May 1, 2007
HDMI Is Here
Are You Ready?
By Joanne Bandlow, Time Warner Cable
Editor's note: We usually look at high definition (HD) from a network and encoding perspective. How cable operators are going to put several dozen more HD channels and hundreds of additional hours of nonlinear HD content in front of their subscribers is a large engineering task, indeed. But there's also action on the subscriber side. And not just the HDTV sets or set-top boxes. The HDMI cable and its associated protocols deserve attention, too. TWC's Joanne Bandlow makes that clear in what follows.
What it is
HDMI stands for high definition multimedia interface and was developed by the HDMI Founders, a group of consumer electronics (CE) companies including Hitachi, Matsushita Electric (Panasonic), Royal Philips Electronics, Silicon Image, Sony, Thomson and Toshiba. CE manufacturers pay fees for HDMI ports to HDMI Licensing LLC.
HDMI is a method for connecting a source device such as a set-top box or DVD player to a display device using a single cable. HDMI carries three channels of uncompressed digital video as well as digital audio. The HDMI specification describes both the physical connection in terms of connector style and pinouts as well as the signaling that travels down the wire.
The HDMI connector contains 19 pins. It is a member of the "D-Sub" connector style.
HDMI allows for three channels of digital video as well as clock, data and dynamic display configuration (DDC) information. The DDC channel is an I2C serial bus, while the video runs on multiple pairs of LVDS/TMDS. (See Figure 1.)
FIGURE 1: HDMI connector pinouts
The video circuits inside an HDMI or digital visual interface (DVI) device use a signaling method called transition minimized differential signaling (TMDS). This is a flavor of low voltage differential signaling (LVDS). An LVDS signal is 300 mv peak-to-peak into a 100-ohm twisted-pair, very similar to the twisted-pair that we see inside an Ethernet cable, except that the HDMI cable uses a separate shield around each video pair to minimize crosstalk.
To see what "transition minimized" means, note the difference between the following examples:
LVDS - 10110101001010101010 (unencoded video)
TMDS - 11110000110011001100 (ones and zeroes bunched)
A TMDS signal employs a method of encoding the bits that clumps ones and zeroes together to minimize the number of 1-to-0 transitions to improve electromagnetic interference (EMI) and skew tolerance. Because the rising edge of a square wave (a transition from 1 to 0 or vice versa) contains boatloads of harmonic energy, it helps to contain EMI by keeping the number of those sharp transitions to a minimum; hence, "transition minimized" in the name. The eight-bit video signal is encoded into a 10-bit symbol at the transmitting end and decoded at the set.
DDC and Hotplug
Did you ever wonder how your computer knows what kind of monitor is plugged into it? That video cable going from the PC to the monitor has a two-way data path in it for something called display data channel (DDC) It is also called dynamic display configuration.
Designed by the Video Electronics Standards Association (VESA), DDC describes a protocol that enables a source device such as a set-top box or a computer's video output card to query the display that it's attached to and set its scan rates accordingly. A DDC query from the source device prompts the display to respond with its manufacturer name, model number and display capabilities.
DDC was designed to allow computers to be connected to cathode ray tube (CRT) monitors without the fear of running the computer's resolution at something higher than the monitor can handle. If this weren't in place (or, in some cases, if it were overridden by the user), it would be possible for the computer to run at a resolution that burns up the horizontal output transistor of the monitor. All modern digital and VGA display connectors run DDC for this reason. HDMI and DVI also use DDC to manage the display resolution settings.
Hotplug detect is a pin on the HDMI and DVI connectors that allows the source device to sense when a display device has been connected to it. When a monitor is plugged into a source, the monitor provides a +5 VDC signal on the hotplug-detect pin. This causes the source device to initiate a DDC query to the set to ask about its display capabilities, as well as querying for high-bandwidth digital content protection (HDCP) compliance. (To see the HDMI initialization sequence, see Figure 2.)
FIGURE 2: HDMI initialization sequence
Three things that can interfere with an HDMI connection are attenuation, crosstalk and skew. You're already familiar with attenuation and crosstalk. What about skew?
HDMI uses three separate TMDS channels for the primary colors. Those digital RGB signals scream down the HDMI cable at a rate of 165 megapixels per second, or nearly 5 Gbps. At that speed, even a small change in the propagation characteristics of the copper can cause problems in the display.
Intra-pair skew is defined as a difference in propagation between the + and - lines of a differential pair. Ideally, the + and - lines of a signal traveling down a twisted-pair will remain in sync. In reality, small differences in conductor length, twists or kinks in the cable, and other factors contribute to skew, causing the + and - conductors to deliver their signal at slightly different moments at the other end of the cable. (See Figure 3.) This causes the data bit to get mangled and can cause it to fall out of the decision boundary for that bit, causing ones to turn into zeroes and vice versa.
FIGURE 3: Intra-pair skew
A typical HDMI cable has a skew characteristic of 34 picoseconds/meter (ps/m), based on the bit rates used for HDMI. Thus a 5 meter cable clocks in at 170 ps. At 10 meters, that jumps to 340 ps, greater than the maximum skew of 303 ps required by the manufacturers of the HDMI interface chips. This can translate into no picture or a very unreliable one. Fortunately, the manufacturers of the HDMI chipsets have been making improvements in their designs and are getting better at recovering weak, noisy or skewed signals.
One way to extend the maximum distance of an HDMI connection is to use an HDCP-compliant repeater, an active device that regenerates the HDMI signal. This includes the current generation of HDMI home-theater source selector devices. In a case like this, the maximum distance is taken to be the distance from the HDMI source to the repeater, and then from the repeater to the TV set. Thus, a 5 meter cable can be used between the set-top box and the repeater, and another 5 meter cable used from the repeater to the TV set while maintaining proper HDMI specs for attenuation, crosstalk and skew.
Developed by Intel and embraced by the Motion Picture Association of America, HDCP supports federal law requiring that all digital content be protected.
When you plug an HDMI source into an HDMI display, the source device queries the display to see if it supports HDCP. If the device doesn't reply with a "yes" within a certain number of milliseconds, the source device assumes that the display is not compliant, and you'll get an "informative display" message stating so and suggesting that you use your analog ports, instead.
In some cases, this can happen if the set-top box is plugged into an HDMI repeater/switcher/home-theater device and that device is not passing the HDCP transaction from the set-top to the set correctly - in such cases, bypassing the repeater. The HDCP spec considers all of these HDMI black boxes as repeaters because they essentially repeat the data between the set and the set-top box.
Troubleshooting and tradeoffs
While the latest versions of HDMI are more stable than their predecessors, they're still not trouble-free. (HDMI 1.3, released June 2006, is the most current; see the sidebar "HDMI Fact Sheet.")
Most HDMI problems can be traced to HDCP issues. In their zeal to protect high-value content, HDMI's creators have produced a protocol that always errs on the side of shutting the connection down.
The workaround involves tracing the source of the problem. Is it outdated firmware in the set-top, TV set or HDMI repeater? Is it a badly kinked or excessively long cable? Is the cable running too close to a source of EMI? Is the TV set tearing down the HDMI handshake when a different input is selected?
The simplest fix is always to encourage customers to use their analog component connections (YPbPr) until the issue has been resolved.
Component connections have less bandwidth than HDMI, so there might be a (very small) loss of quality on HD channels. On the other hand, the SD channels might actually look a little better. Given the very small difference in picture quality vs. all of the baggage that the HDMI port carries, it's a pretty good tradeoff.
Barring that, cable techs can try swapping the cable with a known-good test cable. In addition, if there's an HDMI switcher or repeater in the path, bypass it temporarily and go directly from the set-top to the HDTV set.
If this works, chances are that there's an HDCP issue between the set-top and the repeater. Either the repeater isn't "repeating" the HDCP info from the set to the set-top, or the set-top is running a version of HDCP prior to 1.2, which doesn't recognize active repeaters. This can often be fixed via a firmware update to the set-top box. (For more troubleshooting tips, see the sidebars "HDMI FAQs" and "Troubleshooting HDMI.")
Get a handle
HDMI is here, isn't going away, and will likely become more prevalent as more subs upgrade to HDTV. It offers simplicity of installation wiring and great throughput from the set-top to other devices, but is not yet bug-free. HDMI cables themselves may have problems, and compatibility issues remain, largely with HDCP and older customer premises equipment (CPE) such as DVD players.
If operators want to keep gold-plated subs happy and be prepared for the next video wave, it makes sense for their technical teams to get a handle on HDMI as soon as possible.
Joanne Bandlow is a network engineer for Time Warner Cable. Reach her at email@example.com. The views expressed in the article are those of the author and not necessarily those of Time Warner Cable.
Sidebar 1: HDMI FAQs
The cable box won't work with my HDMI cable, but my DVD player does. Is the cable box is broken?
No. Currently DVD players do not require an HDCP-compliant display, but the set-top box does. If the box doesn't see a valid crypto key from the display, it won't work.
Will I get a better picture from HDMI than from my analog YPbPr ports?
It depends largely on the type of display. If your display is natively digital (LCD, DLP, plasma), then you might. If you have a CRT, you may or may not because the CRT has to convert a digital signal back to analog anyway, and the fewer analog-digital-analog conversion stages, the better.
My screen occasionally pops up the "unauthorized" message when viewing via HDMI. Then everything goes back to normal, and my program continues. Why?
The HDMI spec requires a continuous dialog between the source and the TV set in order to manage content protection. For instance, the encryption status of the picture is checked on every single frame, so this decision is being made 30 times a second. If your HDMI connection is marginal (poor cable, too-long cable, kinked cable), that dialog might suffer from occasional interruptions, and your set goes into "informative display" (denied) mode. The problem clears when the signal re-establishes itself.
How long of an HDMI cable can I use?
HDMI uses TMDS signaling. As a general rule, TMDS signals will travel over typical Category-1 HDMI cables reliably for up to 5 meters (15 feet). Some of the better Category-2 cables may perform at longer lengths, but all cables eventually cause distortions over length, so the rule of thumb is KISS (Keep it shorter, silly). Besides that, the newer Cat-2 cables are intended to enable higher bit rates, and the faster the speed, the shorter the maximum distance.
Sidebar 2: HDMI Fact Sheet
HDMI 1.1 (released May 2004)
• Color palette of 24 bits per pixel
• Maximum bitrate of 4.9 Gbps and 165 megapixels/second
HDMI 1.2 (released August 2005)
• Added support for SACD audio
• Added support for PCI-Express signals
• Added RGB (PC) colorspace (TV uses YCbCr colorspace)
HDMI 1.3 (released June 2006)
• Increases single link bandwidth to 340 MHz
• Supports a maximum bitrate of 10.2 Gbps
• Adds 30-bit, 36-bit and 40-bit "deep color" palette support
• Increases colorspace for wider color palette
• Supports new HDMI mini connector for camcorders and other small devices
• Supports automatic lip-sync capability
• Supports new lossless compressed audio formats
For more on HDMI, DVI and TMDS, see www.hdmi.org and www.ddwg.org. For info on HDCP, check out www.digital-cp.com.
Sidebar 3: Troubleshooting HDMI
The latest versions of HDMI are more stable than their predecessors, but they're not yet trouble-free. Most of the HDMI problems stem from HDCP issues. The simplest fix is to encourage the sub to use analog component connections (YPbPr) until the issue has been resolved. Otherwise:
• Swap the existing cable with a known-good test cable if possible.
• Remove any HDMI switch boxes or repeaters and try again.
• Restart the TV, then power-cycle the set-top box in this order to force a new HDCP query from the box to the set. Sometimes the hotplug detect scheme doesn't work properly, and this procedure helps to identify this situation.
• As a last resort, flip the cable around. I've had one case where a cable exhibited a defect that caused it to be "directional." Apparently one end of the cable was exhibiting excessive crosstalk and only worked when it was plugged into the end that had a more forgiving chipset inside the device!
Warning: An older or poorly made HDMI cable might not work correctly on HDMI 1.3 because of the increased clock and throughput speed. Look for "HDMI 2.0 1080p capable" cables to insure compatibility with the newer specs.