US1498568A - Equalization of carrier transmissions - Google Patents

Description

.Hume 24 1924.

3,498,568 H. s. osa-.ORNE

EQUALIZATION OF CARRIER TRANSMISSIONS Filed July 9. 1920 Zowlaas 20 22 I I mVENToR. /fei\\ Y M IV A TTORNEY 1 number of different causes.

Patented June 24, 1.924.

um'rsu STATES] PATENT orrlcs.

nanou: s. osBoaNE, or NEW Yonx, N. Y., AssmNon To Aumont Turmalin AND 'rnLEeiuuHl COMPANY, A coomrnon or NEW You.

l y, EoUALrzA'rIoN or cAnnInn 'rnANsmssIoNs Application led July 9, 1920.

To all whom it may concern."

Be it known that I, HAROLD S. OsBoRNE, residing at New York,'in the county of New York and State of New York, have invented certain Improvements in VEqualization of Carrier Transmissions, of which the following is a specification. v j This invention relates lto transmission Asystems in which carrier currents are employed for the transmission of signals and more particularly to methods land means whereby the transmission over systems of this character may be maintained substantially constant with changes in the condition of the circuits,

Ina carrier transmission system the' eiiiciency of the circuit for transmission purposes will vary from time. to time due to a Since the carrier currents employed for transmission are of fairly high frequency, changes in the attenuation are due mainly to variations in. the leakage conditions of the open wire lines upon which the carrier channels are superposed. Consequently during wet weather when the leakage increases the circuit will be much less efficient than during dry` weather. This is one of the principal causes of variatlon 1n the transmisslon eiliclency of carrier circuits and is quite distinct from the cause of transmission variations in long cable c ircuits used for the transmission of Aunder different weather conditions.

ordinary telephone frequencies Where the resistance variation with temperature .is the controlling factor.

One of the objects of the present invention is to provide means and methods whereby the. transmission eiiiciency of a carrier.

circuit may be maintained substantially constant under different conditions particularly a nother object of the invention is to provide methods and means whereby the above results may be accomplished automatically and preferably without the adjustment of mechanical devices.

These objects together with other objects of the invention more fully appearing hereinafter are. realized by taking advantage of a peculiar property of certain types of modulating and demodulating devices such as vacuum tubes. It has been discovered that if a vacuum tube is used as a modulator, or as a demodulatoroperating upon the homodyne principle, the eiiciency of the Seriallo. 395,111.

tube varies with the quantity or amplitude of the carrier frequency supplied. As the amplitude of the carrier current is increased the modulated output current increases up to a certain point after which the output current decreases asthe amplitude of the carrier current is further increased. If then the carrier current which is to be supplied to the modulating or demodulating tube is transmitted over the carrier circuitsand if under normal conditions of the circuit the amplitude of the carrierv current be made of such value thatthe tubeisfoperating'at a pointl beyond the peak of its output eiliclency, the efficiency of the transmission circuit will tend to vremain constant. This follows fromthe fact that if, ldue to wet weather or other abnormal conditions along the line, the transmission efficiency ofthe line circuit itself is decreased the amplitude of the carrier current transmitted over the a corresponding increase`in the modulated output current. This increase in the volume of the modulated output-tends to compensate for the decrease in the modulated carrier current transmitted overthe line and thus an automatic regulation is provided which is purely electrical.

The invention may now be more fully understood from. the following description when read in connection with the accompanying drawing, Figure 1 of which is a schematic circuit diagram of the terminal apparatus of a carrier transmission system and Fig.' 2 of which is a curve illustrating the variations in modulated output current of a modulating or a demodulating tube with the amplitude of the supplied carrier current.

Referring to Fig. 1. ML represents an ordinary transmisslon line, such as a telephone line, said line terminatingin a terminal line LT, including the usual composite set whereby the telephone circuit may be composited for Morse operation, and the usual phantom coil Y and vcomposite ringer Z. lSince the elements and Z are well known in the art they arefsimply indicated schematically upon the drawing and'no further description is deemed necessary.

In order that a carrier system may be superposed upon the transmission line LDL, a carrier branch CB is connected to the main line ML at the junction point of said main line with. the terminal line TL, said carrier branch leading to terminal carrier apparatus. ln order to prevent the transmission of the carrier frequencies to the terminal line LT and the low frequency apparatus associated therewith, a broad band filter LF is inserted in the terminal line LT said filter being so designed as to transmit frequencies 'below the upper limit of telephonie transmission and to substantially suppress frequencies above such limit. ln order to prevent the transmission of low frequency currents, such as ordinary telephone or Morse currents, to the carrier branch CB, a broad band filter HF is inserted in said branch, said filter being so designed as to transmit frequencies lying above the upper limit of ordinary telephonie transmission -while suppressing frequencies lying below said limit'` These filters may be of the general type -disclosed in United States patents to George A. Campbell, Nos. 1,227,113 and 1,227,114 dated May 22, 1917. The filters, however, may be of any other type, the only requirement being that they transmita band of frequencies having the limits above referred to.

The carrierbranch CB is associated with a carrier transmitting circuit TL and a carrier receiving circuit RL to which the various carrier channels are connected. The circuits TL and RL are associated with the carrier branch CB in such a manner as to be conjugate with respect to each other, that is, carrier oscillations transmitted from TL to CB will be without effect upon RL and vice versa. This result is secured by providing a balanced transformer arrangement 10, of well known type, and a balancing circuitN.

The carrier transmitting and receiving circuits TL and RL are associated with a plurality of carrier channels, which will now be described. L1, L2, L3 and L, represent low frequency telephone or telegraph lines over which telephonie or other signals are transmitted, which signals are to -be impressed upon carrier currents and simultaneously transmitted over the main line ML. The line L1 is associated with a carrier transmitting circuit TL1 and a carrier receiving circuit RL1 through a balancing transformer arrangement 11 and a balancing network N1, the connection being such that the circuits TL1 and RL1 will be conjugate with respect to each other. In a similar manner, line L2 is associated with circuits TL2 and RL2 through transformerv 12 and network N2; line L3 is associated with circuits TL3 and RL3 through transformer 13 and network N3;'and line L.,L is associated with circuits TL4t and RL4 through transformer 14 and network N4. Each of the transmitting channels TL1, TLZ, TL8 and TL,l is connected to reeaeea the common transmitting circuit TL while each of the receiving channels RLI, RL2, RLS and RL4 is associated with the vcarrier receiving circuit RL.

Tn order to supply the carrier currents having dierent frequencies for each channel, a frequency control system of-the general type disclosed in the application of B. W. 'KendalL Serial Number 130,350, filed November 9, 1916, is provided. The frcquency control system comprises an oscillator FG, which may be of any well known type, such as, for instance, the well known vacuum tube oscillator, said oscillator serving to generate 'a basic carrier frequency illustrated, by way of example, in the drawing as being 5,000 cycles. 1n order to translate the basic frequency into harmonics for the different channels, an harmonic producer schematically indicated at HP is provided. This harmonic producer may be a vacuum tube arrangement for distorthig the waves generated by the oscillator FG- so as to produce harmonics in the output circuits of the distorting tube. Harmonic producers of this character are shown and described in the above mentioned application of B. W. Kendall as well as in an application of B. W. Kendall, Serial Number 139,530 filed December 29, 1916. The harmonics produced by the harmonic producer HP, which may be for example of 10,000, 15,000, 20,000 and 25,000 cycles, are filtered out and separately transmitted to the several carrier channels through filtering devices CF1, CFg, CE, and GF4. These filtering devices may'be broad band filters of the type disclosed in the above mentioned applications of George A. Campbell or may be simple tuned circuits of any well known type. The fundamental frequency here illustrated as 5,000 cycles may be transmitted through a similar filtering arrangement FF to an amplifier FA. The amplifier FA may be of any known type, such as, for instance, a vacuum tube amplifier. The amplified fundamental frequency may be then passed through another filter FF and impressed upon the transmitting carrier circuit TL. The harmonics used as carrier frequencies after being filtered by means of the filters CF1, CF2, etc., may be impressed upon carrier amplifiers C1, CA2, CA3 and C1114. These amplifiers may be similar to the amplifier FA already referred to.

Each of the transmitting channels TL1, TL2, etc., is provided with a modulating arrangement schematically illustrated at M1, M2, M3 and M4. This modulating arrangement may be of any desired character but is preferably a vacuum tube arrangement, such as is illustrated in the U. S. patent to John R. Carson Number 1,343,307, issued June 15, 1920.

with a demodulator or detector, such as 1) l illustrated and described in the U. S.

D2, D2 and D2. These demodulating or detecting arrangements may be of any WellV known type, but are preferably of the ttyp a en tol J. R. Carson, Number 1,343,308, lssued June 15,1920. The detecting or demodulating arrangement of the said vpatent to J. R. Carson employs the sorcalled homodyne method of detecting, ythereby necessitating that the carrier frequency of that particular channel be supplied to the demodulating arrangement. Accordingly, the carrier frequency of eachchannel after beino vamplified by the carrier amplifiers UA1, A2, etc., is supplied in parallel to the modulating arran' ement of the transmitting channel and t e demodulating arrangement of the corresponding'receiving channel.

The carrier oscillations imressed upon the modulator, as for instance 1, are modulated by means of the low frequency signal 'waves transmitted from the low frequency line, such as line L1, to the carrier channel TL1, and are then filtered and amplified. For this purpose broadband filters TF1, TF2, TF2 and TF2' are provided inthe output circuits of the modulators M1, M2, M3 and M1.' These filters may be of thetype disclosed in the patentsto Campbell above referred to and each filter should be capable of transmitting a band of frequencies whose Width is equal to the range of frequencies employed in 10W frequency signaling. In the case of telephone transmission, the filter should be so designed as to transmit a band equal to the telephonie range to the exclusion of frequencies lying Without this range. The actual frequencies transmitted by the filter of each channel will be different` for each channel'. The filter of the channel TL1 transmitting a band of frequencies in the neighborhood of 25,000 cycles, the filter in the next channel transmitting a band in the vicinity of 20,000 cycles, etc. The modulated carrier oscillations transmitted through the filters may be amplified by amplifiers TA1, TA2, TA3 and TA, arranged in the several transmitting' channels. These amplifiers may be of any Well known type but are pref'v erably vacuum tube amplifiers. -In order to further increase the selectivity, filters TF1', TF2', TF2 and TF4 may be provided in the output circuits of the amplifiers.

The several carrier frequencies are modulated in accordance with the lou7 frequency signals transmitted over the lines L1, L2, L1 and L4 and after being filtered and amplified are simultaneously impressed upon the common carrier transmitting circuit TL, the fundamental frequency here illustrated as being of 5,000 cycles, being at the samv time impressed upon the common circuit.

, These multiple frequencies are then transmitted to the transformervlO and over the carrier branch CB' to the mainline ML.

Owing to the balancing arrangement of the ltransformer 10 and the network N, the refilters TF1 or TF1", etc., of the associated transmitting channels. The modulated carrier oscillations transmitted over the main line ML from a distant station pass through the filter HF, being excluded from the terminal line by filter LF, and are then impressed through the transformer 10 upon the' common receiving circuit RL. Modu- .lated currents corresponding in frequencyv to the different channels are filtered out by means of the filters RF1, BF2; etc., and imressed upon the demodulators`D1, D2, etc. lnmodulated carrier frequencies are at the lsame time impressed upon the demodulators from the control circuit already described so thatthe lov7 frequency signal Waves, in accordance with which the high frequency carrier waves were modulated, appear in the output circuits of the demodulators.

In order to amplify these low frequency Waves, low frequency amplifiers BA1, RA2,

RA3 and RA,1 are provided in the several receiving channels. These amplifiers may be of any Well known type but are preferablyl vacuum tube ampliers. In order to purify the lovv frequency currents from any residual high frequency components, low frequency filters, such as RF1', BF2', BF2 and RF1 are provided in the output circuits of the amplifiers. These filters may be of the same character as the low frequency filter LF in the terminal line LT and are of the general type disclosed in the patents to Campbell above referred to. The amplified lorsT frequency signal waves from the several receiving channels are then impressed through the transformers 11, 12, 13 and 14 upon the respective low frequency lines L1, L2, L2 and L2.

It will be remembered that in connection with the production of the various carrier frequencies, a fundamental frequency of some desired value, such as 5,000 cycles, for example, Was generated and transmitted through the filters FF and FF and the amplifier FA to the common transmitting circuit TL. This fundamental' frequency was then transmitted together with various modulated carrier frequencies through the transformer arrangement 10 and the highv frequency filter-HF to the main line At a distant point in the main line the various carrier frequencies may be separated from the low frequency signaling currents,

i uen() such as telephone currents, by means of a high fre uency composite set similar to that already escribed and comprising high freand low frequency filters similar to e fi ters HF and LF. rlllhe .carrier frequencies may then be impressed upon terminal carrier apparatus similar to that shown in Fig. 1. The terminal carrier apparatus may dier, however, from that shown in Fig. 1 with respect to the production of the carrier frequencies. In order to illustrate the manner in which the system will be modified at the distant station in this respect, the apparatus of Fig. 1 has been shown with a circuit 2f leading from thev common receiving circuit RL to an harmonic producer (not shown). The filtering arrangement, such as a tuned circuit FR, is'included in the circuit 21 to filter out the fundamental frequencyv of 5,000 cycles transmitted over the line. This frequency is then Y:impressed by means of the circuit 21 upo an harmonic reproducer which will be in general similar to the harl monic producer HP and the general principle of this arrangement is described in the application of B. W. Kendall, Serial Number 139,530, filed December 29, 1916, already referred to. The general Aarrangement whereby the harmonic reproducer is associated with the receiving circuit is shown and described in the application of B. W.

Kendall, Serial Number 130,350, led .No-v

vember 9, 1916, to which reference has already been made. it is sufficient for the purposes of this application to state' that the harmonic reproducer functions to produce harmonics of the basic frequency in a manner similar to the harmonic producer HP already described, the several harmonics being filtered out, amplified to a desired is derived from this fundamental frequency.

Consequently, the amplitude of the carrier frequencies supplied to these tubes at the distant terminal station will vary as the amplitude of the fundamental frequency varies, which fundamental frequency in turn varies with changes in the transmission efficiency of the main line due to dierent weather conditions and the like.

messes As previously stated the output of modulated current from the vmodulating or demodulating tubes varies in a peculiar manner with increase in the amplitude of the carrier current supplied to .the tube. rlhis variation is illust-rated by the curve 20 in Fig. 2. It is apparent from an inspection of thecurve that as the amplitude of the carrier-current is increased the modulated output current also increases until the point 21 is reached after which the output of modulated currer." decreases with an increase in amplitude ot current. If then with the line operating under normal conditions the amplitude of the fundamental frequency supplied at the generating station be made such that the carrier currents derived therefrom atthe vdistant station will be of the amplitude indicated at 22. the modulated output current of the modulators and demodulators at the distant station will be less than the maximum. Should the transmission efficienc of the main line decrease due to wet weat er, for instance, the amplitude of the fundamental frequency arrivlng at the distant station will decrease, with a corresponding decrease in the amplitudes in the carrier currents supplied to the various modulating and demodulating tubes. For a given modulating current, whether high frequency or low frequency, supplied to the tube, the modulated output current would be decreased.

Let us consider the effect of this as applied to transmission from the station at which the fundamental frequency is generated to the distant station. The modulated current supplied at the generating station to the line as' a result of the modulation of a carrier current by a signal will have the same amplitude when applied to the line regardless of conditions on the main line since the fundamental frequency genera-ted at that station together with the carrier currents derived therefrom will be the same. lf the attenuation of the main line is greater than normal, however, the modulated carrier currents will be decreased in amplitude to a greater degree than normal upon arriving at the distant station. rllhe fundamental frequency transmitted over the main line will, however, arrive at the distant station correspondingly diminished thereby tending to increase the output of the modulating and demodulating tubes. The output of the demodulating tubes, however, will not actually be increased because the modulated carrier currents supplied to the tube after transmission over the line have been decreased by corresponding amounts. Consequently, the actual low frequency output of the demodulating tube will be substantially the same as under normal conditions.

In the case of transmission from the distant station to the terminal station at which the fundamental frequency is generated, if

the attenuation of theqmain line is greater` than normal the amplitude of the fundamental frequency will be decreased below its normal value upon arriving at the distant station. This results in a carrier current of increased amplitude being supplied to each of the modulating tubes and assuming that the low frequency signals applied to the modulators -are of normal amplitude the modulated high frequency output currents supplied to the line Will be of increased amplitude as compared with normal. This increase in the amplitude of the modulated carrier currents supplied to the line from the distant station Will be about sullicient to make up for the increase in the attenuation of the line so that the currents will arrive at the enerating station with their amplitudes a out normal.v Since the amplitudes of the carrier frequencies supplied to the demodulating tubes at the generating station do not vary With changes in the condition of the main line the low frequency demodulated output currents supplied to the terminal loW frequency lines Will have substantially the same values as under normal conditions.

It will beobvious that the general principles herein disclosed may be embodied inI many' other organizations widely different from those illustrated Without departing from the spirit of the invention as defined inthe following claims.

What is claimed is:

1. The method-of maintaining the transmission of a carrier circuit substantially constant for all carrier frequencies transmitted thereover regardless of changes in theline condition, which consists in subjecting a pilot carrier current to the same transmission conditions as those to which the various carrier currents employed for the transmission of signals are subjected,

and controlling the amplitudes of the car.

rier currents employed for signaling transmission 1n accordance With variatlons 1n the amplitude of the pilot carrier current inv tion, which consists in generating at one terminal station a fundamental frequency from which all other carrier frequencies for use at said station are derived, transmittmg said'ifundamental frequency over vthe mainline to the distant station, deriving from said fundamental frequency at said distant station carrier frequencies for use at said station, and maintaining the amplitude of the generated fundamental at such values that under normal line transmission conditions the amplitudes of the lderived carrierkfrequencies at the distant station Will be greater than the amplitude necessary for maximum output of the modulating and demodulating devices.

3. In a carrier system including a line circuit and modulating and demodulating devices operating most eiiiciently at a certain input amplitude and less efciently for greater or lesser amplitudes, the method of maintaining the transmission of a carrier circuit substantially constant for all carrier frequencies transmitted thereover regardless of changes in the line transmission condition, which consists in generating at one terminal station a fundamental frequency from which all other carrier frequencies for use at said station are derived, transmitting said fundamental frequency over the main line to the distant station, deriving from said fundamental frequency at said distant station carrier frequencies for use at said Station, and maintaining the amplitude of the generated fundamental at such values under normal line transmission conditions that the amplitudes of thecarrier frequencies supplied to the demodulating devices at the distant station Will be greater than the amplitudes necessary for maximum output of said demodulating devices by an amount such that the efficiency of the demodulating devices Will increase as the attenuation of the line increases to offset the increased attenuation of the modulated carrier currents transmitted over the line to the distant station.

j 4. In a carrier system including a line circuit and modulating and demodulating devices operating most efficiently at a certain input amplitude' and less efficiently for greater or lesser amplitudes, the method of maintaining the transmission of a carrier circuit substantially constant for all carrier frequencies transmitted thereover regardless of changes in the line transmission con-4 dition, which 'consists in' generating at one terminal station a fundamental frequency from which all other carrier frequencies for use at said station are derived, transmitting said fundamental frequency over the main line to the distant station, deriving from said fundamentalfrequency at said distant station carrier frequencies for use at said station, and maintaining the amplitude of the generatedl fundamental under normal line transmission conditions at such values that thefamplitudes of the carrier frequencies derived therefrom at the distant sta- @3 Lacasse Ymodulating devices, whereby as the attenuation of the line is increased the eiiciency of the modulating devices will be correspondingly increased so that the output currents in the modulators when transmitted over' the line Will arrive at the station at which the fundamental frequencies were generated with substantially the same amplitude as under normal conditions.

5. The method of maintaining the transmission of a carrier circuit substantially constant regardless of changes in the line trans- -mission condition, Which consists in transmitting the carrier current over said circuit and in translatingthe transmitted carrier current into another current so related to the transmitted current that a decrease in the amplitude of the translated carrier current will produce an increase in the amplitude of the signaling current detected from the translated current.

6. A carrier Wave transmission system in which signal variations of carrier Waves are produced and transmitted to a distant station over a suitable transmitting medium subject to changes in transmission eiiiciency, modulators associated With said transmitting medium at the terminal station, said modulators being so arranged that for normal transmission conditions of the transmitting medium the unmodulated carrier component supply exceeds the value for which said modulators operate `at maximum efiiciency, whereby poor transmission conditions of the transmitting medium and a consequent decrease in applied unmodulated carrier comonent are accompanied by an increased efciency of the over-all transmission of the circuit including the transmitting medium and modulators.

7. A carrier Wave transmission system in which signal variations of carrier waves are produced and transmitted to a distant station over a suitable transmitting medium subject to changes in transmission efliciency, demodulators for said Waves associated with said transmitting medium at the receiving station, said demodulators being so arranged that for normal transmission conditions of the'transmitting medium the unmodulated carrier component supply exceeds the value 'for which said demodulators operate at maximum eliciency for reproducing signals, whereby poor transmission conditions of the transmitting medium 'and a consequent decrease in applied unmodulated carrier component are accompanied by an increased efficiency in the reproduction of the signals.

8. In a carrier Wave transmission system inwhich signal variations of carrier Waves are produced and transmitted to a distant station over a suitable transmitting'medium subJect to changes in transmission eclency,

modulators and demodulators for said Waves,

said modulatorsv and demodulators being so arranged that 'for normal transmission conditions of the transmitting medium the unmodulated carrier component supply exceeds the value :tor Which said modulators and demodulators operate at maximum eliciency for reproducing signals, whereby poor transmission conditions of the transmitting medium and a consequent decrease in applied unmodulated carrier component are accompanied by an increased eliciency in the reproduction of signals.

9. A carrier Wave transmission system in which signal variations of carrier Waves are produced and transmitted to a distant stav tion, a pair of translating arrangements corresponding to each carrier Wave, one of said translating arrangements comprising a modulator and the other comprising a detector, one of said translating arrangements being so arranged-that for normal transmission conditions of the `transmitting medium the unmodulated carrier component supply exceeds the value for which said translating arrangement operates at maxi mum eiiclency, whereby poor transmission conditions of the transmitting medium and the consequent decrease in applied unmodulated carrier component are accompanied by an increased efficiency of the over-all transmission of the circuit, including the transmitting medium and translating arrangements.

ln testimony whereof, I have signed my name to this specification this 30th day of June 1920. 'f

HAROLD S. OSBORNE

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