I think some people gets confused about the difference between Dolby Digital and dolby TrueHD.
Both formats can be in any channels, such as mono or 1 channel, stereo or 2 channels, and multi-channels such as 5.1, 6.1, and 7.1.
The major different between Dolby Digital and Dolby TrueHD is that one is a "lossy" compression while the other is "lossless" compression format which can be decode into 100% "uncompressed" quality.
Think of it like mp3 and pcm. MP3 is a "lossy" compression format, while PCM is "uncompressed" format.
Another difference is the spec between the two. Dolby Digital supports only 20 bits sound quality at 48 kHz as its highest, and the compression quality was 15:1 for most Dolby Digital 5.1. Meaning, 15 times the original size of the audio data has been thrown away to save space and cannot be recover since the data been remove are too high to recover and its a "lossy" codec.
Thanks to it much higher storage capacity that both Blu-Ray and HD-DVD offer, Dolby True-HD by the book can supports up to 192 kHz while I think 96 kHz are overkill, and at 24 bits depth at its maximum and up to 7.1 channels. the compression ratio was about 2:1 lossless compression.
That\'s why i am raving about this two lossless codecs Dolby True-HD and DTS-HDMA.
Even most Blu Ray movies that came out recently are recorded with PCM 5.1 since PCM is uncompressed. The reason why they choose PCM for first generation movies is because no A/V receivers have lossless audio codecs yet and the space on Blu Ray movies are sufficient enough for it...