[lchiu7]

Originally Posted by SomeJoe

..

Why would only DD+ be so fundamentally different that it would be the only audio codec that's not at all interchangeable?..

As noted in a Dolby White paper

Unique Aspects of Dolby Digital Plus on Blu-ray Disc vs. HD DVD
There is a fundamental difference in the way programs are carried in the two HD disc formats.

Like DVD-Video, the HD DVD format carries audio and video signals in the MPEG-2 “program stream” format, whereas the Blu-ray Disc format carries them in the MPEG-2 “transport
stream” format, the same manner in which ATSC DTV and DVB signals are broadcast. A result of this otherwise subtle distinction is that there are packetizing differences for the audio
bitstreams between the two disc formats. You may recall that DVD-Video is not able to carry Dolby Digital bitstreams of more than 448 kbps. The same remains generally true for HD DVD,
although due to finer granularity in the choice of data rates, the actual maximum rate increases slightly, to 504 kbps. In order to support the higher bit rates and greater number of channels
offered by Dolby Digital Plus, HD DVD discs will use Dolby Digital Plus bitstreams (which have progressively shorter coding frames as the bit rate increases, thereby always fitting within the
defined audio packets on the disc). For example, standard Dolby Digital has a six-block frame, and in that case the Dolby Digital Plus bit rate can reach 0.5 Mbps; a three-block frame can
reach 1 Mbps; a two-block frame, 1.5 Mbps; and a one-block frame, 3 Mbps.

In contrast, the Blu-ray format has no such packetizing constraint, so one immediate result is the ability to transport Dolby Digital at its maximum 640 kbps rate for the first time. That’s slightly higher than the 576 kbps carried on D-VHS.

I read this as being a difference in the packet sizes which is why the pure DD+ stream extracted from a HD-DVD cannot be played on a BD player.

Not sure how a device like the Popcorn Hour would handle it though