Trim and Downmix Editor (Dolby Atmos) Bug?

I wonder if someone can check something for me. This is using the Nuendo internal Dolby Atmos renderer.

I have a Dolby Atmos music mix that correctly plays back to -18LKFS loudness. I am monitoring in 7.1.4. For this track, the 2.0/5.1 downmix works best if set to 0dB for both heights and surrounds rather than the default -3dB. So I set that and “apply”.

Now, however, my mix - still monitoring in 7.1.4 (i.e. no in-renderer downmix applied, I have merely altered the metadata settings) plays back 1dB loud. It measures -17LKFS on playback in the renderer. If I reset the meta-data to default, all is fine again.

I have tried exporting an ADM and reimporting to a new project. That plays back in the same way - correctly if I set default downmix, 1dB high if downmix set to 0dB. So it seems it is purely to do with the renderer output, not what is written to file. Surely the 2.0/5.1 downmix metadata should not affect the 7.1.4 monitoring?

Thanks. :slight_smile:

Steep learning curve… The renderer takes its loudness reading from an internal 5.1.4 downmix, and this is affected by the downmix metadata, even if the actual master 7.1.4 is not.

But it does mean that if you change your downmix settings at the last moment, you have to recheck all your levels. Of course I am now aware that all of this can be finalised in the Album Assembler.

(It’s a shame you in effect compromise the 7.1.4 so that a 5.1.4 will be at the right level, when it seems to me it would be better to store an offset in the metadata so that they both play back at the target. But I am sure there’s a reason!)

This all makes perfect sense once you get your head around it. Spent last night re-reading manuals. :wink:

Update to correct. Loudness measurement is taken from internal 5.1 downmix, not 5.1.4.

From the manual:

Using the 5.1 loudness re-render for loudness measurement

Use the 5.1 loudness re-render for loudness measurement during postproduction. Ensure that content meets delivery specifications by returning content to your chosen DAW, using the your chosen method and loudness measurement plug-in. The loudness measurement display in the Renderer is also fed from this loudness re-render bus.

To reiterate… this means that if you change your downmix settings in the renderer (for instance set the surround mix to 0dB rather than -3dB) your measured loudness will change, even if you have not altered the actual Atmos mix. This takes a few minutes to get your head around.