Basic Cable: Choosing the Right Cables for Your System Page 3

A coaxial cable's impedance - its resistance together with its inductance and, especially, capacitance - plays a role in its performance. Every conductor has some resistance, which impedes signal flow equally at all frequencies. Capacitance, which stores energy in an electrostatic field, arises when two conductors are close together but separated by a dielectric. A coaxial cable is thus a capacitor. Cable capacitance interacts with other impedances in the connection to create a low-pass filter that gradually attenuates the signal above a certain frequency. This is mainly a concern for analog audio connections, where a combination of high output impedance in the source component, high cable capacitance, and a long run could result in audible treble loss. This sort of problem is rare in modern audio and home theater systems, however.

For video and digital audio signals, which have extremely short wavelengths, what's known as the characteristic impedance is important. (It doesn't matter for analog audio connections.) Determined by the ratio of inductance to capacitance, the characteristic impedance should be the same for the output, the cable, and the input at the other end to prevent reflections that can cause signal cancelations. The standard characteristic impedance for video and digital audio connections is 75 ohms.

The consequences of cable losses are different for analog and digital signals. Analog signals tend to deteriorate gradually as the loss increases, whereas digital connections fail abruptly when the receiving device can no longer distinguish the pulses that comprise the signal well enough for it to be reconstructed accurately.

A cable's terminations are as important as its construction. Cheap cables usually have cheap connectors, which can lead to intermittent or failed connections, EMI pickup, or hum. The connectors on high-quality cables are made of a solid, conductive material, typically brass plated with nickel or gold to resist oxidation and corrosion. Connectors are joined to the cable using solder joints or compression fittings. Good cables have clean, complete solder joints or tightly compressed, very flat fittings and connectors that fit securely, but not too snugly, into the jacks to assure maximum surface contact and to prevent them from pulling out to easily.

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