US1438217A

US1438217A - Means for and method of regulating the transmission over electric circuits

Info

Publication number: US1438217A
Application number: US253484A
Authority: US
Inventors: Alva B Clark
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1918-09-10
Filing date: 1918-09-10
Publication date: 1922-12-12
Anticipated expiration: 1939-12-12

Description

A. B. CLARK.

MEANS FOR AND METHOD OF REGULATING THE TRANSMISSION OVER ELECTRIC CIRCUITS.

APPLICATION FILED SEPT. 10, I918- Patented Bea 12, 1922.

3 SHEETS SHEET l.

INVENTOR. flab. 6705/ Y Z, 3 2

ATTORNEY A. B. CLARK.

MEANS FOR AND METHOD OF REGULATING THE TRANSMISSION OVER ELECTRIC CIRCUITS.

APPLICATION FILED SEPT 10, 1918.

1,438,217, I Patented Dec.12,1922.

3 SHEETSSHEET 2.

MIIIIIITI;

lllll-n IN VEN TOR.

A TTORNE Y A. B. CLARK.

MEANS FOR AND METHOD OF REGULATING THE TRANSMISSION OVER ELECTRIC CIRCUITS.

APPLICATION FILED SEPT 10, l9l8- Patented Dec. 12, 1922.

3 SHEETSSHEET 3.

INVENTOR. 63 lar/o BY Z" 561C. ATTORNEY Patented ea. 12, 1922.

ALVA B. CLARK, OF BROOKLYN, NEW YORK, ASSIGN OR TO AMERICAN TELEPHONE I AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

MEANS FOR AND METHOD OF REGULATING THE TRANSMISSION OVER ELECTRIC CIRCUITS.

Application filed September 10, 1918. Serial No. 253,484.

To all whom it may concern:

Be it known that I, ALVA B. CLARK, residin" at Brooklyn,'in the county of Kings and btate of New York, have invented certain Improvements in Means for and Methods of Regulating the Transmission over Electric Circuits, of which the following is a specification.

This invention relates to transmission circuits, and more particularly to transmission circuits of the type known as four-wire circuits, although it is in general applicable to transmission circuits of any kind.

The principal object of the invention is to provide suitable methods and means for maintaining constant the transmission equivalent of a circuit regardless of variations in the conditions affecting the electrical characteristics of the circuit. Other and further objects will, however, be clear from the description of the invention hereinafter given.

In general, the transmission equivalent of a circuit varies, due to a number of different causes. If the transmission circuit be of the four-wire type, in which a pair of separate transmission lines are employed for transmitting in opposite directions, these causes may be divided into two classes: The first class comprises causes which affect both sides of the circuit; or in other words both transmission lines, in the same manner. Among these causes may be enumerated variations in the resistance of the conductors, loading coils etc., variations in capacity, inductance, or other electrical char acteristics of the cable, and variations in the potential of repeater batteries common to repeaters in both sides of the circuit. The second class comprises causes which aliect the two sides difierently, such as variations in the structure of repeater bulbs where vacuum tube repeaters are employed, variations in the potentials of grid batteries, or accidental variations due to the resistance of bad joints, leakage due to poor insulation etc.

In the case of transmission circuits in which only one transmission line is used for transmission in the two directions, the factor of variation with respect to the two sides of the circuit will be absent, but some or all of the above c u es may contribute be made for overcoming the effect of these I variations.

The present invention contemplates over coming these difiiculties by varying some adjustable element of the transmission circuit to compensate for the variations introduced into the circuit by any or all of the above causes. In general, a number of methods may be employed in carrying out the objects of this invention; for example the effects due to some one cause such as temperature variations, may be compensated for by observing the changes produced in one circuit of a cable, and then varying in accordance with such changes, similar elements of a number of circuits which are affected in a like manner. In the case of a four-wire circuit, effects due to all possible variations in the tWo sides of the fourwire circuit may be compensated for by varying like elements in the two sides of the circuit on the assumption that the two sides vary alike. If greater precision is desired, the adjustment may be made for each side of the four-wire circuit separately to compensate for all causes of variation in either side.

These methods may be carried out either manually or automatically, and by making adjustments either constantly as the circuit varies, or at intervals of suitable frequency. For purposes of illustration, apparatus is disclosed for carrying out the last method above mentioned either manually or auto matically, but it will be understood that any of the above mentioned methods are within the scope of this invention.

The objects of the invention are secured as herein disclosed by impressing potential variations of known characteristics upon a suitable receiving apparatus through a transmission element having a transmission equivalent equal to that to which it is desired that the circuit shall be adjusted. The receiving apparatus is then so arranged that a desired efiect is produced thereon. Similar potential variations are then transmitted over the circuit to be regulated, and impressed upon the receiving apparatus. Some element of the circuit, such as an am- 'tage that it may is then adjusted either manually or automatically under the control of the receiving apparatus until the same effect is produced on the receiving apparatus as in the first case. By separately treating both sides of the four-wire circuit in this man-- ner, the circuit may be so altered thatits transmission equivalent is the same in either direction regardless of-the causes producing variations in its electrical characteristics.

The invention may now derstood by reference to the following description when read in connection with the accompanying drawing in which Figures 1 and 2, when placed end to end, constitute a circuit diagram of an arrangement for automatically adjusting a four-wire transmission circuit to a desired transmission equivalent, while Fig. 3 illustrates a somewhat similar arrangement for securing the same result manually.

Referring to Figs. 1 and 2, two distant stations are shown interconnected by a fourwire circuit comprising transmission line L, for transmitting in one direction, and transmission line L for transmitting in the opposite direction. The terminating apparatus at one station is shown in Fig. 1, and that of the other station is shown in Fig. 2. It will be understood that each of the transmission llines L and L, may include one or more one way repeaters (not shown) located at intermediate stations. At the terminating stations, the four-wire circuit is associated by means of the usual threewinding transformers 1 and 2, with terminating two-wire lines L and L said terminating two wire lines being balanced by suitable artificial lines N and N In order to adjust the transmission equivalent of the four-wire circuit, amplifiers A and A,, are inserted in the lines L and L respectively at the terminal stations. These amplifiers as illustrated are of the vacuum tube type, but it will be understood that any type of amplifier, or in fact, any type of adjustable transmission device capable of giving either a loss or a gain may be employed for this purpose. The employment of an amplifier for this purpose, however, has the advanserve to secure or assist in securing the desired amplification for the four-wire circuit as well as for adjusting the transmission equivalent of the circuit. In order to vary the amplification of the amplifiers A, and A potentiometers P and P are provided in the grid circuits of the amplifiers. In order to vary the setting of the potentiometer P an automatic switch S is provided, said switch being adapted to be rotated in either of two directions under the control of stepping magnets 3 or 4; release magnets 5 and 6 being provided to hold the switch in its altered position. A

be more fully un-- similar switch S; is provided for potentiometer P said switch being control-led by stepping magnets 7 and 8, and release mag- .nets 9 and 10.

In order to regulate the transmission equivalent of the four-wire circuit, a switch 11 is provided, as shown in Fig. 2, said switch controlling a simplex circuit over the two sides of line L, of the four-Wire circuit or controlling any other suitable circuit between the terminals of the four-wire circuit to disconnectthe four-wire circuit from the terminating two-wire lines L and L and connect it to the transmission adjusting apparatus. The transmission adjusting apparatus at the station of Fig. 1 comprises a source of potential variations S and a receiving apparatus B A similar source of potential variations S and receiving apparatus R is also provided at the station of Fig. 2. The two sources should have similar electrical characteris tics, and in order to calibrate the receiving apparatus, for instance, receiving apparatus R the source S maybe connected to the receiving apparatus R through a network or transmission element N whose transmisis desired to adjust each side of the fourwire circuit. N may consist of an amplifier when it is desired to adjust the transmission equivalent of the four-wire circuit to a value less than zero (i. e. a gain). The receiving apparatus comprises amplifiers 12 and 13 and a vacuum tube rectifier 14, said rectifier being shunted by a condenser 15 and a polarized relay 1.6. If desired, an ammeter 17 may be included in circuit with the polarizedrelay 16. Alternating current impressed upon the receiving apparatus will be amplified by amplifiers 12 and 1.3 and rectified by rectifier 14 so that current will only flow through the rectifier in the direction indicated by the arrow. A battery 18 is connected in shunt of the rectifier 14 and so poled as to tend to cause a current flow through the polarized relay 16 in a direction opposite to that which flows from the rectifier 14 through the polarized relay. The alternating current from the source S is transmitted through the network N, to the receiving apparatus R and the amplifiers 12 and 13 are adjusted so that the rectified current passing through the polarized relay 16 from rectifier 14 is just equal to that from the battery 18, so that no efiect is produced on the polarized relay 16. Circuits are arranged whereby the polarized relay 16 may control the setting of the switch In a similar manner the receiving apparatus R, at the station of Fig. 2 may comprise amplifiers 19, 20, rectifier 21 shunted by condenser 22, polarized relay 23, ammeter 24 and battery 25, these elements being similar in all respects to the work N corresponding elements ofFig. 1. The p0- larized relay 23 controls circuits for stepping the switch 8,.

Suitable switching arrangements are provided at the two terminating stations so that when the receiving arrangements have been calibrated, the source of potential variations at one station may be connected to the four wire circuit, transmitted thereover in pne direction, and applied to the correspondlng receiving apparatus at the other station. Should the transmission equivalent of the circuit for transmission in that direction be above or below that of the network N or its corresponding network N current will flow through the polarized relay of the receiving apparatus to throw its armature in one direction or the other, thereby causing the automatic switch to adjust the potentiometer until current no longer flows through the polarized relay, the transmission equivalent being then equal to that of the network N or N as the case may be.

Assuming that it is desired to regulate the transmission equivalent for transmission from the station at Fig. 2, to the statlon at Fig. 1 over the line L the sources of potential variations S, and S, are first adjusted by means of the rheostats 32 and 33 so that potential variations of the same characteristics are supplied by each as measured by the alternating current ammeters 34 and 35. Switches 36 and 37 are so thrown as to connect the source S, to the receiving apparatus R, through the net- B adjusting the potentiometer of the ampli er 12, the receiving apparatus may be so calibrated that no current flows through the polarized relay 16, whose armature will consequently be in a neutral position. Switches 36 and 37 are then restored to the position shown in the diagram, and the switch 11 is thrown so as to connect negative battery to the simplex circuit over the transmission line L Current then flows from negative battery over the switch 11 through. polarized relay 26, relay 27, conductor 28, thence in parallel over the two sides of line L over conductor 29, through relay 30, polarized relay 31 to ground. Relays 27 and 30, as well as polarized relay 31, are a tuated by the current flowing 1n thls circuit, but the direction of the current is such that polarized relay 26 is unactuated.

The relay 27 disconnects the four-wire circuit 7 from the terminating line L, at the station of Fig. 2 and connects the four-wire circuit to the source S, over the front contacts of relay 27, back contacts of relay 38, contacts of switch 42 to the winding of transformer 39, through which current is transmitted from the source S, to the four-wire circuit. At the station of Fig. 1 the actuation of the polarized-relay 31 completes a circuit from battery, through the winding of relay 40,

wlnding of stepping over the contact of relay 31, through the winding of relay 41 to ground. The actuation of relay 41 completescircuits over its front contacts from the opposite contacts of polarized relay 16 through the stepping magnets and release magnets of the switch 8,. By the energization of relays 30 and 40, the four-wire circuit is disconnected from the two-wire line L and is connected over the front contacts of relay 30, front contacts of relay 40, and contacts of switch 37 to the receiving apparatus R Alternating current is now transmitted from the source 8, through the three-winding transformer 2, over transmission line L,, through the ampllfier A over the front contacts of relays 30 and 40, the contacts of switch 37 to the receiving apparatus R The current thus transmlt-ted is amplified by amplifiers 12 and 13, and rectified by rectifier 14. If the transmission over the circuit L is too low, current will flow through the polarized relay 16 in such a direction as to throw the armature of said relay to its upper position whereupon current flows from battery, through the interrupter 44, which may be of any well-known type, such as a rotating commutator, upper contact of polarized relay 16, left-hand front contact of relay 41, magnet 7 and winding of release magnet 10 to ground. Each pulse of current through the circuit just traced causes the release magnet 10 to Withdraw the pawl carried by the armature of stepping magnet 8 from the corresponding ratchet wheel and causes the stepping magnet 7 to advance its ratchet wheel one step. The switch S is thus advance? step by step until the potentiometer is s adjusted that the amplification of the amplifier A, brings the transmission equivalent of the circuit up to a point where It is equal to that of the network N When this condition is reached, current no longer flows through the polarized relay 16, and the desired adjustment for the transmission line L, is attained. Should the transmission over the line L be too great, current will flow through the polarized relay 16 in the opposite direction, thereby throwing its armature to its lowest position and actuating stepping magnet 8 and release magnet 9 to step the switchS, in the opposite direction until the proper transmission equivalent is obtained.

In order to regulate the transmission over the line L in the olpposite direction, the receiving apparatus 3 will first be calibrated in a manner similar to that already described, and the switch 11 will then be thrown to connect positive battery to the simplex circuit through polarized relay 26, relays 27 and 30, and polarized relay 31. The current flowing through this circuit is now in the opposite direction, so that the relays 27 and 30' are actuated as before, and

polarized relay 26 is operated instead of polarized relay 31. The operation of polarized relay 26 completes a circuit for relays 38 and 45, so that the receiving apparatus R is connected to the four-wire circuit and the circuits of the stopping and holding magnets of the switch S, are connected to the contacts of the polarized rela 23. At the station at Fig. 1, the source 5 is connected to the four-wire circuit and transmission now takes place over the line L The operation is now in all respects similar to that already described for the transmission line L and no further description is necessary. Upon restoring the switch 11 toneutral position, thereby opening the simplex circuit, the fourwire circuit will be again connected to the terminating two-Wire lines L and I1 Instead of arranging the apparatus so that the adjustment of the four-wire circuit is performed automatically a manual apparatus such as illustrated in Fig. 3 may be employed. In this figure the apparatus shown at one terminal station only of the fourwire circuit is illustrated, it being understood that similar apparatus will be rovided at the opposite terminal station. he fourwire circuit comprising transmission lines L and I is related through the usual threewinding transformer 1 to a two-wire extension terminating in a jack J, whereby connections may be made to two-wire lines. For adjusting the transmission equivalent of the circuit amplifiers A, and A are associated with transmission lines L, and L respectively, and potentiometers P and P are provided for the purpose of adjusting the amplification of the amplifiers. The setting of the potentiometers may be controlled by manual switches 46 and 47. The amplifier A and its potentiometer P are shown at the same station as amplifier A,, and potentiometer P,, but it will be understood that these devices may be arranged at the other terminating station, as shown in connection with the automatic arrangement of Fig. 2.

A source of potential variations similar in all respects to that shown in Fig.1 is provided, a plug P adapted to cooperate with the jack J being employed for associating the source with the four-wire circuit. A receiving apparatus R similar to that shown in Fig. 1

may be associated with the four-wire circuit by means of a plug P adapted to cooperate.

with the jack J. The receiving apparatus R, differs from that of Fig. 1 in that no polarized relay is provided, but instead, an indicating arrangement such as the indicating meter 48 is associated with the rectifier 14. Transmission elements M and N, are provided for purposes more fully hereinafter apparent. For controlling the various connections, keys K and K are shown. When the key K is operated with the key K in its normal position, the source of pocuit through the plug 1 \Vith the key K unactuated and the key K thrown to the right, the source S, may be applied to the receiving apparatus B, through the transmission device lVI, for purposes of calibration.

In order to calibrate a receivin' apparatus, the transmission arrangement 4 will be given some arbitrary setting as previous experience or the exigencies of the particular situation may determine. The key K is then thrown to the right, thereby closing a circuit over its contact 49 and through relay 50 to battery. Relay 50, upon being energized, at its lower front contact completes the series filament circuits of amplifiers 12 and 13 and rectifier 14, thereby rendering the receiving apparatus operative. The source S, is now connected to the receiving apparatus R through the transformer 51, back contacts of relay 52, transformer 53, back contacts of relay 54, through the transmission device M and back contacts of relay 55. The potentiometer 56 of amplifier 12 may now be adjusted so as togive a proper reading of the indicating meter 48.

If now it is desired to regulate the transmisison equivalent of the transmission line L the key K, at the station shown is operated, the he K is thrown to the left and the plug 4 is inserted in the jack J, The actuation of key K completes energizing circuits for relays 54 and 55 over its lower contact and completes enerigzing circuits for relays 52 and 57 over its upper contact. Relays 54 and 55 disconnect the transformer 53 and receiving apparatus R from the transmission device M and connect them to the transmission device N Relay 52 connects the source 8," over its front contacts to the left hand contacts of key K while relay 57, over its front contacts, connects the transformer 53 to the plug P The actuation of the key K disconects the source S, from the plug P The receiving apparatus R is now connected to the fourewire circuit over the front contacts of relay 55 through the transmission device N front contacts of relay 54, transformer 53, front contacts of relay 57, plug P and jack J.

At the opposite terminal station a source similar to S is associated with the fourwire circuit by operating the key corresponding to K leaving the key corresponding to K, in its normal postion. The source corresponding to S, will then be connected to the four-wire circuit through the transformer corresponding to 51 over the front contacts of a relaicorresponding to 52, over the normal left. hand contacts of the key corresponding to K and over the contacts of the plug corresponding to P and the terminating jack of the four-wire circuit at the opposite terminal station. Potential variations are then transmitted from the source corresponding 'to S over the transmission line L, through the amplifier A and thence over jack J and the plug P and the circuit previousl traced, to the receivingapparatus R T e transmission device N 4 is now set so that its transmission equivalent will be equal to the algebraic difference between the transmisslon equivalentof the device M and the desired transmission equivalent for the transmission line L .By operating the switch- 47 the potentiometer P may be adjusted until the indicating meter 48 gives the same indication as when it was calibrated. Under this condition, the transmission line L, will have a transmission equivalent equal to the difference between the transmission equivalents of transmission devices M and N 'If, instead of regulating the transmlsslon of the transmission line L it is desired to determine its actual transmission equivalent, the circuits may be arranged asjust described and the adjustable transmission device N adjusted until the same indication is given by the ammeter 48 as when it was calibrated. The device may be provided with a scale so that under this condition the transmission equivalent of the transmission circuit may be read directly in terms of some suitable unit.

When it is desired to determine the transmission equivalent, or to regulate the transmission of the circuit of the line L, the apparatus of Fig. 3 will be so set as to connect the source S, to the four-wire circuit over plug P and jack J while at the opposite termlnal station the apparatus w1ll be arranged to connect the receiving apparatus to the four-wire circuit. Since the operation is in all respects similar to that already discussed, no further discussion thereof is deemed necessary.

It will be apparent that by means of the apparatus and methods herein disclosed, the transmission equivalent of a circuit may be maintained constant at any desired value, regardless of variations due to any cause whatever. It will also be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated, without departing from the spirit of the inventlon as defined in the following claims.

What is claimed is:

l. The method of adjusting the efliciency of a transmission line extending between two signaling stations, which consists in applying potential variations of known characteristics upona receiving apparatus through a transmission element, adjusting the circuit includin said transmission element and said rece ving apparatus until a desired effect is produced upon the receiving apparatus, mpressing potential variations of similar characteristics upon said receiving apparatus through sa1d transmission line and adjusting an element of the trans mission line until the same effect is produced upon the receiving apparatus.

\ 2. The method of adjusting the efliciency of a transmission line including amplifyin means Which consists in applying potentia variat ons ofknown characteristicsupon a receivlng apparatus through a transmission element, adjusting the circuit including said transmlssion element and said receiving apparatus until a desired efi'ect is produced upon the recelving apparatus, impressing potential variations of similar characterist1c s upon said receiving apparatus through said transmission line, and adjusting the amplification of said amplifying means until the same efiect is produced upon the receiving apparatus.

3. The method of adjusting the efliciency of a transmission line extending between two signaling stations, which consists in applying potential variations of known characteristics upon a receiving apparatus through a transmission element having a transmission equivalent ,equal to that to which it is desired to adjust the transmission line, impressing potential variations'of similar characteristics upon said receiving apparatus through said transmission line, and adjusting an element of the transmission line until the same effect is produced upon the receiving apparatus.

4. The method of adjusting the eflicien'cy of a transmission line including amplifyin means, which consists in applying potentia variations'of known characteristics upon a receiving apparatus through transmission element having a transmission equivalent equal to that to which it is desired to adjust the transmission line, impressing potential variations of similar characteristics upon said receiving apparatus through said transmission line, and adjusting the amplification of said amplifying means until the same effect is produced upon the receiving apparatus.

5. The method of adjusting the efliciency of the two sides of a four-wire circuit including a pairof transmission lines for transmitting in opposite directions, which consists in calibrating separate sets of receiving apparatus adapted to be associated with said transmission lines, so that a desired effect will be produced upon each receiving apparatus when similar potential variations of known characteristics are impressed thereupon through a transmission path having a desired transmission equivalent, separately impressing potential variations having the desired characteristics upon each receiving apparatus through the corresponding transmission line, and adjusting elements of the transmission lines until the desired efiect is produced in the receiving apparatus.

6. In a signaling system, a transmission line, a receiving apparatus, means to apply potential variations of known characteristics upon said receiving apparatus through a transmission element havlng a desired transmission equivalent thereby producing a desired effect upon the receiving apparatus, means to associate the receiving apparatus with one end of said line, means to apply potential variations of similar characteristics to the other end of said line, and means to adjust the transmission equivalent of said line until the desired efiect is produced upon the receiving apparatus.

7. In a signaling system, a transmission line, a receiving apparatus,'a transmission element having a desired transmission equivalent, means to associate said receiving apparatus with said transmission element, means to apply potential variations of known characteristics upon said receiving apparatus through said transmission element, means to associate said receiving apparatus with one end of said line, means to apply potential variations of similar characteristics to the other end of said line, and means to adjust the transmission equivalent of the line until the same effect is produced upon the receiving apparatus as when potential variations were impressed thereupon through said transmission element.

8. In a signaling system, two stations, a transmission line interconnecting said stations, similar sources of potential variations at each station, receiving apparatus at one of said stations, a transmission element having a desired transmission equivalent at said station, means to apply the source of potential variations at said station upon said receiving apparatus through said transmission element, means to apply the source of potential variations at the other station upon said receiving apparatus through said transmission line, and means to adjust the transmission equivalent of said line until the same effect is produced upon the receiving apparatus under the latter condition as was produced under the first condition.

9. In a signaling system, two stations, a transmission line lnterconnecting said stations, amplifying means for said line, similar sources of potential variations at each station, receiving apparatus at one of said stations, a transmission element having a denaaaarr produced upon the receiving apparatus under the latter condition as was produced under the first condition.

10. In a signaling system, two stations, a transmission line interconnecting said stations, similar sources of potential variations at each station, receiving apparatus at one of said stations, a transmission element having a desired transmission equivalent at said station, means to apply the source of potential variations at said station upon said receiving apparatus through said transmission element means to apply the source of potential variations at the other station upon said receiving apparatus through said transmission line, and means controlled by said receiving apparatus for adjusting the transmission equivalent of the line until it is equal to that of said transmission element.

11. In a signaling system, two stations, a transmission line interconnecting said station, similar sources of potential variations at each station, receiving apparatus at one of said stations, a transmission element having a desired transmission equivalent at said station, means to apply the source of potential variations at said station upon said receiving apparatus through said transmission element, means to adjust the receiving apparatus so that no effect is produced thereon when potential variations are so applied, means to apply the source of potential variations at the other station upon said receiving apparatus through said transmission line, andmeans responsive to the actuation of the receiving apparatus under the latter condition to adjust the transmission equivalent of the line until the receiving apparatus is unactuated.

12. In a signaling system, two stations, a transmission line interconnecting said sta tion, amplifying means for said transmission line, similar sources of potential variations at each station, receiving apparatus at one of said stations, a transmission element having a desired transmission equivalent at said station, means to apply the source of potential variations at said station upon said receiving apparatus through said transmission element, means to adjust the receiving apparatus so that no effect is produced thereon when potential variations are so applied, means to apply the source of potential variations at the other station upon said receiving apparatus through said transmission line, and

means responsive to the actuation of the receiving apparatus under the latter condition to adjust the amplification of said amplifying means until the receiving apparatus is unactuated.

13. In a signalling system, a circuit extending between two stations for transmitting alternating currents between said stations, said transmission circuit including an 10 adjustable transmission device, and means operating automatically under the control of a change in transmission to adjust said transmission device so that the transmission circuit will have a predetermined transmission equivalent.

In testimony whereof, I have signed my name to this specification this sixth day of' September, 1918.

ALVA B. CLARK.