In several recent reviews I’ve talked about the importance of supporting 8-channel LPCM over HDMI. More specifically, you’ll see this as a feature listed with AMD’s Radeon HD 4800 series and more recently the 4600 series. Intel has quietly toted 8-channel LPCM support as a feature of its integrated graphics chipsets since the G965, yet I’ve never done a good job explaining what this feature is and why you should even care.

Honestly, it took my recent endeavors into the home theater world to really get an understanding for what it is and why it’s important. So without further ado, I present you with a “quick” (in Anand-terms) explanation of what 8-channel LPCM over HDMI is and why it matters.

Grab some popcorn.

The Necessity: Enabling 8-channel Audio on Blu-ray Discs

Movies ship with multi-channel audio tracks so that users with more than two speakers can enjoy what ultimately boils down to surround sound. Audio takes up a lot of space and studios keep trying to pack more data onto discs so most multi-channel movie audio is stored in a compressed format.

In the days of DVDs the studios used either Dolby Digital or DTS encoding for their audio tracks, but with Blu-ray (and HD-DVD) the stakes went up. Just as video encoding got an overhaul with the use of H.264 as a compression codec, audio on Blu-ray discs got a facelift of its own. Dolby Digital and DTS were both still supported, but now there were three more options: Dolby Digital TrueHD, DTS-HD Master Audio and uncompressed LPCM.

Dolby Digital and DTS, as implemented with the original DVD standard, had two flaws: 1) They were lossy codecs (you didn't get a bit for bit duplicate on disc of the audio the studio originally mastered when making the movie), and 2) they only supported a maximum of 6-channels of audio (aka 5.1 surround sound: right, left, center, left surround, right surround and LFE/sub channel).

DVDs could store 4.5GB or 9GB of data on a single disc, so using lossy audio codecs made sense. Blu-ray discs are either 25GB or 50GB in size meaning we can store more data and higher quality data at that, for both audio and video.

A standard 5.1 channel audio setup. Copyright Dolby Laboratories.

Both Dolby Digital TrueHD and DTS-HD MA improve upon their DVD counterparts by: 1) being lossless (when decoded properly, you get a bit for bit identical copy of the audio the studio originally mastered for the movie), and 2) currently supporting up to 8-channels of audio (aka 7.1 surround sound: right, left, center, left surround, left rear, right surround, right rear, and LFE/subwoofer channel; both specs actually support greater than 8-channels but current implementations are only limited to 8).

A standard 7.1 channel audio setup. Copyright Dolby Laboratories.

These standards are lossless, which is great. While we're not quite there on the video side, the fact that we can store and playback the original audio track from a movie is an incredible feat and a feather in the cap of technology in general.

The support for 8-channel speaker setups is also a boon, because currently the way people with 8-channel audio setups get those extra two channels is by some form of matrixed audio. Dolby ProLogic IIx and DTS Neo6 generate one or more additional channels of audio from existing surround sound channels; the downside is that these methods never sound good. While they make audio come out of all speakers, generally the original 5.1 audio track produces better sound in that it is less muffled and more distinct.

Now we have these wonderful audio codecs to give us the benefits of fully uncompressed audio without the incredible space requirements, but there is indeed a problem: decoding them on a PC.

The Invention: The Protected Audio/Video Path


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  • gramboh - Thursday, September 18, 2008 - link

    Copy protection on Blu-ray and HD-DVD has been cracked for a while. As predicted by many, it was a big waste of time. You can download full disc images (25-35GB on average) or x264 based encodes with DD5.1/DTS 5.1 sound tracks (4-6GB for 720p and 8-12gb for 1080p) from torrent sites already. It did take a while to fully break (I think AACS on Blu-ray was the challenge) but it has been done, so people are playing these disc images and encodes on HTPC type setups already.

    Basically the industry is just punishing early adopters. The copy protection is irrelevant (especially HDCP) to people who are going to pirate the material anyway.

    The only possible protection they can ever hope for is some online database of keys that players authenticate against before playing a title, but I doubt that level of connectivity (people having their players hooked up to internet 24/7) is closer than 5 years away, at which point uncompressed HDstreaming may be here (current HD streaming is useless and heavily compressed).

    So dumb.
  • jmurbank - Wednesday, September 17, 2008 - link

    I agree. Studios still in the brick and mortar business. Today's world is about buying media from the internet and downloading the bought media like from itunes or Then be able to freely be viewed or heard with out any annoying problems in between. People think downloading movies and tv shows for free right thing to do, so studios are forced to hire encryption specialists to help them find a a way to protect their investment from torrent servers at a cost of stating customers guilty for every action they take. There are rumors going around in the open source community that HDCP and other encryption methods that studios have placed in will soon be reversed engineer, so I or others do not have spend an arm and a leg to upgrade all equipment that support HDCP or other encryption tactic. Studios have to spend more time inventing new protection schemes, but when will people stop downloading illegally.

    7.1 surround sound is for bragging rights and even 5.1 is for bragging rights too. Majority of movies rarely uses the rear speakers in each scene. Also people incorrectly setup surround sound, so they do not get the full effects. Two channels is just enough for movies but people are too brain wash to think that surround sound will be better. In order for surround sound to be better, the listener have to setup it up correctly.

    Dolby Laboratories shows diagrams what 5.1 and 7.1 looks like on paper. Again on paper. In the real world paper does not stand ground for setting up surround sound. A good surround sound takes a lot of time to setup. It can take a day or a whole weekend to setup surround sound correctly. Rear channels for example should always create a null area at the listening area, but the sounds should be reflected to provide ambient sound. Using regular speakers as rears can be done but they have to be directed away from the listening area and point they directly at a material to diffuse the sound that will scatter the sound. Fancy rear speakers can be used such as bipole and dipole to ease awkward setups. Rear speakers should be at a height higher than head level while standing, but not at the height while sitting. The front speakers can be directed at the listening area. These are just small tips, but can fill a whole entire book.

    Buying the best amps and sound cards should not be stopped there. Buying the best speakers also have to be looked at although do not have to buy the best amplifier and sound cards to get good sound. IMHO, BOSE cube/satellite speakers are not the best. Sound is very psychological. This means your sound system might sound very good one day and the next day sound horrible. It can take a month to setup a good sound system, so do homework or research which store provides a good return policy when you do not like the speakers.

    The quality of cable depends on the person. I use OEM oxygen free copper audio cable because it has the less resistance which means more power can pass with out the chances of it overheating. I also build speakers and soon build amplifiers, so I just need to spend the money on surround sound processors. I am thinking of creating a project to provide the open source community to use the GPGPU to decode multimedia content because Linux does not have that support. nVidia removed XVmc support from GeForce8 cards and probably up, so I and many others are left to use high speed multi-processor setups running at 3 GHz or faster to watch HD movies in Linux.
  • pattycake0147 - Wednesday, September 17, 2008 - link

    You mentioned that your home theater is what really got you to take a closer look at this. Are there updates coming soon on the progress? I'm particularly interested on the software side of the project. Reply
  • Anand Lal Shimpi - Thursday, September 18, 2008 - link

    I've definitely got a lot of updates that are in the pipeline. The projector actually *just* died so that put a hold on things, not to mention that Omaura getting out of the market threw a wrench in my plans for the HDD/main chassis split.

    I'll get cracking on some updates in the next week though :)

  • Guuts - Thursday, September 18, 2008 - link

    Click the "Blogs" link, upper left, then on "Anand's Theater Construction" link on the right... I think there's a section or two on his HTPC setup. Reply
  • darckhart - Wednesday, September 17, 2008 - link

    The big issue here is working as advertised. If the bluray disc comes with all the super-bloated lossless junk, then by golly I want all that super-bloated lossless junk played back perfectly. I don't want "Oh well see your HDMI version doesn't quite support that so we downsample...etc,etc," or "Oh sorry that part isn't in the 'protected path' so it won't work...etc,etc," or "See all your hardware components didn't support hdcp using hdmi-this or new-fangled-acronym-cable-that." Get your crap together Hollywood and hardware manufacturers before selling me some half-baked product. Oh, and get rid of that DRM junk and FBI warning junk and unskippable trailers junk and any other crap that hinders my viewing experience. I bought the movie to see the movie, not solve 10 jumping puzzles before seeing my movie. Reply
  • fuzz - Thursday, September 18, 2008 - link

    couldnt agree with you more.. HDCP, AACS, CSS.. it's all just a big waste of time that hurts the industry a lot more than it helps.

    but if you are a HTPC user with these same concerns, you really should send a couple bucks to the boys who make AnyDVD.. not only does it remove any need for HDCP compliant BS and allow you to rip any movie (DVD/HDDVD/BD), it has options for stripping all PUOPs from a disc dynamically as well as jumping straight to the movie upon insertion :D
  • Fant - Wednesday, September 17, 2008 - link

    As far as I know even the PS3 doesnt support outputting DTS-HD 7.1 via HDMI. I believe they downsample it to 5.1 in order to output it over HDMI. Reply
  • jnmfox - Wednesday, September 17, 2008 - link

    It does support DTS-HD 7.1 but there are reported problems with some blu-ray disks from New Line, Lionsgate and some others, that the PS3 downsamples to 5.1.

    Follow the link for a list:">
  • PatMeenan - Wednesday, September 17, 2008 - link

    The biggest problem with a repeater solution is keeping the video and audio signals in sync (a lot easier when they're going through the same device). I hope all of the repeater "HDMI audio" cards support adjusting the timing to compensate. Reply

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