A unit of power ratio used for telephone transmissions by American Telephone & Telegraph until about 1922 and the British Post Office until 1923. Abbr., m. s. c. One mile of standard cable is the ratio between the powers of an 800-hertz signal at the two ends of a loop of cable one mile in length having¹
These values were taken from a mile of B&S #19 copper telephone wire.
In the United States, the m. s. c. had the same values, except that weight, inductance and leakance were not defined.
Because the mile of standard cable is frequency-dependent, it cannot actually be converted to units of power ratio that are not frequency-dependent, such as the decibel. Nonetheless, 1 m.s.c. is approximately 0.9221 decibels.
1. T. E. Herbert and W. S. Procter.
Telephony. Second Edition.
London: Sir Isaac Pitman and Sons Ltd., 1934.
Volume 1, page 811.
It is customary to say that entirely satisfactory local telephoning is done if a subscriber may talk to another with a quality equal to that through ten or twelve miles of standard cable (No. 19 B. & S.-gage copper, with an electrostatic capacity of 0.06 microfarads per mile). Long-distance transmission is practical for the general public if it is as good as that through 30 miles of such cable. Speech is possible through much more than 30 miles, but it is comfortable for normal persons at that value.
S. G. McMeen.
Notes on an Automatic Telephone System Recently Built in San Francisco.
Engineering News vol. 67, (January 4, 1912). Page 27.
The “mile of standard cable” has been used in telephone engineering in this country for over twenty years, and during that time has been adopted in other countries, as the unit for expressing the transmission efficiency of telephone circuits and apparatus. In the present state of the telephone art, this unit has been found, however, to be not entirely suitable and it has recently been replaced in the Bell System by another unit which for the present, at least, has been called simply the “transmission unit.”
For a given loudness of spech sounds entering the transmitter at one end of the circuit, the loudness of the reproduced sounds given out by the receiver at the other end can be varied by changing the amount of standard cable in the circuit. Also, the amount of cable in the circuit can be used to express the ratio of the power of the reproduced sounds to that of the impressed sounds. Due to the dissipation of electrical power in the cable, this ratio and consequently the loudness of the reproduced sounds become less as the amount of cable is increased and greater as the length of cable is decreased.
This circuit then became the measuring or reference system for engineering the telephone plant and the “mile of standard cable” became the unit in which the measurements were expressed. This circuit was used to set the service standards in designing and laying out the telephone plant. Thus, the reproduction obtained over this circuit with a length of cable of about twenty miles was found suitable and practicable for local exchange, that is, intracity service, and that corresponding to about thirty miles for toll or intercity service.
Any telephone circuit was rated by its comparison with the standard circuit. This comparison was on the basis of a speaker talking alternately over the circuit to be measured and the standard circuit and a listener switching similarly at the receiving ends, the amount of cable in the standard circuit being adjusted until the listener judged the volume of the sounds reproduced by the two systems to be equal. The number of miles of cable in the standard circuit was then used as the “transmission equivalent” of the circuit under test.
W. H. Martin.
The transmission unit and telephone transmission reference systems.
AIEE Transactions, vol. 42 (June 1824) pages 797-798.
In order to express numerically the quality of various pieces of telephone apparatus with respect to the transmission of speech, a certain standard testing circuit has been agreed upon and is universally used in this country and in England. This circuit consists of a standard transmitter, receiver, induction coil, and condenser at each end of the line, all of definitely specified “Bell” types, and connected in a standard agreed manner to each other and to a “standard” artificial cable through repeating coils.
The standard cable is represented by a sectional arrangement of resistors and condensers equal to the resistance and the capacitance of 32 miles of a certain agreed telephone cable. At the present writing the standard cable for purposes of measurement is defined as one having a linear resistance of 88 ohms per loop mile; linear capacitance between wires, 0.054 m.f. per loop mile; linear inductance and linear leakage, zero. Artificial-cable boxes for testing are made by a number of manufacturers. The cable units are conveniently arranged with switches so that desired lengths can be readily cut in or out of circuit. Such artificial cables are often designed with linear constants different from those of standard cable, in which case conversion factors are used to change tested results into miles of standard cable.
Measurements are made with this circuit by compensating losses in speech transmission with an equivalent length of standard cable (Fig. 369). If it is desired to know the transmission equivalent (efficiency) of a certain telephone circuit, as compared to that of the standard circuit, the latter is handicapped by adding lengths of the standard cable until the transmission through both circuits is the same. If 6 miles of standard cable is required in the standard circuit to get a balance, then the circuit under test is said to have a loss of 6 miles of standard cable. In cases where the circuit under test has a higher transmission efficiency than the standard circuit, the artificial standard cable is added to the circuit under test. If 6 miles of this standard cable is required for such a balance, then the circuit under test has a gain of 6 miles of standard cable.
The same test circuit is used for comparing the efficiencies of particular pieces of telephone apparatus with the corresponding pieces of apparatus used in the standard circuit. For example, if a transmitter is to be tested, it is substituted in the test circuit in place of the standard transmitter, and the quality of transmission is compared to that of the latter. If it is necessary to add 2 miles of artificial cable to the test circuit when using the standard transmitter, in order to bring the speech quality down to that of the transmitter under test, the latter is said to be below standard by 2 miles of standard cable.
Experimental Electrical Engineering Manual for Electrical Testing for Engineers and for Students in Engineering Laboratories. Vol. 1. 3rd edition.
New York: John Wiley and Sons, Inc., 1922.
[Some examples of the use of the m.s.c. as a standard]
Tentative Standards of Telephonic Transmission. (Telephone Engineer, 14. No.6. pp. 277-279, Dec., 1915. Electrician, 76. pp. 464-466, Dec. 81, 1915.) —This paper embodies the recommendations of the Standardisation Committee appointed by the Independent Telephone Association of America.
The standard of transmission adopted is that obtained between two subscriber's sets connected together through repeating coils and a variable length of standard artificial cable having a resistance of 88 ohms per loop mile and a capacity of 0.06 mfd. per mile. The limit of satisfactory commercial transmission has been fixed as that obtained with the standard terminal apparatus and 32 miles of the cable above mentioned.
The total h.f. loss in the central office between two local subscribers connected to that office shall not exceed 1.5 miles of standard cable.
The total h.f. loss in the central office equipment on a local or inter-office trunk connection shall not exceed 1.5 miles of standard cable. This shall include all losses from the outgoing trunk multiple jack through the trunk circuit to the plug or jack in the position where the trunk terminates, the trunk loop having zero resistance.
The total h.f. loss in the long-distance switchboard equipment between two non-loaded open-wire long-distance lines shall not exceed 1 mile of standard cable.
The h.f. loss in central office equipment on a long-distance switching trunk circuit shall not exceed 1 mile of standard cable.
The h.f. loss in the long-distance connection due to the effect of bridging the operator's set for monitoring a long-distance connection shall not exceed 1 mile of standard cable.
The transmission efficiency of C.B. subscribers' station equipment shall be such that no loss is introduced when two sets of the type under test are substituted for the two sets of the test circuit, with 4 miles of standard cable in each subscriber's loop.
Science Abstracts. Section B. Electrical Engineering. vol.
London: E. & F. N. Spon, 1916.
SYSTEM REFERENCE STANDARD.-Due to the radical differences which exist between the problems and requirements of power and telephone transmission, the same terms used for each field have grown to have different meanings. In telephone work, volume efficiency is usually expressed as a “loss below” or as a “gain above” some standard condition, and as a result the terms “efficiency” and “loss,” or “gain,” have essentially the same meaning. This loss or gain has ordinarily been expressed in miles of “standard cable” (ls.)—due to the fact that the number of miles or distance to which a person can talk with satisfactory volume is, when other things are equal, the logical measure of the efficiency of any telephone system.
It is customary to measure the volume transmission efficiency of any telephone system by comparing it with the volume obtained over the System Reference Standard. This standard is shown below:
This System Reference Standard is seen to consist essentially of two common battery subscribers' circuits directly connected to each other by repeating coil cord circuits and a variable length of trunk of standard cable.¹ The System Reference Standard is therefore electrically similar to a system in which two common battery subscribers on zero loops are talking to each other over a non-loaded cable trunk. The circuit is simplified as much as possible by the omission of all apparatus such as ringers, supervisory relays, etc., which is not necessary from a transmission standpoint. The transmitters and receivers used in the System Reference Standard are required to meet certain volume and quality requirements, and together with the repeating coils and induction coils must have certain definite electrical constants at telephonic frequencies.
The comparison of the volume efficiency of any given system with that of the System Reference Standard is made by adjusting the amount of standard cable in the System Reference Standard until the two observed volumes are equal. The number of miles of standard cable which have to be inserted in the System Reference Standard in order to make the volume of sound equal to that obtained over the system under test is said to be the transmission equivalent of the system in question.
The limiting transmission equivalent which it is feasible to impose on a commercial telephone system depends upon a good many factors. For local service an equivalent of eighteen to twenty miles is usually regarded as reasonable. On the other hand, for trunk or long-distance service, the equivalent may be permitted to go as high as thirty miles. This latter figure is usually regarded as being approximately the upper limit of commercial transmission.
The transmission equivalents of typical trunks may vary from ten to twelve miles—the remaining eighteen miles or so being allowed for terminal losses at the two ends of the circuit.
Although the standard mile, or mile of standard cable, is used for the measurement of telephonic volume efficiency almost universally, the logarithmic attenuation of a uniform line is sometimes taken as a measure of efficiency or loss. Unit attenuation is equivalent to the loss in that part of a uniform line of infinite length in which the current has decreased to 1/eth of its initial value. The relation between miles of standard cable (ls) and attenuation units (A) is as follows:
where R is 88 ohms, and C = .054 × 10⁻⁶ farads, and f is the frequency under consideration. Also ω = 2πf.
1 Standard cable is cable with uniformly distributed constants of R = 88 ohms resistance per loop mile and C = .054 × 10⁻⁴ farads mutual capacitance per mile. The leakage G and the inductance L are ordinarily taken as zero—although it is the practice of some countries to assume an inductance of .001 henry per loop mile.
Richard Glazebrook, editor.
A Dictionary of Applied Physics. vol. 2.
London: MacMillan and Co., 1922.
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