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Applying advanced analog and digital techniques to long video-cable equalization

(Editor’s note : We also have on-going series of “dialogues” between these authors; there are links to them at end of this piece, immediately above the “About the Authors” section. There are also two “Desert Island Design” articles, by one of the authors.)

The challenge in transmitting video over distances is to get more information down longer cables faster.The longer distance video images must travel, the longer the cable must be. As cables lengthen, resolution is lost.These losses affect any type of transmission, but they are definitely most complicated at the higher data rates required by video.

Techniques like equalization can help, but distance remains a challenge once information needs to move more than about 300 meters, or about 1,000 feet. For example, Ethernet in all its forms is only specified to go 300 feet. HDMI on an entertainment center is usually limited to a few meters unless a special cable is added. USB peripherals need to be within five meters a desktop PC to avoid losses.Security cameras, an increasingly popular and necessary application today for video, need to be within 300 meters of the console or the picture will suffer.

These distances are not long enough for larger commercial buildings, security system applications, and other innovative uses for distributing video around locations such as airports, malls and stadiums. . Video feeds in these kinds of facilities need distances much greater than in the past. It’s critical to break the 300-meter barrier.

Applying advanced analog and digital techniques to the challenge of long cable equalization involves automatic sensing of video signal parameters and implementing appropriate correction and equalization.

Equalization

The circuitry used to compensate for losses in a system, called equalizers, ensures that a signal coming out of the cable is exactly the same as the signal going in.The equalizer reverses losses and makes the output signal “equal” to the input signal. But cable length challenges remain. The 300-meter distance is simply not long enough for the applications required today.For example, commercial buildings are larger now than ever, and video displays in airports need to reach longer distances.

Equalization is not the only way to deliver videoaccurately at longer distances, but it is the better way. Briefly, the other way is to use a higher voltage at the input to boost a signal at the source end with the assumption that more ‘information’ will get to the furthest end of the system. To send a video signal down a mile of Cat 5 cable (24AWG twisted pair), with 70 dB attenuation measured at the critical 3.58MHz color subcarrier frequency, the required source voltage will reach 3000 volts. That requirement ends up driving 30 amps into a very thin 24 AWG twisted pair, which generates too much heat for the wires.

Equalization is better because it is performed at the receive end and is designed not to ramp voltage, but instead to restore a signal degraded by cable length.For example, a signal needs a 70 dB boost at high frequencies (the complete video signal takes a bandwidth of 5 MHz or so) to deliver an adequate picture.That’s still a factor of 3000, but now it is a gain factor instead of a voltage.Instead of forcing the source end of the cable to a very high voltage, equalization allows the signal to drop by 70dB, and then amplifies the signal to boost it at the output with no stress to the cable and no potential for exploding wires. A single stage equalizer can boost about 14 dB, so if five are placed in a row, it generates the required 70 dB of boost.

This solution is generally reliable, but there remains the threat that ‘noise,’ or unwanted interference from electronic amplifiers, which can swamp the video signal.Noise at the input of a circuit receives the same gain as the signal. To get acceptable video at the far end, equalizer noise must be at least 30dB (about 32 times) smaller than the video at the input to the equalizer. The dynamic range amounts to about 100dB, or about 100,000:1.

Equalizer Circuits

The simple circuit in Figure 1 is a high boost circuit designed to equalize 300 meters of Cat 5 cable. The amplifier is a low-noise op amp with sufficient gain-bandwidth to create the required 14 dB of boost.


Figure 1: Single-stage equalizer for 300 meters of Cat 5 cable.
(Click on image to enlarge)

For shorter transmissions, the amount of boost must be reduced to match the cable length. Figure 2 shows a feedback resistor as a potentiometer.


Figure 2: Variable equalizer for lengths from zero to 1000 feet
(Click on image to enlarge)

To control the equalization from a microcontroller or digital logic, it is necessary to incorporate a digital input for that variable resistance.One solution is to use a digitally-controlled potentiometer (DCP), as in Figure 3 .The ability to build this circuit on one piece of silicon can provide significant advantages in cost, functionality and programmability. 


Figure 3: Variable equalization stage using DCP.
(Click on image to enlarge)

Figure 4 shows a simplified diagram of a complete automatic equalizer.


Figure 4: A complete (simplified) equalization system
(Click on image to enlarge)

There are eight blocks. 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.Trim codes for the equalizers and filters ensure that each chip behaves the same way.

Block 2 is a simple addition.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 two-wire twisted pair, the 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. The automatic gain control (AGC) amplifier, Block 4 , adjusts for variations in the overall video level if as usual, cameras and other sources vary.

The core of the equalizer is responsible for boosting the high frequencies that typically are lost in transmission over the long cable.These stages need to be very low noise, that is, they should not generate much of their own noise) and they need to reject any power supply noise.Block 5 consists of five of the equalizer stages discussed above.

Block 6 is a noise filter. When the equalizer is configured to boost 5 MHz, it may continue boosting all the way to about 10 MHz before beginning to roll off.This extra boost does not enhance the signal; it amplifies noise so it is necessary to include a sharp cutoff low pass filter to reduce the effect of noise above 5 MHz.

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.

Figure 5 shows a high-quality source image after one mile of Cat 5 cable, with and without equalization.Unaided, as the length of the cable increases, color information is lost.As the cable gets even longer, the images smear and text becomes unreadable.


Figure 5: Source image after one mile of Cat 5 cable, without (left side) and with equalization (right side), using the Intersil ISL59605.
(Click on image to enlarge)

A New Solution

Applying advanced analog and digital techniques to the challenge of long cable equalization involves automatic sensing of video signal parameters and implementing appropriate correction and equalization.The complete single-chip equalization solution described above allows picture details to be restored — even after the signal has traveled more than 5,000 feet.

The Intersil ISL59605 represents the application of advanced analog and digital techniques to the problem of long cable equalization.  Its MegaQ approach provides automatic sensing of video signal parameters and implements appropriate correction and equalization.  It integrates all necessary circuits into a single device, requires no user intervention in most applications, and provides full bandwidth video output from a variety of cable types and lengths.

Previous “dialogues” in this series:

Other related articles by Dave Ritter:

About the authors
Dave Ritter grew up outside of Philadelphia in a house that was constantly being embellished with various antennas and random wiring. By the age of 12, his parents refused to enter the basement anymore, for fear of lethal electric shock. He attended Drexel University back when programming required intimate knowledge of keypunch machines. His checkered career wandered through NASA where he developed video-effects machines and real-time disk drives. Finally seeing the light, he entered the semiconductor industry in the early 90's. Dave has about 20 patents, some of which are actually useful. He has found a home at Intersil Corporation as a principal applications engineer. Eternally youthful and bright of spirit, Dave feels privileged to commit his ideas to paper for the entertainment and education of his soon to be massive readership.

Tamara Schmitz grew up in the Midwest, finding her way west with an acceptance letter to Stanford University. After collecting three EE degrees (BS, MS, and PhD), she taught analog circuits and test-development engineering as an assistant professor at San Jose State University. With 8 years of part-time experience in applications engineering, she joined industry full-time at Intersil Corporation as a principal applications engineer. In twenty years, she hopes to be as eternally youthful as Dave.

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