US1840113A - Telephone transmission system - Google Patents

Description

Patented J an. 1932 KENNETH E.LATIME R, on ALnWYcH, LONDON, ENGLAND, ASSIGNORTO "WESTERN ELEC- rarc COMPANY, rncoaroan'rrn, or NEW YORK YORK, N. Y., A CORPORATION .OFNEW TELEPHONE rnA srrrssIoN SYSTEM Application filed, February 28, 1930, Seria1 No. 432,203, and. in Great Britain March 11, 1929.

This invention relates to telephone transmission systems and-n1oreparticularly to methods of loading long transmission'lines to avoid transient distortion and echoes.

' British Patent 181,328 gives the theory 10 the said patent that the loadingmust be reduced as the length of the lineis increased. The present invention may be regarded as a development of the above principles in order to malretheir application more practical and economical. f

Over any large" and populous areasuch asin the large European countries at least two or three different types of cable are required to accommodate economically the type to give satisfactory transmission up to a diii'erent limiting length of cable. 0 In other words, cables of at least two or three different transmission distance ranges are required.

should be chosen as standard andthe "manner in which circuits of the chosen ranges should be loaded, having regard to the quality and efficiency of transmission.

It was found from considering the conditions represented for example by the more important European countries and considering the manner in which circuits of required lengths may be loaded. that the cost of an adequate system of circuits depends very considerably on the standardranges ofcir cuits and their loading. This will be made more clear fromaconsideration ofthe statistics which will be given hereinafter.

a large number of different lengths which vary over a wide range. are loade'dfwith a give minimum cost of the system'as. a whole, for a given'grade of transmission.

According to one feature of the invention transmission of telephonic messages, each The problems encountered are what ranges In accordance with the invention. cables of a loaded telephone transmission system comprises two types of loaded cable circuits whereinthe high grade circuits have double the loading spacing of the other circuits. Thus the loading coils of the high grade or long distance circuits may be disposed at the alternate loading points on the lower grade or short distance circuits.

According to another feature a telephone transmission system comprises a plurality of circuits loaded with different loading spacings wherein the loading coil inductance is varied as the reciprocal of the loading spacings. I

It has been'found'that supposing w, and 2 to be the-costs per mileof circuit of three types of circuit, and a, b, and 0 to be the per-.

centage of the total miles of circuit lying.

between the ranges from zero to X, from X" to Y,and from Y to 3000 miles, substantially similar values represent the cost of maintenance; and'according to a feature of the invention, by suitable lengths of circuits and suitable loading design thesecosts are made a minimum for a sion efiiciency. I

For instance, circuits bearing a ratio of ideally 1:2.2z5 or approximately 1:22:42 are economical standards and in this case the loading spacing of the longest circuits may be advantageously double that of the shorter range circuits. For example, suitable conditions are made by three types Whose ranges are 250. 550 and 1250 miles respectively and the loading spacing for the 1250 mile circuits would be twice that of the 250 mile circuits. The loading spacing may be staggered.

Fig. l 'is a curve for a system of circuits, connecting the number ofcircuits to be provided and the maximum distances'over which they must give satisfactry transmission,

Fig. 2 gives curves with range or length as-abscissaand cost per mile of circuit as ordinates.

-Fig. 3 shows curves relating relative cost of 100 circuits. per mile, (as ordinates) and range or length as abscissasl, and is used to illustratefa principle of the present invention.

given standard of transmis- Fig. 4 shows curves connecting total annual cost per 100 circuits per mile as ordinates with range as abscissa.

Fig. shows curves similar to Fig. 3 except that the ordinates represent relative annual costs for 100 circuits.

From the Formula 4 given in the above mentioned patent, it will be observed that the transients depend upon the time of direct transmission t and a function involving the cut-off frequency and the highest frequency to be transmitted whereas the echoes are dependent solely upon the time of direct transmission. From this, it is apparent that if t is kept constant there is no need to change the cut-off frequency. Therefore, if some method can be devised for raising the velocity of transmission without affecting the cut-off frequency, this would be ideal from an economical aspect since it is usually accepted that the cost of a circuit of given velocity is partly dependent upon its cut-oil frequency.

In contemplating the practical application of the above principles it was found that by choosing lines of considerably different lengths and instead of maintaining a uniform loading spacing designing the coils and coil spacing according to said formula, it was possible to economize in loading coils. Thus, if lines of say 500 mile lengths have a coil spacing of 8, lines with a length of say 1000 miles may have a coil spacing of 28, the inductance coils of the latter case being approximately half the value of those in the former case.

The following statistics reveal how it is possible to determine the type of system most suitable for telephonic conditions.

1 shows the relation between the number of circuits to be provided and the distance over which they must give satisfactory trans mission. This distance will be referred to as the range. The curve is prepared on the basis of the product of the number of circuits and their length. For example. approximately of all the miles of toll circuits in Europe consists of circuits each having a length less than 500 miles. Now the cost per mile of providing a circuit of a given range depends on that range; moreover, it is found as a result of practical experience that this cost is almost a function of the range alone in a well designed system, so that whatever range is required a circuit having a'cost per mile approximately as indicated by a curve between cost per mile and range canbe obtained, sometimes in several alternative ways. Such a curve drawn with range as abscissas and cost per mile of circuit as ordinates is given in Fig. 2, in which A is the curve for side circuits and B the curve for phantom circuits. It may be found that circuits exceeding 3000 miles in length be best provided by means of some specialized form of construction which would embody the use of screened pairs (i. e. shielded pairs of conductors) with repeaters on a fairly long spacing. According to the present invention the bulk of the circuits in the system should be provided with a relatively few different types of circuits, say three. If the first type of circuit be used up to a range of X, the second for ranges between X and Y and the third for ranges between Y and 3000 miles, and if as, y and 2 be the cost per mile of circuit of the three types of circuit, and if a, b and c be the percentages of the total miles of circuit provided by the three types of circuits, respectively, then as before mentioned the total cost S=aa1+by+cz is a measure of the total cost of the system which should be a minimum.

The relative values of S for dilferent values of X, Y are plotted in Fig. 3, in which the abscissas represent the range and the ordinates the relative cost for 100 circuits. Curve a represents relative cost for 100 circuits when X=100 miles; curve 1) cost when X=150 miles; curve 0 cost when X=200 miles; curve d when X=250 miles; curve a cost when X=300 miles; and curve f cost when X=350 miles. These curves have been prepared from actual statistics. It is seen from the curves that S is a minimum when X=250 miles (curve (I) and Y=550 miles (abscissa). Similar constructive Figs. 4 and 5 represent similar curves plotted in terms of annual charges, instead of first cost. In Fig. 4, curve A is the curve for side circuits and B that for phantom circuits. It has been assumed so far for simplicity that the ranges of the phantom circuits are the same as those of their respective side circuits although as a matter of fact the range of the phantom circuits is somewhat greater for the first two groups in most practical systems, and therefore much greater in the case of the long distance group. If the range of the side circuits of the long distance group is 1250 miles, the number and range of the phantom circuits would be adequate to take care of a few circuits which extend to 3000 miles. Due account of this has been taken in preparing the curves. Consequently the ranges to be aimed at in the practical system for the side circuits are 250, 550 and 1250 miles respectively.

Now, as more reliable data is now available regarding the constants to be used in the formulae given in the above mentioned patent, it is desirable to recapitulate these constants in order that the nature of the final solution may be appreciated. The following are the up to date constants The value of T is that for fourwire circuits assuming no echo suppressors. If echo suppressors are used,-higher values ,(say .060 sec.) may be employed, but it is desirable that the above value benot greatly exceeded in order that similar loading may be used for two wire circuits (upon which it is difficult to use echo-suppressors) and in order that a reasonable number of circuits may be able to operate without echo suppressors Using the above constants and the formulae in the above mentioned patent, itis I found that the range of the loading system using 177 n1.H coils on a spacing of 2000 yards, as at present widely adopted, is about 500 miles on a four wire basis. The corresponding range of 0.9 mm. two wire circuits with this loading is about 300 miles. From this it would seem that 0.9 mm. two wire circuits loaded with 177' m.H coils on a spacing of 2000 yards would be suitable for the first type of circuit, while the same typeof circuit on a four wire basis would be suitable for the second type.

Applyingthe formulae to the design of a 1250 mile side circuit the following solution is obtained fc=2960 cycles per L =4.65 henries spacing=224 miles second The phantom circuits having about the same attenuation will have a range of about 3000 miles.

The solution thus approximates to the use of coils of half the usual inductance placed on a double spacing. The estimated range of the 1.3 mm. two wire circuits using this type of loading is about 650 miles. There are thus two possible methods of providing circuits of the second type, with 0.9 mm. four wire circuits with 177 m.H coils, or 1.3 mm. two wire circuits with 83 IILH coils; the second method is more economical and is preferred. It will be noted that the use of the double spacing is one feature of systems designed in the manner described. Existing'systems us-- ing 44 m.H coils on a 2000 yard spacing have a range of 5000 miles on a four wire basis with echo suppressors and 750 miles on a two wire basis with correspondingly increased cost as compared with the proposed system.

This is considered undesirable, particularly as it is not possible to use the full range of the four wire circuits owing to maintenance difliculties. Other features are theabsolute values and the ratios of the ranges of the various types of circuits, which are considerably difierent from systems so far designed.

The 83 m.Hcoils applied to the various long distance circuits would preferably be staggered so that the number of 17 7 m.H

and 83 m.H coils contained in-each "loading coil-case would be'approximately the same.

It istobe understood that slight changes.

ing coilswill he arrived at which will enable." the coils for each type ofcircuit to be located in the same pots. For instance, using T'for the'timeof transmission per unit length and t for the building .uptime per unit length, both'for the lowergrade or short distance circuits, some "value maybe adopted for the spacing on the high grade or long distance circuits whichiwill giye the desired result. u

By giving the values to the time of transmission per unit length andbuilding up time per unit length respectively, of the higher grade circuits, at being aconvenient whole number and ,beingequivalent to a used for the number of loading sections in theFormula (1) of the above- A mentioned. British patent, theloading coils willbe spaced apart on eachtype of; circuit in such a'mannerthat both types of coils will be in the same pots. In choosing a value for n for the preferred embodiment of the invention, wherein double spacing is employed on the high grade circuits, n will have a valueof2.. I i I ,-Whatis claimed is:- i

comprising two types of loaded toll cable circuits for transmission over relatively long distances and relatively short distances, re-

spectively,the circuits intended for therelatively long distances. havingdouble the loading spacing ,of those for the relatively short distances, a

' 2. A-lo aded telephone transmission. sys:

tem comprising. lower. grade circuits having; a t me of transmissionper unit length of IT and buildingup time per unit length of t, andfcompr sing' h gher grade circuits whose lZlIl'lQ/ Of transmission per unit length and bullding up time per unit length are respectively where n is a convenient number.

3. A loaded telephone transmission system A loaded telephone transmission system comprising lower grade circuits having a time of transmission per unit length 0 T and building up time per unit length of t, and comprising higher grade circuits whose time of transmission per unit length'and building up timeper unit length are respectively' andn n where n=2.

4. A loaded telephone transmission system comprising two types of loaded toll cable circuits for transmission over relatively long distances and relatively short distances, re-

spectively, the circuits intended for the relatively long distances having double the load-.

ing spacing of those for the relatively short distances and the loading coils of the longer circuits being disposed at the alternate load ing points of the shorter circuits.

5. A loaded telephone transmission cable system comprising relatively high grade toll circuits and relatively low, grade toll circuits in which the higher grade circuits are divided into two groups, the loading points on one group occurring at even loading points on the lower grade circuits and the loading points on the other group occurring at the odd loading points of said lower grade circuits.

6. A loaded telephone transmission system comprising two types of loaded toll cable circuits for transmission over relatively long distances and relatively short distances, respectively, the circuits intended for the relatively long distances having double the loading spacing of those for the relatively short distances and the loading coils of the longer circuits having approximately half the inductance value of the coils in the shorter circuits.

7. A loaded telephone transmission system comprising a plurality of circuits loaded with different loading spacings, the loading coil inductance varying as the reciprocal of the loading spacing.

8. A telephone transmission system comprising two types of phantom groups of circuits for transmission over relatively long distances and relatively short distances, respectively. and loading means for both the side circuits and the phantom circuits of said groups, the circuits intended for the long distances having double the loading spacing of those for the short distances.

In witness whereof I hereunto subscribe my name this twelfth day of February, 1930.

KENNETH E. LATIMER.

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