems with random channel routing after that.

/M.E.

Posted by Richard May (Member # 2627) on 06-01-2018, 07:31 AM:

Thanks for the replies guys. It is indeed a "KNOWN" issue with the 750. Only one tech I spoke to at DOLBY told me about it. I can't believe this was not widely put out there before this.

If you configure Non Sync to use Format 71 (Center only), you are basically screwed until you reboot the processor. If you're switching between Dig 1 and Non Sync, center channel will randomly route to the Left Surrounds in the theater and sometimes distort, as per Dolby. A reboot fixes it until the next time it is rebooted.

There is no fix for this other than to not use Format 71. [Frown]