Guest Editorial: The Secrets of HDMI
By Raj Nair — on page 160-162 of the March 2007 issue
Keywords: HDMI
What the consumer doesn't know just might hurt him. Have you come across DVD players at $29, or even as low as $19, and wondered why cables to connect them to HDTVs are 10 times as expensive? Finding a digital versatile disc player with its sophisticated optoelectronics and mechanical systems for $19 seems like magic.
It's not magic though. It's volume production, the beauty of commerce, benefiting the consumer. What doesn't make sense to me is the cost of the cable to connect such a useful device to a television. It is akin to having to pay 10 times what you pay for a soft drink for the straw you use to sip it.
It Began with USB
Do you remember advertisements indicating that if you buy the "better" USB cable for your printer, print quality will be better? All digital data transmissions are prone to bit error rate (BER).
That relates to picture quality, and presumably, the "better" cables offered lower BER. But when USB 2.0, at 40 times the 12-Mbp initial rate came along, the specification asserted that the very same cables could be used for the much higher speed version!
Any cable designed for 12-Mbp transmission of digital data that is also found good for 480 Mbps probably lacked nothing in quality in the first place, and a "better" cable couldn't have given the consumer any more discernible value.
As for audio cables, the art form of duping simple consumers has descended into great depths of sleaze.
High-definition multimedia interface (HDMI) evolved from another interface termed digital visual interface (DVI) developed primarily as a connectivity link between personal computers and digital displays such as LCDs. DVI and HDMI are promoted by select groups of companies.
There is perhaps no consumer representation in the development of these interfaces, nor are there any regulations controlling the development of yet another consumer digital, serial interface. Companies claim to know what consumers want, which is perhaps why we have HDMI cables that cost $199.95 for a 16-foot length.
The lack of industry-wide standardization of such interfaces and, more specifically, the lack of cable/interconnect performance standards for such interfaces creates confusion, and opportunities to profit at the consumer's expense.
For example, when DVI 1.0, was released by the Digital Display Working Group in 1999, it effectively eliminated signal pre-conditioning, restricting transmitted signal quality while requiring very high data rates over long cable lengths. It also did not require well-known electronic compensation techniques within the receiver end electronics. In other words, the specification appears, by accident or design, to burden the link cable.
So how was the consumer affected? A confusing variety of cables for a wide range in cost appeared, all claiming conformance and many claiming to be "better." Cable manufacturers claimed the need for sophisticated materials and manufacturing, driven by a specification placing the burden upon interconnects.
Vendors also confuse consumers with their co-axial, twin-axial, dual-shielded and triple-shielded cables. But do HDMI cables have to be double- and triple-shielded in such manner, when data is transmitted through wire pairs within in low-swing, differential, low-EMI manner?
Studies conducted very recently at ComLSI show that unshielded Cat 5e cabling can be just as good from a signal transmission and reception perspective over 25-meter lengths. Prior work disclosed by Intersil goes further, discussing SXGA video transmission over 300 meters of Cat 5 cabling.
Some companies even tell you that silver-plated wires within cables make video signals flow better. But silver is only marginally more conductive than copper! And plating only helps with what is called skin effect, which is a minor factor at very high data rates.
On the Flip Side …
Ahead of the fatuous marketing efforts discussed this far, a recent development also includes an effort to stuff sophisticated electronic circuits into the assembly of an HDMI cable. A video of this "innovation" may be viewed at CEPro.tv. The company featured would have you believe that HDMI-qualified cables from established vendors generate hundreds of pixel errors they incorrectly call "pixelation" continually visible on-screen.
Sorry. Video images with hundreds of consistent pixel errors correspond to a BER of at best 100 in 2 million, at least three orders of magnitude worse than the DVI specification of one error in 1 billion.
Could a manufacturer have supplied an HDMI-certified cable with such performance? And no, I do not think removing sophisticated electronic circuits easily integrated into chips at either system end of a typical HDMI link and stuffing them into connectors in the cable is lower cost, higher reliability, easier to use or of better quality. Do you?
The HDMI specification has come a long way from its DVI roots, including necessary circuit techniques such as source-termination and sink-equalization that can effectively compensate for cable inadequacies at HDMI 1.3 maximum data rates.
There is, therefore, no need to believe cable vendors touting strange advancements in their cables … simple Cat 5 cables can do the job. In fact, Cat 5 and Cat 6 cables have the necessary standardization and design maturity ensuring optimality for high-speed data transfer. As a practical matter, such Ethernet cables have addressed mechanical locking to systems chassis and snag-free use, aspects unaddressed as of yet by HDMI.
10GBase-T developments also indicate feasibility of HDMI data rates over 100m of Cat 5e, supporting the argument that HDMI cables should be cheap. Recent cable and system studies conducted at ComLSI support this conclusion.
Rather than a DVD player free with an HDMI cable, every HDTV system ought to come with a free HDMI cable!
Raj Nair is an engineer with ComLSI, Inc. (
http://www.comlsi.com), a Mesa, Ariz.-based company developing Analog IP for the high-speed communications and power management areas.
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