Musings on HDMI Audio

Previously, on this blog (see below), I discussed the upgrades that HDMI 1.3 offered for video. Most of them, in my opinion, were nice to have as a hedge against future improvements in sources and displays, but did not offer any real benefits with present and foreseeable video formats, both standard and high definition. As far as video is concerned, then, I saw no reason to toss out your present gear or hold off a purchase until there's a wide range of sources, switchers, and displays with HDMI 1.3. That will likely be a long wait.

The audio side is a little more complicated. There are some potential advantages here to HDMI 1.3, though question marks remain as to how they might be implemented. To minimize the confusion here, in this discussion I will use the term AV receiver, or simply receiver, to mean either a one-piece AV receiver or a separate surround preamp-processor.

There are two major benefits claimed for HDMI 1.3 audio. First, automatic lip sync compensation, and second, support for direct carriage of the new, higher resolution audio formats such as Dolby TrueHD and DTS HD Master Audio Lossless.

Lip Sync When the audio and video in a signal get out-of-whack by even a few tens of milliseconds, the program starts to look like a badly dubbed Italian western. While some people are more sensitive to small sync errors than others, anyone can see them if the delay becomes long enough.

The most common error has the picture lagging the sound. Typically, this is caused by video processing delays. All modern video displays use processing for such things as deinterlacing and scaling. And those operations take time. Again, we're talking milliseconds of delay in the video, but milliseconds are all it takes to cause a perceptible problem.

There is audio processing going on as well, such as Dolby Digital decoding, but in general it's dramatically shorter in duration and not, at present, a significant part of the issue.

If the picture does lag the sound, it's a simple matter (says he who doesn't have to design this stuff!) to delay the sound. But there are many different video-processing technologies available to display manufacturers, all of them with different inherent delays, so a fixed audio delay won't solve the problem. Many of us have a manually adjustable digital audio delay in our AV receivers, but if you've fiddled with one you know it can be tricky to find just the right setting. Wouldn't it be nice if the correct delay could be performed automatically? With HDMI 1.3, it can. Well, maybe.

To implement this HDMI 1.3 lip sync feature, the display must be designed pass the delay information (how much lag is in its video processing) back to the AV receiver on the HDMI 1.3 connection. That means that the source, switcher (in the receiver) and display must all be HDMI 1.3. That goes for any other device between the source and the display as well, including scalers, converters, or outboard switchers. In other words, not only must the source and display be HDMI 1.3, but the entire route from source-to-display must be fully HDMI 1.3 as well.

Furthermore, to access the auto lip sync feature, you must be using the audio that's carried on the HDMI 1.3 connection. A separate digital audio link will probably not be served by this feature (I can imagine ways in which it might, though they'll add considerable complexity to the design of an AV receiver). That means that the AV receiver itself must be able to use the audio that's on the HDMI 1.3 link.

Many current pre-pros, and some AV receivers as well, aren't designed to do this. Their HDMI inputs are video switchers only—you must use a separate audio connection. And among those receivers that can tap the HDMI audio, some cannot perform bass management on an HDMI digital audio source.

Of course, none of these receivers are designed for HDMI 1.3 at present, but when their HDMI 1.3 replacements do arrive, you will need to confirm that they are fully ready to use the audio from their HDMI 1.3 inputs and have the lip sync feature.

If past experience is any indication, in fact, HDMI 1.3 on a product will be no guarantee that the product will make use of every HDMI 1.3 feature. And that caution applies to all HDMI 1.3-ready products, not just receivers.

The HDMI 1.3 lip-sync feature will be limited, from all current indications, to the playback end. That is, to the user's system. But sync errors can also occur in the source itself, particularly broadcast HD. If you watch enough HDTV, you've likely already experienced this. HDMI 1.3's lip sync feature can't account for such broadcast errors. If manufacturers of HDMI 1.3 gear drop the manual lip sync feature (as they well might, to save money and simplify operation), you'll have no recourse when the automatic lip sync feature fails to compensate for all possible sync errors.

In short, HDMI 1.3's lip sync feature is promising, and I'll be the first cheerleader if it lives up to the claims. But it's complicated and system dependent. We'll have to wait to see how it works in practice.

Advanced Audio Formats Looking first at new Dolby formats on Blu-ray and HD DVD, there are two ways to get both Dolby Digital Plus and Dolby True HD from the player to your AV receiver. One is from the multichannel analog connection. The other is the one we're interested in here—HDMI. (You cannot get them over a source's standard SPDIF coaxial or optical digital outputs.) If the player can decode these formats and convert them to multichannel PCM (not all players can), pass that PCM-on-HDMI signal to the AV receiver, and the receiver can make use of that audio signal, you're home free (though you'll be more home free if the receiver will also provide full bass management on that HDMI audio source!).

What HDMI 1.3 offers is the ability to pass not just this converted-to-PCM multichannel information to the receiver, but passage instead (or optionally) of the native bitstream Dolby Digital Plus and Dolby TrueHD signal. In that case, the DD+ and TrueHD bitstream would be decoded in the receiver rather than in the player.

The same applies, in general, to the new DTS formats, including DTS HD Master Audio 5.1 Lossless (say that fast, three times). But DTS HD MALOS (Like that acronym? I thought not.), now available on many Blu-ray titles, particularly from Fox Home Video, cannot at present be decoded in any Blu-ray player. According to a knowledgeable industry source, this is because the processing power required to perform this decoding is so high that it is not currently available in any dedicated Blu-ray player (or, presumably, in HD DVD players either, though I know of no HD DVD titles that use this new DTS format).

What you get in current players from these new high-resolution DTS tracks is simply standard DTS, available as a core track imbedded in the DTS HD Master Audio Lossless data. This is the same DTS we've always had, though here it's recorded at 1.5Mb/sec. The DTS soundtracks available on some standard DVDs, apart from the early days of that format (and on DTS Laserdiscs), are recorded at half that data rate. The improvement in DTS audio from this doubling of the data rate is obvious, but it isn't the Full Monty.

So even with sub 1.3 versions of HDMI we can get (or in some cases can't get, as described above) the new audio formats via multichannel PCM over an HDMI link. But what additional benefits might we get from passing the native bitstream for these formats directly to an AV receiver over HDMI 1.3? It all comes down to the quality of the processing. Will this processing and decoding be better if done in the player or in the receiver?

There is no single answer to that question. It depends on the relative quality of the processing in the player (or other source) and the receiver. Assuming a tossup there (which won't always be the case) I can imagine possible advantages to doing the decoding in the receiver. There will be the room, and perhaps the budget, to include a higher level of digital decoding and processing, particularly in high-end receivers. This might involve such things as reclocking the audio data to minimize common digital demons such as jitter. Reclocking can also be done in PCM, of course, but (and I speculate here from very limited knowledge of the intricacies of digital processing) perhaps it could be done more effectively when you start with the native bitstream data.

Also, as mentioned above, there's the extra decoding power required for DTS HD Master Audio Lossless. Rather than add this to DVD players and increase their cost (at a time when the cost of players is still a major drag on the Blu-ray format), it could be put into the receiver.

There do appear to be some potential advantages to HDMI 1.3, though in my opinion they will be mainly on the audio side for as far into the future as my thrift shop crystal ball will reach. And they won't be fully available (or reviewable!) until we're well down the road with all sorts of front- and back-end HDMI 1.3-equipped products. Only then will we be able to even start to see how much HDMI 1.3 offers in the real world.