EDN Access -- 08.17.95 Color video travels on twisted-pair cable
Design Ideas:August 17, 1995
Color video travels on twisted-pair cable
National Semiconductor, Santa Clara, CA
Telephone lines and local-area networks commonly use inexpensive twisted-pair cables. Video-system designers can also take advantage of this low-cost cable to transmit composite-color-video signals. Using the circuit techniques in Fig 1, you can transmit video anywhere phone lines exist. Although the circuit has more electronic components than the traditional single amplifier used to drive a coax cable, you can easily justify the additional electronics required to drive twisted-pair cables. Four-wire, twisted-pair cable typically sells for 7 cents/ft, yet RG-59 cable can be priced over 20 cents/ft. If just 500 ft of cable is necessary, the cost difference is $65, which more than covers the cost of a few LM6181 amplifiers.
The system consists of two circuits. The first converts the composite video signal to a differential signal using amplifiers IC1 and IC2. Using a differential signal reduces line loss and distortion that could occur from driving the twisted pair single ended. Converting the signal to differential also removes possible ground-plane errors that occur when there is a difference in the ground potential between two pc boards.
The circuit has a minimum signal gain of two to compensate for the terminations' 6-dB signal drop. You can easily adjust the gain of IC2 by decreasing the value of RG to make up for the line losses caused by various twisted-pair and coax cable lengths. RG serves as a single system adjust and as an optional contrast adjustment for the video system.
In the second circuit, IC3 converts the differential signal back to single ended. This circuit has a gain of two to drive a back-terminated RG-59 coax cable out to a monitor. The video amplifier you choose for this application must have high-output-drive capability. The LM6181 is guaranteed to drive a back-terminated 75ê cable over the full industrial temperature range.
This circuit treats the twisted-pair cable as a transmission line that is back-terminated with 75ê resistors. This termination method is superior to using the 600ê characteristic impedance of the twisted-pair. A 600ê termination results in smearing and blurring caused by the RC time constant of the cable capacitance and the termination resistance. Because an increasing RC time constant degrades sharp signal transitions, this circuit uses standard 75ê terminations to maintain a clear, sharp picture. This circuit does not use shielded twisted-pair cable because of its high distributed capacitance, which contributes to the RC time constant.
No visible difference exists between a reference picture and a transmitted one. Using a Tektronix-type 520 NTSC vector-scope, the measured differential gain and phase of the entire system--which comprises three LM6181 amplifiers, a 100m twisted-pair cable, and a 50-ft RG-49 coax cable from amplifier IC3--was less than 0.5% and 0.6o, respectively. Keeping the current-feedback amplifiers' feedback resistors equal to the recommended value of 820ê gives the op amp its proper frequency compensation. The feedback resistor sets amplifier bandwidth and is always required for proper operation, even in unity gain.EDN Magazine. EDN is a registered trademark of Reed Properties Inc, used under license.