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Do We Have A PAP? Is 7.1-Channel LPCM The Answer?

The HTPC / Windows 7 Chronicles: You Asked, We Answer!
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We had an anonymous reader ask if the Maui platform facilitates a protected audio path, and the short answer is: no.

Why did the reader ask about a PAP? Because, in order to play back today’s high-definition lossless audio formats, such as Dolby TrueHD and DTS-HD Master Audio, a Protected Audio Path must exist between the content itself and the software application decoding it. In essence, the AACS (Advanced Access Content System) wants to make sure a user can’t snoop the bus and copy the lossless stream.

AMD is convinced that the MSI Media Live board meets the AACS requirements. Though it could be argued that PCI Express is a user-accessible bus, AMD says only the D2Audio can read it. Nevertheless, our Maui platform is not yet able to put out lossless TrueHD or DTS-HD MA audio, because the software is passing down-sampled audio to the hardware. Slowly but surely, software vendors like Cyberlink and ArcSoft are working with the audio controller and codec folks to certify protected paths where they can (there is currently one discrete sound card able to do HDMI pass-through and two Reaktek codecs capable of 96 kHz/24-bit DVD playback across four to eight channels).

Even if you're able to decode DTS-HD and pass LPCM audio, you aren't getting the 24-bit resolution offered by the Live Free or Die Hard soundtrackEven if you're able to decode DTS-HD and pass LPCM audio, you aren't getting the 24-bit resolution offered by the Live Free or Die Hard soundtrack

Currently, the only way to enable a PAP and play back 24-bit/96kHz multi-channel audio is using Asus’ Xonar HDAV 1.3, which we’ll be looking at on the following page. Auzentech has been working on an X-Fi Home Theater HD card with similar functionality (paired with PowerDVD 9), but as of this writing, it remains unavailable.

The best you can hope for from MSI’s Media Live board, then, is the equivalent of DVD-quality audio—Dolby Digital and DTS. So, for the reader who said he’d upgrade as soon as he could get a Protected Audio Path via HDMI: you can, but not (yet) with the D2Audio chip onboard.

The good news is that you’d be truly hard-pressed to tell the difference. Also, you'd need to test with the right source content if you ever hoped to try to hear 24-bit versus 16-bit resolution. Not all movies take full advantage of the capabilities of Blu-ray. Many natively employ 16-bit/48 kHz sound anyway. And very, very few even offer 96 kHz sampling. Concert DVDs are the exception, and I purchased The Police: Certifiable in Bueno Aires (24-bit, 96kHz) as sample content.

Played back as multi-channel LPCM audio on a G45- or GeForce 9300-based system, it'd be converted to 16-bit/48 kHz. Now, I can't claim my equipment is high-end enough to expose the difference between CD-quality and high-resolution playback, but I do crank the volume in the music and movies I enjoy, so if the higher resolution sound is expected to demonstrate advantages in the noise floor at significant levels, I'd hope to hear that. In truth, it remains difficult to qualify the improvement. The Police concert DVD also included a pair of CDs, enabling a comparison to stereo ouput of the same songs. But the mixing was so different as to render the two incomparable, in my opinion.

The Workaround…Sort Of

So, you’ve likely read that multi-channel LPCM is the way around this whole confusing audio situation. In a way, it really is—especially if you need eight-channel output rather than six. You can build an HTPC based on Nvidia’s GeForce 9300 chipset, use PowerDVD 9 to decode the Dolby TrueHD or DTS-HD signal, and output 7.1 channels of linear PCM audio the same way a Playstation 3 does. The receiver simply plays back the discrete audio streams, oblivious to the original encoded format.

Note the DTS-HD being output as 48kHz/16-bit LPCM audioNote the DTS-HD being output as 48kHz/16-bit LPCM audio

However, you lose the bit-for-bit lossless quality as soon as the software decoder passes the signal out over HDMI. What’s more, there’s a fair chance that, even on a GeForce 9300 or G45, multi-channel LPCM will never yield 24-bit audio (or 96 kHz sampling, if you have one of the few discs offering that). You see, the same restrictions that apply to Maui playing back lossless audio through the D2Audio amp limit PowerDVD and TotalMedia Theatre to playing back CD-quality sound currently.

Both vendors naturally know there are audiophiles who still clamor for such a capability, but of course, their hands are tied by the standards bodies licensing them the technology to do any of this. Thus, as of today, the only real answer for bitstreaming lossless audio is Asus’ Xonar HDAV 1.3. Every other alternative gives up something—regardless of whether you can actually tell the difference.

Here's what your receiver displays if your playback software does the decoding and outputs multi-channel LPCM sound...Here's what your receiver displays if your playback software does the decoding and outputs multi-channel LPCM sound...

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