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


View All Comments

  • plonk420 - Wednesday, September 17, 2008 - link

    you could rip the disc with the slightly expensive AnyDVD HD and play it with Media Player Classic... Reply
  • sprockkets - Wednesday, September 17, 2008 - link

    I've used mplayer on windows. Even without any acceleration, seems to play it very well. VLC is my next choice.

    Oh, we are talking about the 12GB video files here.
  • jnmfox - Wednesday, September 17, 2008 - link

    PC hardware and Home Theater are 1a and 1b for my personal interests. Your article was well written (like almost everything on anandtech), basic enough to understand but detailed enough to give the needed information.

    I have my PC hooked up to my HT and considered getting a Blu-ray drive in my PC. But after looking into it I read about the problem you are describing. I ended up getting a PS3, I'm glad I did so I don't have to worry about all this junk (don't you love the fandangled content protection schemes they come up with ;)).
  • Demon-Xanth - Wednesday, September 17, 2008 - link

    It seems that they were so gung ho on protecting their own "content" that they completely left their own customers out in the cold. Rather than giving the customers something easy to use and setup, they proceeded to require manufacturers to include numerous other (futile) hoops to jump through so someone that bought the movie can enjoy it.

    Hollywood, the problem isn't your paying customers. Quit taking pot shots at them.

    With the rise in HTPCs and that sort of thing the studios should've worked WITH manufacturers to create a solution, even if it's a low cost standalone HDMI output card (a la DVD decoder cards), to support BluRay and HD-DVD on day one.
  • kymas - Wednesday, September 17, 2008 - link

    This is exactly the problem: "Hollywood, the problem isn't your paying customers. Quit taking pot shots at them."

    People buying Blueray hardware or movies are paying to have their rights taken away and they are paying more for it due to the increased hardware and software cost to make these ridiculous protection schemes work. This is another example of content owners implementing a protection system that is at best a minor annoyance to the people actually stealing the content but is a significant detriment to their paying customers. Systems like this encourage paying customers to find alternative methods of acquiring the content they want or to just do with out.

    I have been a HTPC enthusiast for about four years, which was shortly after I purchased an HD TV. I love the flexibility and convenience the HTPC allows and I would also love to be able to have HD content on my HTPC. Unfortunately, even if I were willing to support Blueray (obviously I am not :) it is completely unsupported by the open source software I use and it is unlikely it ever will be. As far as I know there is no software currently available that would allow a Blueray movie to be transferred to a media server for play back on a HTPC or other computer/device. At this point my only hope for legally purchasing HD content is through Internet distribution, hopefully in the near future someone will provide consumer friendly HD content online or I will have to sell or scrape some expensive but useless equipment.

    Sorry for the rant but this stuff really irks me!
  • sxr7171 - Monday, September 22, 2008 - link

    If you chose an open source OS for your computing needs then I suppose Blu-Ray on PC is not in the cards for you. Have fun with your downloads. Reply
  • Cincybeck - Tuesday, September 30, 2008 - link

    I'm pretty sure the open source program he's talking about is a DVD server/client software. That allows the DVDs to be ripped and stored on the server. This allows his HTPC to act as the client and seamlessly access his whole movie collection with out ever touching an actual disc. Just point and click. Or in case of some of the more advance setups I seen. Where there's an interface which shows a screen full of the DVD covers combined with a touch screen. All you have to do is flip threw the movies then press on the DVD cover and the movie automaticly begins to play. With out ever having to touch a mouse or keyboard. Reply
  • nilepez - Sunday, September 21, 2008 - link

    Give me a break. Increased h/w costs? Blu-Ray drives are CHEAPER than DVDs were at this point in the the DVD life cycle. If you'd owned DVDs in 1997-1999, you'd know that.
    Onkyo's THX-Ultra Certified 905 receiver has been available for as little as $550.00, and it comes with more bells and whistles than 99% of the consumers will ever use.

    Software is a bit more, but prices will fall, as will the h/w, though given that prices are as low as $230 for a player, it's hard to understand why you're complaining about price....unless, of course, you didn't get into dvd until 2000-2001 when prices were much lower.

    My first DVD player (the dvd-414) was around 300.00 (and that was about as cheap as you could get in Q1 99).

    Here we are at roughly the same period with Blu Ray, and prices are at least 25% lower. Adjust for inflation, and the price of this tech is dramatically cheaper.

    Finally, it's really annoying when people pretend that DVD and VHS didn't have copy protection. The vast majority of people didn't have a way to copy video tapes and, until Dvd John wrote DeCSS, they couldn't copy DVDs.

    Conclusion: The ridiculous schemes ain't costing us anything. To take advanatage of blu ray, you'd have to buy a new receiver and drive, and both are available at prices that cheap compared to what they would have cost 2 years into the DVD life cycle.

    As for you linux based HTPC, AFAIK, there's no licensed software for playing DVDs on Linux either....that's why, in theory, DeCSS was written (which is illegal if you're in the U.S.)

    As for online HD, it'll happen in 10 or 20 years. Unless you have FTTH, you don't have enough bandwidth. Even if you have the bandwidth, it's unlikely that the place that sells the movie is going to have the bandwidth to fill your 40-50mb/s pipe.

    What you'll get is highly compressed video that isn't as good as blu ray. Inferior video and audio, inconvenient download times and you still have all the DRM you profess to hate. Sounds like a winner to me.
  • kymas - Thursday, September 25, 2008 - link

    I am afraid you have missed the point of the article and my comments which is HTPC's. I am well aware that CE Blu-Ray drives/players are cheaper than DVD drives/players were at this point in their life cycles ... I paid just shy of $800.00 for my first DVD player. It cost PC hardware manufactures more to produce a product that supports the "Security Features" of Blu-Ray because they have to add additional components to their hardware for that support and they have to pay to have their hardware certified before they will be granted the keys required for Blu-Ray playback. This situations is virtually the same for software developers as they need to add additional code to support the Blu-Ray security and pay for the certification process to receive the keys.

    I never said VHS or DVD did not have copy protection. I indicated that the security features of Blu-Ray prevent me from exercising my rights as a consumer and does very little to prevent the thefts it was intended to stop. At the time I wrote my previous comments I was unaware of the AnyHDDVD (sp?) program but like DeCSS this program is likely illegal in the US. It is very sad that people have to resort to using illegal tools in order to exercise their rights.

    As for your comments about online HD content, you are obviously unaware of upcoming transport technologies. You will be able to download Blu-Ray quality HD content to your mobile phone in reasonable times in less than 10 years probably closer to 5 years and wired technologies will be even better. You are correct that, at least for the near future, the bottle neck will move to the provider.
  • sxr7171 - Monday, September 22, 2008 - link

    Thank you. For once, someone who knows what he's talking about. The cost of admission at launch was half of DVD at $600 -> PS3 vs. $1200 -> Sony DVP-S7000 DVD Player in 1997.

    We won't be ready for true HD downloads for at least 5 years.

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