Jeff Bocaccio's Tips When Using HDMI
Cable Issues
When installers call me with problems, the first thing I ask is: how long is the cable? Typically they're pushing the limits – 5, 10, 15 meters or more. Many of these longer cables we have tested show signs of deficiencies with the ever so critical TMDS integrity. I had one installer who had five separate HDMI Cables wired in series with couplers. Every time you add another connector like this will degrade the integrity, and some of the couplers out there are just horrible. You've just got to tell them: "you can't do that with this interface, it's not going to work." Sometimes they'll say "I'm using a three-meter cable and it works fine, but I hook up a six-meter cable and it fails." The first thing I ask them is: is the 6-meter cable fatter than the 3-meter cable? No sometimes is the answer which tells me that the gauge is probably too small for that length. Other recommendations to this are to add a decent digital equalizer to the back end of the cable which will just about guarantee the system to perform normal.
I will also recommend that they carefully run any cables that need to make corners with a broad curve rather than a 90 degree bend. Again, they are working with high speed data that requires some extra care.
Connector Issues
Many installers gripe about the connector, especially the tendency to fall out of its socket if pulled or strained. I will explain to them that the connector is design that way for best integrity and ease of operation. Some of the cable that venders put on these cables is so large it goes beyond the weight limits the connector was designed for. The connector's "holding power" is very similar to a USB connector and yet you never hear anybody complaining about that! After you have explained some of these issues the response is "Oh, I see. I guess I should be more careful." You have no idea how many people call, saying a component has failed, when the problem is actually that the cable is not fully seated. Especially the 5 volt line, since that pin is positioned halfway back in the connector to insure power being the last connection made – and it has to be securely seated.
I2C Implementation Issues
When the problem is at the component level, the cause we see most often is a corruption problem in the I2C interface. I2C is a serial interface developed by Philips, used for decades, very reliable. HDMI uses it on the DDC line – a great idea, a known interface, but not always implemented well. The I2C issues we've seen have primarily been rise time issues. The spec on this is no more than 1 microsecond rise time using the 80/20 rule. When wave forms get close to the edge, the complete data instruction will never follow through. So sometimes we see no audio or wrong color or no video due to HDCP errors. HDCP data comes right after EDID, and it's refreshing every couple of seconds, and if all that HDCP data – the key – doesn't get through and match up, it's going to shut down.
Many misdiagnose this as a video problem but it's not – typically it's either EDID or an HDCP handshake failure. Now this is definitely getting better, because people are building better transmitters and receivers, more robust boards, better cables, designing more dynamic range into the performance of their products. And in the short term, the problem can often be solved with a repeater box - repeating the I2C signal and cleaning it up.
Design & Manufacturing Problems
We're often hired to do a PDR – preliminary design review – before a product goes to compliance testing. That process exposed a lot of products that were being built badly, and would never pass compliance testing. It taught me that people weren't doing their homework, they weren't sourcing products that performed at the level HDMI requires. As I researched this and talked to more vendors, I came to realize that many of these people were from the old days, the analog days, and suddenly they are faced with a new interface nothing like they have been working with in the past. I think what happened was that they treated it as if it were analog. There have always been standards - in the past we followed the NTSC standards - but those standards were very loose. In many cases you could be as much as 20 percent off the standard and the unit would still work. It might not look right, but it would work, and as long as it worked, many end users never knew there was a problem. But HDMI is a much more critical interface, much less forgiving, and you've got to adhere much more closely to the spec. That's why the need for compliance testing is so important. But still, even with testing, I don't always see manufacturers watching and making sure that what's coming out of production is exactly the same as what passed HDMI compliance. They need to go back to the books and make sure production is right. I believe it's just sloppiness, and it has nothing to do with the interface. If you use high-quality hardware, your chances of success are extremely high.
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not necessarily represent the views and opinions of HDMI Licensing, LLC,
the HDMI Founders or any of their respective parent organizations or
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