Could there be a use for demixing parameters with only a scene based audio element?

[jwcullen]

Hi this question was based on an idea I had when reviewing #435. I mentioned I would revive this conversation in the last meeting.

What is supposed to happen for a bitstream with the following properties?

• 1 scene based Audio Element
• A Mix Presentation OBU with a stereo and 7.1.4 layout.
• The listener wants to listen to 7.1.2.

Section 8.3.2.2 tells me that

The IA renderer used is selected according to the following rules:
...
Else, use implementation-specific renderer.
If there is no implementation-specific Ambisonics renderer, use the EAR HOA renderer to render to the next highest

The above rules mean that the scene based Audio element should be rendered to 7.1.4 (this is OK). But the next sentence:
layout compared to the playback layout, and then downmix using implementation-specific renderer.

This rule means that now the 7.1.4 should be down-mixed to 7.1.2 to support to user's request. But restrictions in section 3.7 num_parameters prevent scene based audio elements from having any parameters (e.g. demixing).

In this case the implementation specific rendering would have to guess demixing parameters, but maybe it would make sense to at least allow the option to supply optional demixing parameters for use in this path of the rendering logic?

[sunghee-hwang]

@jwcullen,

I still believe that this is a new scenario which we have not been considered. As suggested, let's postpone it to V2.

Anyway, channel-based is under the same situation. When 7.1.4ch is delivered and listeners want to listen to 7.1.2, the spec recommends to use implementation specific renderer.

The general usages of demixing parameter and libear as renderers is:
If demixing parameter is delivered, it is intended for devices to use it as a renderer for down-mixing. Otherwise, it is intended to use libear.
When we delivers Ambiosnics with demixing parameter, the situation is complicated and the general usages will not work for this case so that it may require lots of discussion. Please consider it.