Film-Tech Forum ARCHIVE: DCP-o-Matic and Barco Alchemy

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Topic: DCP-o-Matic and Barco Alchemy

Mike Blakesley
Film God

Posts: 12767
From: Forsyth, Montana
Registered: Jun 99

posted 03-11-2018 08:05 PM

quote: Monte L Fullmer

I go to the audio and set the audio to -10,for you don't want 0.0, for you'll just might blow the speakers out.

From BluRays, I started at -6, then kept experimenting downward until finally settling on -1.5, which renders a DCP that's still somewhat softer than a typical studio DCP.

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Monte L Fullmer
Film God

Posts: 8367
From: Nampa, Idaho, USA
Registered: Nov 2004

posted 03-12-2018 05:51 AM

And i'm working with MP4,AVI and MOV files...where these are recorded hot. Thus, i really have to scale down the audio track.

It all works great though, no matter what production techniques you use.

Plus, I do like making Audio only DCPs, and there, I scale the sound down to -25 for good background music output.

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Leo Enticknap
Film God

Posts: 7474
From: Loma Linda, CA
Registered: Jul 2000

posted 03-12-2018 09:47 AM

The DCI spec for audio says that it should be normalized to -20dBFS. That is equal to unity (on the AES scale) on a cinema audio processor.

I've found that as a very rough rule of thumb for DCP-o-Matic, adjusting the audio gain such that the sustained peak is around -10 on the audio plot will result in a DCP that plays at around the same fader setting as a studio-mastered one. For example, in this illustration, if you usually play studio DCPs at around 6.5, you'd want to play this one at 4 to 4.5.

quote: Carsten Kurz
I guess it will come one day when Carl has found a safe way to do it - you don't want any existing drive to be touched by such a function, so it has to be absolutely idiot proof. Now think about the Linux method of accessing drives using the dev/sdx and mountpoint schemes...

Exactly, hence the reason this would be much easier to do in Linux than in Windows or Mac OS. One alternative approach would be a feature that creates a hard drive image file of a correctly partitioned and formatted drive, which the user then writes to blank media in a separate operation.

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Carsten Kurz
Film God

Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009

posted 03-12-2018 10:26 AM

Note that DCP-o-matic offers EBU R128 audio loudness analysis. While 'LUFS' created from that analysis is not the same as LEQ(m), it is still very effective in setting the right audio levels for cinemas.

Neither channel peak nor channel RMS is suitable for estimating actual loudness from audio levels. RMS being a bit more useful than peak, though. Peak is actually only useful to identify clipping/distortion.

You need to enable R128 analysis in DCP-o-matic prefs. It will take a bit longer to analyze audio files, but it's worth it. LUFS is targeted at home users/broadcast, so it applies slightly different weighting functions than LEQ(m), but it still works for cinema applications. LUFS standard for broadcast is -23LUFS. In cinema, higher levels are expected/tolerated, so, something between -17 and -20 LUFS will usually work well. I have done a lot of testing around this, and it is dependable.

Why is there EBU R128/LUFS in DCP-o-matic, but not LEQ(m), when LEQ(m) is the proper loudness measurement method for cinema? Because the underlying ffmpeg framework that DCP-o-matic uses currently offers EBU R128 as a filter, and DCP-o-matic simply uses that available function. Writing your own LEQ(m) meter is a lot of work (and may need a Dolby license or NDA), so, as long as there is no LEQ(m) filter in ffmpeg, DCP-o-matic will use EBU R128/LUFS.

- Carsten

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