Film-Tech Forum ARCHIVE: Questions about DCP audio and Dolby LFE levels

This is topic Questions about DCP audio and Dolby LFE levels in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Stephen Angelini (Member # 5741) on 05-11-2010, 12:19 PM:

Hi all. This is my first post here. I'm a composer and sound designer and I've been working with 5.1 surround for while but I'm now involved with a project that has brought me behind the projector so to speak and into the world that most of you see every day. 2 weeks ago I didn't even know what a DCP, or a DSS100, or a CP500/650 were, so bear with me [Wink]

To be succinct:

1. Is a DCP always pure PCM audio? Is any other format ever used on a DCP?

2. When a CP500 is set to "External 6-channel" is all Dolby Processing bypassed? This would mean that the LFE plays back at unity gain, as opposed to the +10 DD applies decoding the LFE from SRD.

The cinema where we're trying out stuff plugs a Blu-Ray via HDMI through a Yamaha Receiver and then connects the 6 analog outs to the "External 6-channel" inputs of the CP500.

3. Since the CP500 analog inputs are consumer line standard 300mV, can I assume that if the Yamaha is set to 0dB gain and that there's no other processing done by it (Straight Mode), the level I'm getting out of the system is roughly representative of the audio levels I'd get by putting Blu-ray PCM audio onto DCP?

4. How does the the CP500 handle clipping? Is there any built-in management or compression applied to a clipping signal? Or is there any additional protection typically included into the cinema output chain to protect speakers from damage (compressors or limiters)?

Thanks for any insights!

Stephen

Posted by Carsten Kurz (Member # 5396) on 05-11-2010, 02:35 PM:

Only Linear PCM, 24Bit. Only Choice is 48 or 96KHz.
No compression ever.

What the Cinema processor does to it is not precisely standardized - usually the minimum would be that it applies the same equalization to the channels that the theater room/sound system requires for 35mm sound formats.

But on modern DSP based cinema processors, even that could be bypassed or adjusted through a special 'Digital Cinema' preset if the installer or operator wants it. Typically you would use the same equalization for ALL sources since it's meant to adjust for sound system and room, not source. But if the system was only adjusted for SR and now you are playing uncompressed linear PCM - you would think that some adjustment could be necessary.

Levels should be passed through linearly, but not necessarily with the same cross-channel balance, since channel level adjustment is also part of the initial audio setting and equalization.

Note that, depending on the BluRay Player, there might be audio settings on the player itself that deal with audio levels. Compression, level, speaker size, delay settings, etc. Better consult the player manual. You just don't know how it is configured.

The manual section on film-tech has a document showing the block diagram of the CP500 - if your techie enough, you might want to look it up.

'Dolby CP-500 reference diagrams'
pages 14/19/20

In general, once you went through some of this initially, just like before digital, you should not care about a specific cinema processor when doing your work.

- Carsten

Posted by Stephen Angelini (Member # 5741) on 05-11-2010, 11:42 PM:

Thanks for the information, Carsten. I would much prefer not having to worry about all this too, but unfortunately it feels like I'm trying to shoot a moving target when I consider all the variables that can exist between what I hear in my studio and what comes out in the theatre.

We recently printed a local commercial on DCP that ended up sounding anemic in the theater. I was told by the projectionist that he has no control over the level of the DCP, which leaves me responsible for a fix. It sounded fine off the Blu-ray, so I'm trying to pin down exactly why.

Lots of reading before I sleep... [Wink]

Posted by Carsten Kurz (Member # 5396) on 05-12-2010, 05:47 AM:

Actually, the sound track in the DCP is Broadcast WAVE wrapped into MXF - you may want to unwrap it again after the DCP packaging and compare it with the input files.
VLC can play MXF audio directly (if it's not encrypted).
Maybe during the DCP mastering, the channel assignment went wrong, that could account for some very spooky audio in the theatre. Easy to check that.

Best thing would be to check levels of all 6 channels in the DCP and BluRay version in the projection booth and compare them.

Was the DCP played over the same CP500 as the BluRay? Because then the sound would have entered through the same input as the BluRay and should have received the same processing.

If it's different - first you would think that the DCP was played over a modern digital installation which should be the reference. But I wouldn't blindly assume it actually has been setup properly. Maybe the CP500 received a better setup.

You should check wether the BluRay Player and AV-Amp and CP500 had been set up more or less linear without to many alterations to the levels and EQ. Sorry that could involve more reading, since depending on player and amp, there could be a lot of options hidden somewhere in the audio setup.

There is also a thread here about DCP test files for image and audio - you may want to make your own testfile that has signals of different level and channel assignments to give you an idea what to expect from the different speakers and levels in the auditorium, eg, F/L whisper, medium, loud, surround whisper, medium, loud, instruments with different harmonic content, LFE, etc.
Then you can compare auditoriums with your studio.

- Carsten

Posted by Sean McKinnon (Member # 612) on 05-14-2010, 09:56 AM:

The other thing to keep in mind here is how is your mixing studio eq'd. In the cinema world the mixing studio's are eq'd to the same standard that the theatres are eq'd to. If your mixing environment is set-up to a different standard that could explain some of the difference. ISO 2969 states that you want to use pink noise and a 1/3rd octave realtime analyzer to acheive a flat response out to 2khz dropping 3db per octave to 8khz. The cp-650 manual (available here) has a very good explanation of the procedure. The sound pressure level of each channel should also be set using a SPL meter set to a slow response with C weighting to 85db using a pink noise generator. Setting your mixing studio's monitoring to match the theatre's eq will ensure that what you hear in your studio is what you will hear in the theatre. Also having a cinema tech come in and check the "B-Chain" of the theatre is a good idea to confirm that the theatre itself is correctly set-up.

Does this blu-ray player have a spidf output? I would be more likely to use a spidf or aes digital audio output into a DMA-8 or the cp-650 itself if it is set-up for it.