US2082357A - Telephone system - Google Patents

Description

L. E. RYALL June `1, 1937.

TELEPHONE SYSTEM Filed NOV. 9, 1934 3 Sheets-Shea?l 1 1 2 m @NM vom AJune 1, 1937. L, E RYALL 2,082,357

v TELEPHONE SYSTEM Filed Nov. 9, 1954 3 Sheets-Sheet 2 L. E. RYALL Jung l, 1937.

TELEPHONE SYSTEM s sheets-shea 5 Filed Nov. 9, 1954 In ue nlor,

Patented June 1, 1937 UNITED STTES TELEPHONE SYSTEM Leonard Ernest Ryall, Eastcote, Ruislip, England 9 Claims.

This invention relates to alternating current signalling systems having a plurality of paths through which alternating current signals in rectified or unrectifled form may pass, for instance, one or more of the paths may take the form of a main transmission path and/or one or more of the paths may take the form of a control path.

One example of such a system is a two-way 10 telephone transmission system in which the main transmission paths are the go and return paths; the invention, however, is also applicable for instance to telephone systems in which the main transmission paths are both go paths.

The principal object of the invention is to ensure that no harmful effects can take place when one transmission path is to be used in place of another.

One of the features of the invention relates to arrangements whereby when a transfer is effected from one transmission path to another the order in which the successive changes are made is predetermined so as to preclude the possibility of interaction taking place, while another feature of the invention relates to arrangements vwhereby the determination of when one transmission path is to be used in place of another is determined in a more positive manner even with weak currents in the path which is being taken into use.

According therefore to the first feature of the invention attenuation networks connected in one or more paths are jointly controlled by changes in the applied control voltage and each network is given a bias, for instance by a normal biassing current owing therethrough, so as to ensure that when the applied control is varied other than instantaneously the critical changes in the attenuation of the networks take place 4o in a predetermined order depending upon the relative bias of the respective networks so as to ensure that conditions deleterious to the operation of the system cannot occur.

According to another feature of the invention in addition to controlling the substitution of one transmission path in place of another in accordance with the relative effective strength of the signals in the two paths, a third controlling effect is introduced which is dependent upon the strength of the signals in the path in which through transmission is taking place, which is arranged to effect the relative effective strength of one set of signals in such a way that the changeover is effected in a more positive manner even with weak currents in the path which is being taken into use.

By way of example, in a two-Way telephone system in which the return transmission path is closed so as to prevent through transmission during transmission in the go transmission path, according to the present invention a voice operated device which may also be an amplifier either in the main transmission path or in a control path is provided controlling the closing, i. e., the preventing of through transmission and opening, i. e. the permitting of through transmission, of the return path which has a main input to which signals derived from the input side of the go path are applied to operate the device to close the return path and/or open the go path, a reply input to which signals derived from the input side of the return path are applied during signal transmission in the go path to operate the device in the reversef direction to open the return path and thereby enable reply signals to break in, and a third input which in this example might be termed anti-echo input to which signals derived from the go path are applied to prevent any signals, such as echo signals, created in the return path due to .signals in the go path effecting the reverse operation of the device to open the return path.

A further feature of the invention relates to the timing arrangements which enable Weak signals in one path to cause transmission in that path to be substituted for transmission in another path with the minimum of delay and this is effected by arranging that the normal changeover or hangover time should just exceed the normal time which elapses between words and syllables while the time during which the effect of echo or like signals derived from signals in the path in which transmission is taking place is only just longer than the time during which echo signals must be received so that Weak signals oc- Y curring in the path in which through transmission is prevented can be effective after the cessation of echo or like signals and before the cessation of the normal changeover time to hasten the time at which the changeover will take place.

Operation of the said voice operated device is preferably dependent on the voltage across a condenser, a rectifier being arranged to rectify the signals applied to the main input and to apply the rectified signals to the condenser. A further rectifier may be arranged to rectify the signals applied to the reply input and to apply the rectiiied signals to a condenser on the voltage across which the operation of the said voice operated device is dependent. An amplifier would preferably be arranged to amplify the signals applied to the reply input before rectification. In addition a rectifier may be arranged to rectify the signals applied to the third or anti-echo input and to apply the rectified signals in opposition to the signals derived from the reply input whereby rectiiication of echo signals is made more or less non-eifective. The rectified signals from the third or anti-echoI input may charge a condenser having a discharge time slightly greater than the time taken for echo signals to pass round a circuit and through the reply input, and appreciably less than the discharge time of the condenser to which rectified main signals are applied. The condenser or condensers may be arranged in the grid circuit of a therrnionic valve to control the grid bias of the valve. -This valve may also be arranged in o-ne of the transmission paths to amplify the signals therein.

A similar voice operated device may control the opening-and closing of the go path, the go path being closed during transmission in the return path. The opening and closing of the transmission path may be effectedrbry varying the attenuation of attenuation networks arranged in the paths and adjusted Yto prevent instability in either the quiescent or transfer state due to coupling between the paths and the voice operated device, the arrangement being such that operation of a device to close one path results in an increase in the attenuation of that path as well as decrease in the attenuation of the other path. Thus a pair of attenuation networks may be arranged in series in each of the transmission paths, one network of each pairhaving a low attenuation and the other a high attenuation in the quiescent state and the steady decrease of the attenuation of a path may then be effected by decreasing the attenuation of the network having a high attenuation. Alternatively, both transmission paths may be closed in the quiescent state and the voice operated devices so arranged that operation of either of the devices to open one of the transmission paths effects the closing of the main input to the other voice operated device, whereby the operation of the said other device to close the open transmission path is prevented by echo signals or signals derived from the open transmission path applied to its main input. Thus an attenuation network which has low attenuation in the quiescent state may be arranged in the main input to each voice operated device and the said closing of the main input effected by increasing the attenuation of the network. Each attenuation network preferably comprises one or more dry plate rectiiiers arranged in series and/or shunt arms, the attenuation being varied by passing direct current through the rectifier or rectifiers. In the case of a system having two voice operated devices, each including a thermionic valve, the said direct current may be dependent on the difference of the anode currents of the two thermionic valves. VWhere more than one attenuation circuit is controlled by a single variation `of the controlling voltagepor controlling Vcurrent it is possible by giving a normal bias to each network, whichis different to Vall the other networks, to arrange that one network isl closed before another is opened so as to ensure that conditions are not created which would result either inthe creation of intermittent interruption or howling conditions or other effect deleterious to the operation of the system.

It will be understood that in certain cases no biassing condition might be applied to either one or more of the networks. The absence of a biassing condition would serve to distinguish from networks which have a biassing current or voltage and to enable the order of opening and/or closing of the respective networks to be determined.

The invention will now be described by way of example with reference to the accompanying drawings of which:

Fig. l is a schematic diagram of a loudspeaker telephone equipment for attachment to a telephone line.

Fig. 2 shows the circuit represented by Fig. 1 in detail.

Fig. 3 is a schematic diagram showing the application of the invention to a two-wire repeater circuit, while Fig. 4 is a schematic diagram showing the arrangement by which the different biassing effects are applied to the attenuation networks of Fig. 1.

As shown in Fig. 1, a microphone I and a loudspeaker 2 are coupled to a telephone line 3 through a differential transformer or hybrid coil 4 having the usual balance network 5. The path from the microphone l to the transformerV 4 extends through a first stage amplifier 6, a second stage amplifier 'I and a normally closed attenuation network 8. The path from the transformer 4 to the loudspeaker 2 extends through a volume control device 9, a iirst stage amplifier I0, a normally closed attenuation network I I and a second stage amplifier I2. In the arrangement shown in Fig. l, the second stage amplifier 'l and the iirst stage ampliiier I in addition to serving as amplifiers in the main signal paths also act as voice operated devices to control the attenuation of the networks 8 and II and for this purpose they are operated by varying the grid potential in such a way as to vary the mean anode current. It will be evident, however, that entirely separate valves may be used for this purpose if so desired. Such an arrangement is shown in Fig. 3. For the purpose of controlling the amplifier as a voice operated device 'I it is provided with a main input I3 in which are arranged a normally open attenuation network I6 and a rectiiier I1, a reply input I4 and a third or anti-echo input I5. The signals applied to the main input I3 are derived from the output side of the amplifier IG. The signals applied to the reply input I4 are derived from i the output side of the amplifier 'I. The signals applied to the third or anti-echo input I5 are derived from the output side of the amplifier I2.

The amplifier or voice operated device lil similarly has a main input I8 in which are arranged a normally open attenuation network 2I and a rectifier 22, a reply input I9 and a third or antiecho input 2U. The signals applied to the main input I8 are derived from the output side of the amplifier 'I. The signals applied to the reply input I9 are derived from the output side of the amplifier IU. The signals applied tothe antiecho input 2E] are derived from the output side of the amplifier 'I.

The attenuation of the networks I E and 2l is also controlled by the voice operated devices I0 and '1. Signals applied to the main input I3 tend to operate the voice operated device 'I so as to close the network 2I and open the network II successively. Signals applied to the reply input Cil I4 tend to operate the device 1 in the reverse direction so as to close the network II and open the network 2| successively. Signals applied to the third or anti-echo input I5 tend to prevent this reverse operation of the device I in dependence upon the value of the signals applied tothe anti-echo input I5 so that only if the amplitude of the signals applied to the reply input I4 are suiciently great can they have any effect upon the voice operated device 1. Similarly signals applied to the main input I8 tend to operate the voice operated device Ill so as to close the network I 6 and open the network 8 successively. Signals applied to the reply input I9 tend to operate the device I0 in the reverse direction so as to close the network 8 and open the network I6 successively. Signals applied to the third or antiecho input tend to prevent this reverse operation of the device I0 in dependence upon the value of the signals applied to the third or antiecho input 20 so that only if the amplitude of the signals applied to the reply input I9 are sufficiently great can they have any effect upon the voice operated device II).

In the quiescent state the networks 8 and II are closed. Instability due toelectrical coupling of the transformer 4 and acoustic coupling between the loudspeaker 2 and microphone I is thus prevented. When speech signals are received `from the microphone I they are amplied by the ampliers 6 and 1 and applied to the main input I8 of the voice operated device Ill. 'Ihe device I0 is thereby operated to close the network I6 and open the network 8 successively. The signals can thus pass through the network 8 and transformer 4' to the line 3. Signals from the microphone I which may reach the -amplier I6 due to echo eiiect or due to lack of balance of the transformer 4 and incoming signals from the line 3, are prevented from reaching the loudspeaker 2.by the closed network II and are prevented from operating the device 'I by the closed network I6. When speech signals from the microphone I cease the voice operated device I0 restores to the quiescent state condition to close the network 8 and open the network I 6 successively. There is however a hangover time which must elapse before this restoration takes place, this hangover time being suiicient to bridge the gaps between words. If, now, whilst the apparatus is in the quiescent state, reply signals are received from the line 3 these signals are amplified by the amplier Ii) and applied to the main input I3 of the voice operated device 1. The device is thereby operated to close the network 2I and open the network I I successively. The reply signals can then pass through the network II and amplifier I2 to the loudspeaker 2. Originating and' echoing signals applied to the microphone due to the acoustic coupling between the loudspeaker 2 and the microphone I are prevented from reaching the line 3 by the closed network 8 and are prevented from operating the device I0 by the closed network 2 I. When the reply signals cease the voice operated device I restores to the quiescent state condition to close the network II and open the network 2I successively. A hangover time is also provided in this case to bridge the gaps between words.

If during the transmission of speech signals from the microphone I, reply signals are received from the line 3, these reply signals pass through the amplifier I0 to the reply input I9 and during a falling away in the strength of the speech signals from the microphone, for instance due to a weak syllable cause reverse operation ofthe device IU to close the network 8 and open the network I 6. As soon as the network I6 opens the reply signals can pass through the main input I3 and effect operation of the voice operated device 'I to close the network 2| and open the network I I successively. The reply signalsare then able to pass through the network I I and amplifier I2 to the loudspeaker 2. It will thus be seen that during the transmission of signals from the microphone reply signals may break in and cease to wait for the control time to elapse.

Since echo signals are also applied to the reply input I9, it is necessary to prevent such signals operating the device Ill in a similar manner -to the reply signals to effect a break in. This is achieved by means of the signals applied to the third or anti-echo input 20, which signals arey derived from the same source as the echo signals. It is arranged that the signals applied to the third or anti-echo input 20 always have a greater eiect than the corresponding echo signals applied to the reply input I9. echo signals are prevented from operating the device I0, whilst reply signals, which have a greater amplitude than the anti-echo signals, are not prevented from operating the device I0. The eiect of anti-echo signals applied to the antiecho input 20 is maintained for a time which is suicient to guard against the echo signals which pass round the circuit to the reply input I9 but which is considerably less than the hangover time of the device II).

In a similar manner, during the reception of reply signals from the line 3, signals applied to the microphone I may break in by causing a reverse operation of the device 'I through the reply input I4. Echo signals applied to the microphone I are prevented from effecting a break in by anti-echo signals applied to the anti-echo input I5.

The parts shown in Fig. 1 are shown in greater detail in Fig. 2 where like parts have like reference numerals. As shown in Fig. 2 the attenuation networks each comprise a plurality of dry plate rectifiers arranged in shunt and series arms. The attenuation of each network is controlled by direct current passing through the rectiers which thus determines the alternating current impedance of the rectiiiers.

Attenuation networks of this kind are described in British specification No. 413,383. It will therefore suice for the purpose of this description to say that the attenuation of each network is varied by the D. C. control current which passes through the rectiiiers in such a direction that the attenuation of the network is varied by varying the value of the control current, and it should be pointed out that under certain critical conditions a slight change in the value of the control current will cause the attenuation to vary within very wide limits. One of the features of the invention concerns the utilization of this property for the purpose of ensuring that the attenuation of the networks is varied in a predetermined order. The microphone is connected to the leads 40, and the loud-speaker to the leads 5I The attenuation network I5 is represented by two networks in series in Fig. 2, the combined networks having a pair of control terminals 23 and 24 to which the D. C. control current is applied. Similarly the attenuation network 2| is represented by two networks in series in Fig. 2 the combined networks By this means the having apair-of control terminals 25 and 26. The network 8 has a pair of control terminals 21 and 28, the network has a pair of control terminals 29 and 3U.

.f5 For convenience in order to enable the supply of the control current to the different networks to be readily followed reference Vmight be had to Fig. 4 aswell as Fig. 2, Fig. 4 showing exactly Ythe same arrangement as Fig. 2 but eliminating as far as possible all details so that the .networks are only shown as rectangles, and certain equipment whichis not necessary for the understanding of the control circuits is eliminated.

The two voice operated devices 1 and I0 each of which consists of a valve, control the attenuation of the networks by means of a bridge arrangement, the valves 1 and I0 forming two complementary arms of the bridge and the resistances 3| and 32 forming the other two complementary arms of :the bridge. The high tension battery 33 is connected across one diagonal of the bridge, having its positive Ypole connected to the junction point of the resistances 3| and'32 and its negative pole .or earth connection con- 5 nected to earth in thelament circuit of the valves, the control circuits of the networks being connected in'series across the other diagonal between the points 34 and 35. The path of the control current may be traced from point 34 0 through terminals 25, 26, 21, 28, 3B, 29, 24, 23,

choke 36 to point 35.

The choke 36 serves to reduce alternating current ripple in the control circuit. In the quiescent state the bridge is balanced by adjusting the re- 5 sistance 32 so that the points 34 and 35 are at the same potential. The attenuation .of the networks in the quiescent state is adjusted to the desired values by biassing current applied to the control circuits. For this purpose see Figs. 2 and 0 4. Terminals 25 and 21 are connected to earth through a resistance 31. Terminals 24 and 23 are connected to earth through a resistance 38 and terminals 28 and 30 are connected to the positive pole of battery 33 through a resistance 39. 5 T hus in the quiescent state biassing current ows through the control circuits of the networks |6 and 2| in such a direction that` these networks have a low attenuation and may be considered as open. At the same time biassing current flows 0 through the control circuits 8 and in such a direction that the networks have a high attenuation and may be considered as closed. n Y

Variation of the attenuation of the networks is Veiectecl by unbalancing theY bridge so that the 5control current flows in one direction or the other between the points 34 and35. This current might.

be considered for the purposes of description as being quite separate from the biassing currents, the biassing currentsl being substantially steady 0 and comparatively small in value compared with the control current which is superimposed on it. If the .bridge is unbalanced so that control current ilows from the point 34 to the point 35, then in networks 2| and the control current ows 5 in the same direction as the biassingcurrent so thatthese networks'remain opened and closed VArespectively, but the controlcurrent in the networks 8 and I6' flows in opposition to the'biassing current and thus closes the network I5 and opens 0 network 8 successively.V It is important that the network I5 should be closed before the network 8 opens as otherwise in the case of a transfer from one direction to the other it is possible' for speech passing through the attenuation network 8 being able to cause, due to echo or out of balance eiects, the back' action through the network I6, which would interrupt the operation'of the amplifier 1 and so create a condition whereby the speech transmitted was intermittently interrupted due to the network 8 being again closed. The presence of the different biassing currents ensures that the changeover takes place in the correct order. Due to the presence of condenser and inductance in the control current bridge circuit the changeover does not take place instantaneously and the fact that the biassing currents of the two networks 8 and I6 are in different relative directions as well as being' possibly different in value, ensures that the network I6 is closed in advance of the opening network 8. That is to say the critical condition whereby the change takes place from high or low attenuation or vice versa is rst reached in network I6 and is subsequently reached in network 8.

The presence of inductance or condensers in the circuits ensures that the critical condition is reached at different times in the different networks, and also ensures that if the change in the control voltage is of a small nature then the tendency would be for both networks to be closed rather than for them both to be open at the same time, which latter condition interferes as explained above with the proper operation of the system. It will be understood that the control is not limited to a bridge circuit but is applicable to any arrangement in which a number of networks are jointly controlled, that is bythe same control voltage or the same control current and the biassing currents or potentials of the different networks vary.

If the bridge is unbalanced so that the control current flows from the point 35 to the point 34, then in networks IG and 8 the control current flows in the same direction as the biassing current so that the networks I@ and 8 remain open and closed respectively, but the control current in the networks and 2| flows in opposition to the biassing current and thus closes the network 2| and opens the network in the order mentioned. The value of theresistance 39 again ensures that the biassing currents are such that the network closes before the network 2| opens. The unbalancing of the bridge so that the control current iiows from theY point 34 to the point 35 is effected by operation of the voice operated device Hl. The device |0 is operated by decreasing the negative grid bias of the valve so as to increase the mean anode current and thereby lower the potential of the point 35 with respect to the point 34. The negative grid bias is decreased by signals applied to the main input I8. These signals pass through the network 2|, are rectified by the rectiiiers 22 and applied Vto a pair of condensers 42 so as to charge them in opposition to the grid bias batteries 43. When signals are removed from the main input |8 the charge on the condensers 42 leaks away through the resistance 44 so that the grid bias of the device Il) returns to that which exists in its Vquiescent state, the value of the resistance 44 being so chosen as to give the necessary hangover time, that is, to ensure that transfer will not take place from one direction to another between syllables.

A similar restoration of the grid bias of Ythe device I8, as already explained, may be eiected byY signals applied to the reply input I9 without waiting for the hangover time to elapse. These signals are rectied by a rectiiier 45 and applied to the condensersY 42 so' as to discharge them ..10 densers 42.

. -15 .the kreply input I9.

rapidly during a weak syllable of the signals applied to the main input I 8,. and thereby cause the grid bias of the device I to restore to that which exists in its quiescent state. Signals ap- .5 plied to the third or anti-echo input 20 are rectified by a pair ofv rectiers 46 and applied to a pair of condensers 41 so as to charge them in opposition to the voltagey derived from the input I9, which voltage tends to discharge the con- The voltage across the condensers 41, due to anysignal applied to the anti-echo input 20, is always greater than the maximum voltage of the corresponding echo or other signal derived from the original signal and applied to By this means the echo or like signal can never be rectified by rectifier 45 to cause a discharge current to ow from condenser 42. The condensers 41 are provided with a discharge resistance 48, chosen so that the 20 hangover time of the condensers 41 is slightly greater than the time taken for the echo signal to reach input I 9. When a reply signal is applied to the reply input I9 it will be rectied by the rectier 45, and so discharge the condensers 42 25 provided its maximum voltage exceeds the voltJ age across the condensers 41.

If the echo time is short or practically instantaneous, the hangover time of the condensers 41 need only be a few milli-seconds so that the 30 `condensers 41 discharge between syllables. Reply signals are thus not prevented by the condensers 41 from discharging the condensers 42. The unbalancing of the bridge so that the control current flows from point 35 to the point 34 35 is eected by operation of a voice operated device 1. The manner of operation will be evident from the description given above of the operation of the device I0. Signals applied to the main input I3 are rectied by the rectiers I1 so as to charge a pair of condensers 49 in opposition to the gridy bias batteries D. Signals applied to the reply input I4 are rectified by the rectier 5I so as to discharge the condensers 49. Signals applied to the anti-echo input I5 are rectified by a pair of rectil'lers 52 so as to charge a pair oi condensers 53 in such a direction as to prevent the discharge of the condensers 49 by echo signals applied to the reply input I4.

50 The application of the invention to a two-wire repeater circuit is Yshown schematically in Fig. 3; circuit details are not shown as these will be evident to those versed in the art from the description of the previous circuits. Parts cor- 55 responding to those in the previous circuits have like reference numerals. As shown, the line 3 is coupled to the go and return paths through the hybrid coil or differential transformer 4. The go path comprises an equalized 60 four-Wire repeater 53, normally closed attenuation network 8 and a transmission amplifier 54. The return" path comprises an equalized four- Wire repeater 55, a normally closed attenuation network II and a transmission amplifier 56.

65 The attenuation of the networks is controlled in a similar manner to that of the circuits already described. The method of operation will therefore be clear from this previous description. In this case however the voice operated 70 devices 1 and I0 While they may amplify, do not serve the additional purpose of transmission ampliers; the signal transmission paths and the control paths are entirely separate.

It will be noted that the voice operated devices 75 1 and I0 each have three input circuits as before and these serve exactly the same purpose. The inputs can be described as follows, namely, the main inputs I3 and I8, the reply inputs I4 and I9 and the third or anti-echo inputs I5 and 20.

It will be understood that the invention is not specically limited to the particular arrangements described as several modications can be made in the circuit details Without departing from the spirit olf the invention. For instance the invention is not limited to two-way transmission circuits as it can be readily applied to other transmission circuits, for example two one- Way circuits which should not overlap, while the arrangement for controlling attenuation networks in a denite order is `capable of application in any circuits Where the properties referred to might be used to advantage.

I claim:

l. In an alternating current signalling system having two transmission paths for signals of voice frequency, the combination comprising an attenuating device arranged in one of said transmission paths, said device serving to control the attenuation of said path, and a voice operated device associated with said attenuating device and having a main input connected to the input side of one of said transmission paths, a first auxiliary input connected to the input side of the second of said transmission paths, and a second auxiliary input connected to' the input side of the rst of said transmission paths, signals applied to said main input serving to operate said voice operated device in one direction to change the attenuation of said attenuating device, the signals applied to said nrst auxiliary input serving to operate the device in the reverse direction to reverse said change of attenuation, and the signals applied to said second auxiliary input, provided that they are suiiciently strong, serving to prevent said reverse operation of the voice operated device.

2. In an alternating current signalling system having two transmission paths for signals of voice frequency, the combination comprising a voice operated device, said voice operated device serving to control the attenuation of at least one of said transmission paths to prevent echo' effects, means for operating said device to change the attenuation of said one transmission path, said means being responsive toy signals in one'of the said transmission paths, and first auxiliary means for operating said device to reverse said change of attenuation, said rst auxiliary means being responsive to signals in the input side of the second of said transmission paths during transmission in the first path, and second auxiliary means for suddenly rendering said first auxiliary means eiective, said second auxiliary means being responsive to signals in said first transmission path.

3. In an alternating current signalling system having two transmission paths for speech signals and in which echo' signals derived from signals in one of said paths are liable to produce an effect resembling that produced by signals in the other of said paths, the ycombination of change-over means for permitting signal transmission to change from one of said paths to the other, delay means for preventing said changeover means operating during the interval which usually exists between words, preventive means for preventing said echo signals producing said eiect, and further delay means for ensuring that said preventive means are only elective for a .time which is less than said interval between words and slightly longer than the time taken for said echo signals to produce said effect.

4; In an .alternating .current signalling system having two transmission paths for speech signals, the combination comprising an attenuation device arranged in cneof said transmission paths, said device serving to control the Vattenuation of said path, a condenser associated with said attenuation device, the attenuation of said device being dependent on the condenser voltage, a rectier interposed between the input to one of said transmission paths and the condenser, said rectiier serving to charge the condenser with rectied signal current, a high resistance discharge path for said condenser, Ya second rectifier interposed between the input Yside of the second transmission path and the condenser, said second rectier serving to apply signals from said second path to discharge said condenser rapidly, and a source of biassing voltage appliedto .said second rectifier and derived from the input to the rst transmission path, said biassing voltage vbeing dependent on the strength of the signals in the input to the iirst transmission, path and serving to prevent the rectier applying signals to discharge said condenser so long as these signals are not suiiicient to overcome said bias so that the ,condenser is not discharged by echoI signals.

5. The combination claimed in claim 4, wherein the said source of bias voltage comprises a second condenser, a third rectier interposed between the input tothe rst transmission path and said second condenser, said third rectifier serving to charge the second condenser with rectii'led signal current, and a discharge path for said second condenser, the resistance of the dis-r charge path of said second condenser being sufcient to maintain the eiect of a signal for a time which Ais just suicient for the echo of that signal to reach said second rectier.

6. The combination claimed in claim 4, wherein an amplifier is interposed between the input side of the second transmission .path and said second rectifier.

7. In an alternating current signalling system having a plurality of signal paths, the combination comprising at least two attenuation networks arranged in the Vsignal paths, the .attenuation of each of said networks being controlled .by an applied control voltage, the attenuation varying considerably when the applied control voltage passes through a critical value, a source of control voltage arranged to be .applied to' control points in said networks, `said voltage jointly controlling the networks, means lior varying said control voltage, at least one source of biassing voltage applied Vto said networks, means for differently biassing the networks of the respective pathsso that when the applied control voltage is varied the critical changes in the attenuation in the,

different networks of a signal path take place consecutively.

' 8. In a two-way telephone system havingtwo transmission paths, the combination comprising a normally closed .attenuation network arranged i in one of said transmission paths, attenuation control apparatus, said apparatus having vone input associated with the input side of the one transmission path and a second input associated with the input side of the other transmission path, a second normally open attenuation network arranged in said second input, saidrcontrol apparatus applying a control voltage to the two networks to control the attenuation thereof jointly, signals applied to the first input tending to open the rst network and close the second network and signals applied to the second input tending to close the iirst network and open the second network, and a source of bias voltage applied .to at least one of said networks, said bias ensuring that the second network closes slightly in advance of the opening of the rst network to prevent application of signals to the 4second input to close the first network.

9. In an alternating current signalling system having a plurality of signal paths, the Vcombination comprising at least two attenuation networks arranged in the signal paths, each of said networks having a control circuit the voltage applied to which controls the attenuation of the network, a bridge circuit, the control circuits of said networks being connected in series in a diagonal circuit extending between a pair of opposite corners of the bridge, at least one connection between `an intermediate point .of said diagonal circuitand .the potential of some point of the bridge, said connection serving to bias said networks differently, and means for unbalancing said bridge circuit in response'to signals in at least one of the signal paths.

LEONARD ERNEST RYALL.

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