The Invention: The Protected Audio/Video Path

Remember all of the garbage the PC industry went through with trying to enable HD-DVD/Blu-ray playback? Unfortunately, most of those efforts were spent on enabling protected video playback, and most of the companies involved didn't spend much energy on enabling protected audio playback.

There are two forms of content protection that help secure both audio and video when playing back a Blu-ray disc: HDCP and AACS. HDCP protects the data as it leaves the PC; it's why you need an HDCP compliant graphics card, graphics driver and monitor if you want to play an HDCP enabled Blu-ray disc on your PC with a digital video output (DVI or HDMI). The idea behind HDCP is that a user should not be able to easily intercept the decoded signal and make a bit-for-bit copy of the audio and video before it reaches the display.

AACS protects the data on the disc itself, much like CSS did in the days of DVDs. In order to play back an AACS encoded Blu-ray, you have to decrypt the content coming off of the disc and hand off the decrypted content to the application so it can be decoded and sent to your graphics driver, and from there to the graphics card for display.

The problem is that the movie studios wanted a way of securing the content between the time the AACS was decrypted and the HDCP encryption took over. Once the AACS was decrypted the encoded movie was sitting in main memory and could be intercepted by any other application, so something had to be done.

The solution was to re-encrypt the data once it was pulled off the disc (I'm not kidding). This time the encryption would be done by the application and decrypted by the GPU itself, creating a protected path that couldn't easily be compromised.

The graphics driver would be able to pass along the encrypted data to the GPU, which would then decrypt and decode it in hardware and then the entire framebuffer would be HDCP encrypted by the GPU before sending it out over DVI/HDMI. Again, I'm not kidding.

A simplified encryption/decryption diagram for Blu-ray playback on a PC.

This intermediate stage of encryption/decryption is called a protected path, and two such paths need to exist: a Protected Video Path, and a Protected Audio Path.

The Protected Video Path needed to exist in order to get any sort of HD playback out of a Blu-ray disc, so it was enabled right away. The Protected Audio Path was only necessary if you wanted to use any of these lossless audio formats (Dolby TrueHD/DTS-HD MA), so none of the companies involved actually spent any time on enabling it (not to mention that the HDMI 1.3a spec, which enabled the streaming of these codecs over HDMI wasn't completed at the time that these GPUs were created). I should mention that there doesn’t appear to be a problem with passing a lossy version of TrueHD/DTS-HD MA (48kHz 16-bit vs. 48kHz 24-bit), but most content is authored with lossless audio so this is a moot point.

Every single modern day GPU today lacks support for a Protected Audio Path; that's true for the Radeon HD 4800 series, NVIDIA's GeForce GTX 200 series, and Intel's G45 GMCH. What this means is that there's absolutely no way to output a compressed Dolby Digital TrueHD or DTS-HD MA signal over HDMI from any PC today.

Index The Fix: 8-Channel LPCM over HDMI


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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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