Hi-Def Audio on a PC?
True, graphics cards and motherboards that include high-definition multimedia interface (HDMI) outputs among their various ports and attachments are readily available. And true, you can get good audio results from installing a Blu-ray or HD-DVD player in a PC, and then using an HDMI output port to pipe the audio and video into an HDMI-equipped audio/video (A/V) receiver or a TV set.
But even then, you won't get all the different audio options you'd expect. The bottom line for system builders: The full range of audio options most likely will not be available through an HDMI port on a PC.
In this Recipe, I'll discuss the benefits of HDMI, along with its associated standards and requirements. Then I'll explain how audio is most commonly picked up and passed into a PC's HDMI port—and why it won't work with the latest high-definition multi-channel surround sound formats, such as Dolby TrueHD and DTS-HD Master Audio.
THE LOWDOWN ON HIGH-DEF MULTIMEDIA
HDMI's big draw is that it offers salvation to those whose entertainment centers have become a veritable rat's nest of cabling at the back of their A/V receivers or TV sets. A single HDMI cable can replace as many as nine cables. It does this by combining multi-channel surround-sound audio and high-definition video signals in a single compact cable with an equally compact connector.
Here's what that single HDMI cable can replace: One cable for video, which may have been a three-strand component audio or a DVI cable at the top-end of the resolution spectrum before HDMI. Eight cables for 7.1 analog audio, which uses a separate cable for each audio channel. Then add two more cables, same as before for high-end video, with optical or coaxial cables for digital audio.
As the following photo shows, when viewed side-by-side, a Type A HDMI connector (left) isn't much bigger than a Type A USB connector (right). Also, the Type A HDMI connector is electrically compatible with DVI-D:
As shown in the following diagram, HDMI pinouts are considerably more complex than USB. They include 19 leads, along with a maximum bit rate of 10.2 gigabits per second (Gbps), as opposed to the 480 megabits per second (Mbps) offered by "old-school" USB.
HDMI PRIMER
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. THE PIPE IS TOO NARROW !
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).