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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  • Anand Lal Shimpi - Thursday, September 18, 2008 - link

    Actually I believe both TrueHD and DTS-HD MA include the lossy DD/DTS tracks as a part of their spec. If you can't decode the lossless version, it should default to the lossy version. This is how it works on CE devices but admittedly I haven't played with it enough on the PC side.

    Sigh, there's so much work to be done here :)

  • jnmfox - Thursday, September 18, 2008 - link

    True, on the PS3 you have to make sure you have LPCM selected as your output, but if the original poster has it hooked up properly than he should be getting the lossless version.

    That is also part of the problem, so many blasted formats. I understand what they are and why we have them but to the un-Home Theater educated, i.e. my parents, it is just a bunch of mumbo jumbo.

    This is a time when the movie industry should be trying to make things simpler instead it is just getting more and more complicated and as we can see from the posts it turns a lot of people off. A lot of people that may have been paying customers.

    There is a lot of work to be done and it is sad we are so far away.
  • jnmfox - Wednesday, September 17, 2008 - link

    Obviously I don't know your set-up but if you have your PS3 set to transcode the audio to LPCM and have it hooked up to your AVR via HDMI then you should be getting a lossless audio track not a downsampled DD signal.

    "but my HTPC has a much better quality picture due to GPU acceleration magic"

    Are you talking about SD-DVD PQ or Blu-ray PQ?

    The Blu-ray jukebox would be nice.
  • sprockkets - Wednesday, September 17, 2008 - link

    This is 2000 all over again: trying to find a sound card that actually passed on a DD signal via spdif with a dvd software program that properly talked to said sound card was a PITA. Then VLC came out and ended all the BS with the a52 codec and it being a free program. I remember buying a $20 sound card and finally having the right WinDVD to work with DD, even if it was the analog ports. What sucks is they wanted $60 for the same stupid program separately.

    Of course, why bother using cyberlink and paying them $$$ for the program (the version bundled with blue ray drives is crippled) when you can buy...
  • fri2219 - Wednesday, September 17, 2008 - link

    I fail to see what problem 8 channel audio solves, aside from "how do audio vendors sell more equipment?".

    Human brains are lousy sound locators, this just isn't needed- 6 channel audio is pushing it as it is.

    When you factor in the fact that most people in the G8 under 40 have damaged their hearing, it's even nuttier.
  • sxr7171 - Monday, September 22, 2008 - link

    Seriously, just like the megapixel race the number of channels race is simply moronic. Reply
  • fuzz - Thursday, September 25, 2008 - link

    don't know if there's any racing going on. i don't think i've got a single movie (regardless of format) that does anything over 5.1ch.. Reply
  • nilepez - Sunday, September 21, 2008 - link

    I'm not sure about 7.1 (since virtually nothing is encoded at 7.1), but 6.1 provides a rear center, which can help with pans for people who aren't in the center of the room. 7.1 does the same thing, in theory, but I don't htink there's much advantage unless the movie is encoded that way.

    I"m actually a bit surprised that BD movies aren't encoded in 6.1 or 7.1
  • fuzz - Thursday, September 18, 2008 - link

    true where audio is concerned, thats why nobody has made this a priority.. the point though is not that 8ch 24/192 is so much better than 6ch 16/48, rather that these ineffective and costly practices are in place when they shouldnt be..

    the fidelity argument is also largely true of HD video.. a waste of time if you don't own a HD projector and view on a 100" screen. you won't see sh*t-all difference between your DVD and a HD movie on a 32" display if you're sitting further than a meter away.

    well okay you might if you *really* pay attention but then you'd be missing the movie ;)
  • npp - Wednesday, September 17, 2008 - link

    You couldn't be more right.

    But people like big numers - well, 7.1 can't be worse than 5.1, right, just like megapixels, horse power, cores, and everything else.

    This aside, I find the "bit perfect" hype to be the next stupid thing. In my eyes, it's simply that most people don't want to admit that their ears and brain are imperfect, and can be fooled (by means of frequency masking). The word "lossy" seems to be a bad one, but I've heard plenty of "lossy" sound that was better than studio-mastered CD-s... And a lot, yes I mean A LOT of people can't hear any difference between properly compressed and uncompressed tracks at all... And yes, human hearing degrades rapidly with time, to a point when even a 10 Khz sound can't be heard - but you can rest assured that you have all your frequencies up to 48 Khz untouched, it's lossless.

    You just have to swallow your ego to admit this, and there are plenty of people who aren't prepared to do it. You have the guys at which can hear not only differences between cables, but also between their wall sockets, ladies and gentlemen. A separate power line gave an amplifier something like more vivid and punching sound, for example.

    I don't know who is crazy in this case but I think that people got what they needed long time ago and anything beyound that (read: all the 24bit/192Khz, 7.1, etc. stuff) provokes more imaginative than objective, quantitative effects... And of course you'll hear a difference if you've paid an amount enough to feed a small african village just for equipment, you have to.

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