Merge pull request #837 from AOMediaCodec/expanded_layouts_loudness

Clarify layout for loudness when using expanded channel layouts

index.bs

@@ -1278,7 +1278,7 @@ class MixPresentationOBU() {
 
 <dfn noexport>loudness</dfn> is an instance of the [=LoudnessInfo()=] class, which provides the loudness information for this sub-mix's [=Rendered Mix Presentation=], measured on the layout provided by [=loudness_layout=].
 
-The layout specified in [=loudness_layout=] SHOULD NOT be higher than the highest layout among the layouts provided by the [=Audio Element=]s. In other words, rendering from an [=Audio Element=] with the highest layout to the [=loudness_layout=] SHOULD NOT require an up-mix. The exception is when the [=Audio Element=] is a zero-order Ambisonics or Mono channel; they MAY be rendered to Stereo. In this exception case, the [=loudness_layout=] for a zero-order Ambisonics or Mono channel [=Audio Element=] SHOULD NOT be higher than Stereo.
+The layout specified in [=loudness_layout=] SHOULD NOT be higher than the highest layout among the layouts provided by the [=Audio Element=]s. In other words, rendering from an [=Audio Element=] with the highest layout to the [=loudness_layout=] SHOULD NOT require an up-mix. In the case of a CHANNEL_BASED [=Audio Element=] with an expanded channel layout (i.e., [=loudspeaker_layout=] = 15), the [=Audio Element=] is considered to be providing the reference layout that it is a subset of. The exception is when the [=Audio Element=] is a zero-order Ambisonics or Mono channel; they MAY be rendered to Stereo. In this exception case, the [=loudness_layout=] for a zero-order Ambisonics or Mono channel [=Audio Element=] SHOULD NOT be higher than Stereo.
 
 Each sub-mix SHALL include [=loudness=] for Stereo (i.e., a [=loudness_layout=] with the [=sound_system=] field = [=Loudspeaker configuration for Sound System A (0+2+0)=]).
 - If a sub-mix's [=Rendered Mix Presentation=] is Mono, its [=loudness=] for Stereo SHOULD be measured on the Stereo signal generated using the equations: