As a broadcast engineer, you are automatically expected to be the expert in all things electronic. For that matter, aren’t you also considered the go-to expert for advice about all things electrical and mechanical? So, it should come as no surprise, that in addition to your regular broadcast engineering duties, you may be asked a question or two about HDMI by colleagues, your general manager or your mother-in-law.
While single- and dual-link HD-SDI is the professional serial digital interface, supporting 16 or 32 channels of embedded audio and time code on a single piece of coaxial cable, a rapidly growing number of products you are likely to encounter include a HDMI interface. In fact, with more than 1000 licensed HDMI adopters and a worldwide installed base of more than 1.5 billion HDMI products, it is clearly the de facto standard in most consumer electronics applications, including HDTV displays and many computer video cards.
HDMI stands for high-definition multimedia interface. It is not a true serial interface like HD-SDI, but rather a parallel format consisting of three color signals with embedded audio on three individually shielded cable pairs and a timing signal on a fourth shielded pair, with each pair at 100 ohm impedance. It is the first and only consumer industry-supported, uncompressed, all-digital audio/video interface, and it’s electrically backward-compatible with DVI. So while there is no audio on a DVI signal, the quality of the digital video signal is untouched when converted by a connector adaptor from HDMI to DVI or vice versa. However, be aware that because there is no Line 21 in the HDTV format, HDMI does not pass traditional Line 21 closed-captioning, even though it does support many forms of digital closed-captioning, which is often performed at the source device instead of the display. HDMI also doesn’t support traditional time code.
The most ubiquitous question about HDMI is how to choose an interface cable. A visit to the Best Buy website, for instance, reveals 146 varieties of HDMI cables, ranging from the least expensive 3ft cable at $8.99 to a 10m cable that sells for $2200.99. Other than length, why is there such a dramatic price disparity and what’s the difference in cables?
To begin, let’s look at what’s on each end of a HDMI cable. Typically, you will find standardized Type A, 19-pin connectors; however, HDMI specifies some other connectors as well. The Type B connector is physically wider (21.2mm vs. 13.9mm) and is essentially a dual-link system capable of two times the video bandwidth of Type A, but the Type B connector became obsolete after HDMI 1.3. Type C is a 19-pin miniconnector measuring 10.42mm by 2.42mm. Type D is the smallest 19-pin HDMI connector. Resembling a Micro USB connector, the Type D connector measures 2.8mm by 6.4mm. The HDMI wire itself consists of four individually shielded twisted pairs plus another seven miscellaneous conductors.
Consortium founders Hitachi, Panasonic, Philips, Silicon Image, Sony, Technicolor (formerly Thomson) and Toshiba introduced the HDMI 1.0 specification in 2002. The agent responsible for licensing the specification and promoting the standard is HDMI Licensing, which works with the HDMI consortium companies to promote the interface.
Since its inception, several revised versions of the HDMI specification have been introduced. For TV professionals, the most important revisions are HDMI 1.3, 1.4 and 1.4a. HDMI 1.3 supports the IEC 61966-2-4 xvYCC color standard for a wider color gamut. This standard is not supported by DVD-Video or Blu-ray, but it is supported by the AVCHD recording format. HDMI 1.3 introduced automatic lip-sync that allows HDMI 1.3 or greater devices to perform this automatically. It also authorizes the Type-C connector, which may be found in camcorders and other small electronic devices. Most importantly, HDMI 1.3 promises 4.46Gb/s, which supports up to 1080p, 8-bit, 60Hz Blu-ray systems and can handle in excess of 10Gb/s.
Introduced in 2009, HDMI 1.4 was designed to include an HDMI Ethernet channel, an audio return channel, 3-D over HDMI for movie and game content, and 4K by 2K video resolution. HDMI 1.4a, introduced just before the 2010 NAB show added 3-D formats (side by side and top and bottom). It also supports the next format on the horizon: 3-D 1080p/60fps. Could a HDMI 1.4b or 1.5 be at the next CES or NAB?
HDMI cables are not defined by HDMI versions; rather, they are defined by bit rates. The HDMI bit rate is the sum of the bit rate of all three video channels, so a 10.2Gb/s cable, for example, actually carries 3.4Gb/s per video channel. All cables showing the trademarked HDMI logo are required to be certified by the manufacturer to comply with the HDMI Compliance Test Specification. While cable length is not specified, better cables can pass the test up to lengths of 30ft; cheaper cables may not.
In the real world, virtually all HDMI cables, with a couple of exceptions, are made in China. Therefore, there may be some knock-offs to be found. To avoid disappointment and embarrassment, stick with reputable, trustworthy manufacturers.
Generally, HDMI cables are categorized at two performance levels. Standard cables, also known as Cat 1, are certified to speeds up to 2.25Gb/s and recommended for up to 720p or 1080i applications. Cat 2, also called High-Speed HDMI, is certified to 10.2Gb/s and recommended for 1080p, 4K, 3-D and deep color applications. Cat 1 and 2 cables are available with or without an Ethernet channel, which only works when devices on each end are HDMI Ethernet channel-enabled.
How will you know if you’ve got the wrong HDMI cable? The first electronic symptoms are digital dropouts or sparkles, followed by poor colorimetry, picture digitization and, finally, no picture. If you do not observe any of these symptoms, you’re above the digital cliff; however, more expensive cables may be more physically reliable under stress.
Some equipment, such as video projectors, may include an HDMI receiver chip that automatically equalizes the received signal for runs longer than 30ft. But if you see problems that you don’t see with a short cable and your run exceeds 30ft, you need a repeater, amplifier, Cat 5 or Cat 6 system, or a fiber solution.
The good news for the consumer electronics world is not such good news for broadcast engineers. HDMI also contains a feature not found in HD-SDI called High-Definition Digital Content Protection (HDCP). When a signal is sent over an HDMI connection, if the transmitters and receivers cannot confirm digital rights management permission, the signals will not be allowed to pass. This verification process is known as the HDMI handshake. Most HDMI-to-SDI converters will not allow a HDMI handshake because most HDMI TV sources, such as Blu-ray players and direct-to-home satellite receivers, use HDCP. Sometimes older DVI-based HDMI equipment will also fail the handshake. If the handshake fails, you might see a flashing picture, at best, but it’s more likely you will have no pictures or sound. At that point, your only alternative is to use the analog I/Os, which, depending on the urgency of the situation, may be better than nothing at all.
Steve Venuti, president of HDMI Licensing, contributed to this article.
New "Transition to Digital" writer Ned Soseman was formerly technical editor of Broadcast Engineering, editor of Video Systems magazine and director of engineering at KCTV5, the CBS affiliate in Kansas City, MO. He is currently chief operator of KRBK-DT Osage Beach/Springfield, MO, and owner of LAKETV.