For Better Analog Video, Try Differential Signaling

The compensation can be added by replacing the Zt external resistor, which defines receiver gain, with a complex RC network. The compensation network consists of a dc-gain term set by R only and three ac terms set by R1*C1, R2*C2, and R3*C3. The parallel combination of these terms becomes the complex impedance, Zt, as it appears in the gain equation:

V_OUT/V_IN = K*(Rl/Zt)

where K is the current gain (internally set to 1) and Rl is the external output resistor (Fig. 8). This combination can efficiently compensate high frequency loss for up to 1000 feet (300 m). For some applications, it could be up to 1500 feet (450 m). Zt isn’t efficient above this length, and increases in gain and boosts in the high frequencies actually destroy the signal-to-noise ratio (which drops below 45 dB) and K factor (>3%).

We’ll need to use some kind of modulation for longer distances. In the example shown, the EQ network is a module that covers a certain frequency range (Fig. 9). The EQ network is based on three R*C components, but the user can achieve more accurate frequency compensation by increasing the number of R*C components. The figure shows typical values for the EQ network. The waveforms in Figure 10a through 10d show the NTSC multiburst signal after 0 feet, 500 feet (150 m), 1000 feet (300 m), and 1500 feet (450 m) with appropriate compensation.

EMI considerations
Designers who are familiar with coax cable know its performance and what to expect. The CAT5 cables in the wiring of a building, on the other hand, are unknown in terms of their ability to isolate between signals in adjacent twisted pairs. This uncertainty is reflected in the cable’s specifications for EMI emission and RF immunity. To determine isolation performance, you can run tests that compare traditional single-ended coax with CAT5 differential pairs (Fig. 11).

The typical results were generated by injecting a CVBS signal into a TV via a single-ended RG-59 coax, then a differential CAT5 unshielded twisted pair (UTP), then a standard UTP with single-ended amplifier. Next, the cables were irradiated with an RF signal, and they were examined for interference. When visible interference was seen, the signal level was reduced until the interference was no longer visible, and the interference level was recorded in volts per meter. As the figure shows, UTP cables delivered better performance.

Conclusion
One cannot compare coax and UTP directly. Yet performance comparisons show that differential signaling performs as well as coax, and for some applications, it may even be better while costing less. The cost advantage and performance benefits tip the scale in favor of differential signaling. It remains, however, for equipment manufacturers to incorporate differential signaling in their products.

Footnotes:
ⁱ Amplifier Applications of Op Amps, J. Graeme, Chapter 6: Differential Output Amplifiers
ⁱⁱUsing CAT5/5e/6 for Audio and Video Applications, S H Lampen, Belden Cable
ⁱⁱⁱVideo and UTP, S.H.Lampen, SMPTE Journal, Feb. 1996
^iv Video and UTP, S.H.Lampen, SMPTE Journal, Feb. 1996
^vwww.maxim-ic.com/appnotes.cfm/appnote_number/2045
^vi Chapter 6, “High-Speed Signal propagation,” Graham & Johnson, Prentiss-Hall
^vii “Video and UTP,” S. Lampen, SMPTE, Feb. 2, 1996
^viii “High-Speed Signal Propagation,” Graham & Johnson, Prentiss-Hall
^ix “Balanced LVD SCAI Drivers and Receivers,” Sept. 1997, SCSI Trade Association
^x “High-Speed Signal propagation,” Graham & Johnson, Prentiss-Hall
^xi “Transmission Line Design Handbook,” B.C. Wadell, Artech House, Chapter 4
^xii “Transmission Line Transformers,” J. Sevick, Noble Publishing