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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  • Mastakilla - Wednesday, September 17, 2008 - link

    for movies and music the sync issue can be solved on a decent receiver...

    the big problem is for people (like me) who also want to play games on the same system...

    you cannot sync your keyboard / mouse input with a delayed sound / image!
    that is the real issue for me and exactly what i would like to see some focus on in a decent (anand) article
    the forum link i posted above touches the same topic...
  • Mastakilla - Wednesday, September 17, 2008 - link

    Been waiting for something like this for a long time now...

    I hope the final in depth article will cover all aspects of sound on a PC (movies, music and gaming)

    I have started a thread about the lack of decent information on this topic awhile ago on your forum:">;thre...

    this includes a request to review the Auzentech soundcard toghetter with Onkyo or alike receivers as well

    Hope you can make this mess a little more clear :)

    thanks in advance!
  • Anand Lal Shimpi - Thursday, September 18, 2008 - link

    I don't have the Auzentech card yet but I did just get the ASUS card in and will be working on that shortly. As soon as I get the Auzentech I'll add that to the queue :)

  • Zefram0911 - Wednesday, September 17, 2008 - link

    Anyone know when this card is coming out? I've been waiting for it since it's press release in June that said this month. Reply
  • AnnihilatorX - Wednesday, September 17, 2008 - link

    Protected path, all the bells and whistles would utterly fails at the end point. To make content viewable, ultimately you need to have it decoded. There is a weakest link between the image processor on your monitor and the HDCP chip. Surely uou can eavesdrop on the electrical signal coming from the monitor's driving circuit. Reply
  • Xenoterranos - Wednesday, September 17, 2008 - link

    Well yes, but the mechanism to do that would be hardware based, and thus, somethign you can't download from, which the studios are most affraid of. Remember Css? That didn't go to well for them.
    (Besides, this is moot. Read the above posts about AnyDVD HD: the encryption has already been broken)
  • Golgatha - Wednesday, September 17, 2008 - link

    I'd be happy if the Cyberlink PowerDVD 8 software would just transcode the lossless audio to 5.1 DD and call it a day. Also, I wish the part of the Blu-ray spec was to require a DD track on the disc along with the lossless format.

    Case in point. Transformers on Blu-ray only has a TrueHD track on the disc. My HTPC cannot playback the audio in anything but stereo because of the limitations listed in this article.

    The idiotic thing about this situation is even if I bought a receiver which supported TrueHD, there is no way for me to get any kind of 5.1/7.1 signal to the receiver due to the DRM implementation, lack of hardware support on the PC side, and lack of software transcoding in any commercially available playback software. Sure my PS3 can do the transcoding and I can watch Transformers with TrueHD downsampled to 5.1 DD, but my HTPC has a much better quality picture due to GPU acceleration magic. Not to mention my HTPC is a nice Blu-ray jukebox (thank you AnyDVD and yes I own the original discs), which can start the movie directly at the beginning of the movie (thank you again AnyDVD) without the ironic "don't steal our content public service message" or any previews (which are outdated and unwanted within 6months anyway and are a waste of my time).
  • michal1980 - Thursday, September 18, 2008 - link

    i'm sorry, can you please explain to me how your htpc has a better picture then the ps3?

    and please make up better bs then 'gpu acceleration', becasue that is just 'bs'. The ps3 is one of the better blu-ray players on the market, and I have never seen a review mention any problems with its picture quality.

    GPU acceleration is required when your CPU is too weak to playback the video, if system a is able to decode the video file, and system b is also able to, but one uses a cpu and one a gpu, then unless there is some post processing (ie changing of the final output after decoding), the picture will look the same.

    also, if your reciver can play back true-hd, or dts-ma, then it can playback LPCM. and you know what the difference in sound quality between the 2 is? nearly zero. All that happens is that in the 1st case your receiver decodes the file, while in the 2nd your pc does.
  • fuzz - Monday, September 22, 2008 - link

    well theres probably ZERO difference between htpc and ps3 when it comes to hddvd/bd, but for playing back standard dvd the htpc wins hands down (thanks to the maturity of the technology)

    you can get around pretty much all the quality-reduction features hollywood could come up with.
  • erple2 - Monday, October 06, 2008 - link

    Maturity of what technology? The tech used in the PS3 for all of that is what boils down to clever software decoding. The question is simply whether the hardware on the PS3 is capable of doing all of the clever algorithmic magic to make the DVD output look good. Some "high end" (read expensive) systems do that, but they also can't put large General Purpose Processors in their components, so they use embedded systems (which are VERY inexpensive) to do the crunch work on those DVD upscale algorithms. I think that the PS3 is more than capable of doing it in software (particularly given it's nVidia G70 based GPU, plus the Cell Processor). Reply

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