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DCP doesn't play in 5.1

Posted: Tue Mar 03, 2020 2:59 pm
by mike
Hi,

I've got a film screening at a local theater in two days. I made a DCP for the showing from a .mov with 5.1 sound. When the theater tests my DCP, we see the film but only hear sound from (I think the left) channel. First, I think DCP-o-matic is creating a 5.1 file. It creates two .mxf files, the one that starts with pcm seems to have 6 channel audio. I'll also post screenshots of how the project looks when I create it. Could I be messing this up somehow? Are there any settings I need to fix? Attaching my metadata.xml as well.

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Finally, I'm not entirely sure what software is being used to play the files, but I believe the theater uses Christie projectors and the interface I was shown said TightVNC Viewer. Can I recommend anything to them to get 5.1 working? Apparently they use DCP-o-matic to convert trailers for some of their films and they've never had 5.1 sound on them. This makes me think there is a problem with how they ingest the files.

Any help would be greatly appreciated.

Re: DCP doesn't play in 5.1

Posted: Tue Mar 03, 2020 3:04 pm
by carl
From the audio plot it looks like your left channel is a fair bit louder, on average, than the right and center channels. How does your MOV file sound if you play it back and listen over headphones?

Re: DCP doesn't play in 5.1

Posted: Tue Mar 03, 2020 9:59 pm
by Carsten
Yup. Left RMS/Average is a lot louder than right or center. So much louder that it may occur to you as if only left is playing.

Are you sure that you didn't mix left and center in that Quicktime file? Because, if you compare the green (center) and red (right) RMS plots, they appear a lot more to me as a typical stereo L/R combo. Whereas black/Left is louder and more decorrelated - looks to me more like an actual center signal. Maybe you need to adjust the channel mapping to bring 3 to L (DCP left) and 1 to C (DCP center), and all could be well.



- Carsten

Re: DCP doesn't play in 5.1

Posted: Wed Mar 04, 2020 2:09 am
by mike
Well crap. Just listened to the thing and the damn left channel is louder than all the others.

I didn't do the edit, but perhaps the audio channels are out of order. Will definitely check with the editor. If they are wrong, I'm sure I can just use the matrix to change which channels are assigned to what? I'll remake it and see if that fixes it, but I'm pretty sure we heard absolutely nothing from any of the other channels when it played.

If the center channel (currently left) is too loud and that is my problem, what would be the best settings to adjust it down?

Re: DCP doesn't play in 5.1

Posted: Wed Mar 04, 2020 9:02 am
by carl
You can adjust channel assignments and change relative levels in the audio matrix, but if two of your original channels have been mixed in the ProRes export there's not a lot that DCP-o-matic can do.

Did you go right up to the silent speakers when you listened in the cinema? Ones ears can play tricks!

If you right-click on the green square for the centre channel in the DCP-o-matic audio matrix you can set its gain to whatever you like.

Re: DCP doesn't play in 5.1

Posted: Wed Mar 04, 2020 10:09 am
by Carsten
I would first try to re-route left and center (swap). See if that fixes it. If necessary, you can adjust the center channel in the audio matrix individually by right-clicking on it's matrix connection point.

See here:

https://dcpomatic.com/forum/download/file.php?id=602

- Carsten

Re: DCP doesn't play in 5.1

Posted: Wed Mar 04, 2020 3:12 pm
by mike
Left and center channels were definitely swapped by the last edit. Don't know how in the hell that can happen, but here we are. Easy enough to fix on the DCP. Will be testing again at the theater. Fingers crossed for 5.1. And I will put my head right next to the back speakers to see if anything is coming out.

Re: DCP doesn't play in 5.1

Posted: Wed Mar 04, 2020 3:42 pm
by carl
Cool. Also up in the booth there may be some metering on the sound processor that you could check.

Re: DCP doesn't play in 5.1

Posted: Thu Mar 05, 2020 12:32 am
by mike
So sound test failed. I was standing right under the surround speakers and heard nothing during a part where I know there is surround sound. We will check the meters as soon as the next movie finishes.

Are there any settings I might be missing that would cause a Christie projectors not to recognize 5.1? The projector recognizes the folder and ingests the video, but seems to only play the Right and Left channels.

Next I’ll try reducing the gain on the front channels to see if that balances it out. Maybe the front channels are washing out the back channels?

Re: DCP doesn't play in 5.1

Posted: Thu Mar 05, 2020 1:41 am
by Carsten
Surrounds do not show much signal in the above graph. The overall loudness, -22.40LUFS, though, is in the right ball park. Depending on the material, maybe a bit on the soft side, but that's hard to judge without knowing the character of the mix.

The audio/pcm mxf that goes into the DCP folder can be opened in some audio editors - I use Audacity with the FFMPEG plugin installed, as it's free and straight.

You can easily see the individual tracks, listen to them, solo/mute, etc. What's in there will definitely be played in the cinema


There are typical visual patterns to be recognized for L/R, Center, LFE, Left/Right surround, as you can see in the example below. This is from a 5.1 trailer.
Bildschirmfoto 2020-03-05 um 02.39.56.png
Bildschirmfoto 2020-03-05 um 02.39.56.png (152.13 KiB) Viewed 1102 times


The individual surround channels in cinema are calibrated 3dB softer than the screen channels. So, for either L or R, you lose 3dB there already. If the signal is too soft, that will play an additional role.

Not all cinema processors have individual channel meters. The very popular CP750 has a somewhat cryptic display. See legend below.
Depending on the knowledge of the projectionist, he could temporarily switch off screen channel amps, so only surrounds stay audible.
Bildschirmfoto 2020-03-05 um 02.48.57.png
Bildschirmfoto 2020-03-05 um 02.48.57.png (88.3 KiB) Viewed 1098 times
- Carsten