US1570772A - Reduction of distortion in multiplex repeaters - Google Patents

Description

Patented Jan. .26,- 1926.

UNITED' STATES PATENT [OFFICE:

HARRY NYQUISL'OF ELMHURST, NEW YORK, ASSIGNOB TO AMRICAN TELEPHONE .AND TELEGRAPH COMPANY, A. CORPORATION F NEW YORK. l

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Appncationmea Haren 20.1922. serial no. 545,29a Y To all 'u1/tomI t may clmcem."

Be it known that I; HARRY NrQurs'r, residing at Elmhurst, in the county of Queens and State of' New York,have invented certain Improvements in theReduction of Distortion in Multiplex Repeaters, of which the following is a specification.

This invention relates to multiplex signaling, and more .particularly to means for reducing the eii'ect of distortion, in repeaters or other translating devices used in connection with such a system.

Heretofore, in the operation of multiplex vcarrier systems, it`has been found that the repeater tubes employed at points along the line for transmitting carrier frequencies produce certain undesirable effects where the same tube is used to transmit the frequencies of a number ofV channels. This is for-the reason that the tubes do not have a straight line characteristic, andhence produce a certain amount of distortion which causes what might he termed mutual crowding. of the channels. 'By this is meant that Where one frequency exists in -a repeater or' other form of translating circuit along with other 'frequencies,.it decreasesthe amplitude of all of the other frequencies by a certain amount, as compared Withthe amplitudes the frequencies would have if each were'on the circuit alone. This eiiect at the receiving end of Ythe circuit is very similar to the effect Y which would be produced by the introduc- 'tion of a loss in' the circuit by any other means, such', for example, as a resistance shunted acrossv the circuit. AThis eiiect is very undesirable in a carrier telegraph sys- Vtem involving a large number of channels employing carrier frequencies 'within the usual voice range, for 1n such a system 1t is almost essential to economic'working of the system that the channels be narrowand closely spaced, vso that all the channels may be amplified bythe same repeater tube.

i In accordance with the present invention, it is proposed to overcome this difficulty by so arranging each carrier channel that when the carrier frequency is applied to the channel, the normal impedance of the circuit will be reduced by an amount" suiiicient to compensate for the reduction inl amplitude to which the frequencies then existing on the circuit would be subjected.

The invention may now be more fully understood by reference to the following description, when read in connection with the accompanying drawing, Figure 1 of which illustrates in simplified form the circuit arrangement of a. carrier system embodying the present invention, and Fig. 2 of which shows a number of curves illustrating the principles of the invention.

Referring to Fig. 1, Le and Lw designate two transmision lines forming a four-wire .circuit for the transmission of carrier frelective device or lilteryRFs, a detector and a receiving relav RR.

The filter RS may be of the Well known Campbell type and is illustrated as consistyingA of two sections, each having a series resonant element and a shunt anti-resonant element. The filter at the side adjacent the bus-bars terminates in a full series element in order to present the greatest impedance to frequencies lying Without the range of free transmission. The detector l) maybe v ofy any well known type, such, for example, as a vacuum tube detector. It will be understood that-each of the receiving channels includes apparatus similar to that illus.-

'trated in connection with the channel RLS,

and that the lters'of each channel will be so designed as to freely transmit a range of frequencies differing from the other channels.

The .receiving relay RR is arranged .to A transmit vthe direct current impulses detected by the detector D toY an ordinary telegraph line L,Ywhich is balanced by means of anetwork N, the transmitted impulses beingi' connected tors. neutral point between the line L and the network N in order not to operate the transmittingrelay TR associated with the telegraph line L.

' The input side of therepeater Rw is connected to bus-bars 11, from which transmit- .ting channels TL TL TLS, TL TL5,etc., l

are branched. Each channel, such as the channel TLB, includes a filter TFv which may be of thegwell known Campbell type. As illustrated, it, like the filters associated with the receiving channels, consists of' two sectionseach comprising a series resonant element and al shunty anti-resonant element.

The section adjacent the bus-bars terminates .mitting carrier oscillations. `Thesource S3 for the channel TL3 is associated therewith through a transformer 12, thesecondary circuit of which is included in the channel TL3 and may be opened or closed under the control of the armature 13 of the transmitting relay TR. Direct current impulses from the line L will, therefore, be translated into groups of carrier oscillatioris from the source 3, which may be transmitted through the channel TL3 to the line Lw. The transmit- I ting apparatus-associated with each channel v fas is illustrated -at 5-6-7 i form indicated at 202122 andas seen,

will be similar to that illustrated in connection with the channel TLB; f l

Before describing the arrangements of the invention designed to overcome-the condition known as crowding, it will be explained "how thi-s condition arises in connection with a repeatenfor example. Referring to Fig.

i 2, 1-2-'3 is a curve illustrating the characteristic of a vacuum tube repeater. It will be noted that the curve is nota straight lme and kthat its angle with the axis 2-4 becomes less as the curve recedes from saidA axis. They condition represented by the curve 1--2-3 is, of course, exaggerated in 45, order to morel clearly show what takes place,

and in practice it will b understood that the characteristic curve of a vacuum tube repeater" will more nearly approach a .straight line. However the phenomenon known asv crowdin will exist, though to a lesser degree than lllustrated in connection Let us suppose a'simple sine .wave such and having a definite frequency and constant amplitude is impressed upon the repeater. The waveI appearing in the output circuit will take the 7 the amplitude of the wave in `the output .circuit will be proportional to that of the input wave and the frequency will be the same. For theI sake of simplicity, lthe am- .'plitude of the output wave is made equal to' that'of the input wave which would co'rrespond to a condition in wh1c h no actual .amplification takes place. Thls, however,

has no effect upon the principle about to be explained. y

' Assume now that in addition to the wave having the same frequency and amplitude as the wave '5-6-7, another wave of the same amplitude but of different frequency be impressed upon the amplifier. The two.v

waves now impressed upon the amplifier will take the form indicated at 23-24. The two waves, when combined in the same circuit, however, produce a combined wave having the form illustrated at 7-25-26. Owing to the non-linear characteristic illustrated at 1-2-3, the Goutput wave will take the form 22f 27-4. This wave, it will be noted, is substantially of the form of the wave 7-25-26, but its amplitude is less than that of the wave 7-25-26. If the wave v22---.274 be resolved into its components,

it will 'be foundto comprise two waves 28 and 29, corresponding in frequency to the waves 23 and 24, respectively, but each Vbeing of smaller amplitude than the original waves impressed upon the amplifier. Thus it will be seen that while the wave 24, if impressed upon the amplifier alone will produce a wave of the same amplitude -in the output circuit, when another wave, such as 23, Iis impressed upon the amplifierat the same time, the wave 29 in the output circuit corresponding to the applied Wave 24, will have an amplitude less than it would have vif the wave 23 were not applied.

In order to overcome this diiiiculty, a condenser 15 is included in the transmitting ychannel TL and a high impedance alternating current relay 16 is shunted about the condenser. The contacts of the relayv 16 control a resistance 17 shunted across the channel TLsin parallel with the side of the filter TF3 adjacent the bus-bars 11.. Whenever the carrier frequency is applied to the channehthe relay 16 disconnect-s' the resistance 17 shunted across the channel, and

'when the carrier frequency is no longer applied to the circuit, 'the shunt resistance 17 is again bridged across the channel. The condenser .15 acts as a shunt for the high impedance relay 16, Vso that the inclusion 0f this relay in Ithe circuit does not materially affect the impedance ofthe lter as viewed from the source of carrier oscillations S3. An arrangement, similar to that just described in connectionwith the'channel TL,x is associated channels. l .i l .A

If we consider the action of the transmitting channels on the assumption that the shunt -resistances, Such'as 17, are notv providedgit `will be seen that when a frequency withV each of the transmitting is applied' to a single onev of the channels,

the limpedances of the filters in the other channels will be so large that 'very little of the energy transmitted from-the active channel will be diverted from the lineL, into l comes active and a carrier frequency isthe other transmitting channels. When, however, shunt resistances such as 17 A are provided, as illustrated, and but one channel is act-ive, a certain percentage of the energy from the Achannel vwill be diverted from the line Lw into each of the other.

channels, owing to the reduced impedance of the channel due to shunting the resistance 17 across the terminal of the filter. This reduces the amplitude of the carrier frequency supplied from the' active channel to the linel Lw by an amount which may be made by proper proportioning of the resistance 17 justequal to the reduction in amplitude due to crowding.

Consequently, when a second channel betransmitted therethrough the opening of the shunt 'through the resistance 1n the second channel increases the impedance of the second channel to the frequency transmitted from the first active channel, so that agreater proportion of the energy vfrom .the first channel is transmitted to theline L. The

increased amplitude -of the current transmitted to the input side of the repeater in y the line will, if the shunt resistance is properly proportioned, be sutlicient to make up for the reductionin* the output of the repeater due to crowding, and consequently, the amplitude of the frequency from the Y iirst channel appearing in the output circuit from the spirit of the invention as de of the repeater will be substantially the same whether any other channel isv active or not.l It will be at once apparent that any desired combination of channels' may be transmitting without substantial variation in the amplitude of the frequency transmitted froml each channel as it a pears at the receiving end. The reduction 1n amplitude which this which a plurality of frequencies lfrom a pln-` rality of channels may be transmitted over a circuit including. a common repeater, the

method of overcoming the eiects ofcrowding of the channels in the repeater when more than one frequency is transmitted at the same time, which consists in increasing the transmission of the circuit for the fre-` quencies of the other channels lwhen a given ft "the energy thus diverted into the impedance associated with an inactive channel when it becomes active.

channel i's l transmitting. ff

.'2. In a multiplex signaling whicha plurality of frequencies from afpliil rality f channels may" be transmitted through a common repeater, the method of overcoming the effects of crowding of the V*channels in the repeater when more than one frequency. is transmittedV at the same time, which consists in diverting a certain amount `of the energy transmitted by a given channel for each of the other channels which are not then active, and reducing the amount of energy thus diverted for any of said other `channels when they become active.

3. In a multiplex signaling system in which a plurality of frequencies from a plurality of channels may be transmitted through a common repeater, the methodY of overcoming the edects of crowding of the channels in. the repeater when more than one frequency is transmitted at the same time, which consists in diverting a certain amount of the energy from a channel which is actively transmitting into each of the inf active channels, and increasing the imped-v ance of the inactive channels to frequencles outside the range of the particular mactive channel when the inactive channel actively transmits.

,4. In a multiplex 'signaling system in which Aa plurality of channels maybe transmitted through a common repeater, the method'of overcoming the eiectsV of crowding of the channels in the repeater when I more than one frequency is transmitted at the same time, which consists in diverting from the repeater a certain amount of the -energy from a channel4 which isactively' transmitting, controlling the amount diverted by impedances in shunt with the other channels, and reducing the amount thus diverted as other channels become active.

5. In a signaling system, a transmission line including a repeater, a plurality of 'channels transmitting at diiferent frequencies through said repeater, means operating Awhen a given channel is actively transmitting to divert from the repeater ahportion of the energy `from the transmitting channel, and means controlled bythe other channels as they become active in transmitting through said repeater, terminal apparatus 1 for the transmittingV channels, said terminal apparatus including 'means operating when 'a given channel is transmitting to partially4 divertthe energy from said channel into an for reducing the amount of energy which is impedance associated with the, inactive j ransmitting channels, and means to reduce 7. In. a signaling system,- a transmission line, a repeater therefor, a plurality of transmitting channels for transmitting through said repeaterat different frequencies, and means controlled by each channel for increasing the transmission of the repeater circuit for frequencies of the other channels when said channel is transmitting.

8. In a signaling system, a transmission line, a-repeater therefor, a plurality of transmitting` channels for transmitting through said repeater at different frequencies, and means associated with each channel for controlling an impedance associated with 'that channel as regards frequencies transmitted by the other' channels when that channel istransmitting.

9. In a1 signaling system, a transmission line,.a repeater therefor, a plurality of terminal transmitting,` channels connected in multiple to transmit through said repeater,

the im edance of the channels being such that when any channel is transmitting a portion of its energy will be diverted from the'line to'the parallel channels, and means associated with each channel .to decrease the amount of energy of other channels diverted thereto `When'the particular channel is transm'tting v 0; In a multiplex signaling system involving transmission loss due toV mutual w crowding of the channels, means to produce a variable transmission loss, and means for y varying the transmission loss Jproduced by said means in a manner complemental to the variation in transmission loss dueto crowd- March, 1922. HARRY` NYQUIST.

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