Originally Posted by Gary McCoy
does 8-channel LPCM-over-HDMI compromise audio quality at all if the original format on the disk was Dolby TrueHD or DTS-HD with 7.1 channels?
First let's clarify some nomenclature. Dolby and DTS have both introduced new audio codecs. The lossy ones are DD+ (Dolby Digital Plus) and DTS-HD (High Definition). The lossless codecs are Dolby TrueHD and DTS-HD MA (Master Audio).
Think of Dolby TrueHD and DTS-HD MA as zipping a computer file to save space. None of the data is discarded, just packed more efficiently to take up less storage space. When you unzip the file, 100% of the data is still there, and you get a bit-for-bit copy of the original.
If you had a zipped document that you wanted to send me on disc, you would have two choices. You could unzip it on your computer before putting it on the disc. Or you could send it to me as a zipped file (would take up less space on the disc) and I could unzip it on my computer. Either way, I end up with the exact same document, down to the last letter.
Likewise, decoding (unpacking) a soundtrack in the player or in the receiver will yield the exact same results. It's not like high end receivers have a special secret version of TrueHD decoding reserved for them that cheap players aren't allowed to have. It's just format decoding. If certain audio data is flagged for the left front channel, then decoding in the worlds most expensive receiver won't place that data somehow "more" into the left front channel than decoding in the world's cheapest player.
Going back to the zipped document analogy. If you wanted to change anything in the document, from simple correction of spelling mistakes to complex re-formatting for a better look, you would first need to unzip that document. You wouldn't be able to manipulate it while it was still zipped.
Similarly, everything a receiver does to the soundtrack, upto and including D/A conversion, requires the soundtrack to be in uncompressed PCM form. In fact, when you send your receiver a DD or DTS bitstream, the first thing it does is decompress the soundtrack to linear PCM. Only then can it apply things like bass management, time alignment, etc.
Soundtracks on HD DVD (and eventually on Blu-ray, when it goes interactive) operate very differently than they do on DVD. With current DVDs, you need entirely separate soundtracks for things like foreign languages and filmmaker's commentary. This is actually a pretty wasteful approach.
With HD DVD, soundtracks can be authored in the 'Advanced' mode, which allows multiple content streams to be live-mixed (mixed in real time). You don't need another soundtrack for foreign languages. Just swap out the English centre channel stream with one of the foreign centre channel streams. You don't need another soundtrack for commentary. Just reduce the level of the main soundtrack and mix in the commentary stream. Same with button sounds and other interactive features, like picture-in-picture.
Just like editing the document requires unzipping the file first, doing any of this live-mixing to the soundtrack requires decoding it to linear PCM first. This is why it has to be done in the player. They're not going to transmit every option to your receiver, just one soundtrack. You choose what you want to hear, it is mixed in the player (i.e, the soundtrack you want to hear is literally built in real time inside the player) and transmitted as a final mix to your receiver.
Current HDMI allows 8 channels of 96/24 PCM to be transmitted (more than enough resolution for any soundtrack), but not the new codecs in their native form. When HDMI 1.3 arrives, it will allow the new codecs mentioned above to be transmitted in their native bitstream, but only if they were authored in 'Basic' mode
(no interactivity). If the soundtrack was authored in Advanced mode, then it cannot be transmitted in undecoded form; decoding in the player is mandatory because of live mixing.
So far, all HD DVD soundtrack have been authored in Advanced mode. Which means nothing will change when new receivers arrive on the market. Despite having HDMI 1.3 transmission and decoders built into the receiver, decoding will still have to take place in the player.
Currently, Blu-ray discs are authored in Basic mode, since they haven't gotten interactivity yet. As soon as BD Java is up and working, they'll all be authored in Advanced mode too. At that point, what are the decoders in the receivers going to do? Decode the relatively few BD titles that were released before interactivity? Most of those titles will be re-issued anyway.
Personally, I'm glad that decoding is shifting to the player. I wish it had always been that way. Since receivers need the data in PCM form anyway, that's what every player should be outputting (irrespective of what format is used to store the data on the disc). As mentioned before, when new audio codecs and formats arrive, you'll have to buy a new player. But as long as the players keep outputting the audio in PCM form, current receivers will always remain compatible with anything that shows up in the future. How elegant is that!