US1853070A - Equalization of power level in alpha signal transmitting system - Google Patents

Description

D. MITCHELL April 12, 1932.

Filed Dec. 29, 1928 ATTORN EY I outrun stares;

Patented Apr. 12, 1932' PATENT OFFICE DOBEN MITCHELL, OF -NEWYORK, N. Y., ASSIGNOIR TO AMERICAN TELEPHONE ,AND TELEGRAPH VC OMEANY, ACORPORATION' OF NEW, YORK EQUALIZATION, or rownn nnvnn IN A SIGNAL TRANSMITTING SYSTEM 2 Application filed December 29; 1928. Serial No. 329,203.

mitting system to efiect adjustment of a condenser charge applied so as tocorrect a tendency for the signals to depart from a deslred power level at that point. Another object indicated injthe appended claims.

of my invention is to provide for keeping the power level adjuster to a desired standardvalue in a telephone j transmission system, In one aspect, a device-accordmg to my in- Venti'onoperates by virtue of the )power in the, signaling Currents andthereby adjust-- ment of a suitably connected condenser is efg fected to hold the power levelat a desired value. My invention may be practiced by means ofthree-electrode vacuum'tuhes with- ;out the use of relays having make-and-break contacts. V

p The accompanying drawing is adiagram- V maticrepresentation showing an example of the practice of'my invention. It'will be understood thatthe following specification relat-es principally to this particular example ofthe invention and that its scope will be "Referring to the drawing, this shows a four-wire telephone system which may be'a link connecting with a two-way radio telephone system. Transmission from west to east is over the lineL through the amplifier A' and on over the 7 line L Transmission from east to west is similarly through the line L' amplifier A and line ,L' The amplifiers A and A constitute a four-wire repeater.

V1 and V afford a shuntp athacross theline The system shown in the drawing tests the power levelon the output side ofthe ampli iier A and, shunts more or less of theenergyj passing from L to L; soas to correct for a,

departure from proper standard value in the energy level at the output of the amplr:

fier A. y c 1 i It will be seen that the two vacuum tubes L and'the adjustment referred to is :made by varying thedynamic impedance of the shunt'path through these two tubes, as will be'explained farther along in this specification. For reasons whichwill appear, a reverse adgustment is made in a correspondv ing shunt in the line L' L The four-wire system shown in the drawing is arranged so that transmission in one direction cuts oii' the system for transmission in the opposite direction, and vice versa. Thus objectionable echo effects are prevented Normally, the line LI L is short circuited at 21. so that the system'stands in read ness only'for transmission from eastto. West. \Nhen'transmlsslon from east to west occurs, a little of the voice current energy goes by a. branch circuit to the amplifying d'etectorAD which operates a relay to shortcircuitthe line L L at22. The delay circuit D0 gives assurance that p the relay 22 will, be operated before energy fromeast to west can get around and operate the amplifying detector AD a path for suchenergy might be afforded by some degree of unbalance at the three-winding transformer or hybrid coil H.

When voicecurrents'are put on the systenr from west tofeast, theyjopera tethe amplifying detector AD andoperaterbothrelays at21 and 23, thus short-oircuiting theline from east to west at 23, and opening, the

short circuit from west to east at '21. The

delay circuit DC, gives assurancethat the short-circuit at 21 will be opened beforethe voice currents reach'this point on their way from west to east on the line L1 v Proceedingfrom west to east along the line L1 L2, the repeating coil RC gives a high ratio step up and the repeating coil RO gives an equal ratio step down. Hence, between these two coils the circuit is of high imped ance. Normally, direct currents'are flowing: inthe plate circuits of the two vacuum'tuhes V and Vi, and the path from, plate to fila ment in V and'from filament to plate in V is a high impedance shunt path across the two sides of theline'between the two repeating coils RC and RC ,The shunt impedance here in view is a dynamic impedance and is the ratio of'voltage increment to current in-V crement. This ratio is the slope on thevoltamperecharacteristic of the tubes Vyand V5,-

The point of this characteristic that is in volved is determined by the voltage on the grids of the tubes V and V and this is determined by the charge on the condenser C Normally, there will be no charge on the condenser C This condenser might receive a charge in the plate circuit of either tube V or V but the grid of the tube V is at such a negative value that no condenser char ing current willfiow in the plate circuit of Also, as will be shown presently, there is a potential drop in the resistance R that puts a sufiicient negative voltage on the grid of the tube V to prevent ny charging of the condenser C in the plate circuit of that tube.

Normally, a considerable current is flowing in the plate circuit of the tube V; and this gives the potential drop in the resistance R that was mentioned above. Although the grid battery for the tube V; is negative, its electromotive force is counter-balanced in the grid circuit by an opposite potential drop along the resistance R due to current flowing in the plate circuit of the tube V This is the normal condition for the apparatus mentioned in the foregoing description. Now, suppose voice currents come from the west over the line L L These currents are amplified in the tube V and affect the detector tube V Due to the blocking condenser C and resistance R in the grid circuit of the tube V their only action is to reduce the plate current of the tube V This allows the condenser C to discharge and reduces the potenial drop through the resistance R, thus permittting the grid circuit battery of the tube V to become effective.

The tubes V and V and the transformers between the line L and the tube V and between the two tubes are so chosen that the discharge of the condenser C can be just brought about by the peaks of speech current from the weakest voice for which the system is designed to operate, but not by the noise current-s normally present. Stronger voice waves can produce no greater change than this because the space current of tube V cannot become negative. The change of potential of the grid of the vacuum tube V;,, due to the removal of the drop in the resistance R due to the space current of tube V thus occurs suddenly on the arrival of speech waves but is substantially independent of the magnitude of these waves.

But the voice currents from amplifier A to line L go through the repeating coil RC into the grid circuit of the tube V and affect the tube V as a detector and counteract the effect ofthe negative battery in the grid circuit mentioned in the preceding paragraph.

Thus, if the power level is correct at the output from the amplifier A, and if adjustments have been made accordingly, re-

moval of potential drop across the resistance R will be counter-balanced in the grid circuit of the tube V by the impulses received through the repeating coil RC and the current in the plate circuit of the tube V will remain unchanged. Thus, there will be no tendency for the condenser C to become charged by current in the plate circuit of the tube V The adjustment of the plate circuit battery of the tube V, is such that currents in the repeating coil RC due to a normal power level at the output of amplifier A, will not be sufficient to overcome the negative bias on the grid of the tube V and therefore the condenser C will not tend to charge by current in the plate circuit of the tube V Now, suppose the power level is too low at the output of amplifier A, as, for example,

if the speaker at the station on the west 5 speaks in too low a voice. The result will be a subnormal transfer of energy into the grid circuit of the tube V through the repeating coil RC accordingly, the grid battery for that tube will put higher negative potential on the grid, and the plate circuit current in the resistance R will be decreased, thus cutting down potential in the grid of the tube V and permitting a charging current for the condenser C to flow in its plate circuit. The condenser C will charge slowly. This will make the grids of the tubes V and V: more negative and increase the dynamic impedance in the shunt branch through the two tubes V and V 2 in series. Hence, more of the speech current energy on the line Will be transmitted through the repeating coil RC into the amplifier A, and the tendency of a low power level on the output side of this amplifier A will be corrected.

On the other hand, suppose the power level on the output side of the amplifier A is too high, this will have no effect in the plate circuit of the tube V but will operate on the tube V as a detector through the repeating coil RC4, and accordingly a current will flow in the plate circuit of the tube V and char e the condenser C slowly in a direction to taEe negative potential off the grids of the tubes V and V This will lower the impedance of the shunt through the tubes V and V- and hence more of the voice current energy coming in over the line L will be shunted and less will go through the repeating coil RC into the amplifier A, and thus a compensatory adjustment will be effected on the output side of the amplifier A.

' The line from east to west, from L through the amplifier A to L has a high impedance section like the line the other way, with two tubes V' and V corresponding to V and V In the line L -L we have seen that adjustment is effected by varying the potential on the grids of the tubes V and V It will be seen at once from the the system works as follows; If the person at the west end stops talking,the shunt at I 23 willopen after thelapseof a certain hangover interval, Suppose that at once voice currents come from east. Due to the likelihood of some degree of unbalance at H, some of the energy of these voice currents will get into the line L and with high over-all amplification from L to L there would be v a tendency to diversion of energy through AD and anundesirable operation ofrelays '21 and 23. But when thereis high over-all amplification from Li to L there will-be low over-all amplification from I1 to 1/ and thus the tendency to cause such false operation will be'counteracted. Y a

It willbe understood that all impedance elements, batteriesjetc. are adjustable; certain of these for which 'adjustment is most important have the adjustability indicated diagrammatically in the drawing.

Itmay be noticed that with suitable adjustment of the apparatus shown in the drawing no action'will take place whencorrect volume is being delivered at the output from the amplifier A. The proper adjustments in-c volve certain time constants and the relative ains in the system. According to these ad justmnts, when any speech whatever is going through the system from west to east the tube V will be blocked, thus tending to interrupt the plate current in "the tube V and permitting the charging current to flow in the plate circuit of the tube V to the con denser C However, the condition for correct volume is that the speech current impulses operating in the grid circuit of the tube V through the repeatingcoil RC shall be just strong enough to overcome the grid bias so that the current in the plate circuit 7 of the tube V shallbe maintained and keep a potentialdrop through theresistance R and thus prevent the flow of a charging cur rent for the condenser C in the plate circuit speech currents of correct volume.

7 As to the time actions, the measure ofthe capacity of the condenser Cgdeterminesthe speed with which regulation accomplished If-thevolume departs from standard bya large amount theinitial regulation "will take place faster than if the volume departs 'to a less degree. Thisadvantageous feature or operation is due to the fact that the impedance through which the condenser will charge will be less if the volume departs from I the standard by a'large amount than if it departs by a small amount. The system shown in the drawingis back acting; this means that regulation is accomplished bythe use of'energytaken from the point at which standard, volumes isdesired, that is,; at the output; from the amplifier A,and adjustment to preserve constancy of volume occurs when the volume at this pointtends to change. I a l 1 The system shown in the drawing is also full neutral?,i,that is,yit does not-regulate unless speech currents (or currents due to interference, for example) are entering the syste'm, butthev system stays at the gain last set until such currents again enter the sy tern, or until the charge on condener G has slowly leakedawayi -c However, by changing the grid bias on either of the tubesV or V; it is easy to make these tubesdraw a slight plate currentunder normal conditions and-thus cause the device to creep back to a predetermined setting Forthis purpose the principal adjustments will be madeat the resistance R and attlie c grid battery of the tube V4. In this way the.

system can bemade to assume a desircdad- V j ustment'of normal gainafter theend of any 1; In thetransmission ofjsignals by .elec tric currents, the method of adjusting over H all ga n at an amplifier'whlch consists in i shunting part ofthe current adjacent to the amplifier through a vacuum tube, varyingits cgrid potential to adjust the proportion of the current shunted, through the tube and cordance with the output from said amplifier.

2.1111 the transmission of signals by electric currentsover apairof conductorsj the method of ad ustmgloss or gain wh-1chcon efi'ecting the variation of said grid potential; by varying the .chargeona condenser in act-t sists, instepping the voltage up over at short stretch of the conductorpair and then down,g p and in thisshort stretch shunting part of the, a cui'irent through a vacuum tube, andivarying its impedancebyqjvarying its grid potential:

ed (to thatctransinitte id'.

Qto'adjust the proportion of the current shunt,-

tials in opposite directionsfor the two shunts to increase or 'decreasetheimpedance of the shunt andslmultaneously decrease or increase;

the impedance of the'other shunti j 1 .;.;Inafour wiretWo-way signaltransmit-r ting system, means for cutting off transmission one way while it occurs the other way, amplifiers respectively for the two ways, vacuum tube shunts respectively on the two pairs of the four-wire system, and means to increase the grid potential in one such shunt and simultaneously decrease it in the other so that as the over-all amplification for one pair is increased or decreased, the over-all amplification for the other pair will be simultaneously decreased or increased.

5. A conductor pair for transmitting signals, step-up and step-down transformers to increase the impedance between them, a vacuum tube shunt across the pair between these transformers and means to vary the grid potential in this shunt to adjust the shunt impedance and thereby adjust the proportion of current that is shunted to the proportion transmitted.

6. A signal transmitting circuit comprising a pair of conductors, a vacum tube shunt across the conductors, means to test the amplitude of the signaling current on the output side of said shunt, and means operated thereby to adjust the grid potential of the vacuum tube to compensate for changes of power level acording to the test.

7. In a four-wire two-way signal transmitting system, vacuum tube shunts across each pair of wires, means to test the power level on the output side of one such shunt, and means to adjust the grid potentials of the vacuum tubes to compensate a tendency to change the power level for the pair on which it is tested and to effect an opposite change in the power level in the other pair.

8. In a four-wire two-Way signal transmitting system, means to cut off transmission one way when it is in course the other way comprising amplifiers respective to the two ways of transmission, two vacuum tube shunts, one associated with each amplifier, means to test the power level on the output side of one amplifier, and means to adjust the grid potentials in said "acuum tubes shunts thereby so as to keep the power level at the point of test substantially constant butto vary the power level by opposite adjustment in the other pair of the system.

9. In a four-wire two-way signal transmitting system, a voice operated detector-amplifier set to block transmission one way on one pair of wires when signal transmission the other way on the other pairs is in progress, vacuum tube shunts for each pair of wires and means to change the grid potentials in oppo site directions for the two shunts to increase or decrease the impedance of one shunt and simultaneously decrease or increase the impedance of the other shunt.

In testimony whereof, I have signed my name to this specification this 28th day of December, 1928.

DOREN MITCHELL.

Images