The Use of Pre-Emphasis With HDMI

Transcription

1 The Use of Pre-Emphasis With HDMI By Jeffrey Boccaccio Senior Engineer for InVisions Technology Inc. 555 W Granada Boulevard Suite F-3 Ormond Beach FL jeff@invisionstech.com January 2006 As we grow and learn more about the Real Life Effects of the HDMI environment, we also have learned to adapt our thinking and our future engineering models towards solutions in creating a more robust and reliable system interface. This is nothing new in any new technology that happens to stroll down the road. Clearly, the most volatile area in this new HDMI interface, unfortunately, is the cable. Tests have proven time and time again that cable companies basically, at least in my opinion, have been basing their cable products on the old punt and guess method. We have tested cables from firms where their shorter cables are not as good as their longer ones. Which of course is a complete inverse of the norm. Why? Simple. The HDMI cable and connector is not one that a typical cable company can produce in a small, under 5 million dollar, company. There just is not enough capital available to put towards the machines necessary for soldering, molding and terminating cables of this caliber. There are just a handful of firms in this country that have actually put their best foot forward in the effort to build and control their output. To those firms, I commend them. The old rule of thumb with any cable system is the line loss and the response curve. That, to some extent, is exactly the problems with many HDMI cables. This is why compliance testing is so important. At least you can operate in a safe window of operation when using a true compliant HDMI cable. The tests we are about to show you are tests that were taken on known HDMI compliant cables. 1

2 With that you will be able to see how, by just adding some correction electronics, an even better signal integrity can be achieved. Here are some curves that may represent this very issue. These curves are not related to actual cables. They are for reference only. 0 db _1db _2db _3db _4db _5db 6db 20 Meters 28 Gauge 500Mhz 1Ghz 1.5Ghz 0 db _1db _2db _3db _4db _5db 6db 20 Meters 26 Gauge Electronic Correction 0 db _1db _2db _3db _4db _5db 6db Meters 24 Meters Gauge 500Mhz 1Ghz 1.5Ghz Electronic Correction 500Mhz 1Ghz 1.5Ghz 2

3 Notice that after we add the necessary correction, the curve becomes more uniform in shape with only a 3db loss factor over the range of the curve. But also notice that for every gauge change the correction becomes less and less effective. Now, through the efforts of some semiconductor companies, electronic correction is introduced into the HDMI hardware itself. We use these advantages with the HDMI 5X1 switch. This Pre-Emphasis boost that is offered can be enabled or disabled depending on the type of cable and its length. The only way to accurately measure this is by using cables that are known to be within compliance. This will give you a much better understanding as to how all this works in the real world. 1m 28AWG HDMI 2m 28AWG HDMI 3m 28AWG HDMI 5m 28AWG HDMI These cables are made from the same material, which gives us the consistency that we need in order to make any type of realistic test base line. The following will demonstrate the use of Pre-Emphasis in the HDMI 5X1 in relationship with the eye patterns that are generated through each device. This tells the entire story: 3

4 1-Meter Cable with No Boost Touching the Light Blue Eye Mask is considered Failed Notice that the light blue mask area is almost touching parts of the Eye Curve. Enabling the boost, the Eye is much more defined with the mask more uniformed within the eye closure. 4

5 This next pattern is that of a 2-meter cable. Notice that the eye is closing down on the blue mask area demonstrating a poorer response curve and an increase in jitter. Same cable only with Boost. Here, notice the eye again becomes more defined and opening away from the mask area. 5

6 Here a 3-meter cable is tested. Again, the eye is closing down on the mask worse than the 2 meter, but at least passable. Adding the Boost option again opens the eye up with a more uniformed eye and better distancing from the mask. 6

7 Now you can really see what has happened to our 5-meter sample. The mask has now touched the eye and is considered a failure. Adding Boost has corrected for the response curve and jitter problems keeping good distance from the eye. The cable now becomes a Pass grade and the integrity is back. All this can be interpolated by using different cables and different gauges. Knowing that the bigger gauge will offer better response, these numbers can be interpolated to better lengths. Of course all of them would have to be individually tested due to the wide variety of characteristics found with so many assortment of cables. 7

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