Film-Tech Forum ARCHIVE: "Robosound" on a CP-750?

This is topic "Robosound" on a CP-750? in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Jim Cassedy (Member # 4115) on 08-27-2016, 08:43 PM:

I got a call from a theater today that they had to cancel a show
and move a movie out of their big auditorium due to some strange
noises coming out of the surround speakers.

I went down there and there was definitely a lot of distortion
and a low 'garbling' sound coming out of the surrounds, especially
during one of the trailers that had a lot of surround info.

Surrounds were silent if playback was paused, or when the CP-750
was switched to an analog or non-sync input.

Pink noise & the 'thump track' out of the 750's internal
generator were OK too.

Since the noise was especially obvious during one of the trailers,
I put that trailer into "loop" mode and the more I listened to it,
the more the offending audio began to sound a bit similar to the
familiar "robosound" that sometimes happens with a CP-650.

I finally shut EVERYTHING down, including some equipment they
usually leave on 24/7. While powered down, I also pulled & re-sat
all the digital & analog connections between the (Christie) projector,
CP-750 processor, amps, etc.

After this, and a complete re-boot, sound was OK. I 'looped' that
trailer and watched it at least a dozen times at higher-than-normal
volume. Then re-loaded their playlist for today and watched all their
pre-show stuff and the first 20min or so of the feature, and it was OK.

So, I'm not sure what it was- - but as I said, it had more of a 'digtal
problem' sound to it than some analog issue. But I've never had a
problem with any of the CP-750's I've encountered.

They've had two shows since I did the "repair" earlier today, and all
is still good.

Any ideas, anyone? I hate it when I "fix" something and I'm not sure
of the exact cause. And, I'm leaving for my annual projection gig at
the Telluride Film Festival tomorrow, so I'm hoping whatever I did
stays "fixed" for them, at least till I get back.

Posted by Mike Olpin (Member # 1115) on 08-27-2016, 09:24 PM:

Sounds like the D/A went a little wonky and simply needed a power cycle. Do you use the 750's internal D/A or do you use an external D/A and then come from that into the EXT input?

I'm surprised the theater called out their tech before doing a simple cycle on the sound equipment.

Posted by Jim Cassedy (Member # 4115) on 08-28-2016, 08:35 AM:

Hi Mike-
In answer to your question- - the audio at this location is handled
by the 750's internal D/A circuitry.
I'm not surprised they called me. They were in 'panic mode', and
operate with a bare-bones staff, so taking someone off the floor,
even to do something as mundane as a re-boot, would have been
a problem for them. That's how I make 'the big bux' LoL! [Roll Eyes]

>U at Telluride this year? I remember meeting you there a couple
of years ago. This is my 10th year there.

Posted by Carsten Kurz (Member # 5396) on 08-29-2016, 05:50 AM:

Sometimes, noise from the surrounds can be an indication of a dying CP750 power supply. I know a few places where this has been the case.

- Carsten