the bitrate demands placed on HDMI cable are severe. At 1080i, the pixel clock runs at 74.25 MHz, and each of the three color channels sends a ten-bit signal on each pulse of the clock, for a bitrate of 742.5 Mbps. What's worse, some devices are now able to send or receive 1080p/60, which requires double that bitrate.

Impedance, in particular, becomes a really important concern any time the cable length is more than about a quarter of the signal wavelength, and becomes increasingly important as the cable length becomes a greater and greater multiple of that wavelength. The signal wavelength, for one of the color channels of a 1080p HDMI signal, is about 16 inches1, making the quarter-wave a mere four inches--so impedance is an enormous consideration in getting HDMI signals to propagate along a cable without serious degradation.

As it happens, some types of impedance variation are easier to control than others, so depending on the type of cable architecture we choose, the task of controlling impedance becomes harder or easier. Coaxial cable, in this area, is clearly the superior design; the best precision video coaxes have superb bandwidth and excellent impedance control. Belden 1694A, for example, has a specified impedance tolerance of +/- 1.5 ohms, which is just two percent of the 75 ohm spec; and that tolerance is a conservative figure, with the actual impedance of the cable seldom off by more than half an ohm (2/3 of one percent off-spec). Twisted pair does not remotely compare; getting within 10 or 15 percent impedance tolerance is excellent, and the best bonded-pair Belden cables stay dependably within about 8 ohms of the 100 ohm spec.