US1617392A - Carrier-wave-transmission system - Google Patents

Description

J. s. JAMMER Filed Maren 4, 1924 CARRIER WAVE TRANSMISSION SYSTEM L1' La Fe'b. 15 1927.

IIIII tems.

y Patented eb. A15,1927.

UNITED ,STATES JACOB SUTER JA'MMEB, 0F NEW YORK, N. Y., ASSIGNOR T0 WESTERN ELECTRIC COM- PANY, INCORPORATED, OIEl NEW YORK, N. Y., A l(.'IORPORATION OF NEW YORK.

CARRIER-WAYE-TRANSMISSION SYSTEM.

Appncaaon fiiea March 4, 1924. serial No. 696,775.

The present invention relates to the regulation ,oftransmission in an electrical system of distribution, and has part1cular applicatlon to carrier Wave transmission sys- It is an object of the invention to enable regulation of the transmission through such a system in an eflicient and convenient manner. v

'A feature of the invention comprises transmission controlling devices arranged to give the desired transmission level for the entire channel vor line and to permit changes in the transmission level to be readily made sate for line changes.

This will be more clearly understood if it be assumed that a certain over-all transmissionlevel is to be maintained between the two distantly separated low frequency terminals of a carrier wave channel. Such a channel, as is well known, includes a high frequency transmission section and portions over which low frequency currents, such as speech or other signal currents are transmitted. Variations in the transmission .characteristics of the channel (due to weather or other conditions) are compensated for by potentiome- `ters vor other adjustable transmission control 30 devices. These control devices are commonly situated at such apoint kin the system that they vary the high frequency load on the detecting and receiving circuit. Over a limited range of load values` changes in the high frequency load produce correspondingl changes in the detected output current. For other loads, however, considerablyl outside such range. an altogether different relation exists between the changes in high frequency load and the detected output. For example, for a certain region of load` a change in the high frequency load corresponding to a change of one mile of standard cable in the transmission equivalent may produce a change in the detected output also corresponding to a one mile standard cable change in lou7 frequency equivalent. With a differient value of load, however. it has been found in practice that a one mile change in high w frequency load produced as much as a thirtymile change in thesdetected output,lwhile at still othe'values of high frequency load. a thirty mile Vchange in high frequency load was required to produce aone-mile change in detected output.

' for regulation purposes such as to compen- It is seen, therefore, to be advantageous to "fix the hlgh frequency load at such' value that the potentiometer or other input control all transmission equivalent of a system to some specific value. Forinstance. if a channel has a zero equivalent, the low frequency detected arrival current has the same amplitude as the current which is put into the channel at the distant terminal. This means, of course, that the gain introduced by the various circuit elements ofthe channel just make up for the transmission losses. Since the transmission characteristics of lines differ widely among themselves and the characteristics of the same line may differ Widely with conditions` and also the channel transmission characteristics differ widely'among channels on the same line, it will often, if notv generally, happen that the transmission level of a channel cannot be brought to the required value by changing,

the high frequency load on the receiver withi out carrying the load outside the relatively small region discussed above in which the desired relation exists between load change and change in detected output. 4This diiiculty is overcome in accordance with the invention by providing a low frequency transmission control element which is adjustable to change the strength of the received detected current within relatively wide limits. This element, being in the low frequency circuit, does not disturb the high frequency portion of the circuit, so that with the arrangement provided by the invention, the desired fine adjustment can be effected by the high frequency input control means.

These and the other features and objects of the invention will be more clearly understood from the following detailed descripdrawing.

Fig. 1 is a schematic circuit drawing of so much of a carrier wave system as is necessary to an understanding of the invention.

Fig. 2 shows curves illustrative of the action of the regulating devices of the invention.

In Fig. 1 there is shown the west terminal of a multiplex carrier transmission system. This System is of the general type shown in Fig. 42 of the paper by Colpitts and Blackwell on Carrier current telephony and telegraphypublished in vol. 40 of the Journal of the American Institute of Electrical Engineers in a series of installments beginning in April, 1921, and reference may be had to that figure for a more detailed disclosure of the actual circuit arrangements of the various parts of the system. In Fig. 1 of the drawing, a carrier line ML is shown terminating in two grouping filters GF1 and GF2 which serve to separate the oppositely directed carrier waves. For example, all of the carrier waves employedfor transmission from the station shown in Fig. 1 to the main line ML may be comprised in an upper frequency range and these are selectively transmitted by the filter GFI. The carrier waves received at the station in Fig. 1 from .the main line ML maybe comprised in a lower range to which the filter GF2 is made selective. If preferred, the separation between transmitted and received waves may be effected by the used of a hybrid coil as shown in Fig. 42 of the Colpitts and Blackwell paper.

In Fig. 1, three transmitting channels and three receiving channels are indicated schematically. The transmitting filters comprise a modulator M andl a band filter MBF for transmitting from the ymodulator to the line the desired components of the modulated wave such, for example, as the carrier and one of the side bands. Each receiving channel, likewise comprises a band filter DBF for selectively transmitting the components intended for the particular channel and a demodulating circuit D for deriving from the received waves the low frequency signal components such as speech. In the drawing, the apparatus belonging to each channel is designated by a suitable subscript.

The terminations of the low frequency lines adapted for connection with the carrier system are not shown in detail in Fig. 1 but are merely indicated by the jacks 1 to 6 inclusive and the respective line branches, such as L1 to L3 indicating the transmitting line branches andLl to L3 indicating the receiving line branches. The association of these low frequency line branches with the low frequency telephone lines may be made in any suitable manner, such, for example,l

as that indicated in Fig. 42 of the Colpitts and Blackwell paper.

One of the receiving channels is illus- -pad 10 may be omitted in some cases.

trated more in detail in Fig. 1 and this will now be described in connection with the devices for regulating the transmission in the channel in accordance with the invention. The detecting circuit\generally indicated at D8 comprises the high-frequency pad 10, a potentiometer 11, the amplifier 12, the detector 13 and selecting circuits 14 and 15 for separating the detected speech from the in the U. S. Patent to Venues, No. 1,472,501,

issued October 30, 1923. i

The operation of this portion of the circuit will be clear from the drawing and from the above designation of the various l portions of the circuit. The high frequency Waves selectively transmitted to the filter DBF 3 are impressed on the amplifier 12 and the detector 13 with a desired amplitude as determined by the settings of the hgh frequency-pad 10 and the potentiometer 11. As explained hereinafter the high frequency The detector 13 serves in well known manner to producein its output circuit low frequency variations corresponding to the signal being transmitted over the channel, such, for example, as speech variations. selectively transmitted through the low pass filter 15 which is so designed as to prevent passage of currents of higher frequencies such as the carrier frequency. The speech currents after traversing the filter 15 pass through the low frequency pad 1G to the receiving jack 17 from which they may be impressed on the desired low frequency line by the insertion of the proper plug into the jack 17.

The currents of the carrier frequency appearing in the output of the detector 13 are selectively transmitted through the high pass filter 14 to the rectifier 18. This filter 14 is designed not to pass currents of speech frequencies. Only current of the carfier frequency is therefore impressed on the detector 18 and rectified high frequency current appearing in the output circuit is indirlhese are cated by a suitable meter 19 and is utilized to actuate the relay 20." The relay 20 may be used in accordancewith known practice to transmit a call signal to the operator at the receiving terminal at the station shown in Fig. 1 or may be relayed to a further point. For example,` if the operator at the distant station interrupts the normally transmitted carrier wave, relay 20 becomes deenergized and closes a circuit through the calling lamp 2l to indicate to the operator ofthe station in Fig. l that a call is incoming on channel 3.

Itv will be understood that each of the other' receiving channels may be arranged in substantial duplicate of thei channel 3 so that each receiving channel will haveits individual high frequency and low frequency pads corresponding to 10 and 1G. its individual amplifying, detecting and rectifying devices. calling relay and calling lamp. Each of the transmitting channels is adjusted and 'supervised either by an attendant or in any known manner so that the output current is maintained at a constant value. The only variations in the received current strength are therefore those due to variations in transmission characteristics of .the system.

As indicated above. the strength of the detected speech currents will vary in some non-linear relation with the high frequency load impressed on the detecting circuit. Re

ferring to Fig. 2. for example, the full linecurve A may indicate the relation between the high frequency arrival current at the terminals of the band filter DBF3 and the strength of the detected speech currents at the terminals of jack 17 as compared with the strength of the speech currents impressed on the transmitting jack at the distant ter min-al of this station. That is. values of low frequency equivalent above the zero axis indicate a gain in overall 'transmission between the transmitting jack at the distant station and the receiving jack 17 atl the station of Fig. l. while values of .low frequency equivalent belowthe Zero axis indicate an over-all loss. As the high frequencv load impressed on the detector'is varied in any manner due to the high frequency line pad l0 or the potentiometer 11 or the transmission line ML, the low frequency equivalent of the Asystem is changed by the relation shown by the curve A.

The main function of the potentiometer 11 is to secure a fine adjustment of the low frequency output detecting circuit to compensate for the variations in the transmission characteristics of the line ML which occur from time to time and which are in general relatively small. It is convenient in practice to set the potentiometer l1 at 'about its midpoint for an average condition of the line ML so that the potentiometer will give a substantially equal variation in either directionfrom the average condition to. compensate for corresponding variations in the transmission `characteristics of the main line. If it be assumed that a 4zero transmission: equivalent is desired between the low frequency yterminals of the channel a'the high frequency loadimpressed on the receiving terminal when the main line ML has its normal characteristics should be that indicated by the dotted line 25 in Fig. 2. If the system is adjusted to have a zero low frequency equivalent for this normal characteristic of the main line and with the'potentiometer adjusted to its midpoint, the conditions will be as indicated in Fig. 2 with the potentiometer in position I. The range of variation of the high frequency load by the potentiometer is indicated in this figure by the length of the horizontal line marked potentiometer. The amount of variation in the low frequency output produced by varying the potentiometer setting from its midpoint in either direction to the left or to the right is shown by the corresponding ordinates of the curve A. Within the range of the potentiometer, therefore, the low frequency equivalent may be either increased or decreased in a substantially linear manner as shown by the curve A Within these limits.

In setting up a system or in adjusting a system to proper working conditions it may be found that varying the potentiometer 11 from one end of its scale to the other produces practically no effect in the detected speech currents. It may also happen that the detected 'speech currents are much stronger than is desired, that is, the overall gain between the transmitting low frequency jack and the receiving low frequency jack is too great.

This condition may be as represented in Fig. 2 by position II of the potentiometer ll and by the curve A. On account of the small slope which the curve A has in its upper right hand portion, no amount of adjusting the potentiometer is effective in producing any substantial change in the detected. output. This is due to thefact that the high frequency load iinpressed on the detecting circuit is too great. By inserting a high frequency artificial line or pad l() such as to introduce a loss in the high frequency channel, the effect of the potentiometer in controlling the low frequency output may be changed from that indicated in position II to that indicated in position III. In position III, the potentiometer gives a substantially linear regulation in the detected low frequency output. If still greater losses are introduced in the high frequency arrival currentsq the effect of the potentiometer 11 may be still further increased since' its position with respect` to the curve A will be moved further to the left. However, no amount of change in the high frequency pad l0 will bring about the relations indicated by curve A andby the potentiometer in position I, and it will he impossible. therefore, by the use of the high frequency pad alone to adjust the system to `give a zero low frequency equivalent and atthe same time give a fairly linear relation between the potentiometer setting and the output current withI the desired sensitivity of the potentiometer settings.

In accordance with the invention, thedesired regulation is effected by introducing enough losses by the highvfrequency pad to reduce the mean high frequency load on the amplifier to the value indicated by line of Fig. 2. The required low frequency equivalent is then secured by introducing a low frequency pad 16 which produces the required loss between the detector output and the jack 17 to reduce the strength of the detected speech currents to the value indicated by the zero axis in Fig. 2. By means of the high frequency pad 10 and the low frequency pad 16, therefore, the regulation may be readily changed from that indicated by'the potentiometer at position II and curve A to that indicated by position I and curve A.

Y The low frequency pad 16 does not, of course, affect the high frequency load on the detecting circuits. The use of the separate high frequency pad and low frequency pad enables the high frequency load impressed on the detecting circuit to be adjusted to the point at which the detector operates most efficiently without regard to the actual strength of the detected currents. The latter may be regulated by means of the low frequency pad 16.

The curve B in Fig. 2 indicates the relation between the high frequency arrival current and the readings of the meter 19, that is, the rectified carrier component. With the high frequency load indicated by the curve A it is seen that curve B is'practically symmetrical with respect to the line 25. For a considerable region each side of the line 25 both curve A and curve B have slopes of about This relation permits the attendant to judgev of the transmission characteristics of the main line ML from time to time by observing the reading of the meter 19. If the regulation Were that corresponding to the curve A with the potentiometer at position, II, ythe attendant would not observe a substantial change in the reading of the meter 19v by any change in setting of the potentiometer 11. The variation in the meter reading With a change in setting of the potentiometer 11 may, therefore, be taken as a convenient indication of the proper regulation of the channel.

The use of the high frequency and loW frequency pads, as described, not only per- ,mits the desired transmission level to be seunder thel the highest frequency channels. The separate high frequency and low frequency regulating pads readily permit all channels regardless of length or frequency employed to be broughtI to the same over-all transmission level and still keep the potentiometer settings the same. A given change in the potentiometers, Whether made manually or automatically, and separately or in common,

will produce substantially the same change in detected output and Will therefore serve to compensate in all channels for a given change in line attenuation.

lVhat is claimed is:

1. In a system in which high frequency Waves are transmitted between distantly separatedv stations V,and are detected to yield desired low frequency components, the method of regulating transmission comprising lowering the high frequency amplitude, rcgulatingthe high frequency amplitude bcfore detection to obtain a required relation between changes in the high frequency amplitude and the detected low frequency current, and independently regulating the -detected low frequency current to obtain the required low frequency amplitude.

2. In a carrier Wave transmission system, a line the transmission level of which is subject to change, a receiving carrier Wave channel, including a detector and a low frequency termination, means associated with said detector for regulating the high frequency load impressed on the detector to compensate for changes in the line transmission level, means to control the sensitivity of said regulating means, and means in the low frequency terminating circuit for independently regulating the detected output current.

3. In a regulating system for a carrier signaling channel having a detecting circuit, a low frequency output circuit, a high frequency input circuit and an adjustable potentiometer for varying the high frequency load impressed on the detecting circuit` the combination `of an artificial line in the high frequency input circuit for controlling the sensitivity of the potentiometer, and an artificial line in the low frequency output circuit for determining the over-all transmission level of the channel.

4. In a multiplex carrier Wave transmission system, a line. a plurality of carrier channels supcrposed on the line, a detector ineach receiving channel for deriving low frequency signal currents from the transmitted high frequency Waves, an adjustable potentiometer individual to each channel for controlling the high frequency load on the respective detector and the resultant low frequency output of the detector, an artificial line individual to each channel between the multiplex transmission line and the respective potentiometer for making the sentector for adjusting the low frequency output to the same level in all of the channels for a substantial mid-scale potentiometer setting.

5. In a regulating system for a carrier 10 signaling channel having a detector and a vhigh frequency input and 10W frequency output therefor, the combination of means in the high frequency input circuit for lowering the high frequen'c for regulating the high frequency amplitude in the input circuit, and means for independently regulating the low frequency amplitude in the low frequency circuit.

In Witness whereof, I hereunto subscribe amplitude, means my name this 29th day of February A. D., 20

J ACOB SUTER J AMMER.

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