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.

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.