The transmission of serial-digital video above 1Gb/s (as required by the DVI, HDMI, and DisplayPort video-interface standards) has elevated the requirements for performance in the cables that connect PC and HDTV monitors. As a consequence, the traditional suppliers of analog audio/video cable must now learn the pitfalls already known to makers of datacom serial-digital differential cables (e.g., for Infiniband and PCIexpress at 2.5Gb/s, CX4 at 3.125Gb/s, and FibreChannel at 4.25Gb/s).
This article highlights the phenomenon of data jitter caused by conversions between differential and common-mode components of the video signal. It also exposes myths about intra-pair skew, and proposes cable tests for the purpose of predicting jitter. (For the relevant measurements, go directly to the section "Measuring the conversion from differential mode to common mode.") Differential cables with good performance need not be expensive, just well balanced!
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Figure 1: Common types of differential cable include Twinax, STP, UTP,
and twin coax.
The most common type of differential cable for digital-video signaling in the range 0.25Gb/s to 3.40Gb/s, as required in DVI/HDMI systems, is 100 ohms shielded twisted pair (STP). One-hundred-ohm Twinax is an alternative, and is already a mainstay in datacom applications (Figure 1).
Keeping your balance
DVI, HDMI, and DisplayPort systems each include four lanes of differential interconnect for digital-video signaling. Provided the differential paths maintain the transmitted signals in differential mode, with little or no conversion to common mode, and provided they are balanced (i.e., the two lines have symmetrical effects on the signal), the signal can be recovered with inexpensive receive electronics, even for long high-loss cables.
A cable that maintains signal energy in the differential mode produces predictable phase delays and skin-effect losses across the frequency spectrum. Both effects are easily compensated. Otherwise, the signal may not be recoverable by an ordinary receiver. Indeed, conversions between differential and common modes in a coupled differential cable (STP or Twinax) is an aberration that destroys the ability to predict phase delay and signal loss.
A related source of digital-signal corruption is "intra-pair skew" (different propagation delays on the two lines of a differential pair). To illustrate intra-pair skew, consider a pair of twin-coax cables cut to different lengths (Figure 2). The input is differential, i.e., no common-mode voltage is present. The output, however, contains intra-pair skew equal to the difference in propagation delays. Hand-in-hand with intra-pair skew you find common-mode energy, in addition to reduced differential-mode energy.
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Figure 2: Simple intra-pair skew converts some differential signal to
common-code (CM) signal.
The stimulus used in this example is sinusoidal, rather than a digital NRZ waveform. Skew delay for the simple twin-coax cable of Figure 2 is constant over frequency, but that is not true for STP or Twinax cables. Instead, each sinusoidal (Fourier) component of a digital NRZ waveform propagating in STP or Twinax cables suffers a different amount of skew!
Next: Myths of intra-pair skew