# Video Transmission Solution

Author:
Dave Ritter, Senior Designer and Tamara Schmitz, Ph.D., Senior Principal Applications Engineer, Intersil Corporation

Date
06/01/2011

PDF

Click image to enlarge

Figure 1: Single Stage Equalizer for 300 meters of Cat 5 cable

What happens if the cable is shorter than 300 meters? For shorter transmissions, we would need to reduce the amount of boost to match the cable length. Figure 2 shows a simple approach—make the feedback resistor a potentiometer.

But we may wish to control the equalization from a microcontroller or digital logic, so it would be nice if there was a digital input for that variable resistance. We can solve that problem using a digitally-controlled potentiometer (DCP), as in Figure 3.

Building the Whole System The ability to build this circuit on one piece of silicon gives great advantages in cost, functionality and programmability. Figure 4 shows a simplified diagram of a complete automatic equalizer. There are eight blocks, so let's take it a stage at a time and investigate the function of each. The loop control and user interface are implemented with a modest number of gates of digital control logic (Block 1). The logic determines how much the signal needs to be equalized and how to adjust the equalization. Since no two chips are exactly the same, trim codes for the equalizers and filters ensure that each chip behaves the same. Block 2 is a simple addition that makes installation easy. Since the video cable in this case is twisted pair, it is possible to swap the wires, effectively inverting the signal. This circuit notices if the wires have been switched and inverts the signal to compensate. Also, since the video cable is a twisted pair (2-wires), our input stage must be differential. Block 3 shows the differential input amp that receives the signal from the twisted pair wire. At this point the signal has differential video (the intended signal) and common mode noise (corrupting signal from power lines and other nearby equipment). The differential input amp removes the common mode noise, leaving only the video signal. This video signal is expected to be a 1V peak-to-peak signal. Of course, cameras and other sources may vary. The automatic gain control (AGC) amplifier, Block 4, adjusts for variations in the overall video level. The core of the equalizer is responsible for boosting the high frequencies that were lost in transmission over the long cable. These stages need to be very low noise (they should not generate much of their own noise) and they need to reject any power supply noise (they should block noise from outside sources). Block 5 is simply five of the equalizer stages we discussed in the previous section. Block 6 is a noise filter. When the equalizer is configured to boost 5MHz, it may continue boosting all the way to about 10MHz before beginning to roll off. This extra boost does not enhance the signal, it just amplifies noise! Therefore, we need to include a sharp cutoff low pass filter to reduce the effect of noise above 5MHz. Once the signal travels through the noise filter, there is one more op amp stage (Block 7) before reaching the output. This output amplifier provides sufficient gain and drive current to interface with other video equipment. Finally, Block 8 is an analog-to-digital converter. This circuit samples the output and feeds it back into the digital control circuitry. If anything goes wrong, the control circuitry can adjust the other blocks in the equalizer to fix it. For example, if the output starts to droop, the gain amplifier (AGC) could be turned up to compensate.

Visual Results Figure 5 shows a high quality source image after 1 mile of Cat 5 cable, with and without equalization. Remember that an unaided video signal can travel down around 300 meters of cable. As the length of the cable increases, the color information is lost. As the cable gets even longer, the images smear and text becomes unreadable. With the complete single-chip equalization solution (ISL59605), just described, all the picture details have been restored even after the signal has traveled over 5,000 feet (>1500m). Conclusion The ISL59605 (MegaQ) represents the application of advanced analog and digital techniques to the problem of long cable equalization. It provides automatic sensing of video signal parameters and implements appropriate correction and equalization. It requires no user intervention in most applications and provides full bandwidth video output from a variety of cable types and lengths. www.intersil.com

Jan 23,2023

Jan 4,2023

Dec 31,2022

Jan 1,2023