This article provides in-depth information on component video standards and amplifier characteristics necessary to achieve those standards. The trade-offs of differential line driver and receiver topologies are discussed in detail. This paper also presents an array of common video problems along with an explanation of the source and a suggested solution. This array includes: ghosting, color matching, common-mode noise, powersupply noise, mismatches in channel timing and equalization.

With a focus on CAT-5, we'll take an especially close look at video equalization techniques, an inevitable result of the lower bandwidth of the cables. While circuit specifics are presented, they involve manual tuning to compensate for the exact length of the cable used. Since manual tuning requires professional installation, two new systems based on automatic cable compensation are also presented.

Video Example – SXGA
We have chosen to discuss the main aspects of a video system with respect to the SXGA video standard. Table 1 presents key parameters of 76Hz SXGA video signal so we can calculate the bandwidth and slew rate needed in the driver and receiver amplifiers. If not provided, the signal bandwidth is calculated with the following equation:

BWS = 1/2 [(K*AR*(VLT)2*FR)*(KH / KV)] = 51.9MHz

K = Kell factor, Visual information is lost when video information passes during the retrace rather than the active portion of the scan line. Assuming 30% of the visual information is loss, K = 0.7.
AR = Aspect ratio (the display width divided by display height) =1.33
VLT = Total number of vertical pixels = 1067
FR = Frame rate or refresh rate = 76
KH = Ratio of total horizontal pixels to active pixels = 1720/1280=1.34
KV = Ratio of total vertical lines to active lines = 1.04

Table 1 Parameters of SXGA Video Standard

The signal bandwidth is calculated to ensure adequate system bandwidth. When choosing an amplifier, remember that the reported bandwidth is measured as the halfpower point, also known as the 3dB frequency. The 50% loss of power at this frequency is not acceptable in any part of the video signal bandwidth. Therefore, it is customary to determine the 0.1dB bandwidth of the amplifiers chosen for the drivers and receivers of the video system. The correlation between 3dB bandwidth and 0.1dB bandwidth will depend on the number of poles in the amplifier. A single-pole amplifier needs to have a reported 3dB bandwidth of 337MHz (~6.5 times more than 51.9MHz) to have sufficient 0.1dB bandwidth. A multiple-pole amplifier (including most modern high-speed amplifiers) must have a 3dB bandwidth of at least 156MHz (~3 times more than 51.9MHz) to ensure the proper 0.1dB bandwidth.

The second crucial characteristic is slew rate. The slew-rate can be calculated from the signal amplitude and pixel rate. To maintain video signal integrity for a pixel rate of 139.5MHz and a swing of ~1V transition during ¼ of a clock period:

Therefore, the video driver and receiver must have bandwidth in excess of 337MHz (worst case) and slew rate greater than 558V/s.

CAT-5 Cable Characteristics
Standard CAT-5 cable consists of 4 twisted pairs of AWG 24 cable, which has a characteristic impedance of 100. The DC resistance is 10/100m with a capacitance of 4.6nF/100m. One important characteristic of SXGA video transmission is high frequency cable attenuation, which increases exponentially over frequency and distance. Figure 1 shows the effects of signal frequency and cable length on the signal attenuation. As Figure 1 illustrates, the losses within CAT-5 cable are significant at 51.9MHz, the calculated signal bandwidth of SXGA.

Figure 1. CAT-5 Cable Attenuation Characteristics

Next: Differential Line Driver Topologies