Hi-Def Audio on a PC?

There's a problem with using S/PDIF to transport audio from the audio chipset or an expansion PCI sound card to the graphics card or motherboard, so that it may be combined with video signals for transport through the HDMI channel.

In a nutshell, here's the problem: High-resolution, high-definition audio schemes, such as Dolby TrueHD and DTS-HD Master Audio, require more bandwidth than S/PDIF can deliver.

So the best audio that most PC-based HDMI connections can deliver for playback involves using "old-school" compressed Dolby Digital or the DTS formats. These have been used for multi-channel surround sound on DVDs since the 1990s.

Too many system builders believe that a PC with added HDMI capability can handle the latest high-resolution audio formats required in HDMI 1.3. But the most advanced version of HDMI supported in all PC equipment that supports the interface is version 1.2. That's because vendors are not obliged to support high-resolution audio, at least not now.

The root of the matter lies with S/PDIF and its inability to support the bandwidth necessary for such schemes. This bandwidth is usually in excess of the 640 Kbps. This represents the maximum that the Dolby Digital AC-3 codec can handle. It also exceeds the maximum carrying-capacity for S/PDIF, even when compressed.

PCs equipped with Blu-ray or HD-DVD players can use software from Cyberlink (PowerDVD Ultra) or InterVideo (WinDVD 8 Platinum) to play back and decode high-resolution audio codec formats such as Dolby TrueHD and DTS Master Audio. But they can reproduce high-resolution sound accurately only if they deliver that audio via analog outputs to a pre-amplifier or receiver.

If these PCs use their HDMI connection to carry audio and video, they will be restricted to the formats the S/PDIF connection that brings audio into HDMI can handle. Namely, old-school, compressed 5.1 Dolby Digital or DTS.

VENDOR FEEDBACK

In talking with vendor representatives from Intel, AMD, and Nvidia, I've asked how and when this mismatch of older audio channels and new audio formats will be resolved. I've mentioned that a new audio interface to replace S/PDIF will need to be developed in order to bridge the current gap. Unfortunately, they didn't give me much in the way of specific answers.

But the vendor reps did admit that the clamor raised by unhappy buyers—including yours truly—is pushing their firms to offer more satisfying solutions. Look for these solutions over the next year or two, the reps add.

Meanwhile, system builders should understand that just because high-definition DVD formats (such as Blu-ray and HD-DVD) can interpret and hand off high-resolution audio from the media they read, doesn't mean PCs endowed with such equipment can deliver that audio through an HDMI connection. Listen up!

ED TITTEL is a freelance writer and trainer in Austin, Texas, who specializes in Windows topics and tools, especially networking and multimedia matters. Ed is also a co-author of Build the Ultimate Home Theater PC (John Wiley, 2005).