From the
Dolby Digital Plus FAQ:
Is Dolby Digital Plus content backward-compatible?
Because Dolby Digital Plus is built on core Dolby Digital technologies, content
that is encoded with Dolby Digital Plus is fully compatible with the millions
of existing home theaters and playback systems worldwide equipped for
Dolby Digital playback. Dolby Digital Plus soundtracks are easily converted
to a 640 kbps Dolby Digital signal without decoding and reencoding, for
output via S/PDIF. The 640 kbps bit rate, which is higher than the standard
448 kbps used on DVDs, is fully compatible with all existing Dolby Digital
decoding products such as A/V receivers, and can provide higher-than-DVD
quality from Dolby Digital Plus soundtracks when played back through
existing systems.