Film-Tech Forum ARCHIVE: Strange behaviour from Dolby DMA8Plus.

This is topic Strange behaviour from Dolby DMA8Plus. in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Stephen Furley (Member # 1277) on 04-19-2016, 08:25 AM:

Some of our films have HI and/or VI tracks on channels 7 and 8 on the DMA, but most do not. At present these channels are not connected to anything, but we may use them in the future. At present our HI system is driven from the HI output on the CP-65.

A couple of months ago the LEDs for these channels started showing sound on these channels when there shouldn't have been any. It doesn't always happen, but when it does it is there even when there is no content playing. If I switch to any input other than Digital 1 then these lights go out, as might be expected.

I looked with the DMA8Plus setup software and can see nothing in the configuration which would account for this. The signals on these channels are shown as being at a high level, as high as can be shown on the histogram. Any ideas as to what might cause this?

My thinking is that the next things to try are to disconnect the 4xAES cable from the server to see if this is a real signal being received from the server, or some spurious effect being generated within the DMA, and then to swap these two channels onto left and right so I can listen to what is actually on them.

Posted by Marco Giustini (Member # 4544) on 04-19-2016, 06:16 PM:

I'd be very careful in playing those channels in case it was just 0dB digital garbage. I'd begin with volume set to 0.

What version is your dma8+ running?

Posted by Stephen Furley (Member # 1277) on 04-20-2016, 05:48 PM:

Marco,

Don't worry, I would always take the fader right down before listening to any unknown signal.

I was projecting this afternoon. We don't normally screen on Wednesdays, but this was a special screening for a local mental health charity of a play they have performed.. Of course, when I was looking for it the problem didn't show up. The DCP for this show had only 2 channel, L and R sound, nothing at all on the other channels. I wonder if the problem only happens when all of the normal channels are in use. I'll have to look at it again when I'm next there on Tuesday morning, setting up next week's films. No, thinking about it the problem happens even when there's nothing playing, so it can't be that.

The mystery signals are much higher than 0 dB; they're right to the top of the histogram on the DMA setup software, which is labelled as +20 dB, so if the software is to be believed they're at least at this level, and possibly even higher, and off the scale.

When there is a proper VI track on a film it is silent most of the time, with quite brief bursts of speech from time to time. This mystery signal is quite different, it's continuously at the same, very high level..

Software version is 1.4.4.1. This isn't origiginal, I upgraded it about two years ago when we re-opened the cinema. I was having problems controlling the DMA from Ethernet at the time, and this upgrade seemed to resolve that issue. No other changes were made at around the time when this present problem started, a couple of months ago.

Hardware revision is 3.

Posted by Carsten Kurz (Member # 5396) on 04-21-2016, 04:54 PM:

Our AP20 actually has a VU demo mode. It drove me nuts occasionally. Now that I know it's there, it doesn't upset me any more. One of Datasat's techs told me he strongly advised against implementing it in a professional cinema environment, but the sales people wanted it. Then how many cinema processors do you actually sell in a shop?

- Carsten