Heavy Delay in the Surround Channels

Anything and everything to do with DCP-o-matic.
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Herbert
Posts: 8
Joined: Sun Mar 14, 2021 4:14 pm

Heavy Delay in the Surround Channels

Post by Herbert »

Dear friends,
I just came back from testing my 5.1 mixed feature in the local cinema.
There is a heavy delay in the surround channels.
Can´t be, because when I mixdown the 5.1 back to stereo
(I do not have a surround setup to check)
is almost indistinguishable from the seperately made stereo mix.
I think the projectionist is to blame as he does not seem to
dig his system but just to rule things out:
We CAN delay ALL tracks in DCP-O-Matic AT ONCE.
But we can never delay SINGLE TRACKS, correct?
Best,
Herbert
carl
Site Admin
Posts: 1468
Joined: Thu Nov 14, 2013 2:53 pm

Re: Heavy Delay in the Surround Channels

Post by carl »

You're right - it shouldn't be possible for DCP-o-matic to adjust the delay of individual tracks (if they are imported in a single file).

Another check you could do would be to load the PCM MXF file from your DCP into Audacity and look at the sync there.
Herbert
Posts: 8
Joined: Sun Mar 14, 2021 4:14 pm

Re: Heavy Delay in the Surround Channels

Post by Herbert »

Dear Carl,
thanks for the tip with Audacity, here you can see the tracks,
some percussion heavy music, the grid is 10 Samples.
Absolutely perfect aligned. I Even tickled LFE a bit
Their feedback was that someone did check the soundsystem
yesterday and everything was fine - but the projectionist seems to be overtaxed.
I guess they just checked Prologic files but not discrete 5.1 tracks.
To rule another thing out:
The DCP does not "tell" the projection system in what format audio is?
If it is discrete tracks or Dolby Surround Encoded?
As we can only choose between discrete tracks or an stereo upmix,
nothing can be done wrong from our side?
Best, Herbert
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Last edited by Herbert on Tue Oct 12, 2021 1:52 pm, edited 1 time in total.
carl
Site Admin
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Re: Heavy Delay in the Surround Channels

Post by carl »

There's no instructions about upmixing in the DCP, it's more-or-less just a bunch of channels - if someone wants to do some Prologic or something after the playback system that's up to them.

How long was the delay we are talking about? Is it a big auditorium?
Herbert
Posts: 8
Joined: Sun Mar 14, 2021 4:14 pm

Re: Heavy Delay in the Surround Channels

Post by Herbert »

It is small, 60 seats.
I did test a longer version of the film
in another cinema of the same size in March -
sound was perfect.
I mixed Surround, no Upmix.
To the ear I would say delay is even 1/4 of a second.
It is a "slapping" delay, audible in the back rows of the theatre.
Default of the cinema seems to be 7.1.
I checked some settings with the overtaxed projectionist.
We found an 80ms delay as preset, reduced it to 0,
the slapping echo was still there.
When i get it right 80ms are 26meters - the room is maybe 20.
But the Dolby Logo worries me...
Carsten
Posts: 1888
Joined: Tue Apr 15, 2014 9:11 pm
Location: Germany

Re: Heavy Delay in the Surround Channels

Post by Carsten »

Setting a front/sub audio delay is always part of the setup, so, it is very likely that something has been set wrong. On many movies with just atmospheric surround beds, this is not immediately evident. The moment you play the same content front and surround, like dialog, or dialog with room components in the surrounds, it becomes a problem.

The standard in digital cinema is plain 5.1, no surround decoding, Prologic or such things. So, if it is actually wrong with your DCP, it is wrong for every DCP.

It is very easy to create a sync test DCP that plays the same beep or tick sound on all or special selections of speakers to show the problem.

The trouble is, if there is no one on location trusting you, or to make the proper decision, you are out of luck even if the problem is obvious.

The typical projectionist is not able or allowed to correct such issues on his own. Only few, very dedicated projectionists have the tools and knowledge to deal with equipment setup.

Where do you see the Dolby logo? There may be a range of Dolby devices in the cinema - playback servers, and audio processors. But that doesn't mean that Prologic is active during DCP playout.


But - if you have the same percussive signals in both fron/center and surround, it is unavoidable that you hear a slap-delay both at the very front, and the very back seats. If the sound leaves front and surround (back) speakers at the same time, the will only be in sync right in the middle of the auditorium. Therefore it's important to configure the surround-delay properly. Some echo is unavoidable if identical signals come from front and surround speakers, as the surround delay can only cater for a specific average seating position.

- Carsten
Last edited by Carsten on Thu Oct 14, 2021 10:48 am, edited 1 time in total.
Herbert
Posts: 8
Joined: Sun Mar 14, 2021 4:14 pm

Re: Heavy Delay in the Surround Channels

Post by Herbert »

Dear Carsten,
thanks for the great explanation!
So the percussion could be the reason - but
I did have the problem in a second
cinema that has roughly the same size - 60 seats.
A dedicated upmix plugin was used for the stereo music.
I am also surprised that it is not expected that projectionists
need to know their sound system - but yes, other people told me the same.
In the German Democratic Republic, becoming a projectionist was a three
years training - those people could build a cinema and repair the projectors.
But only had to deal with few formats.
I guess the best way to defeat 7.1 is to but in 2 mute tracks for
the rears...
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