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May 1, 2007

HDMI Is Here

Are You Ready?

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.

Connector pinouts

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

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

FIGURE 2: HDMI initialization sequence

Intra-pair skew

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

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.

Content protection

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 joanne.bandlow@twcable.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.

Comments (14) for "HDMI Is Here"
Now you got your HD system, lets make sure you are getting the most out of it.
I've seen too many people who have nice HDTV settups and then not optimize the system because they are not using the correct connections. Its not surprising because of all the choices.

The best thing you can do are:
Step1) GET HDre program!. You don't get HD quality unless you view an actual HD channel.
You can do this buy subscriing to the HD channels from your cable or satellite service.
Use HD-DVD or Blu Ray instead of standard DVD. The pictu

If you watch Standard Definition video on a HD set, it will actually look worse than on your old CRT because of all the extrapolation of pixels to fill the extra pixesl on the HDTV display.

HDMI is an all digital video connection. It will essentially deliver the EXACT information from the HD sources to your Display with 0 artifacts or degradation! If you have several HDMI sources you can connect it to your HDTV with a HDMI switch.

Posted by berni on Saturday, October 6, 2007 @ 08:47 AM
Article mentions that electromagnetic interference can cause hdmi/hdcp problems. this prompted me to move my hdmi switch away from an unshielded subwoofer. this corrected the hdcp problems i was getting!

Thanks to the author!!

Posted by Chris Birdsmith on Tuesday, January 6, 2009 @ 09:03 AM
I am not getting any HDMI output from my Xbox 360. I use an HDMI SWitch 3T01 which connects 3 HDMI cables to the TV at a time - one of them is working but the Xbox one is not. Help.
Posted by Sam on Sunday, June 7, 2009 @ 08:05 AM
I can not get an audio signal when I pass the hdmi cable thru a switcher - just the video. I\'ve tried reversing the cables and using ones that I know work. Any ideas??
Posted by Lorenzo on Friday, July 31, 2009 @ 09:12 PM
same problem Lorenzo reports. The cables all work, the dvd works when connected directly to the video but when run through the switcher (a top line one) no audio.
Posted by Bob on Monday, August 17, 2009 @ 08:33 PM
I have tried two HDMI switch boxes on both a SONY VEGA LDC TV and an LG wireless. With the SONY, the switch boxes work perfectly, but with the wireless LG, either switchbox transmits video but not audio. Is this a known problem with some switch boxes?
Posted by David on Monday, January 4, 2010 @ 09:19 PM
i have a dell hdtv that i purchased several years ago...hd4200 plasma.i\'am currently upgrading my system to use hdmi cables.i\'ve got a hdmi port on my cable box and on the t.v....i connected them properly but the t.v. will start blinking on and off for some reason.i can unplug the power cable on the t.v. and plug it back in and it stops...got any answers.
Posted by jim brown on Monday, January 18, 2010 @ 10:20 AM
bought new windows 7 desktop and hdmi cable, hook it up on olivia plasma tv (bought in 2008), switch to hdmi channel, picture appears on pc, loads up and get an error message on tv \"invalid format\" -black screen-. really appreciate any help.
Posted by kris on Friday, January 29, 2010 @ 03:25 PM
im having problems with my projector video.im using a 50 ft cable.video cuts in and out when bluray player is set to 1080p but runs fine when set to 1080i.im using a hdmi amplifier that suppose to extend signal up to 100 ft.any help would be apreciated
Posted by matt on Thursday, February 18, 2010 @ 03:40 AM
Both my Dish satalite box and my Playstation PS3 connect separately, but when I run them through a GE switch (22714) neither works - I receive a\"unusable signal\"
designation - the final HDMI cable is intact.
I\'ve tried 3 different GE HDMI switches without success.
Any thoughts?
Posted by Richard on Thursday, April 8, 2010 @ 10:13 PM
I love HDMI and USB. Kudos to our great engineers behind the scence.
HDMI cable is very important and expensive. It is very difficult to tell. After replace old HDMI cable for my new Sony Receiver, you will see a big different in brightness, color, and video experience. Sales people should share profits with Engineers.
Sony and Microsoft spent too much money and time to prevent prirate in the past, HDCP is one of the worst protection protocol and second biggest annoying issue stopping users to enjoy their audio and video.
We stop buying discs long time ago and love Netflix\'s video streaming at a reasonable pricing, $10/mo. Remember AOL CDs filled up our waste land. No need to dowload and burn video. Lawyers and studios do not have to worry about pirate movies anymore. God create corporate lawyers for a good reason. Apple and similar companies offer great quality musics selection at great price. We do not have to buy junk CD with many song but only paying to enjoy few songs. The Best of Both World.
Apple and Netflix enjoy huge run on their stocks and market acceptance. Stop paying these lawyers.
Posted by Charles Q. Tran on Monday, May 3, 2010 @ 09:21 PM
HDCP is a waist of time an easy crack with a converter box I have, HDMI leads are a righ rip off in the shops I urge people to shop around as a $99 lead DOES NOT do anything differant to a $4.50 lead even the gold plating is a waist of time, granted it looks nice, I stuck a Spectrum analiser and Oscilloscope to check leads out and NO differances were detected between a cheep lead or a dear one, its good that HDMi detects the optimum resoution the display device can handle aso yuo never need to set it up, I have a 15 year old Sony CRT projector displaying 1080P from my PS3 via a HDMI to RGBHV adapter it looks cool indeed and razor sharp better than Plasma or LCD, long live the CRT projector !
Posted by Mr HDMi on Friday, October 8, 2010 @ 03:30 PM
I have a netmedia HDM auto switch, with a DirectTV DVR, and a new Apple TV (patched opSys asof Dec, 2010). Switch does not work with these two devices. The DVR does not auto switch and display video, even when ATV is set to sleep mode. The ATV autoswitches to video, when the DVR is set to sleep. Both devices work fine, with they are the only device plugged to the switch. What? Any ideas?
Posted by Stuart Asher on Sunday, December 5, 2010 @ 11:24 AM
First I would like to say that you web page http://www.cable360.net/ct/operations/bestpractices/23213.html is very informative. However I still have a question. It appears that only 8 of the available 19 pins are utilized. What am I missing here? Are the remaining pins not utilized? If not why not? Thank you.
Posted by Robert D. Bailey on Tuesday, January 1, 2013 @ 04:30 PM

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