DD5.1 is compressed much like an .mp3 is compressed.
Indeed Nick you have described a technical difference between the formats but i was questioning more the subjective term "high definition" sound.
DD 5.1 is not unlike a mp3 at >500 kbps (DVD audio is rated for a similar bitrate although this is only one metric of quality among the formats). I doubt anyone has been able to tell the difference between the compression and non-compressed versions at such a high sampling rate.
Compression does not equate to a loss in audio quality if it can't be distinguished by the human ear anyway. There is another similar discussion at the bluray forums which parallels this one. http://forum.blu-ray.com/showthread.php?t=159982
Understood...
I generally steer away from Dolby TrueHD whenever there is a DTS-MA track, and most of the blu-rays default to DTS-MA as *the* track of preference. Maybe that is why I hear such a big difference. The DTS mix on DVD was always more dynamic than the DD5.1, so when they put it into a lossless format on blu-ray, it still sounds light years better than DD5.1.
I will admit, since we are all being honest here, that I have never done an A/B comparison between a DD5.1 DVD track and the Dolby TrueHD track for the same movie on blu-ray.
You may have challenged me to give it a shot tonight.
One thing to keep in mind is that my gear and your gear are going to have different variables, heck, I have an acoustically treated room that may be the biggest variable in hearing a difference or not. If a room destroys upper frequencies or has nulls in the low end bass, that would suck up any real clarity or bass extension that would would get from a lossless audio track.
Then again, some people have better hearing than others. Even though my wife is younger than me, I hear a lot more detail than she does, but not as much as my teenage kids.
I will see if I still have a DVD of something that I also have on blu-ray, but if not, I will just manually switch between the two Dolby tracks from the same blu-ray and report back tonight. The trick will be to find a blu-ray with Dolby TrueHD on it since most studios went with DTS-MA as the default for blu-ray and DTS in general is more "dynamic" than Dolby, so that wouldn't be a good comparison.