US2081312A - Carrier telegraph system - Google Patents

Description

May 25, 1937. v, R THQRP 2,081,312

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1 Ar- X63 Lan, Emis? L12 Detector Input (db Chang? in Carrier'lwge) jpl/'y2 INVENTOR BYI W A ATTORNEY May 254, 1937'.

V. P. THORP CARRIER TELEGRAPH SYSTM Filed June 5 1934 2' sheets-smet4 2 mvENToR BY 7j? Tho/fp ATTORNEY article 'l a 'reine ysrsrr WaughnP. Thorp, River Edge, N. il., assigner to i r rican Telephone and |ii'elegraph Company,

" corporation ci New Yorh application rune s, wel, serial Nc. vzaisv 6 @la/ima.

This invention relates to carrier telegraph systems and, moreparticularly. to arrangements for reducing the distortion of the telegraph'signals received over such systems.

In carrier telegraph systems ofthe usual type any changes in the received currents are apt to cause distortion in the telegraph signals. These changes in received current are due to a number oi causes and primarily tovariations in line attenuation with weather conditions. It is the practice in certain types of carrier telegraph sys--V tems to utilize sensitive receiving relays, which are biased to spacing by a current which has a magnetic edect on the armature of the relay of one 'half that-of the marking orreceived rectified current. This Vbiasing current is substantially constant and is independent of the line characteristics since it is supplied from a local battery. It is evident, therefore, that any change in the received current from the distant endwill cause bias or distortion in the telegraph signals inthe receiving loop circuit. In certain types of service, such as high speed teletypewriter service, this may be sumciently large to cause trouble. Accordingly, it is one of the primary objects of the invention to provide arrangements whereby variations in the received currents due to changes in line attenuation or to other causes will not cause bias or distortion in the signals in the receiving loop circuit.

While this vinvention will be pointed out/with.

particularity in the appended claims, the invention itself, both as to its further objects and ieatuies, will be better understood from the de vtailed description hereinafter following, when read in connection with the accompanying draw- .1 in which Figure 1 shows an embodiment of the invention given merely for the purpose of illustration, this gure showing the sending and signals may be transmitted to line L1 and a branch line Ls over which carrier telegraph'si'gnals may be received from line Li. Line L1 in this particular cme is aV two-wire line and different frequencies are used for sending to and receiving from the distant station. The same frequency can bev and (Ci. litt-66) often is used for transmission and reception but in the latter case a 4-wire circuit is used, the two pairs being connected respectively to L2 and L3 through suitable transformers. This will be apparentto those skilled -in this art.

For purposes of illustration of the invention Vthe local loop .circuit is 'shown full-duplex, that is, separate loops are used for transmission and reception of signals. Half duplex arrangements may be used equally well in the practice of this invention but these are not shown on the drawings since that is not necessary for an understanding of the invention. For transmission of the telegraph signals, the well known balanced loop is used for actuating the sending relay. This will not. be described in detail as it does not constitute an important part of the invention. I am concerned here particularly in any arrangement' for actuating the sending relay by. means of the operation of the subscribers sending` key K1.

When key K1 is closed,A the armature of the sending relay will rest against its M contact and the carrier current from the carrier supply Will flow `ireely into the sending illter F1, branch line L2, through transformer T3 to line Li and to the distant station. When key K1 is opened, the armature of the sending relay will move to its S contact, which connects resistance R1 in shunt with the carrier supply O and this will reduce the input to the sending filter F1 by a predetermined amount but not to zero. When resistance R1 shunts 'the carrier supply O, a substantial current will still be supplied to the filter F1. Other arrangements may be used for accomplishing this purpose, and these will occur to those skilled in this art, but it will be necessary always to keep in mind that the fundamental method of transmitting signals in accordance with this invention is by means of varying the transmitted current over A a relatively small range rather than by changing Fig. 1. Carrier signals are received from the distant station over line L1, then pass hrough transformer T3 to line L3, where theyA are selected from other signals of other frequencies by the receiving lter F2 and pass into the amplifler A and from there g'o through transformer T1 to the detector tube D. The outputof the detector tube D includes one Winding, i. e., the primary winding of the transformer T2, the other winding, i. e., the secondary winding of which connects to the operating windings of the polar receiving relay. This relay is provided with holding windings H which serve to hold the armature on whichever contact it happens to be resting when there is no current in the operating windings. It is evident that, when either a steady carrier current or zero current is applied to the input to the detector, there will be no current in the operating winding of lthe receiving relay since the direct current component is kept out of the operating windings of this relay by the transformer T2. However, when a change of current occurs in the plate circuit of the detector tube, a current will be induced in the secondary of T2 which will ow in the operating windings-of the receiving relay and its magnitude will be such as to overpower the holding current, thus moving the armature of the relay to the oppositemcontact.- The windings oi.' the relay are so poled that an increase of current in the detector plate circuit will move the relay armature to its M contact and a decrease in detector plate current will move the relay armature to its S contact. Movement of the receiving relay armature from one contact to the other connects alternately positive and negative batteries B4 and B5 to the receiving loop L5 which actuates the subscribers receiving mechanism Sz.

It will be noted that the polar receiving relay is arranged to operate on changes in received carrier current. There is no local bias current supply for this relay, although there is a holding circuit to keep the relay armature on the contact to which it has been moved by the pulse of current, either marking or spacing from the transformer Ta in the detector output circuit. 'Ihe holding circuit is unbiased regardless of inequality in the receiving leg battery voltages and this is because the return circuit from the relay armature is brought to the center tap of a resistance Z connected across batteries B4 andl B5, as shown.

A further feature of the invention is the arrangement of the detector input and output circuits so that the detector characteristic will have a large range over which the output has a linear relation tothe input. .'I'he direct current resistance of both the input and outputof the detector is kept as low as possible for this purpose. This improvement is indicated more clearly upon a consideration of curve B especially if compared with curve C in Fig. 2 which is a normal characteristic for a detector circuit.v

The foregoing explained the operation of the circuits for sending and receiving signals by means ofy small changes in the transmitted and received currents. For an understanding oi' the performance of such a system yunder various line equivalent conditions, reference is made to Fig. 2 of which curve B shows the detector plate current plotted against detector input level. Assume, for example, that thenormal equivalent of the line is such that the input level to the detector for a marlnng signal is Xml and for a spacing signal is Xsi. The corresponding detector plate currents will be IMi and ISI. The change in input level vfrom marking to' spacing is independent of line conditions since it is determined by the resistance R1 and the position of the sending relay armature. This charge in input level is indicated in Fig. 2 by C1. The change in detector plate current for this level is from IM1 to IS1. or vice versa,` and this change in current through the primary of T1 in Fig. 1 induces the operating current in the receiving relay windings as shown by curve S in Fig. 3. If the line equivondary winding of transformer Tn.

alent' changes so that the input level to the detector fora marking signal is now Xmz and for a spacing signal X82, the change in detector plate current will be IMa-ISz when going from the marking to the spacing condition. This change is substantially equal to IMi-IMz so that the eiect on the receiving relay is the same as for the normal level case. The same conditions optain when the line level drops to such a value that the input level for aymarking signal is Xm: and for a spacing signal Xss. It is thus evident that over a considerable range of line equivalents, the telegraph signals will be received free from bias which would ordinarily be present in the ordinary sending and receiving means.

In addition to compensating for line attenuation changes, the circuit arrangements of this invention take care to some extent of the change in battery voltages at the terminal stations. At

4the sending station, for example, the oscillator and sending'amplier which together supply they carrier current will vary in output with charges in battery potentials and at the receiving station the gain of the receiving amplifier will likewise vary with changes in battery potentials. In any case involving the features of this invention, the pulse of current which operates the relay to its M contact will be the same in magnitude as that which operates it to its S contact and signals will be repeated unbiased.

In Fig. 3, the two upper curves `are practically the same. They show that the plate current ilowing through the primary winding of the transformer Ta is independent of large changes in, for example, the line equivalent of the transmission circuit. Curve S shows the current in the sec- 'Ihe square cornered curve H shows the holding current ilowing. through the holding windings H of the receiving relay. 'I'he lowermost curve shows the current in the receiving loop Ls.

.While the invention has been disclosed as embodied in certain specic arrangements which are deemed desirable, it is'understood that it is capable of embodiment in many and other widely varied forms without departing from the spirit oi.' the invention as defined by the appended claims.-

What is claimed is:

1. The method oi' transmitting carrier telegraph signals which consists in continuously and uninterruptedly transmitting a carrier current during signaling, changing the magnitude of the carrier current during its transmission by a small predetermined amount in accordance with marking and spacing signals and maintaining the said changes in magnitude of the said carrier current at the transmitting and receiving terminals substantially free of bias independently of variationsigllze transmitting4 medium within predetermined 2. In a carrier telegraph system, a source of carrier current which is uninterruptedly emitted for conveying signals to a distant point, means for varying the magnitude of the uninterrupted current emitted by said source by' a small, predetermined amount corresponding to telegraph signals, and means for maintaining the said changes .in magnitude of the said carrier current at the transmitting and receiving terminals substantially free of bias independentlyof variations in the transmitting medium within predetermined umits. l I 12' s 3. In a lcarrier telegraphsystem, the method of reducing the eitect of line equivalent changes which consists in transmitting uninterruptediya carrier current the amplitude of which varies by Va predetermined amount inaccordance with signais and maintaining in the uninterrupted car- `rier current a xed. amplitude change during reception independent of variations in the conditions of transmission.

4. The method of signaling which consists in generating a carrier current, varying the magnitude of said carrier current by a small amount in accordance with telegraph signals without interrupting said carrier current, transmitting said modulated carrier current, receiving said modulated carrier current, rectiiying it into current pulses corresponding to the original signals, and maintaining said pulses cf a magnitude which is always independent of the medium of transmission.

5. The combination of 'a source of alternating current of large amplitude, means for tranmittina said alternating current and varying its magnitude by a small amount in accordance with signais, the transmitted signal bearing current always being ci substantial magnitude, a vacuum tube detector upon which said alternating curvrent is impressed, and a. circuit coupled to said detector to which the detected current is transmitted, said detector having input and output circuits oi' low resistance so that the characteristic of the detector will have a large range over which the output circuit current bears a linear relation to the input circuit current. A v 6. In a carrier'telegraph system, a line havin a transmission characteristic which is highly variable-and transmitting current of large but slightly variable amplitude corresponding totelegraph signals, a'detector having low resistance input and output circuits, the input circuit oi said detector being coupled to said line, a circuit coupled to the output circuit of said detector in which pulses oi detected current ow. and means including said detector and the circuit coupled to the output circuit of the detector whereby said pulses will maintain an amplitude variation in said cir cuit coupled tothe output circuit which is always independent of the variations in the transmission characteristic oi' said line.

VAUGHN P. THQRP.

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