US2214804A - Two-way communication system - Google Patents

Description

P 1940- H. w. AUGUSTADT 2,214,804

' TWO-WAY. COMMUNICATION SYST F11! July 18, 1959 INVENTOR I FIG. .3 l-l. WAUGUSTADT ATTORNEY 4,0, being accomplished Patented Se t. 11, 1940 UNITED STATE TWO-WAY com'ruulca'rron srs'rau HerbertW. Angultadtvalley Stream, N. Y., as-

signer to Bell Telephone Laboratories, Incorpoi-ated, New York, Y.,'

York

a corporation of New 1 J llian... July 1%, 1939, Serial No; 2 5,042

' This inventionrelates to two-way communicationsystems and more particularly to communication terminal circuits. Still more specifically. it relates to ieedback'stabilization for a combined signal and control amplifier in voice operated systems with applications to two-way communication;

Whilethe invention has applicability in a wide variety of communication circuits, it will be described more specifically in connection with atelephone conference circuit. In two-way communication terminals, especially it used in conference circuits where the connection between -a plurality of terminals is more or less permanent, it is desirable that when no normal signal is taking place the system shall be as quiet as possible, being substantially immune from room, noise and other background disturbances. Even during periods of signaling, suchas speech signaling, it is desirable that in the pauses of the usual speech the circuits shall be brought to ahighly quiescent condition. It is desirable also'that at no time shall the power level oi the signal rise to such values as to overload any part of the signaling equipment. Furthermore, it isnecessary, if; the terminal is a two-way circuit, that the high gain which is characteristic amplifiers in such circuits shall not lead to singing.

In my invention there is present in a signal path a vacuum tube amplifier, the output load 01 which comprisesa communication line or some signal responsivedevice. In order to maintain the system in a quiet condition during intervals at no signal, there is inserted in the signalpath "a loss device or attenuator which in my invention may be a series resistance or a shunt reslstonce, or a combination of the two. When signal is initiated-in this path the loss is removed, this by a control circuit, the input of which is itself connected in parallel to the main load. Also connected across the output. of the amplifier and in parallel to the main load isa circuit which limits-the power delivered by the plifier to the main load. Thus there are two control circuits in parallel with the load,

However, the cutting out of the loss referred to above substantially alters the impedance into which the amplifier works, causing a drop in output voltage which may be of suiilcient magnitude to seriously afiect the operation of the control circuits. A part of my invention resides .in/sta ,bilizlng the amplifier circuit in such manner that the output voltage is nearly independent. of the fvariations in load impedance.- This stabiliza- I my invention:

- lying a stage amplifier thus causing a drop tion is accomplished bymeans of a negative ieed-. back circuit to be described in greater detail.

. In addition, it the. system is a'tw'o-way terminal having a transmit side -and a receive side, the control circuits on the one side, when subjected 5 to signal, operate to. disable the circuits on the other side;

The inventicnwill be more fully understood by' reference to the following specification and accompanying drawing in which: l0 Fig. 1 is a simplified schematic circuit showing Fig. 2 is a more detailed circuit diagram; and v Fig. 3 is explanatory of the principles underpart of my invention.

Referring more particularly to Fig. 2, there is shown at At an initial amplifier, pre'ierably'of the'variable mu type. While a variety of such tubes is available, the one which I have illus trated inthls figure is that commonly identified a the SL7 tube. The output of A1 is impressed on an amplifier A: which is here shown as a twocomprising the amplifiers As and A3". In the output of the latter stage there is shown a transformer T1 feeding into a line, the line including a loss device Kl. here shown as aseries resistance and a shunt resistance. The loss K1 is of such a magnitude that signal initiated in this path will be so attenuated as to be of relatively small magnitude on arrival at another terminal. For the satisfactory transmission of signal, therefore, it is necessary. to reduce the effective value of this loss andI accomplish this by shorting out the whole or a part of this loss,' in this case the series portion of the resistance being shorted .out by a relay. To thi's'e'nd there is connected across the output circuit of As", in parallelto'the line load a circuit comprising a potentiometer P1 which in turn applies potential to the input circuit of the amplifier Aa- Theal- 4o ternating current output of this last amplifier is rectified with any suitable form of rectifier, such as the tube A1, giving a direct current drop across resistance R1. This drop .may then be applied directly to the winding 1 of a relay R1 or preferably, as shown, may be impressed on the input circuit of an amplifier. Ao-operating virtually as a direct current amplifier which,'inturh, gives power to the relay winding 3. The relay upon excitation closes the contacts 5 shorting out per-- tion oi-the loss K1. .It will be appreciated that the removaloi an appreciable portion of the loss K1 materially 'af-. fects theimpedance of the main load circuit,

in the output voltage-of the.

\ 5 the following:

amplifier A3" and this drop may be suflicient to cause a release of the control circuit operating on energization of coil 3, thus giving rise to a chattering of the relay with accompanying un- 5 desirable eflects.

A second control circuit is connected across the output of the amplifier As", this circuit containing a rectifier such as the diode All. The voltage drop thusset up from the output of diode I Alracross resistance R1; is applied to the grid gs of the variable mu tube Al, the bias being such as to reduce the gain when the signal volume becomes excessive. The operation of this volume limiting device is also subject to variations when 15 the load impedance changes.

To stabilize the action of the circuit as a whole I provide a negative feedback over a portion of the circuit, such as across a part or across the whole of the amplifierv As, the feedback in 20 thi circuit being shown as a network consisting of the blocking condenser Ca and resistance R3. The effect of this stabilizer is that of making the output voltage of A3" practically independent of the variations in load impedance, thus removing 25 the dimculties referred to.

"ter understood-by analysis with reference to Fig. 3, which shows a simplified circuit-diagram of a portion of the circuit under consideration, and in connection with which an analysis on the basis of a numerical example will be'given. 'It is t0 be understood, however, that the invention is in no way restricted by the values used.

For thepurposes of the analysis let R be the equivalent plate circuit generator impedance.

Also for illustrative purposes assume the following impedance effects.

these being reasonable values for the circuits and .tubes which may be used. For convenience also one may use the following notations:

V1=voltage at X required to operate the relay. 45 Vz=voltage at X after relay operates when Va=voltage at X after relay operates when f=20R. Eo=equivalent plate circuit generator to pro- 5 duce voltage V when j= r E=equivalent plate circuitgenerator to produce voltage V when f=20R.

.The analysis of feedback circuits then leads to Voltage V to operate relay is the same in both cases, hence the-percentage voltage change is The action of this stabilizing'circuit will be bet- From this it is seen that without the stabilizing circuit there would be a 45% change "in the output voltage of the amplifier. As" whereas with the stabilizing network this variation is reduced to 4.8% for the particular values used in this 5 illustrative case.

In Fig. 3 one tube only is shown whereas in Fig. 2 the feedback is over the two amplifiers As and A3. Whether the one or the other connection is made will be determined by the sensitivity re- 'quired for suitable stabilization. In either case consideration must be given to the proper phas ing of the feedback connection. Thus in Fig. 3 the particular feedback network should be connected. directly to thegrid of the one amplifier whereas in Fig. 2 the-feedback over two stages should be made tothe cathode for proper pliesing. Also, while in both of these figures a very simple form of negative feedback is shown, it

is to be understood that other more complicated 1 networks may be desirable in certain cases.

If the terminal is to be arranged, for two-way 1 communication as shown in Fig. 2, a similar cir=- cult is provided for the receive side. Thus incoming signal is impressed on the input circuit v of the tube As. which in turn is shown as connected to a second stage A4. To the output of this stage there is connected the transformer T2 feeding into the line through a loss device K:

similar to K1. A control circuit is also provided analogous to the control circuit of the transmit side already described. Operating on the relay R2 this controls the contact 6 to short out a portion of the loss m when signal comes through the receive path. 85 l In order that there shall be no singing around the circuits of the terminal it is desirable that when signal is going out over the transmit side, the receive side shall be disabled. To this, end

two additional sets of contacts 1 and 9 subject 40 to the relay winding 3 are provided and when the relay is operated the contacts 1 close to connect the control grid of A: to ground and the contacts 9 close to connect the grid of tube Aoto ground, thus disabling botlrof these circuits.

Similarly, if signal is passing over the receive side then the operation of its control circuit closes. 1 i I two additional sets of contacts 8 and ID 'of relay R: which respectively connect the grid of the tube .1 A A3" and the grid of the tube As to ground, thus j disabling the transmit side.

In the operation of the rectifiers. it is desirable that they'shall not respond to undesired disturbancesv of relatively small magnitude. This can be accomplished by suitable biasing ofthese rec- 1:1 tifiers. Thus in the rectifier A'l instead of con-' v V necting one terminal of the resistance R/: to

ground it is shown as connected to a point above ground on the battery B5 of As, the'biasing being 4$ such that no current flows through the rectifier" o unless, the impressed voltage is in excess of bias.

limiting voltage to be impressed on the grid as of..

amplifier A it'may be desirable that no limitin 6a m action shall occur until the signalpower level at the output of As" exceeds a certain value. To this end a biasing battery Eumay be inserted in the-circuit of the tube All, the polarity'being such as to preventrectification until the im'- 7ocuit through a hybrid coil H but it-isto be under.- v

I This or similar provision is in general-pro-i 5 vided on all the rectifiers. Thus in the case of the amplifier in shunt with said load circuit and restood, at course, that one may leave the system as a four-wire circuit. Also, while the invention has been described primarily in terms of a terminal circuit, it would serve equally well as a repeater station at any intermediate point on the transmission line. 1

- The schematic drawing oi, Fig. 1 is a simplifled diagram of a two-way telephone system which include a terminal A having circuits such as illustrated in Fig. 2, shown in single line sche-.

matic form, anda plurality of other similar terminals B, C and D which may be associated with 1 each other and terminal A over the two-way line L.

While in the specification. the change in im- Dedance has been described as that due to the insertion or removal of a loss device as a function of time, it will be recognized that the impedance variation may come about in other ways. For example, the impedance of a certain element in the'load circuit might be a function 01' frequency and my circuit arrangement would still be eil'ctive for such impedance changes.

What is claimed is:

1. In combination, an amplifier, a plurality of load circuits connected in parallel to the output of said amplifier, atleast one of said load circuits having a variable impedance, and means to prevent any substantial change in the voltage input to one ofsaid load circuits due to the varying impedance of another load circuit comprising means for negatively feeding back the alternating voltage across the output of said amplifier to the input thereof so as to maintain the amplifier output voltage substantially constant with variation in the impedance of said other load circuit.

2. In a signal transmission system, a signal transmission circuit, an amplifier in said circuit, a load circuit associated with the output of said amplifier, a control circuit having its input associated with the output of said amplifier in shunt with said load circuit and responsive toimpressed signals to function as a control in said system, and means for substantially preventing variation in the input to said control circuit due to variations in the impedance of said load circuit, comprising means for negatively feeding back voltage from the output of said amplifier to,

the input thereof so as to maintain the output voltage of said amplifier substantially independent of variations in the impedance of said load I circuit.

a. In 'a signal transmission system, a ma transmission circuit, an amplifier in said circuit,

I a load circuit containing a loss device connected to the output of said amplifier, a control circuit having its input connected to the output of said ables the amplifier and the control in the impedance of said load circuit sponsive to impressed signals to remove the loss fromsaid load circuit and means for substantially preventing variation in the input'to said control circuit due to variations in the impedance of said load circuit with removal of the loss there from comprising means for negatively feeding back voltage across the output of the amplifier to the input thereof so as to maintain the output the receive side of a twoeway communication system. and further characterized by the fact that eachcontrolcircuit when performing its func-' tion of removing the loss on its one side also discircuit on the other side.

-' 6. In a signal transmission system, a .signal transmission circuit,- an amplifier in said circuit, a load circuit containing a loss device connected to the output of said amplifier, a plurality of control circuits connected to the output of said amplifier in shunt to said load circuit and responsive to impressed signals to remove the loss from the load circuit and to limit the amplitude of the signal impressed on the input of said amplifier,

means for substantially preventing variation in the input of said control circuits due to variations with removal of saidloss therefrom comprising means for negatively'feeding back thevoltage acres the output of said amplifier to the input thereof so as to maintain the output voltage of said amplifier substantially constant with variation in impedance of said load circuit.

7. In a telephone communication system, a plurality of two-way communication terminals adapted to be associated with each other for intercommunication, each terminal comprising atransmit side and a receive side, each side being characterized by the combination of claim 6 and further characterized by the fact that when one of the-control circuits on the one side is functioning to remove loss from its side-it disables. the IMaxie amplifier and control circuits on the other e. a

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