Complete Broadband Video-over-UTP Driver and Receiver Solution for RGB, YPbPr, and More
Signals transported over UTP cable suffer from three major impairments that degrade video quality
- Nonlinear bandlimiting due to the skin effect, resulting in signal dispersion and loss of high frequency signal content. This impairment results in loss of image sharpness and dark streaking.
- Low frequency flat loss due to resistive loss that reduces image contrast.
- Delay skew between the four twisted pairs that stems from the unequal twist rates (lay lengths) which are used to minimize crosstalk between the pairs. Delay skew produces color errors in the received image due to the misalignment in time of the three received signals.
The solution shown in Figure 1 overcomes these impairments by using the AD8122 triple receiver/ equalizer to restore the high frequency content of the video signals while also providing flat gain. The AD8120 triple skew compensating, analog delay line adds delay to the two earliest arriving signals such that the three received signals are properly aligned in time. The AD8147 triple driver provides the required single-ended-to-differential conversion of the source video signals.
Figure 1. Equalized and Delay Compensated UTP Driver and Receiver (Simplified Schematic: All Pins, Connections, and Decoupling Not Shown)
Figure 2. Simplified System Block Diagram for Video-over-UTP System
There are a number of options with regard to drivers besides the AD8147. The AD8146 provides the same functionality as the AD8147 but does not include the dedicated sync-on-commonmode circuitry. The AD8146 is usually used in systems that place the vertical and horizontal synchronization pulses in the blanking intervals of the video signal instead of on the common-mode voltages. The AD8148 is the same as the AD8147 but has a fixed gain of four instead of two, and it can be configured to provide pre-emphasis to drive up to 100 feet of UTP. For systems that require lower power consumption, the AD8133 and AD8134 provide the same functionality as the AD8146 and AD8147, respectively, and consume less power, but they have less bandwidth. Finally, for the lowest cost systems that can run on 5 V, the AD8141 and AD8142 CMOS drivers may be the best choice.
UTP installations vary widely and can cover wide areas, pass through multiple patch bays, and at times have no ground reference. These and other conditions can cause large fluctuations in the received common-mode voltage relative to the local receiver ground reference. Placing a flat gain differential receiver with a wide common-mode range, such as the AD8143, in front of the equalizer can provide up to 21 V of input common-mode range in these demanding situations.
The AD8122 and AD8124 both support coaxial cable as well as UTP cable. The AD8122 can be pin strapped to either mode, and the AD8124 uses a VPOLE control to modify its frequency response to support either cable type.
The following equipment is needed:
- An UXGA video source (laptop computer)
- The EVAL-CN0275-TX-EBZ transmitter evaluation board
- The EVAL-CN0275-RX-EBZ receiver evaluation board
- ±5 V power supplies (two: one for the TX board and one for the RX board)
- Cat-5e cable, 100 feet through 1000 feet in 100 foot increments (Stellar Labs U5E-24-CMR-665, MCM Electronics #24-10510)
- An UXGA video display
The simplified block diagram of the test setup is shown in Figure 11. After connecting the equipment, standard video tests can be used to perform end-to-end testing.
Figure 11. Video-Over-UTP Auto Adjust Test Configuration Functional Block Diagram
|AD8120||Triple Skew-Compensating Video Delay Line with Analog and Digital Control||
|AD8122||Triple Differential Receiver with 300m Adjustable Cable Equalization||
|AD8147||Triple Differential Driver for Wideband Video||