Unshielded twisted pair (UTP)—such as Category-5e (Cat-5e)—originally designed to carry local area network (LAN) traffic, has become an economical solution for many other signaling applications because of its considerable performance and low cost advantages. These applications are systems for transmitting broadband video signals. They use 3 pairs of 4 twisted pairs to transmit red, green, blue (RGB) computer video signals or brightness and two color difference (YPbPr), high-definition component video signals. The required horizontal and vertical synchronization pulses can be embedded in the video signal blanking interval, and these pulses can also be transmitted as common-mode differential signals in 3 twisted pairs. These systems often include video crosspoint switches and are used to distribute a small portion of the video source to many displays, such as digital signage, or a large portion of the video source to a few displays, such as a keyboard-video-mouse (KVM) network. .

Signals transmitted over UTP cables are mainly affected by three impairments that cause video quality to degrade

• The skin effect causes nonlinear band limiting, resulting in signal dissipation and loss of high-frequency signal content. This damage results in a loss of image sharpness and the appearance of dark streaks.
• Resistive loss brings low-frequency flat loss, causing image contrast to decrease.
• The twist ratios (trace lengths) used to minimize crosstalk between pairs are different, resulting in delay skew between the 4 twisted pairs. Due to the time alignment error in the received 3-channel signals, delay skew causes color errors in the received image.

The solution shown in Figure 1 overcomes these impairments by using an AD8122 triple receiver/equalizer, restoring the high-frequency content of the video while providing flat gain. The AD8120 three-way skew compensated analog delay line adds delays to the two first arriving signals so that the three received signals are correctly arranged in time. The AD8147 triple driver provides the single-ended to differential conversion required for video source signals.

The video transmission system shown in Figure 1 uses RGBHV video signals, where RGB represents red, green, and blue video signals, and HV represents independent horizontal and vertical synchronization pulse signals. Therefore, a total of 5 signals are transmitted through 3 pairs of twisted pair cables.

Video system performance is best described in the time domain, and the most important specification is the step response settling time. The transition between two pixels in a video display is usually a step function, with each pixel lasting a specific period of time. Ideally, the video's step response should settle in a fraction of the pixel time (about 6 ns for a UXGA at 60 Hz) and should have a negligibly small error from the final value (less than full Range is about 46 dB, or 3.5 mV). While certain frequency domain performance metrics are important, what is most important is how these metrics affect the video signal in the time domain. For example, the system bandwidth must be high enough to produce a step response with a rise time short enough to meet settling time specifications. However, bandwidth alone is not enough because ringing, overshoot, and sluggish response, even the short rise times associated with wide system bandwidths, can produce significant settling errors. The simplified block diagram of the system is shown in Figure 2.

driver

RGB signals typically originate from a 75 Ω single-ended, source-to-termination voltage source and require a 75 Ω load termination. At the load, the amplitude of a properly terminated signal typically varies between 0 mV and 700 mV. To transmit an RGB signal over UTP, the signal is converted from single-ended mode to balanced (differential) mode and then amplified by a factor of 2 to account for the 6 dB loss due to UTP source and load termination. This is easily accomplished using a three-channel differential driver such as the AD8147.

The AD8147 provides additional features to encode horizontal and vertical sync pulse signals at TTL logic levels , three output common mode voltages (V _{OCM ) according to the following equation:}

in:

K represents the peak deviation between the mid-supply voltage (V _MIDSUPPLY ) and the common-mode pulse voltage. V _SYNC and H _SYNC are unit weighted terms, +1 for logic 1 and −1 for logic 0. This encoding scheme produces a net AC common-mode voltage of zero, thereby minimizing common-mode electromagnetic radiation from the cable.

The driver evaluation board contains all the information needed to implement single-ended to differential mode conversion and synchronized pulse encoding, including adjustment of K.

receiver

The skin effect of UTP cables produces transmission losses that increase with frequency, causing the received signal to appear rounded and spread out, and simple cable resistance causes flat resistive losses in the cable. Figure 3 illustrates these effects by comparing the step response of a 300-meter-long UTP full-swing video to the step signal of the input cable.

The AD8122 three-channel equalizer performs differential to single-ended mode conversion, providing high common-mode rejection and compensation for these signal impairments. Figure 4 shows the corrected step signal at the equalizer output, settling to 1% error in less than 70 ns. Note that the time scale in Figure 4 is in nanoseconds.

For the frequency domain, Figure 5 shows the frequency response of Cat-5e cable lengths from 100 feet to 1000 feet, in 100-foot increments, where band limiting effects and flattening losses are evident.

How the AD8122 effectively recovers the high-frequency content of the received signal as well as flat losses can be seen by comparing the balanced frequency response of the AD8122 output in Figure 6 with the unbalanced frequency response in Figure 5. For a 1,000-foot (300-meter) cable, the unbalanced loss exceeding 50 dB at 60 MHz is 3 dB after optimization with the AD8122 equalizer.

For the last impairment, the AD8120 triple delay line corrects the time skew between the 3 twisted pairs and provides a gain of 2 to drive the video signal transmitted to the display over a double-terminated 75 Ω cable.

The receiver evaluation board includes the AD8122 and AD8120 and all supporting circuitry required, including 5 potentiometers to manually adjust high frequency boost, flat gain, and 3 delay lines. In addition, a serial interface for optional serial control of the AD8120 is provided.

in conclusion

Image quality at the far end of a video distribution system is important. Image quality is determined by the time it takes for the step response to build up to a difference of 3.5 mV from the final value. When this value exceeds a certain fraction of the pixel time, image quality begins to suffer. Figure 7 shows an extreme example of an image received over 300 meters (1000 feet) of Cat-5e cable without equalization or deskew. The black smearing in Figure 7 is extremely severe, the step response is sluggish, and the time skew causes color imbalance. The fully corrected image is shown in Figure 8.

Actual photos of the transmitter and receiver evaluation boards are shown in Figures 9 and 10 respectively. For a complete design support package for the EVAL-CN0275-RX-EBZ Transmitter Evaluation Board and EVAL-CN0275-TX-EBZ Receiver Evaluation Board, including schematics, layout files, and bill of materials, please visit www.analog.com/ CN0275-DesignSupport .