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.

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Colin's picture

Whew! Talk about confusing! I would hate to be in A/V retail right now... I cant even imagine the average consumer being able to understand this stuff, let alone many retail employees! Tom, is there any word on the street as to when we're going to start seeing surround processors that decode the new formats? I have been holding out on a processor specifically for this capability, and it seems the industry has been slow to respond in simply being able take advantage of all of blu-ray and HD-DVD's spiffy new features! This is a brave new world yes, but I dont remember not being able to take advantage of EVERY new feature DVD offered at the format's launch, besides a brief stint of not being able to decode DTS. I really want to support these formats, but in the constantly changing world of HDMI, it seems it will be forever before I can fully enjoy them (that's not to say I haven't been loving my HD-DVD drive for my Xbox 360)!

Les Moss's picture

Your whole discusion of HDMI audio assumes that all sources are routed thru aeceiver. I think a better solution is to route all sources thru the TV with a single unified digital audio output to an amp. Advantages of connecting device directly to TV: 1. You can use individual video settings per device. 2. Simpler cabling 3. Simpler control 4. You can use the TV without turning on the receiver. And when you do that you can take advantage of a TV with surround simulation (like "SRS TruSurrund XT") to get better sound quality. 5. You don't have to worry that an external device you own or may buy can simultaneously output 2-channel on HDMI and 5.1 on SPDIF. 6. You can use your whole receiver budget on a top quality audio amp without HDMI and analog to digital video conversion. 7. You don't need the HDMI 1.3 lip sync feature since the TV can just delay the audio bit stream. Unfortunately, I have to found a TV that accepts DD5.1 on its HDMI inputs. Why is that?

Tom Norton's picture

Colin: We have not yet heard of any HDMI 1.3-compatible pre-pros, but they will most certainly show up in time. Expect to see HDMI 1.3 first in AV receivers. Onkyo, in fact,just this week announced new receivers that will accept the new high resolution audio formats in native bitstream form over HDMI 1.3. Les: The only way to feed a multichannel audio source into an AV receiver today, in digital form, is by an HDMI link. That means that the HDMI from the source must first pass through the video switcher in the receiver on its way to the display. Your suggestions are't too clear, but if I understand you correctly I'll just say that most average TV users will be attracted to HDMI for its convenient single cable hookup. And most serious audio-video philes don't want their TV to become a core component in their audio systems.

Jan os's picture

I wonder if the time has come to seriously entertain the idea, that all of these complex processing functions could best be carried out in a dedicated HT computer. Just think of the available processing power, short signal paths, connectivity to program providers, program recordablity, upgradeability etc. Surely there are ways to effectively reduce the mechanical and digital noise generated by the power supply and other components and take advantage of an elegant and 'simple' one-box solution.

Bruce's picture

I'm with Jan os. It's time for computers to be an essential piece of the home audio/video system. My next computer will be an HTPC. With the way things are advancing it's really the only solution. Of course my dream "imaginary" set up is to have all my music, movies, and tv on the internet stored on some massive super server and all I have to do is push a button and watch or listen on my home theatre, carputer, and cell phone at anytime from any where with no discs to load or store. I want to even be able to access my account in my hotel room or friends house, anywhere. Imagine the possiblilities. Wouldn't have to go to Blockbuster any more and can watch whatever show I want when I want but better than tivo.

Tom Norton's picture

Good thoughts. But you'd better have hard backup copies of everything, so that when the hard drive in that server crashes you don't loose your entire lifetime collection of music, movies, and tv.

Jeffrey's picture

I am not sure a computer or HTPC will ever yield the experience we are all looking for. What really interests me is the maturity of Network Attached Storage (NAS) devices that are becoming common. These devices can be set up as RAID 5 (Redundant Array of Inexpensive Disks) which uses multiple hard drives to store redunant data across multiple drives so that if a single drive fails you do not lose any data. Once the NAS is attached to a wireless router you can access the data from any wireless component. Access your music on the NAS from your laptop, your Squeezebox, Sonos, etc.. This is here today. Tomorrow i'd like to see DVR's make good use of those USB ports to attach to wireless networks via something simple like the Apple Airport and send/receive data to the NAS. Once we separate the data from the application specific device we will start to see real benefits in home. This is the direction large Companies are moving in with the Service Oriented Architecture (SOA). It is the same core is

jeneratör's picture

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maria's picture

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Cheema's picture

After tinkering for a bit I've got it working with my Samsung 42 70-648 exam" - Looking forward to Apple integrating sound through the Mini Display Port in the future 920-805 exam.

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