DCP audio problems

[Aaron12975]

Hello,

Firstly, thanks so much to the team for creating a very intuitive and accessible DCP creation tool! Much appreciated.

I've just recently made a DCP for screening and had quite a few technical issues with audio along the way - hopefully someone will be able to shed some light on what went wrong. I'm no expert in terms of sound mixing but have a general understanding, yet still finding it hard to put my finger on the problem.

The DCP was made from a stereo mix and I experimented with all of the different methods (plain stereo, mid-side, 5.1 upmixing etc.) but no matter what I did, the speakers at the cinema would clip quite harshly on certain dialogue (mainly higher pitched frequencies). What I can't understand, is that the audio from the stereo mix is clean and peaks at -3db, with an average -6db for dialogue, and in DCP-o-matic I experimented with many different volume levels, with versions that peaked at around -10db, and the results were still the same. The cinema is a state of the art Vue with high end facilities, and other DCPs in the same space sounded just fine, and were even louder. Testing the DCP with the DCP-o-matic player, the audio sounded perfectly fine with no clipping whatsoever on any version.

Can anyone shed any light on what may have gone wrong?

Cheers,
Aaron

[Carsten]

Which DCP-o-matic version do you use?

Can you reload your project and show us the audio graph?

- Carsten

[Aaron12975]

Thanks for the reply Carsten. I'm on version 2.12.4 of DCP-o-matic running on macOS Sierra.

Here is a screenshot of the audio graph - this is the lowest volume version I tried and it still caused the speakers to clip harshly throughout. Again, there's no clipping in the original stereo mix and it peaks at -3db. Also sounds clean when coming out of DCP-o-matic and testing with the DCP-o-matic player.

Screen Shot 2018-05-15 at 22.16.44.png

[Carsten]

Hi Aaron -

I'm trying to replicate this.

If possible, install audacity with the ffmpeg plugin on your Mac.

https://www.audacityteam.org/download/mac/

https://lame.buanzo.org/#lameosxdl

With the ffmpeg pugin, audacity will allow to load the audio mxf file from the DCP that you generated as a multitrack file. Looking at the MXF audio waveform will immediately tell you wether there is something wrong with your DCP or the playback system/audio system.

- Carsten

[Carsten]

Okay, DCP-o-matic 2.12.4 in Sierra. I added a stereo file with near full scale peak levels (music/singing) and created three DCPs, one with M/S, one with UpmixerA, one with UpmixerB. Both M/S and UpmixerA create normal no clipping levels for me. Upmixer B creates SOME clipping, but not much (spurious peaks around +1.5dBFS). I can see that clipping already in DCP-o-matic audio graph. Setting -3dB gain solves the issue.

All three audio MXF files look normal in audacity. UpmixerB audio is really hot, but not (yet) broken. It is clearly too loud for cinema playback, but technically okay.

Honestly, I would have difficulties to understand why and how a cinema sound system would create heavy clipping when a standard 5.1 DCP is played back...

Let us see a screenshot from audacity. Of course, you could also load these audio mxf files right into DCP-o-matic and perform another audio analysis on them. What's the sample rate of your audio?

- Carsten

[Aaron12975]

Hi Carsten

Thanks for helping me troubleshoot this and sorry for the short delay. Just received the DCP back from the cinema and loaded it into Audacity. Here are the results...

Worth noting that the projector was set to play back at the standard levels (4.5db I think) and that even when turning these levels down on the projection system, the clipping was still apparent.

The audio was recorded, mixed and exported at 48000 Hz.

Screen Shot 2018-05-18 at 15.58.59.png

[Carsten]

Well, I guess you can see yourself that there is no clipping in the audio. What do you hear when playing back that file in audacity?

I can't see it in that screenshot, but is it possible that what was noticed was not clipping, but 'clicking' - a kind of noise? Because levelwise, it looks okay, even if don't see all the surrounds, but I guess there is not much in it. Maybe something else happened. Can you upload a piece of audio with the problematic noise in it?

IF there is clipping in a DCP, it will be audible no matter what the cinema audio system level is set to. If there is no clipping in the mxf audio, then no clipping should occur at the cinema. Distortion yes, if playing back at high levels, but no clipping.

Clueless...Let's hear what you say when you play back that file on your PC. You can also mute/solo individual channels to find out wether there is something in individual channels. The L/R clearly shows the effect of a mid/side decoder (obviously, a lot of in-phase content on L and R, because most of the signal is transferred into center, and only a little bit is left in L/R. Those spurious signals in L/R, could they be audible as 'noise'?

- Carsten