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Atmos export to Mp4 -- Channel count

I’ve spent some time learning the Dolby Atmos Renderer and am having trouble with the "export to MP4” feature recently introduced in v3.3. The Atmos Master file I created plays back perfectly in the Renderer, but when I try to export the Atmos master file to MP4, the resulting file only appears to have a six channel, "5.1 + Atmos" audio track. I was expecting to be able to test 7.1 plus Atmos object metadata, as present in the Master file. Is this a limitation of the Renderer? Or might I be missing a step? 

Current configuration includes: 

Mac Pro 5,1 3.33 6-core w/24 GB RAM. Mac OS 10.13.6 running off 1TB SSD. 

DAW is Nuendo 10.1

Dolby Atmos Renderer version 3.3

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5.1 for the encoded audio is expected and correct.  The .mp4 contains a Dolby Digital Plus with Atmos Content bitstream.

Dolby Digital Plus with Atmos Content contains all the audio rendered to a 5.1 core along with the metadata used with by a consumer device (AVR, Soundbar, Game Console etc) to render to the listening speaker configuration.

Dolby Digital Plus with Atmos Content is rendered to 5.1 to preserve backward compatibility of the bitstream for playback on non-Atmos consumer devices.

The process of encoding an Atmos master file to Dolby Digital Plus in abbreviated form is:  the Atmos master file is spatially coded which is a process whereby bed and object audio is dynamically grouped together into elements that contain their own positional metadata.   The spatially coded audio is rendered either directly to 5.1 or to 7.1 and downmixed to 5.1.   The metadata for the elements and joint object coding metadata is written into the Dolby Digital Plus with Atmos content bitstream along with the 5.1 audio.    This metadata is used by a consumer atmos device to recreate the elements which are then rendered by the consumer device to the speaker configuration be used by the consumer.

The number of elements used in the spatial coding process is determined by the bitrate of the Dolby Digital Plus with Atmos content.    If the datarate is 384kbps, 12 elements are used in spatial coding.  If the datarate is 448kbps and above 16 elements are used which provide greater spatial resolution and higher quality for the 5.1 audio.   The .mp4 export from v3.3 uses 16 elements in spatial coding.

Hope this helps.



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