Note to readers: this information was current as of early 2010. Since then, ATI, Intel, and Nvidia have all released video/audio solutions to allow bitstreaming of the original Dolby TrueHD and DTS-HD Master Audio streams with the use of playback software such as Arcsoft TotalMedia Theatre. The below content, from early 2010, is for historical reference only.

If you’re a home theater enthusiast, you want the highest-quality audio and video possible, and that means Blu-ray with lossless audio (Dolby TrueHD or DTS-HD Master Audio) and the best video compression codec (VC-1 or MPEG-4 H.264 AVC HP.)

Most people would choose a Sony PlayStation 3 or stand-alone Blu-ray player to deliver this content to their receiver / display, but some people like me prefer to use a PC because it is extremely flexible and allows browsing and playing of movies backed-up on your hard drive. Although it is relatively easy to get full-resolution Blu-ray video from a Home Theater Personal Computer (see this AVSforum thread for hardware details), it is extremely difficult to get full-resolution audio. Here is a run-down on the current state of HD Audio on the HTPC.

Protected Audio Path Required

All Blu-ray movies with AACS protection, which is pretty much all of them, require a Protected Audio Path (PAP) to transmit the original HD audio if the bit depth and sample rate exceeds 16-bit, 48 kHz. Without PAP, all audio must be downsampled (a.k.a. “bit crippled”) to 16-bit, 48kHz per the AACS specification. Unfortunately, there is no Protected Audio Path (PAP) on any computer right now, so all HD Audio is currently downsampled on a PC.

Fortunately, or unfortunately, most Blu-Ray HD Audio soundtracks are 16-bit, 48 kHz anyway, so you are getting the soundtrack in full fidelity. However, there are quite a few Dolby TrueHD / DTS-HD Master Audio soundtracks that are 24-bit, 48kHz, so these are downsampled down to 16-bit, 48kHz. The same is true for the very few 24-bit, 96 kHz soundtracks (the maximum possible resolution of 8 channels.)

As far as when PAP will be available for PCs, no one knows. It is not a hardware standard that can be used across devices, it is not a universal Windows Vista standard (rather it is specific to each driver/hardware config), and there is no industry group working on a standard. The only possible way to have PAP is a proprietary solution specific to each piece of hardware.  Pluse, this hardwarw would have to be supported by uniquely by software players. Hardware and software companies working together — not a good thing! This could take forever.

Does downsampling make an audible difference in Sound Quality?

Every home theater enthusiast wants the best sound quality possible, even if the advantage is only theoretical. Having said this, even at the lower 16-bit / 48 kHz, the sound tracks sound amazing. If you can hear the difference between 8 channels of 16-bit / 48 kHz and 24-bit / 96 kHz, you have a truly golden ear.  I’m won’t go into detail as one could nit-pick this to death, but you won’t get any audible improvement even if there is no downsampling.

Even with the down-sampling, HD Audio is a significant step up in quality from compressed Dolby Digital 5.1/DTS 5.1 on DVDs. This a difference you can hear. Down sampling from 24 to 16 bits is not.

But what if AACS protection is removed?

If the down-sampling restriction applies only to AACS-protected Blu-ray movies, then you should be able to run AnyDVD HD and remove AACS from Blu-Ray. If this occurred, the software player should not downsample/bitcripple the output and existing motherboards with HDMI could play the full resolution signal. Unfortunately, all of the software players currently downsample regardless of whether the media is AACS protected or not due to stipulations within the Dolby True HD and DTS-HD Master Audio codecs. Hopefully this limitation will be removed in the future and currently-available motherboards, even without PAP, will be able to pass full-resolution audio.

Is it possible to rework the movie and get HD Audio using a different player?

Yes, it is, but isn’t easy. Basically you decrypt the Blu-ray disc using AnyDVD HD, split the audio and video into different files, convert the lossless HD audio track into a lossless 8-channel FLAC file (containing identical audio), and the recombine the video and FLAC audio in a MKV container and play it back using your favorite video player.  Unfortunately, you lose all the extras on the disc. See this AVSforum thead for details.

If the open source community develops an audio codec that can decompress these lossless formats, it will be possible to get software such as Media Player Classic to play the unencrypted disc. But once again, this means no menues or other content in the forseeable future.

What about bit-streaming?

The restrictions on bit-streaming are the same as on full-resolution HD audio: not allowed by AACS.

As an aside, there is no benefit to bit-streaming, as the end result is bit-for-bit identical sound. It’s only a difference of the player or the receiver doing the decompression (assuming the player doesn’t mess-up the decompression.)

What are the differences in all of the sound formats?

Good question; MissingRemote has an excellent overview that details which codecs can be sent over the older optical (toslink) connectors and which require HDMI.

Why is the HD Audio Steam so protected anyway?

This is a valid question. Since all the encryption schemes on Blu-ray have been cracked by both Slysoft and also independent developers on the Doom9 forums, there is full access to the unencrypted steams for pirates to copy. There isn’t likely any way to introduce a new form of encryption without breaking all existing Blu-ray players. So the only result from all this protection is that owners of legitimate Blu-ray movies can’t play the full resolution HD Audio soundtrack, but pirates can copy it without any issue. The media companies are ignoring the needs of their legitimate customers, which is disappointing to say the least.

Refrences: http://www.avsforum.com/avs-vb/showthread.php?p=15006998#post15006998