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| Telephone Lines |
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| The transmission lines used in telephony are of two kinds: open- wire and cable. The open-wire lines are gradually being superseded by cable construction, either overhead or underground, for two principal reasons. In the first place, many more circuits can be accommodated-there is a standard cable containing more than 2,100 pairs of wires, and it is common practice to run 900 pairs in overhead cables on a single pole line; while a pole line carrying 50 pairs of open-wire construction would be a monstrosity. The other reason for preferring cable is it affords much better protection against weather hazards and against electrical interference, both noise and cross talk. |
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| The decibel losses and gains in the telephone lines and other equipment are frequently stated in terms of a logarithmic unit. The decibel is defined as |
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| where W1 is the input power and W2 the output power. If W W2 exceeds W1, the value is positive and is spoken of as again. When W1 exceeds W2 the expression will be negative and indicates a loss. |
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| The decibel is also used as a measure of the amount of power absorbed or furnished by any device, by comparing it with a standard power. This standard is referred to as zero level, and in telephone practice, the power at zero level is 1 mill watt. For example, if the power output of an amplifier is 4 W, its power level expressed in decibel is 4. |
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| When so used, with 1 mill watt as the zero or reference level, the unit is sometimes referred to as the dbm. |
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| When used in the broadcast industry and measured by instruments of specified dynamic characteristics, the readings in dbm are sometimes referred to as being in volume units or nu. |
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| The decibel is a very important unit, and it is used extensively in all branches of electrical communications. The fact that it is logarithmic in nature makes it possible to obtain over-all effects, resulting from a combination of lines, amplifiers, and other equipment connected in sequence, by simply adding or subtracting their respective gains or losses. It is also true that the sensitivity of the ear is nearly a logarithmic function of sound intensity, and a change in sound level of 1 db is just barely perceptible to the average person, which shows further that the size of the unit is well chosen. |
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| The smallest conductor used in standard open-wire telephone circuits is No. 12B and S. gauge. A line of such construction will have a loss of approximately 0.06 db per mile. Cable circuits, on the other hand, make use of conductors not larger than No. 19, and a typical cable circuit with wire of this size will show a loss of about 1.0 db per mile. In this respect, cable circuits are at a disadvantage as compared with open lines, especially for long distances, although recent improvements in the efficiency of telephone instruments, have greatly extended the useful range of cable circuits. |
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| In long-distance lines, losses are so high that satisfactory operation becomes impossible without the use of amplification. Amplifiers for telephone service are known as repeaters, and they must of course function in both directions along the line. Repeater stations are ordinarily installed at 50-mile intervals along the line, except in the "newer wide-band transmission systems, for some of which repeater spacing is as little as 5 miles. Because of the total number of stages of amplification in a long-distance transmission, the performance requirements of telephone repeaters are very severe. Any distortion is cumulative, and any appreciable amount would soon result in unintelligible speech. The demands of high-quality programme transmission for broadcast networks are even more exacting. |
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| One method for obtaining two-way repeater service is shown in the figure. Two amplifiers are employed, one for each direction of transmission. Feedback and oscillation are prevented by the hybrid coils, which are essentially three-winding transformers of balanced construction. A signal traveling from west to east encounters the first hybrid coil, where part of the power enters the west-east amplifier by way of the centre taps on the main windings. |
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| Type 22 telephone repeater |
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| The amplified power is fed into the third winding of the other hybrid coil, and divides there into two equal portions, one passing out on the line to the east, the other into the artificial network. This network is constructed to have impedance properties equal to those of the line over the entire frequency band, so that the combination of line and artificial network is equivalent to a balanced bridge. As a result of this balance, none of the output of the west-east amplifier reaches the input terminals of the east-west amplifier, and feedback around the loop is avoided. |
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| It will be noted that power is diverted and lost at both hybrid coils. Raising the gain of the amplifier by an equivalent amount, which turns out to be 6 db in power, compensates for this loss. |
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