. The Bell System technical journal . he telephone, loudness of talking,battery supply, and transmitter efficiency. The subscriber may betalking over a long distance circuit from a distant city, in which casethe loss of the toll line further attenuates the received waves. Figure1 t shows that the range of outgoing speech volumes as measured by avolume indicator at the transatlantic switchboard at New York isnearly 40 db for terminal calls. When via calls and variation in volume * The term volume will be used through the rest of this paper to designate thisquantity and not as synonymous with lo
. The Bell System technical journal . he telephone, loudness of talking,battery supply, and transmitter efficiency. The subscriber may betalking over a long distance circuit from a distant city, in which casethe loss of the toll line further attenuates the received waves. Figure1 t shows that the range of outgoing speech volumes as measured by avolume indicator at the transatlantic switchboard at New York isnearly 40 db for terminal calls. When via calls and variation in volume * The term volume will be used through the rest of this paper to designate thisquantity and not as synonymous with t This curve is plotted on so-called probability paper, in which the scale is suchthat data distributed in accordance with the normal law will produce a straight line. THE COMPANDOR 317 of the individual talker are taken into account, it is even greater than40 db. Volume Range of a Telephone Circuit There are two limits on the range of volumes which a system cantransmit. The upper limit of volume is set by the point at which. -25 -20 -15 -10 -5 0 DECIBELS RELATIVE TO MAXIMUM VOLUME Fig. 1—Volumes of 950 local subscribers at New York transatlantic switchboard, January-April, 193 L overloading appreciably impairs the signal quality or endangers thelife of the equipment. It is an economic limit set by the cost of build-ing equipment of greater load capacity. The lower limit of volume isset by the combination of the amount of attenuation and the amountof interference in the system such that the signal should not be appre- 318 BELL SYSTEM TECHNICAL JOURNAL ciably masked by noise. This also is ordinarily an economic problemdepending on the cost of lowering the attenuation or of guardingagainst external interference. In some cases, however, this limitationis a physical one. A striking case is that of radio transmission in whichwe have no means of controlling the attenuation of the electromagneticwaves in transit to the receiving station. They may arrive at levelsbelow those
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