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Document Scope
Distribution Best Practices
HDMI Installation Best Practices is the third document in a series of whitepapers published by CEDIA. Previous
documents include Introduction to HDMI and HDMI Initialization Sequence. These documents are available in
the CEDIA Marketplace at
www.cedia.org/marketplace. This document discusses best installation practices in dis
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tribution such as active cables, HDMI, Cat 5/6, and fiber solutions, as well as basic HDMI tests and interoperability.
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Long-Haul Cables
Before we start to discuss best practices for long-haul cabling, it is important to know how digital video
signals are affected over long cable lengths. As cable length increases, image quality (signal integrity) is
perceived to be perfect until a certain length. Beyond that length, the image either degrades in quality
(flashing, sparkly dropouts and/or loss of sync) or disappears all together. This is known as the “cliff
effect”
[9]
. This is depicted in Figure 1 below.
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Inter/Intra-Pair Skew
HDMI uses twisted pair cables, which are subject to two types of skews that can create timing errors and
cause data drop-outs. The two types of skew are intra-pair skew and inter-pair skew. Intra-pair skew is the
time differential between the two wires making up itself, while inter-pair skew is the time differential
between separate twisted pair lines in the cable assembly
[13]
. Total allowable intra-pair skew is only
about 4.8 picoseconds, and inter-pair skew should not exceed 7.2 nanoseconds
[13]
. Because of these
small tolerances, HDMI cable construction is of important significance to installers. Some attributes to pay
attention to include the cable gauge, cable material, and its impedance. Since allowable skew is fixed
and the percentage error in cables (normally measured per meter) is constant, longer cables are more
difficult to manufacture. Also, it is worth noting that cabling is a component of the overall system and
one has to take into account the percent error between all source and sync devices including any
switches, splitters or matrix devices.
Even before the cable length reaches the “cliff”, data errors are already occurring in transmission. The
reason we don’t see the errors is because there is a built-in error correction technology used in digital
transmission. The picture will still be perfect as long as the error rate is below the threshold. The
signal recovery mechanism will fail once it exceeds the capability of the display
I guess this says it all, it may work for awhile but inevitably it will fail with continued use. Thank you for this informative knowledge Mark, it really helps me to understand how critical HDMI characteristics have to all work in tandem with all devices they are used with.
Cheers Jeff