HDMI 1.4 Cables: No News is (Mostly) Good News

After a couple of teaser press releases, one at CES and one just a short while ago, the HDMI Licensing organization has released the final version of the HDMI 1.4 spec. This is the first major HDMI spec revision since 1.3 was issued back in June 2006, and there's been a lot of speculation as to what it would contain.

The spec document itself is available for download at www.hdmi.org. However, since it's 425 pages long, including appendices, and since most of that content is actually just a restatement of the material that was already in 1.3a, we'll take a short spin through it to see what's there, with a particular emphasis on the impact of the new spec upon our products: HDMI cables.

New HDMI Connectors

The words "new HDMI connector" send chills up many a spine, as people worry whether their old gear, and old cables, will still be able to plug in to future HDMI devices. The good news is that nothing in the spec unseats the conventional "Type A" 19-pin HDMI connector from its role as the primary way of hooking up the vast majority of HDMI-compatible devices. There are two new connectors introduced--but these are special-application connectors. The first is a "Type D" connector, the "micro-HDMI" type. If you already thought the "mini-HDMI" connector was small enough (or, as we tend to think, too small), then someone down at the HDMI organization disagrees with you. The pin body on the micro-HDMI connector is under 6mm wide, as compared to about 21mm for the full-size type A and 11mm for the type C "mini." This connector is likely to be seen on very small devices - HDMI for your iPod, anyone?

The other, and more novel, connector type is "type E," for use in vehicles. This is an HDMI connector with shell assemblies both on the plug and receptacle, and a locking tab to hold the cable in the receptacle once inserted, similar to the sort of interlock tabs seen on a lot of automobile wiring harness connectors. The object appears to be to protect the connection from vibration, and perhaps from oil and the like. It will be interesting to see these connectors "in the flesh"--at the moment, nothing but the line-drawings in the spec document are available to be seen.

Bandwidth:
The More Things Change, the More they Stay the Same

HDMI Licensing stated, in its CES press release, that new resolutions and 3-D functionality would be introduced in 1.4, and that these would be accommodated by "increasing the upper limit of the HDMI link" -- in other words, by increasing the bitrate allowed to run through an HDMI connection. This particular remark gave us some cause for concern, as we have long felt that the bitrates supported by HDMI are already excessively high for the cable architecture, particularly when the signal has to be run over distance. We were pleased, then, to see that while the new spec does announce support for "4K by 2K" video formats and 3D, these are to be run within the existing 3.4 Gbps/channel (10.2 Gbps combined) link limit. It's not immediately clear why anyone would actually need "4K by 2K," as no consumer content is actually recorded in any such format--but, for the time being at least, if those resolutions are run in HDMI cable, they'll be run at framerates low enough to keep the data rate within existing limits.

What this means--barring any change to the Compliance Testing Spec, which has not yet been revised to account for 1.4--is that a cable which is certified as "Category 2," or "High-Speed" as some call it, under HDMI 1.3 will continue to be compliant with all resolutions, framerates and protocols associated with HDMI 1.4, with the exception of the Ethernet and Audio Return Channel features, which we'll talk about next.

Ethernet and the Audio Return Channel

In general, HDMI spec modifications have been both backward and forward compatible, where cable is concerned. HDMI 1.4 does introduce, however, one area where this will not be so: the provision for an Ethernet connection and an Audio Return Channel.

In HDMI versions 1.3a and before, the HDMI cable and its standard 19-pin "type A" connector had one spare conductor reserved for future use, on pin 14. The cable configuration consisted of four shielded twisted pairs (each accounting for 12 pins) and seven other, unpaired conductors. 1.4 introduces a slightly revised cable architecture: instead of four shielded twisted pairs, there will be five, and the unpaired conductors will be reduced to four. The existing DDC/CEC ground connection, pin 17, will now also function as the shield connection to this pair, and the "hot plug detect" on pin 19 will be twinned up with the "reserved" conductor on pin 14 (now known as the "Utility" pin). This new shielded twisted pair will carry Ethernet and Audio Return functions.

Now, anyone familiar with Ethernet will of course immediately recognize a problem: an Ethernet connection ordinarily consists both of a sending and a receiving pair. Accordingly, HDMI cannot simply route Ethernet signals onto these conductors, and the spec document devotes a full 72-page appendix to explaining just how this will be done. Suffice it to say, for cable design purposes, that all that this requires is that the conductors on pins 14 and 19, which are now run separately, must be twinned up into a 100-ohm pair and shielded by a shield tied to pin 17.

The result will be that HDMI devices with Ethernet capability will be able to interconnect via Ethernet without an Ethernet cable or hub, and that a display which has the capability to decode digital audio from an ATSC or QAM tuner will now be able to pass the digital audio "upstream" through an HDMI cable to the A/V receiver without the need for a separate digital audio cable. Some of this may work with existing HDMI cables, over short distances; in particular, the Audio Return Channel will very likely work just fine with existing cables in many applications. Ethernet, however, runs pretty fast and is unlikely to work on a cable that hasn't been designed to carry it, at least for more than a few feet.

Will devices use the new Ethernet feature? It'll be interesting to see--but we suspect that this is really a case of a complex solution that lacks only a problem to solve. Most home theater devices do not need Ethernet, and when they do, the low cost of conventional patch cables, hubs and wireless adapters makes hookup simple enough. Our suspicion is that it will be less costly for a device manufacturer to embed a wireless adapter in its design than it will be to incorporate the circuitry required to accommodate Ethernet over HDMI while still supporting conventional Ethernet through another port, and allowing the device to act as a hub between those two ports--but who knows? Suffice it to say that it will be a long while before anyone actually needs this feature, as it will have absolutely no application until the user has at least two devices which both incorporate it, and which are not otherwise networked.

The Future of Cable Compliance

At present, the HDMI Licensing organization has not published the new Compliance Testing Spec, which is the document which will provide the actual engineering standards for compliance. It appears that there will now be two types of compliant cable--those which are Ethernet-enabled, and those which are not--and there will presumably be new testing standards for the former. While the "old" HDMI cable will not be compatible with the new Ethernet standard, it's hard to imagine that presenting any problem unless the HDMI-Ethernet standard becomes so common that manufacturers stop putting RJ-45 jacks on back panels and use of the HDMI Ethernet capability winds up becoming effectively mandatory. That, it is fair to say, is a long way off, if indeed it ever happens at all, and effectively, existing cable designs should be forward-compatible for a good long while to come.

With no changes to the bandwidth requirements, the existing Category 1/Category 2 certification system looks to be with us for a while, and a Category 2 cable today should be every bit as good as a Category 2 cable tomorrow, regardless of whether it's a 1.3-certified cable or a 1.4-certified cable. Unfortunately, confusion over the significance of the spec version numbers is likely to lead a lot of people to suppose that 1.3-certified cable is somehow obsolete or inferior to 1.4-certified cable, when the two are actually tested to the same engineering criteria. For the time being, however, with no 1.4 Compliance Testing Spec, there are and will for a while be no 1.4-certified products of any kind on the market, so that bit of confusion is still a few months away.

Many thanks to Blue Jeans Cable for providing this informative article.