A phantom group is composed of three circuits that are derived from two single-channel circuits to form a phantom circuit. Here the phantom circuit is a third circuit derived from two suitably arranged pairs of wires, called side circuits, with each pair of wires being a circuit in itself and at the same time acting as one conductor of the third circuit. The "side circuits" within phantom circuits can be coupled to their respective voltage drops by center-tapped transformers, usually called "repeating coils". The center taps are on the line side of the side circuits. Current from the phantom circuit is split evenly by the center taps. This cancels crosstalk from the phantom circuit to the side circuits.
Phantom working increased the number of circuits on long distance routes in the early 20th century without putting up more wires. Phantoming declined with the adoption of carrier systems.
It is theoretically possible to create a phantom circuit from two other phantom circuits and so on up in a pyramid with a maximum 2n-1 circuits being derived from n original circuits. However, more than one level of phantoming is usually impractical. Isolation between the phantom circuit and the side circuits relies on accurate balance of the line and transformers. Imperfect balance results in crosstalk between the phantom and side circuits and this effect accumulates as each level of phantoms is added. Even small levels of crosstalk are unacceptable on analogue telecommunications circuits since speech crosstalk is still intelligible down to quite low levels.
Read more about this topic: Phantom Circuit
Famous quotes containing the words phantom and/or group:
“It was a favor for which to be forever silent to be shown this vision. The earth beneath had become such a flitting thing of lights and shadows as the clouds had been before. It was not merely veiled to me, but it had passed away like the phantom of a shadow, skias onar, and this new platform was gained. As I had climbed above storm and cloud, so by successive days journeys I might reach the region of eternal day, beyond the tapering shadow of the earth.”
—Henry David Thoreau (18171862)
“Just as a person who is always asserting that he is too good-natured is the very one from whom to expect, on some occasion, the coldest and most unconcerned cruelty, so when any group sees itself as the bearer of civilization this very belief will betray it into behaving barbarously at the first opportunity.”
—Simone Weil (19101943)