Information Technology Reference
In-Depth Information
voltage and the black, ground. The 4-wire cable continued to be used long after the
ringing voltage was moved to the tip and ring and the ground was deleted. For a
time in the 1960s, the yellow and black were used to supply power from a wall-
mounted transformer to the lamp that lit the dial of the famous “Princess” phone.
This was later eliminated when the lamp power was also supplied on the tip and
ring wires. When a second phone line was to be added, the yellow and black wires
were generally available, and that's where it went. Unfortunately, the nonpaired 4-
wire cable can generate serious crosstalk between the pairs, and you may faintly
hear the other person when you are on a different line. This may be an advantage
for parents, but is a problem for LAN circuits and must be avoided.
Nonpaired Shielding
Nonpaired cables may be shielded with foil or braid shields, as with twisted-pair
cable. A typical use for a shielded, nonpaired cable is for microphone cable. Even
flat cable may be shielded. Such cables are useful in meeting electromagnetic inter-
ference (EMI) requirements of regulating agencies.
