Hi-Def Audio on a PC?

HDMI was first released in late 2002 and started to gain market traction in 2005, when it became available on DVD players and high-end HDTV sets. By the end of last year, vendors began to introduce Blu-ray and HD-DVD discs and players that could deliver high-resolution video and audio. That's when HDMI began to go mainstream, at least at the high end of the market.

In between, we had HDMI version 1.1 in May 2004, which added support for DVD Audio, a 5.1 channel surround sound scheme that supported six channels at sampling rates of up to 96 KHz each.

Next, HDMI 1.2 appeared in August 2005. It supported Super Audio CDs, which delivered up to eight channels of 96 KHz audio. This version of HDMI also introduced specifications to enable PCs to use HDMI Type A connectors.

HDMI 1.3 appeared in June 2006 and, as mentioned above, increased the single-link bandwidth to 10.2 Gbps. This HDMI version also added support for Dolby TrueHD and DTS-HD Master Audio multi-channel surround sound schemes. Both are lossless audio codec formats found on Blu-ray Discs and HD-DVDs.

If a disc player can decode these formats into uncompressed audio, then HDMI 1.3 is not required. That's because all versions of HDMI can transport uncompressed audio when it's delivered through that link. But there's a gotcha involved, having to do with yet another format. (I explain this in more detail below.)

For the record, only some DVD players--such as Blu-ray and the newer HD-DVD players--and a small number of HDTV sets and A/V receivers actually support HDMI 1.3. All the other HDMI-equipped PC hardware I've seen, including several motherboards and graphics cards, support HDMI 1.2, but not HDMI 1.3. They sidestep the high-resolution, high-bandwidth audio requirements that Dolby TrueHD and DTS-HD Master Audio entail.

System builders should understand what this means: Today's PCs cannot generally accommodate these audio formats through any HDMI links they may provide.

THE SONY/PHILIPS DIGITAL INTERFACE FORMAT

Known as S/PDIF, the Sony/Philips Digital Interface Format is associated with various hardware and low-level data signaling protocols used to transport digital audio signals between pairs of devices. Usually, this involves a PC or some PC component, such as a sound card or the sound circuitry on a motherboard, on the sending end.

On the receiving end, S/PDIF is used to transport digital audio signals over an audio component, such as a preamplifier or receiver. Sometimes, over specialized playback gear, like the head-end that Logitech provides with its 5.1 PC speaker rigs. S/PDIF may be used to ferry digital signals in a number of other different formats. These include the 48 KHz/channel sample rate used for digital audio tape (DAT) and the 44.1 KHz/channel sample rate used for CD audio. In fact, both are two-channel stereo formats, which makes their total bandwidth 96 KHz (DAT) and 88.2 KHz (CD audio).

S/PDIF is also used to transport multi-channel surround sound. One example is Dolby Digital or DTS-HD, most commonly found in DVD players in either standalone or PC-based devices. There's a total bandwidth limit for Dolby, which is determined by the AC-3 codec: It may not exceed 640 Kbps. Or, in 35 mm. film prints, it has a fixed bit rate of 320 Kbps. Because S/PDIF bandwidth tops out at under 200 Kbps, both formats must be compressed before they can use an S/PDIF channel for transport.

All graphics cards with HDMI outputs use S/PDIF inputs to pick up audio signals for delivery to HDMI. Today these include the popular 7600 and 8600 models from eVGA, Asus, Gigabyte and MSI. These models are built around Nvidia chipsets. But, according to AMD, this will hold true for a similar set of cards to be built around the forthcoming R600 chipsets from AMD's ATI division. It also applies to HDMI-equipped motherboards, including models from Asus and Gigabyte.