The Poor State of HDMI Audio in PCs

It wasn’t that long ago, we HTPC users were left out out of the high definition loop, sure we could get over the air HDTV, but that was it. Slowly HDCP enabled video cards became a common item. Vista and CableCARD support allowed people with the right PC to finally get premium HD content, and HD DVD and Blu-ray playback became possible thanks to cheap processor power and later GPU offloading of the decoding. But then it became clear that while we had the video OK, we were missing the advantages of audio that the set-top players had with HDMI. I’ll outline the history of HDMI on the PC and how the audio capabilities aren’t really up to the level of the set-top player competition. In case you get lost along the way I have also included a rundown of how we outgrew S/PDIF for our digital audio needs and an overview of the audio formats found on HD DVD and Blu-ray discs today.

Overview

Why HDMI?

• The S/PDIF standard doesn’t have enough bandwidth for the high definition audio on HD DVD and Blu-ray discs
• Copy protection: with HDMI comes HDCP
• Ease of using a single cable

EDITOR’S NOTE: If you’re unfamiliar with S/PDIF please see the S/PDIF History and Specs section for more details. Also in the last section is an overview of the various audio compression schemes, it may be helpful to review this as a supplement to my critique.

The PC and HDMI an overview

Initally most PCs just had a DVI port, which could be adapted to an HDMI formfactor, because DVI is the predecessor to HDMI’s video specifications. Of course this didn’t send audio, nor did the video cards have an HDCP key to make the copy protection of next generation content like Vista’s CableCARD support and HD DVD or Blu-ray satisified.

First Generation HDMI video cards – These used a physical loop-through cable from the S/PDIF port of the PC’s sound card into the video card to be combined the DVI signal. This approach is less then ideal because it requires a loop-through cable the use of which hadn’t been seen since the 3Dfx Voodoo days and of course limited the HDMI audio to the same limits as S/PDIF. These cards have dwindled but this design can still be found.

HDMI on the motherboard – HDMI was recently brought down on to the motherboard level with chipsets such as the ATI/AMD 690G and NVIDIA’s GeForce 7050PV. This seemed to be one of the better paths to proper HDMI audio support since the video and sound were all integrated into the same motherboard. It became clear that the motherboards had the same limitations as S/PDIF. It seems very likely that all that was done was to route the S/PDIF interface to the HDMI connector via traces on the motherboard instead of using an external loop-through cable.

HDMI on the video card (Radeon HD 2000 series) – AMD/ATI seemed to be taking the HDMI audio issue seriously when it announced it was integrating an HD Audio compliant codec right on to its’ next generation GPU line. The cards are now released and the audio output appears to be no different then if it were S/PDIF, and in fact the current audio drivers don’t even seem to pass DTS properly!

Why so poorly implimented?

Why is it that even in the case of Radeon HD 2000 series we can’t get anything better then S/PDIF functionality? I don’t understand it. The Radeon HD 2000 series GPUs have a sound chip right in their silicon. The audio chip certainly handles multi-channel audio just fine, because the same basic silicon drives thousands of motherboard audio chips and a person can easily hook up a 7.1 speaker system to the analog audio ports and get all sorts of audio just fine. So why does it appear that the full 8 channel pathway of the HD Audio codec is not wired up to the HDMI connector? If it were it should easily pass the decoded multi-channel PCM audio to your HDMI equipped home theater receiver just as the set-top HD DVD and Blu-ray players do today.

I’m no electrical engineer, so there may be something I’m missing here about how the HD Audio codec is wired up, etc. So please anyone with more knowledge feel free to provide feedback in the comments.

S/PDIF History and Specs

S/PDIF was made as digital transport of CD Audio in high end stereo systems. It transported the 2-channels of PCM a stereo CD had.

So if S/PDIF is made for 2-channels of audio, how is it that we have surround sound?
Easy: compress it. Dolby Digital and DTS are compressed audio formats. These two formats can then easily fit within the bandwidth of S/PDIF. A flag in the bitstream tells properly equiped audio receivers it is going to get a bitstream of compressed data rather then a raw PCM stream.

What is PCM?
Pulse Code Modulation (also referenced as Linear PCM or LPCM) is uncompressed digital audio (think: WAV file). Any time you play audio on your PC it is decoded as PCM for playback. That MP3 file is compressed on disk, but when you hit ‘play’ in WinAMP it is decompressed into PCM audio for processing by your PC’s sound card.

More discussion on S/PDIF
S/PDIF can carry around 3Mbit/sec max, however as far as consumer usage goes the maximum surround sound protocol we can use is standard DTS (1.5Mbit/sec).

Description of Audio Formats and Blu-ray/HD DVD Support Chart

Lossy compression formats 

Dolby Digital (AC3) – Classic Dolby Digital we all know from DVDs. The difference here is that the specification’s maximum rate of 640Kbit/sec is possible. Technical limitations of how standard DVD works limits Dolby Digital to 448Kbit/sec. So even good old Dolby Digital will show improvement over the standard DVD version of the same movie soundtrack.

DTS – We’re all familiar with DTS from DVDs. DTS has a maximum bitrate of 1.5Mbit/sec. However, DTS was typically only 768Kbit/sec on standard DVDs, so as with Dolby Digital, the next generation optical disc formats allow full breathing room for DTS.

Dolby Digital Plus (E-AC3) – Syntax changes, improved quantization, and a few other tweaks render this incompatible with standard Dolby Digital. However a downconversion of DD+ to DD is easily done as the specifications include techniques made to lose as little as possible in the process. The typical bitrates of a DD+ soundtrack are 1.5Mbit/sec or 3Mbit/sec.

DTS-HD High Resolution Audio – Commonly just called "DTS-HD." Essentially this is less compressed standard DTS; the bitrate of DTS-HD is typically 3Mbit/sec, it includes a 1.5Mbit/sec DTS ‘core’ for compatibility sake, with the expanded audio data around it as an extension to the core which will be recognized by a DTS-HD enabled decoder.
For more on the concept of a DTS core check out DTS’s DTS-HD Audio page.

Losslessly compressed/uncompressed formats

Dolby TrueHD – Derived from MLP which was used in the DVD-Audio format. Quickly becoming the defacto standard in lossless compression for next gen formats.

DTS-HD Master Audio – Brand new format, includes a DTS ‘core’ for compatibility sake. The lossless data is contained as an extension around the core which describes the difference between the core and the lossless data version.

*Both Dolby TrueHD and DTS-HD M.A. typically hover in the 3Mbit/sec to 6Mbit/sec range, both are VBR (Variable Bitrate) so there is no constant bit rate, and spikes well past 6Mbit are common during any given second in complex audio tracks.*

Multi-channel LPCM – Linear pulse code modulation: in essence this is raw audio (like a WAV file). The bitrate on a typical 5.1 sound Blu-ray movie soundtrack is 4.6Mbit/sec. Sony’s music concert Blu-ray discs go even higher using a 6.9Mbit/sec rate.

Blu-ray/HD DVD Audio Format Support Chart

Audio Codec	Blu-ray	HD DVD	Notes
Dolby Digital	X	X
Dolby Digital Plus		X
DTS	X	X
DTS-HD
Dolby TrueHD		X	Standard spec for HD DVD, quickly becoming a defacto standard for Blu-ray.
DTS-HD M.A.			Optional on both formats, no set-top player decodes this as of yet.
Multi-channel PCM	X	X
			X = mandatory