An Overview of Active vs. Passive UTP Technology

The practicality of using UTP (unshielded twisted pair) cable for certain specific high-resolution AV applications, such as long able runs, existing infrastructure, and limited space, has already been established. However, once we've made the initial decision to use UTP, we're left with the question of whether to choose active (powered) or passive (unpowered) technology to implement our decision. The issues we'll confront are balanced signal, impedance matching, and signal loss. Cost will no doubt be a factor in our decision, but as AV professionals, signal quality must be our primary consideration. This Tech Corner will examine the factors that will influence our choice.

Balanced Signal

Due to the way UTP cable is constructed, it is more susceptible to noise than coaxial cable. Common mode noise from such things as electric motors, transformers, fluorescent lighting, and other common sources is present in most installation environments and is a problem due to the UTP cable's lack of shielding. Adjacent pair crosstalk, due to the close proximity of several pairs of wires, is also a potential issue. Minimizing this type of interference requires a special type of signal called differential, which requires a dedicated transmitter and receiver.

Figures 1-4

Essentially, here's how it works. In differential mode (Figures 1 and 2), when the transmitter receives a source signal, it creates a complimentary (mirror) signal and then sends this balanced pair of signals to the receiver. The differential receiver receives the balanced positive and negative input signals and uses the difference between the two to form an output signal. Any induced common mode noise from other sources causes the same noise signal to be induced into both wires equally. The receiver processes this common mode noise in the same way it processes the signal; however, since the amplitude and polarity of the induced noise are the same on both wires, the noise cancels out and only the original signal is output by the receiver. This is true for both active (transmitter/receiver pair) and passive (balun transformer) solutions.

For example, in (Figure 3) the transmitter changes the input signal into a balanced analog signal and sends it along the twisted pair cable. The differential receiver uses the difference between the signals to produce the output signal. The result is a noise free signal.

Impedance Matching

In addition to interfacing the physical connectors of the several types of cable involved, consideration must also be given to the issue of impedance matching. Coaxial cable has an impedance of 75 ohms, video signals have 75 ohm impedance, and displays have 75 ohm input impedance. UTP cable typically has an impedance of approximately 100 ohms (Figure 4). If impedance is not matched, the result is a less than optimum power transfer from the source to the destination. This causes energy to be reflected back to the source and results in ghosting and a poor quality picture. Again, both active and passive solutions will resolve the issue.

Signal Loss

Signal loss, also called attenuation, is measured in dB (decibels). The more attenuation there is, the poorer the signal will be at the receiver. Attenuation is a problem with UTP cable due to the cable's inherent lack of uniformity caused by differences in twist tension and rate, bends in the cable, and other inconsistencies, as well as lack of shielding and variances in insulation type/thickness—all problems that are not an issue with coaxial cable. The use of passive balun transformers does not address these issues. By their very nature, passive transmitters and receivers can provide no help for signal loss. Passive devices actually further attenuate the video signal, making the problem more serious. Far more satisfactory are the superior results obtained by using active transmitter/receiver pairs, which do not share this limitation, and overcome the problem of signal loss by providing amplification, complemented by variable level and peaking controls designed to optimize the signal.

To illustrate this, actual oscilloscope shots (Figures 5 and 6) show a comparison of rise times between passive (balun transformer), and active (Extron transmitter/receiver pair) configurations. The test signal is 1024 x 768 @ 60Hz, with an approximate rise time of 1.5 ns (nanosecond). Scope measurements in Figure 5 (passive) and Figure 6 (active) were taken using the same 100 feet and 300 feet of UTP cable. In each case, the reference signal, provided by a video test generator connected directly to the scope, appears as a thin black line, and the test signal (Ch1) appears as a heavy yellow line. Note the inferior response of the passive solution (poor rise time and low amplitude, resulting in a soft, dim image) using either length of UTP cable. At the same time, in both of the active scope shots the active transmitter and receiver pair were able to compensate for the distance by using the peaking adjustment to improve the rise time (sharpening the picture) and the level adjustment to boost the amplitude of the signal to acceptable levels (brightening the picture).

Figures 5-6

Active Solutions

To provide solutions to these problems, Extron has designed a special class of transmitters and receivers especially suited for twisted pair (TP) applications, with specific features designed to address each issue. These products are compatible with Category 5, 5e, or 6, shielded or unshielded cable and are available in configurations suitable for BNC or 15-pin HD video connectors, with 3.5 mm captive screw and/or RCA audio connections. In addition to converting the video and audio signals to differential analog, which is suitable for longer distances, they also provide a mechanical (connector) and electronic (impedance) interface between dissimilar cable types. Finally, they compensate for signal loss by providing amplification, with level and peaking controls to adjust for cable length.

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