US2382588A - Gain control circuits for carrier systems - Google Patents

Description

Aug 9 1945- A. M. THORNTON ETAL 2,382,588-

GAIN CONTROL CIRCUIT FOR CARRIER SYSTEMS FiledMay 20, 1943 4 She'ets-Sheet- 1 Inbentor Aug. 14, 1945,

A. M. THORNTON ETAL GAIN CONTROL CIRCUIT FOR CARRIER SYSTEMS Filed May 20, 1943 4 Sheets-Sheet 2 To LA on LB.

Inventor A. M. THORNTON ET AL Aug. 14, 1945.

- GAIN CONTROL CIRCUIT FOR CARRIER SYSTEMS Filed May 20, 1945 4 Sheets-Sheet s Attorney g- 14, 1945. A. M. THORNLI'ON ETAL 2,382,588

'GAiN CONTROL CIRCUIT FOR CARRIER SYSTEMS Filed May 20, 1943 4 Sheets-Sheet 4 TO chin F To own:

RELAYS I QL RELAYS Inventor A tiomev Patented Aug. 14, 1945 GAIN CONTROL CIRCUITS FOR CARRIER SYSTEMS Alfred Marcus Thornton and Wilfred Frank Baly,

London, England, assignors to Standard Telephones and Cables Limited, London, England,

a British company Application May 20, 1943, Serial No. 487,732 1 In Great Britain July 17, 1942 f 9 Claims.

The present invention relates to gain control arrangements for carrier current transmission systems, and in particular concerns means for dealing with abnormal or faulty conditions in such systems. I

In carrier current transmission systems it is now frequently the practice to provide automatic gain control arrangements for the purpose of adjusting the gain characteristics of the line repeaters in accordance with the changes in the attenuation of the line or other medium over which the currents are transmitted. This is usually done by transmitting one or more pilot currents with the signal currents over the communication path, which pilot currents will be subjected to the same changes in the medium as the signal currents, and are caused to vary the gain characteristics of the repeaters in the proper manner to compensate for these changes.

An increase in attenuation of the communication path will reduce the level of the pilot currents and this will be arranged to increase the gain of the repeaters. When the carrier system is operated over an open wire line, for example, the attenuation may vary between wide limits, and in exceptional circumstances (for example in conditions of sleet or snow) may become so much increased that it is impracticable to provide compensating means to deal with such an abnormal case, even if noise conditions do not render the circuit uncommercial. On the approach of the exceptional condition, or on the occurrence of a fault having an equivalent effect, the gain of the repeaters will increase to a maximum, and unless some action is taken, will remain at a maximum, which is undesirable for several reasons; and moreover it is necessary that an alarm should be given if the faulty condition persists.

Since in these carrier current systems it is usual for a proportion of the repeater stations to be unattended, it is very desirable that the means for dealing with the exceptional or faulty condition at these stations should be automatic, and moreover, that on the clearance of the fault, the unattended stations should return to normal without the necessity of a visit from an operator. In other words the repeaters should be self-aligning.

The present invention solves the above problem in a simple manner, and the solution provides as a by-product a valuable feature whereby a spare line can be provided with repeaters at unattended stations and can be brought into use without the necessity for the stations to be 5 visited to make the necessary adjustments. By means of the self-aligning feature, these adjustments take place automatically at each repeater station in turn as soon as the pilot currents are switched on at the terminal stations, or when a faulty section of line has been replaced.

The arrangements of the invention may also be applied in the receiving circuits of the group terminals in order to provide the self aligning feature and at the same time to prevent the occurrence of excessive noise on the circuit. It also prevents singing round the channels immediately after the last repeater has restored the transmission path while the gain of the group terminal receiving circuit is still high.

According to the invention, there is provided a multi-channel carrier current transmission system provided with one or more repeaters which comprise two oppositely directed one way amplifiers each of which is provided with an automatic gain control operated by one or more corresponding pilot currents, in which means is provided for introducing a large attenuation at the output of each amplifier when the level of any of the corresponding pilot currents departs by more than a predetermined amount from a given level.

According to another aspect, the invention comprises a multi-channel carrier current transmission system provided with one or more twoway repeaters, the transmission paths of which are blocked in both directions, comprising means for transmitting from each end of the system one or more pilot currents to the repeaters, means at each repeater controlled by'the pilot currents for adjusting the gain thereof in such manner that for each direction separately the pilot current levels at the corresponding repeater output are brought to predetermined values, and means for unblocking the corresponding transmission path when the adjustment for the direction concerned has been completed, to allow the pilot currents to be transmitted to the next repeater.

The invention will be more clearly understood from the following detailed description with reference to the accompanying drawings in which:

Fig. 1 shows a block schematic diagram of a carrier current line repeater according to the invention;

Fig. 2 shows a schematic circuit diagram of a transmission cut-oii circuit according to the invention; and

Figs. 3, 4 and 5 which should be placed side by the invention.

alarm and cut-ofi arrangements according to Fig. 1 shows a block schematic diagram of a two-way repeater for a carrier current system showing the application of the invention. The system is of the kind in which different groups of carrier frequencies are used for transmission in opposite directions on the same line or communication path, the two directions of transmission being separated-at the repeater by means of directional filters.

Signals arriving from line A are supposed to be conveyed on a high frequency group of carrier currents and are diverted by the directional filters to the lower half of the circuit and are applied to the regulating amplifier RAA, and then pass successively through-a fixed equalising network NA, a line amplifier LA of fixed gain and a transmission cut-off circuit TCA, to the directional filter- DFB and thence to the line B. Similarly, signals arriving from line B are conveyed on a low frequency group of carrier currents, and pass through a corresponding regulating amplifier RAB, equalising network NB, line amplifier LB, and transmission cut-ofi circuit TCB, to the directional filter DFA and thence to line A. Corresponding apparatus in'the two halves of the circuit may diifer in detail on account of the diiferent frequency ranges concerned, but these diflerences are not significant as regards the present invention.

For the purpose of adjusting the gain of the repeaters, two separate pilot frequency currents are transmitted in each direction, the frequencies being suitably located in the corresponding transmitted ranges so as not to interfere with any of the channels. One of these controls the slope of the gain-frequency characteristic of the corresponding regulating amplifier in accordance with the changes in the form of the attenuation characteristic of the line, and the other controls the general level of the characteristic in accordance with the average increase or decrease of the attenuation. Thus for signals transmitted from the line A, the two pilot currents are transmitted through the regulating amplifier RAA and the equalizer NA and the line amplifier LA, and are there diverted to two pilot control panels SPA and GPA through separating filters in these panels,'the output sides of which are separately applied to the regulating amplifier RAA. The panel SPA controls the slope and the panel GPA controls the level of the characteristic in a. manner well known, which will therefore not be described. Similar pilot-control panels SPB and GPB are provided for the opposite direction and operate in the same way.

Each of the pilot control panels produces a rectified continuous voltage depending on the level of the corresponding pilot current at the output of the line amplifier LA or LB, this voltage being used to produce the necessary adjustment of the corresponding regulating amplifier. This voltage will be subject to small variations (depending on the degree of regulation obtained) while the line attenuation varies within wide limits, unless the gain control fails for any of the reasons previously explained.

According to the present invention, the rectified voltages from the two panels GPA and SPA are employed to control the same alarm panel ARA, in which an alarm relay is operated if the level of either pilot current departs by more than a specified amount from the standard level. In

the same way an alarm relay in a second alarm panel ARB for the other direction is controlled by the other pilot currents. The two alarm relays in tumcontrol a common pulsing circuit PC which generates pulses for the purpose of repeatedly re-testing for the fault condition and cancelling the alarm if the fault does not persist. They also control independently of PC an alarm delay circuit ADC which, if the fault persists, ultimately operates the transmission cut-off circuit TCA or TCB which corresponds to the faulty direction, and gives the alarm. The transmission cut-of! circuits are similar and operate to introduce a very high attenuation while at the same time accurately maintaining the impedance of the circuit at the proper value as seen from both directions. The circuit is thus practically cut although the repeater may be at maximum gain as a result of the fault.

These arrangements according to the invention which have just been briefly summarised will be explained in detail later on. It will be understood that only those details of the well known transmission arrangements necessary for understanding the invention have been shown in Fig. 1, and additional elements such as filters and equalisers may be provided as required. Also, there may be only one pilot current in either direction if slope adjustment, for example, is not required; or alternatively there may be three or more if other characteristics have to bepcontrolled. In such cases there will be a panel such as SPA or GPA for each pilot, and each such panel will control the same alarm relay on either side, the operation of the 7 circuit of "the invention being the same for any number of pilots.

Fig. 2 shows the details of a preferred form of the transmission cut-off circuit 'I'CA or TCB shown in Fig. 1. It comprises a bridge network of four equal resistances R, the input terminals l, 2 being connected to one pair of diagonal corners, and the output terminals 3, 4 to the other pair. Terminals I and 2 are connected to the line amplifier LA, or LB in Fig. 1, and terminals 3 and 4 are connected to the directional filter DFB or DFA. A transmission cut-off relay '1 has two sets of change-over contacts t1 and t: which in the unoperated position shown connect terminals l and 3 together and also terminals 2 and 4 together, short circuiting at the same time two of the resistances R occupying opposite arms of the bridge. The other two resistances are likewise disconnected at one end from the bridge corners connected to terminals I and 2 respectively. The terminals I and 2 are thus respectively joined straight through to terminals 3 and 4 without any intervening attennation.

The winding of relay T is connected at terminals 5 and 6 to the alarm delay circuit ADC (Fig. 1) and when energised in a manner to be presently explained, the contacts t1 and t: are caused to cha ge over and to complete the bridge, the direct connection being new removed. As terminals l, 2 and 3, 4 are now connected to the diagonals of a balanced bridge, transmission from one pair to the other will theoretically be impossible and the circuit is cut-off. At the same time the impedance seen from either pair of terminals will be R. In practice, of course, the bridge will not be exactly balanced on account of the manufacturing limits on the resistance values and also on account of stray capacities which cannot be eliminated altogether. In practice, however, the network can easily be made to introduce an attenuation of more than 60 decibels, which is sufficient for the purpose The value of the resistance R should be chosen to be equal. to the impedance of the circuit in which should be placed side by side with Fig. 3 of the left hand side. In these figures, an

arrow pointing away from a relay indicates a grounded battery of suitable voltage.

Fig. 3 shows part of each of the two pilot control panels SPA and GPA for the A to B direction. In the panel SPA, two similar relays HS and LS have each two equal windings a and b of which the a windings are connected in series to the output of the rectifier circuit which produces the variable continuous voltage, indicated by the signs and for operating the gain control. This is arranged to produce a certain current I1 when thepilot level is normal. Connected to the terminals 5 and 6 there may be a voltmeter MS used for indicating the level of the pilot current.

The b windings of the relays HS and LS are connected in a local series circuit operated between a grounded positive battery connected to terminal l0 and ground at terminal I! as shown. Adjustable resistances R28 and R29 are connected in series respectively with the relays LS and HS. The junction point (terminal ll) of R29. and LS is normally connected through conductor 9 and through a contact of relay FA in Fig. 4 to ground at terminal l0 in that figure, so that relay HS is normally short circuited.

According to a preferred arrangement, which, however, is only one possibility described to make the invention clear, the relays LS and HS are designed to operate on a net current of 311/2 and to release on a current of 11/2. It will be noted that the currents I1 and I2 flowing in the windings a and b of relay LS are in opposition (the inner ends of both windings are designated In), so that the not current effective to operate the relay will be 12-11. The resistance R is adjusted so that for a normal pilot current level, I2=2I:, the'net current being therefore I1, and the relay does not operate. In the case of the relay HS, since the current I3 flowing through its winding b is normally zero as just explained, the net operating current will also be I1 and it will not operate.

Should now the level of the pilot current fall until the current in the a, windings of the relays is I4=I1/2, then the net current in relay LS will be I2--I4=3I1/2, and the relay will operate; and the net current in HS being now I4=I1/2, it still does not operate.

If the level of the pilot current should rise so that'the current through the a windings of the relays becomes I5=3I1/2, then HS operates, but LS does not operate because the net current is now I2I5=I1/ It will thus be clear that with the adjustment described LS operates when the pilot level is low and HS operates when it is high. By suitable choice of I1, the pilot levels at which these relays operate may be fixed as desired. For example, these levels may be 3.5 db. above normal and 6 db. below normal.

. The purpose of the winding 1) of relay HS and the associated resistance R20 is to allow a momentary release of these relays by the pulsing circuit PC (Fig. 5) in a manner to be more fully explained later on. When the pulsing circuit operates, relay FA (Fig. 4) removes the ground connection from terminal H of SPA (Fig. 3), and resistance R29 is adjusted so thatthe current Io which then flows through both relays is equal to I1. This will cause the momentary release of either relay when the pulsing circuit PC operates because the net operating current will in either case be reduced below the releasing limit 11/2. If the fault is still on, the relays will operate again when the ground is replaced on terminal H by the pulsing circuit .PC. The purpose of this arrangement is to allow robust telephone type relays to be used instead of expensive and delicate voltmeter relays which would otherwise be necessary owing to the small margin available between the operate and release currents. Furthermore, since the operation of the alarms depends on the values of the currents I1 and I2, it is very desirable that the battery connected to terminal l0 (Fig. 3) which produces these currents should be a regulated battery.

The arrangements for the other pilot-control panel GPA are the same as those which have just been described for the panel SPA, the contacts of the corresponding relays HG and LG being simply connected in parallel with those of HS and LS so that they operate the alarm panel ARA (Fig. 4) in just the same way. A voltmeter MG may ,also be provided to indicate the pilot level, and when the alarm is given a reference to the meters MS and MG will show which of the pilot currents is at fault.

When either of the, relays LS or LG of Fig. 3 operates, ground from terminal ll) of Fig. 4 is connected through conductors I and 6 to the relay LLA in the alarm panel ARA (Fig. 4), and this relay operates. Similarly if either of the relays HS or HG of Fig. 3 operates, relay HHA in the alarm panel ARA (Fig. 4) is operated from the ground at terminal l0 through conductors I and 8.

Both relays LLA and HHA (Fig. 4) have the same subsequent effect except in one particular to be presently mentioned. The sequence of operations is as follows:

On the operation of either of these relays, the alarm relay AL operates through an obvious circuit from ground at terminal I0. This relay prepares an operating circuit from terminal IQ for the pilot alarm signal relay PAS; it also prepares an operating circuit from terminal M for the relay FA; and it switches on the pulsing circuit PC and the alarm delay circuit ADC by short circuiting respectively terminals l5, l6 and I1, I8 (Fig. 4). This will be further explained presently.

When the alarm delay circuit ADC (Fig. 5) operates after a. predetermined time, the pilot alarm signal relay PAS (Fig. 4) operates and lights a warning lamp PL and may also operate an audible alarm connected to terminal 5 if desired. This relay may clearly have other contacts (not shown) to perform other functions. At the same time the relay T in the transmission cut-off circuit TCA (Fig. 5) is operated through conductor 20, but only if relay LLA (Fig. 4) has operated; the resistance R1 in series with the contacts of HHA being adjusted to prevent relay T from operating when HHA opcrates first, but to allow T to be held by HHA once it has been operated by LLA.

The purpose of this arrangement is as follows:

The occurrence of a pilot current at an abnormally high level usually indicates a fault in the pilot equipment and not a line fault which would prevent the circuit from operating satisfactorily. Accordingly it would not be desirable to shut the system down for this reason. If the fault persists an alarm is, however, given when the alarm delay circuit ADC operates and the situation may be dealt with appropriately.

If the level of the pilot current is low, however.

a line fault i usually indicated and itis necessary to shut down the system. Relay LLA operates in this condition and so the transmission cut-off circuit TCA operates, the alarm being given as before. After rectifying the fault, the pilot current will be restored and may be at an abnormally high level until the gain adjustment has taken place. It is not desirable to pass on the pilot current until the adjustment has been made, and accordingly HHA will operate and will hold the relay T operated through resistance R1 although LLA will have been released. When the pilot current is back to its normal level, HHA finally releases, releases the relay T and switches of! the pulsing circuit P. C. and the alarm delay circuit ADC (Fig.

It will thus be appreciated that by the arrangement described, the circuit is cut on if the pilot level is low but not if it is high, but the cut-off is maintained when a low pilot level is immediately followed by a high pilot level.

If it should be desired to cut oil the circuit also for a high pilot level, then the resistance R1 may be omitted: and if this is done both the relays HHA and LLA perform the same function and one of them, for example HHA, may be omitted, terminals l2 and I3 of each of the panels SPA and GPA being connected together, and conductor 8 being omitted.

The pulsing circuit PC (Fig. 5) is a relaxation oscillator comprising a cold cathode gas filled discharge tube V1 having a cathode y connected to ground at 3, a control electrode a andan anode z. When the relay AL in Fig. 4 operates to short-circuit terminals l5 and IS, the positive terminal I of a high tension supply, shown as a battery HT in Fig. 5, is connected to the anode 2 through a smoothing filter comprising retardation coil L and a shunt condenser C1, and through a resistance R a relay D and conductors l6 and III. A potential dividing circuit R1, R1 is connected between the anode z and the cathode y of the tube V1 and the junction point Z is connected through a resistance R2 to the control electrode :1: which is connected to ground through a condenser C2. As soon as the relay AL (Fig. 4) operates, the condenser C2 begins to charge up until its potential reaches the ionising potential for the electrodes :2: and y. The tube then fires and the anode current operates the relay D. This discharges the condenser C2 through a low resistance R3, and also operates the relay E which cuts the anode circuit of V1 so that the discharge is extinguished. Relay E also operates relay FA in the alarm panel ARA (Fig. 4) (since relay AL is already operated), which releases any of the relays HS, LS, HG, or LG which may have operated. Relay E is preferably a slow release relay so that it does not release immediately on the release of D when the discharge is extinguished. Relay FA is likewise preferably a slow release relay and in addition to releasing the relays in the panels SPA and GPA (Fig. 3) also provides when operated, holding'circuits for the windings b of relays HHA and LLA. The b winding is so designed that although it will not operate the relay alone, it will hold the relay up when once it is operated.

Resistances R5 and Rs are associated with relay D for the purpose of reducing surges in the anode circuit of the tube V1 which might cause erratic operation.

Resistance R1 is shunted across relay E to act as a spark quencher for the contact of relay D, and to increase the release time of relay E.

After the release of relay D, the condenser charges up until the tube V1 fires again, the process being repeated indefinitely. Thus each time the tube V1 fires the relays in the panels SPA and GPA are momentarily released. when the fault condition is cleared, the relays HS etc. will not be re-operated and the corresponding relay HHA or LLA will be released, breaking down the alarm circuit, which then restores to normal.

It will be observed that the relay HHA or LLA is held operated during the pulsing because when FA is released it will be energized from the relays HS etc. through the a winding, and when FA is operated it will be held through the b windings from relay FA. By making these relays slow to release they will hold up during the changeover period, but will release as soon as the corresponding relay HS etc. releases.

The period of the pulsing circuit depends on the time constant of the circuit R2, C2. This may be made variable by making R2 and/or C2 adjustable, so that the period can be set to any desired value, for example 2 seconds.

Thus as soon as a fault or abnormal condition arises, one of the relays HS etc. operates and starts the pulsing circuit PC which continually re-tests for the fault condition, say every 2 seconds, until the fault is cleared.

The alarm delay circuit ADC is in principle exactly the same as the pulsing circuit PC and has corresponding components which will not be again described, except that the slow release relay E of the circuit PC is represented by a quick release relay M which, however, does not on op eration switch off the high tension supply. The tube V1 accordingly only ionises once, giving a single pulse after a time determined by the values chosen for R2 and C2 and remains permanently ionised until the fault condition is cleared. The operation of the relay M operates the alarm signal relay PAS through conductor l9 (Fig. 4) as already explained and also permits the operation of the cut-01f relay T in the circuit TCA, the circuit having already been prepared by relay J. It will be observed that the alarm delay circuit ADC is switched on at the same time as, but independently of, the pulsing circuit PC by the short circuiting of the conductors I1 and it by relay AL (Fig. 4). The values of R2 and C2 in ADC will be chosen to give a relatively long time for the building up of the charge in C2 to the ionising potential (say 4 or 5 seconds) so that the circuit is not cut off and the alarm is not given unless the fault condition persists. This is for the purpose of preventing a shut down caused by momentary fault conditions, such as line surges, which do not persist. During the time when the condenser C2 in ADC is charging up, the pulsing circuit PC is continually retesting for the fault, and ensures that the arrangement will clear down as soon as the abnormal condition has disappeared.

The arrangements have been described so far for the A to B direction of transmission. For the opposite direction, there will be provided a duplicate alarm panel ARB (not shown) multipled to the conductors 14 to l9 at the right hand side of Fig. as indicated. This will have connected to it two pilot control panels SPB and GPB (also not shown) similar to SPA and GPA of Fig, 3. The corresponding transmission cut-off circuit ICE is shown connected to the relay M at the right-hand side of the alarm delay circuit ADC. The operation of the apparatus corresponding to the B to A direction will be as already explained for the A to B direction. It will be noted that on the occurrence of a fault affecting a pilot current in one direction, only that direction is out oil; but should both directions be simultaneously affected, as may frequently occur in bad weather, both directions will be cut off, and it is immaterial whether the faults in the two directions occur at the same or at difierent times.

It will be usual to supply one alarm delay panel for each complete repeater (Fig. 1) in the repeater station. However, the pulsing circuit PC may be shared between any number of repeaters in the station, and Fig. 5 shows how for this purpose the conductors l4, l5 and I6 may be extended to the other repeaters. The conductors l5 and I6 are connected to all the AL relays for the purpose of switching on the PC circuit when required, and the conductor Hi is connected to each of the F relays through the corresponding AL relay contacts as shown in Fig. 4.

pilot current in each direction, one of the pilot control panels SPA or GPA and one of the panels SPB or GPB can be omitted without affecting the operation of the other. Similarly if there are three or more pilot currents, corresponding pilot control panels will be provided for each of them and they will be exactly like SPA or GPA, the terminals ll l2, l3 and 2| of all the panels being multipled together.

When the automatic gain control fails, because of the abnormal increase of attenuation due to bad weather conditions for example, it is frequently the practice to provide means for switching over to manual gain control, or to set each repeater at some predetermined fixed gain. This may allow some of the carrier systems on aroute to be maintained in service. When thisis done, it is necessary to provide means for cutting out the alarm and cut-off arrangements which have been described. For this purpose, the ground connection to the circuit ARA Fig. 4, may be taken through a key or relay contact K, as shown, so that it may be disconnected when the circuit shown in Figs. 3, 4 and 5 is not to function. The contact K may be associated if desired with the arrangements (not shown) for switching over to manual control or fixed gain so that the alarm and 'cut-ofi arrangements will be automatically cut out. It will be seen that when the contact K is opened, the relays AL and T will be prevented from operating.

The arrangements described in connection with Figs. 3, 4 and 5 provide a means whereby a spare line provided with one or more repeaters at unattended stations may be taken into service when required, without the necessity for an operator to visit these stations. Assuming that all the repeaters have been originally lined up, and the automatic gain control arrangements properly adjusted, the spare line may be brought into operation simply by switching on the pilot currents, or by the usual indication of a low level pilot and the current will be cut in both directions. When the pilot currents are switched on from the nearest attended station, the first repeater will line itself up to produce the proper pilot levels at the output. When this has been done, the cut-off circuit will indicate normal conditions and the line circuit will be completed by the release of relay T to enable the pilots to be transmitted at the proper level to the next station. This then proceeds to line itself up in the same way, passing on the pilots to the next station and so on. When one direction has been lined up in this way, the opposite direction may be automatically lined up likewise, or both directions may be lined up simultaneously, after which all the cut-off circuits will be restored to normal.

It will thus be seen that during this process the pilot currents can never be transmitted to line at an abnormal level, and there will be no danger of interference with other systems on the same route during the lining up.

What is claimed is:

l. A multi-channel carrier current transmission system provided with at least one two-way repeater, including means for transmitting from each end of the system at least one pilot current to said repeater, means at said repeater controlled by said pilot current for adjusting the gain thereof in such manner that for each direction separately the pilot current levels at the corresponding repeater output are brought to predetermined values, means for blocking the transmission paths during said adjustment, and means for unblocking the corresponding transmission path when the adjustment for the direction concerned has been completed, so as to allow the pilot currents to be transmitted to the next repeater, said blocking means including an attenuating network comprising a substantially balanced Wheatstone bridge network. the input and output terminals of which comprise, respectively, the two pairs of diagonally opposite corners of the bridge and located in the output sides of said amplifiers, so that they can be blocked by the introduction therein of a large attenuation, and including a transmission cut-oil relay having contacts connected so as to insert said bridge network in the circuit when said relay is operated, and to cut out said network when said relay is released, whereby said relay acts as said blocking and unblocking means.

2. A system according to claim 1, in which the four arms of the bridge comprise, respectively, four resistances, each substantially equal to the impedance of the circuit at the point where said network is introduced thereinto.

3. A system according to claim 1, in which at said repeater there is also provided for each one-way amplifier a pilot control panel correspondin to each pilot current which controls said one-way amplifier, each of said pilot control panels including a high pilot relay which is so connected as to close a pair of contacts only when the corresponding pilot level is high, and a low pilot relay connected so as to close another pair of contacts only when the corresponding pilot level is low.

4. A system according to claim 1, in which a plurality of repeaters are provided, and at each of said repeaters there is also provided for each one-way amplifier a pilot control panel corresponding to each pilot current which controls said one-way amplifier, each of said pilot control panels including a high pilot relay which is so connected as to close a pair of contacts only when the corresponding pilot level is high and a low pilot relay connected so as to close another pair of contacts only when the corresponding pilot level is low, said system including more than one amplifier and pilot control panel at each repeater and means for connecting in multiple, at all said pilot control panels, the contacts corresponding to all said high pilot relays, and also the contacts corresponding to all said low pilot relays, whereby a single circuit is responsive to all the high pilot relays and a single circuit is responsive to all the low pilot relays.

5. A multi-channel carrier current transmission system provided with a plurality of twoway repeaters, includin means for transmitting from each end of the system at least one pilot current to said repeaters, means at each repeater, controlled by said pilot current, for adjusting the gain of said repeater in such manner that for each direction separately the pilot current levels at the corresponding repeater output are brought to predetermined values, means for blocking the transmission paths during said adjustment, and means for unblocking the corresponding transmission path when the adjustment for the direction concerned has been completed so as to allow the pilot currents to be transmitted to the next repeater, said blocking means including an attenuating network comprising a substantially balanced Wheatstone bridge network, the input and output terminals of which comprise, respectively, the two pairs of diagonally opposite corners of the bridge and located in the output sides of said amplifiers, so that they can be blocked by the introduction therein of a large attenuation, and including a transmission cut-oflf relay having contacts connected so as to insert said bridge network in the circuit when said relay is operated and to cut out said network when said relay is released, whereby said relay acts as said blocking and unblocking means, including at each of said repeaters, for each one-way amplifier, a pilot control panel corresponding to each pilot current which controls said one-way amplifier, each of said pilot control panels including a high pilot relay which is so connected as to close a pair of contacts only when the corresponding pilot level is high, and a low pilot relay connected so as to close another pair of contacts only when the pilot level is low, and also including at each repeater an alarm panel for each of said oneway amplifiers, including a high level slow release relay connected so as to be operated by any of the high pilot relays and a low level slow release relay connected so as to be operated by any of the low pilot relays.

6. A system according to claim 5, in which said low level slow release relay is so connected as upon operation to prepare the circuit of said transmission cut-off relay for operation after a predetermined delay, and in which said high level slow release relay is so connected as upon operation to hold said transmission cut-ofi relay operated after the release of said low level relay, said high level relay also being connected so as to prevent the operation of said transmission cut-oil relay if said high level relay operates before said low level relay operates.

'7. A system according to claim 5, in which said low level slow release relay is so connected as upon operation to prepare the circuit of said transmission cut-oil relay for operation after a predetermined delay, including as means for providing said delay a relaxation oscillation circuit comprising a gas discharge tube connected so as to produce a single pulse, and a time delay circuit determining the discharge of said tube,

said time delay circuit including a condenser and a resistance and means for charging said condenser through said resistance, and another relay, said pulse being used to operate said other relay, which latter closes the circuit of said transmission cut-off relay already prepared by said low level relay.

8. A system according to claim 1, including a plurality of repeaters, in which at each of said repeaters there is also provided for each one-way amplifier a pilot control panel, corresponding to each pilot current which controls said one-way amplifier, each of said pilot control panels including a high pilot relay which is so connected as to close a pair of contacts only when the corresponding pilot level is high anda low pilot relay connected so as to close another pair of contacts only when the corresponding pilot level is low, said system also including a pulse generator connected so as periodically to release any high pilot relay or low pilot relay which has operated as a result of an abnormal pilot current level, and so as to allow the periodical reoperation of such relay or relays while said abnormal condition persists.

9. A system according to claim 1, including a plurality of repeaters, in which at each of said repeaters, there is also provided for each oneway amplifier a pilot control panel corresponding to each pilot current which controls said oneway amplifier, each of said pilot ,control panels including a high pilot relay which is so connected as to close a pair of contacts only when the corresponding pilot level is high and a low pilot relay connected so as to close another pair of contacts only when the corresponding pilot level is low, said system also includinga pulse generator connected so as periodically to release any high pilot relay or low pilot relay which has operated as a result of an abnormal pilot current level, and so as to allow the periodical reoperation of such relay or relays while said abnormal condition ersists, said pulse generator including a relaxation oscillator comprising a gas discharge tube connected so as to produce pulses at regular intervals, an interval determining circuit including a condenser and a resistance, and means for charging said condenser through said resistance, and another relay connected so as to be actuated by the pulses from said generator, and operative so as to release said high and low pilot relays.

ALFRED MARCUS THORNTON. WILFRED FRANK BALY.

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