US2236134A

US2236134A - System of transmission of electric signals

Info

Publication number: US2236134A
Application number: US249677A
Authority: US
Inventors: Gloess Paul Francois Marie
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1952-10-17
Filing date: 1939-01-07
Publication date: 1941-03-25
Anticipated expiration: 1958-03-25
Also published as: GB523434A; FR63409E; NL69923C; FR842357A; CH248972A; BE432307A

Description

March 25, 1941.

SYSTEM OF TRANSMISSION 015 ELECTRIC SIGNALS P. F. M. GLOESSS Filed Jan. 7, 1939 5 Sheets-Sheet 1 PH SE Rf'rqR E l SELECT/V5 ATTENUHTOR 1 8 INVERTER 6 MIXER F ig. 4. ECHO MEASURING DEV/Cf PILOT SEPARHTING DEV/CE H [NI/567T MIXER I #1 4 H pg RETHPUER 2 HTTENUATO? 6 O/ VV v7 /P Sol/RC5 or SIGNALS n/vD 'PILOT IMPULSES FEM GL OESS Ana/WE;

March 1941. v P. F. M. GLOESS 6, 34

SYSTEM OF TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. '7. 1939 5 Sheets-'-Sheet 2 50/0 MEASURING pzwc:

PILOTsEPAR/JTIIVG' DEV/CE I0 PHASE RETAR/DER N 7 ,WXER

3 7 l 4 5 c 2 I ATTENUH TOR I sou/Pct 0F SIGNALS AND PILOT IMPULSES Fzg. 7.

MHSURING .D. VICES Pia/415E I? ATTENUH T023 PILOT SEPRRRTOR INVERTER 6 MIXER SOURCE OF SIG/VHLS HIVD PILOT IMPULSES March 25, 1941. GLQESS 2,236,134

SYSTEM OF TRANSMISSION-OF ELECTRIC SIGNALS Filed Jan. '7, 1939 5 Sheets-Sheet 5 R m s M m 5 m V a T w A J m M W I I I I I l I 1 I i I i March 25, 1941. P. F. M. GLOESS 2,236,134

SYSTEM OF TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 7. 1939 5 Sheets-Sheet 4 Fig. 8.

ECHO MEASURING DEV/6E5 sol/gas ors/a/vnw mva PILOT wPuLsss PILOT SEPHEfl/NG DEV/C55 INVL'RTER Pill/95E PETARDEE MIX ER March 25, 1941. P. F. M. GLOESS 2, 6,134

SYSTEI OF TRANSMISSION OF ELECTRIC SIGNALS Filad Jan. 7, 1939 5 Sheets-Sheet S Fig. 9.

JWEEP c/k "7 l0 Mrs/mama 4 PILOT SEPARAT g v Jul/RC5 0F slalvnLs AND PILOT IMPuLs s INVA'RTER SWEEP c/Rcul7'-, l4 7' I BC I I I 10 i HTTENUATORS 5 it PILOT SEPARATOR I I :wmsz RETARDH? F 7 WVE/W'OR Pinszozss ATTORNE Z Patented Mar. 25, 1941 UNITED STATES PATENT OFFICE SYSTEM OF TRANSMISSION OF ELECTRIC SIGNALS Application January 7, 1939, Serial No. 249,677 In France January 25, 1938 14 Claims.

The present invention relates to electric signal transmission systems and particularly refers to arrangements in such systems for reducing and substantially eliminating certain harmful phenomena originating in the said systems.

In electrical systems for the amplification, transformation or transmission of electrical signals it happens in fact that parasitic signals are produced and accompanying the main signal, having the effect of rendering the said signal unclear.

In certain cases the parasitic signal merely consists of a repetition of the main signal but at a lower amplitude lagging behind the main signal. In other cases parasitic signals of this type, differing from each other and lagging behind the main signal by different phase angles of the same order, are grouped together and follow the main signal, and thus constitute'a sort of trail effect on the signal. These two parasitic signals are produced due to defects in the matching of impedances along the transmission system, which defects give rise to backward reflections, the portion reflected back being again reflected forward thus following the main signal with a phase lag, from which there results either an echo, in the case of two reflections, or trail in the case of a greater number of relatively close reflections.

In the case in which a portion of the transmission path is a radio path it often happens that high frequency waves undergo one or morereflections on the ground surface, and/or on more or r less high atmospheric layers, and the receiver then receives in addition to the main wave, one or more secondary waves having followed longer paths and consequently arriving with a phase lag on the main wave. This produces a phenomenon similar to that of echoes the amplitudes of signals of certain frequencies undergoing either attenuation or amplification with respect to the others. This phenomenon varies in intensity in the manner of that known by the name of fading. Consequently, this phenomenon may be designated by the expression selective fading. Its existence is particularly frequent in the case of radio telephone and telephotographic transmission by short waves over long distances.

Consequently, the invention in particular provides arrangements for reducing or substantially eliminating such distortions of transmitted signals. These arrangements may be either predetermined or may be adjustable and in this latter case may be adapted to prevent such defects from occurring, correct existing defects, or to be automatically adjusted by the distortion of the signals themselves before correction, or .by the distortion still remaining in a signal to which correction has been applied.

In accordance with the invention, in order to correct such distortions as hereinbefcre referred to a network'c'o'mprising a direct path and a bye-path is inserted in the transmission line, the distorted signals being transmitted directly in the direct path and if desired amplified, and in the bye-path the distorted signals are retarded, inverted and attenuated to a desired degree, and afterwards the signals from the two channels are combined, the attenuation and the delay imparted to said signals in the bye-path channel being such that the resulting signals from the byepath are opposed to the parasitic signals following the'main signal in the direct path and substantially eliminates them.

In carrying the invention into practice, the'correcting network may comprise certain already existing portions of the normal transmitting path anjd'lin the case in which the amplitude of the trail parasitic signal decreases in accordance with a substantially exponential law, circuits of simple impedances may be employed as corrector networks.

The corrector networks may comprise a device which automatically analyses the intensity and/or shape of the parasitic signals and controls the adjustment of a device for producing attenuation and arrangements are provided for separating the main signals from the parasitic signals. Furthermore, the corrector network may be provided'with a number of retard channels corresponding in number to the actual number of parasitic signals, or to a number arbitrarily chosen in the case of a parasitic trial signal and Q the retard channels may have certain devices in common.

The device for analysing the parasitic signals maybe fed by signals which have previously been approximately corrected so as only to analyse the parasitic signal residue after the approximate correction, and to control the attenuator inorder to'improve the correction. To this end the analyser may be fed from the output terminals of the corrector network.

For the analyser measuring devices and circuits are particularly adapted to be used in systems for the transmission of signals such as television or telephotographic systems.

The nature of the defects and the means provided according to the present invention for correcting or reducing them will be set forth in detail in the following description based on the attached drawings in which:

Figs. 1, 4 and 5 show very schematically corrector networks in accordance with the invention;

Figs. 2 and 3 show circuit diagrams of two correctors embodying the invention;

Fig. 6 shows the circuit diagram of. an embodiment of the corrector schematically indicated in Fig. 5;

Figs. 7 and E. represent in more detail respectively the embodiments of the correct-01's shown in Figs. 4 and 5;

Figs. 9 and 10 show two further embodiments of the invention, employing a measuring device, for instance like that shown diagrammatically in Fig. 11.

In an electrical signaltransmission system, a main signal may be inscribed in the form:

S=f (t), in which S is the instantaneous amplitude of the signal and t is time.

The first parasitic echo of instantaneous amplitude P originating on the first reflection of the signal is of the form:

P=Kf(t9) in which K is the coefiicient of reduction of amplitude of the signal and 0 the time lag of the parasitic signal behind the main signal. The nth parasitic signal is of the form: Pn=K"f(t-7L9). The complete signal will then be a signal of instantaneous amplitude.

vice may consist of a real or artificial transmission line or of any electrical or other system adapted to fulfil the same purpose. The path I also comprises an inverter device 4 for inverting the sign of the applied signal, and an attenuator device 5 for modifying the amplitude of the signals in the ratio K. The two paths I and 2 feed into a mixer device 6 which thus receives the direct signal, and the corrector signal consisting of the direct retarded attenuated and inverted signal.

If the signal at the input terminals I of the network is. of amplitude S'=,f(t),f(t) being different from (t) since the former comprises the parasitic signal; the signal at the output terminals 8 of the corrector network is of amplitude S"= "(t)Kf(t-0). Consequently, if S is represented by the expression given above in terms of fit), S will again become equal to S, that is'to say, that at the output of the corrector network, the initial signal will have been restored without echoes.

In Fig. 1 it is clear that the paths I and 2, although schematically indicated by a simple line, consist of any suitable medium of progagation, for example, of transmission lines with two or more wires, coaxial cables, dielectric guides or even may comprise connections of other types, for example, radio.

In certain cases, for example, in the case of coaxial cables, transmissions; by carrier currents, or short-wave power transmission, the reflections Which are produced have an amplitude which depends upon the frequency. In such cases a selective circuit is associated with the attenuator device 5 so that the artificially retarded signal has as a function of the frequency the suitable characteristics necessary with a view to the desired compensation.

When the parasitic signals proceed from a large number of different reflections and have time-lags of the same order, but different from each other, the trail of the main signal resulting in consequence is the sum of the parasitic signals of which the instantaneous amplitude m is of the form: p1=uf(t0), a being a function of H i. e. a=tp (I9) the total trail will have an amplitude P1 of the form:

P1= 111:E (0) f(t-0) and the complete signal will be of the form: S'=f(t) +E (0) f(t0).

In this case according to the invention the cor rector network will be provided with a plurality of paths similar to the path I of the preceding case, that is to say, a plurality of paths having different time-lags respectively corresponding to each value of a and of a.

The number of paths is equal to the real number of echoes or to a smaller number arbitrarily chosen, and the correction will obviously be all the more exact the greater the number of paths employed.

When the transmission medium is sufliciently long, for example, in a. coaxial cable of great length, the parasitic signal resulting and proceeding from reflections due to irregularities of impedance of the cable approximates to an exponential trail which will be represented by:

in which (p (0) is of the form in which 5 is a constant.

In this particular case, in accordance with the invention, the correction of undesirable trail is effected by inserting in the transmission medium of the system a network to correct an exponential drag which approximates very near to the trail actually existing. Two examples of such networks are shown by way of example in Figs. 2 and 3.

In Fig. 2, the corrector circuit under consideration consists of a network with four terminals AB and DE formed of two resistances RI, R3 in shunt on each side of a series connected capacity C shunted by a. resistance R2. The values of the shunt resistances RI and R3, and C and R2, should fulfil the terms: CR2= R2/R3 fl the resistance RI being of a sufficiently low value that its influence can be ignored in the design calculation of the network.

In Fig. 3 the corrector network comprises four terminals AB and DE connected to the elements of a thermionic discharge valve V preferably having a screen-grid, for example of the pentode type with internal resistance p which is high with respect to the anode circuit resistance Ra. The cathode circuit resistance should be such that:

I. being the slope of the valve characteristic in ohms.

the time of transit through the retard clement 3, I the echo appears at the input of the system and,

For-distortions due to echoes, a tall or "selectivefading of complex form, the invention also provides echo and trail ,corrector arrangement which comprise means for automaticallyadapting themselves to the conditions of transmission,

and consequently for reestablishing a correct signal when the distortion of this signal varies in intensityand/or form, for example between certain predetermined limits.

Such corrector systems are adapted to respond to periodic pilot signals of known predetermined form and contained in the signals transmitted in order to ensure the automatic adaptation of these corrector systems to the conditions of operation for the desired correction. Such pilot signals can be separated from the rest of the signal, by arranging, for example, that they have a peak potential higher than that of the other signals.

Each pilot signal is followed and preceded by a time interval in which thereis no transmission of current variations.

If a transmitted signal has undergone a distortion resulting from one or more echoes or trail, the said periodic pilot signal will be followed by a parasitic signal, the form of which will depend on the one hand upon the original signal, and on the other hand upon the process in accordance with which this parasitic signal is formed.

ping time in the variation of. intensity transmitted.

In carrying out this feature of the invention into practice, means are provided for the automatic analysis of the intensity and shape ofthe' parasitic signals, and for automatically utilising the result of this analysis to correct the undesirable distortion, further means being provided to modifyyif necessary. the signals which are analysed at the time of the variation.

- Fig. 4 represents a schematic example'of a corrector network embodying this feature of the invention in the case in which a simple echo has to be corrected. In such a network the retard path I comprises the retarding device 3, inverter 4 and attenuator 5 as in the case of Fig. 1, but a measuring device 9 and a separating device It are suitably connected in this path for example, the measuring device 9 is connected between the input of the retard path I and the attenuator 5 and the separator device [9 is connected between the output of the retard device 3 and the measuring device 9. I

The separator device ll] consists of any suitable arrangement for effectively separating the pilot impulse from the rest of the signal and the measuring device 9 consists of any suitable device for transmission channel at a moment, which for the sake of convenience is considered as origin of time. A time 0 later, the same impulse appears at theoutput of the retard element 3 andat the input ofv the separator element In. .As the .time 0 corresponds to the delay of the echo andalso to consequently, at the input of the measuring device 9 at this moment.

The measuring device 9 is normally blocked and is put into use by th pilot impulse isolated by the separator device ID; at the time of its brief period of operation this measuring device 9 receives the echo of the pilot impulse and records its amplitude. This indication will be transmittedthus to the attenuator 5 for the adjustment of the latter.

Consequently, the elements of the system being suitably established, for each value of the amplitude of the echo existing during a given period of time the corrector path of the system will be automatically adjusted so as to neutralise or eliminate said echo, and consequently the network will supply at its output a correct signal practically free from parasites.

The measuring device '9 instead of being designed and arranged to register and transmit the measure of the amplitude of the echo, may be arranged to record only the residual echo after correction for the said echo. In this case the said This arrangement permits the use of an attenuator device of more simple construction owing to the fact that the inaccuracies for its normal operation will, to a great extent, be compensated soy or reduced owing to the return connection of i said measuring device.

Fig. 6 shows an example of the application of the system of Fig. 5 to the case of transmission of television or telephotographicsignals.

In this figure the portion framed by the dotted linell represents schematically an amplifier arrangement associated in the transmission system concerned, and incorporated in the correcto-r network. The other devices indicated on the drawing are also enclosed by dotted rectangles indicated by the same references as the corresponding blocks of Fig. 5. The inverter and

attenuator devices

4 and 5 have been shown combined'in a single element.

The retardelemen t 3 has been shown-in the form of an artificial line LA with capacities C and self -inductances L. A commutator with contacts CP at the input permits the adjustment as desired of the number of elements of the artificial line LA. The connection to the separator device It] is taken at G in front of the output connection J to, the attenuator 45, so as tooperate the measuring device during the beginning of the arrival of the echo in accordance with a method of operation of the Whole arrangement which'will be set forth hereinafter.

The separator I0 is of the type commonly employed to separate the image current synchronising impulses .in television systems. This separator is preceded by a device l2 for restoration of the direct current component, which permits it to operate under better conditions of stability, but is not absolutely necessary. This separator may, for example, consist of a screen grid valve LEI and its associated polarization circuits, and the device for restoring the direct component, may consist of a diodeLDand its associated circuits..

r The a screen grid valve LE2 the screen grid of which is fed only during the passage of the pilot impulses selected by the separating device I0, and of a measuring circuit CE of suitable time constant.

The

attenuator device

4, 5 consists, for example, of a valve LPV with variable characteristic and the grid polarisation of which is supplied by the measuring circuit CE. The variation of gain of this valve LPV permits the desired attenuation to be obtained automatically. Said valve at the same time fulfils the function of inverter.

The mixing device is composed of two valves 'LSI, LS2 the outputs of which are interconnected,

one of these valves LSI moreover, forming a part of the amplifier normally existing.

The measuring device is controlled through a coupling circuit l3 consisting, for example, of a capacity circuit. This circuit 13 permits the application to the measuring device not the residue of the echo at the output of the mixer, but the principal from which this echo is derived. The existence of this circuit 13 and that of the device I2 for the restoration of the direct current component permits the use of simpler measuring and separating devices.

The signal-s applied to the input of the retard device 3 are of such polarity that the pilot signal to be separated must be in the form of a negative impulse. Then, the various elements in the circuit shown will operate in a satisfactory manner.

In the case, for example, of transmission of television signals by radio means, it may happen network can still be employed at an intermediate point of the chain of amplification which precedes the modulator, and may be suitably modified for such use. For example, in this case the signals applied to the device for measuring the amplitude of the echoes should no longer come from the output of the mixing device, but from the output terminals of a radio receiver tuned to the high frequency transmission, and the receiving antenna of which is placed in the field radiated by the antenna of the controlled transmitter. Moreover, in order to take into consideration the delay undergone by the signals thus collected and transformed, the input of the separator device should be connected at a suitable point of the artificial line difierent from that previously employed.

When more complicated parasites than an echo are produced in a systemof signal transmission, for example, such as those called in the present specification by the name of trail, corrector networks incorporating characteristics of the invention will be employed and comprise a certain number of retard channels, each of these channels corresponding to different values of and :1 having the meanings defined hereinabove.

Figs. '7 and 8 represent two examples of systems incorporating characteristics of the invention for the correction of trail.

Fig. 7 corresponds in one respect to Fig. 4, all the measuring devices 9 being connected to the retard" device 3 at different points of the artificial line so as to measure and record the amplitudes of the various natural echoes or those artificially separated in an existent trail a single separator l0 alone being necessary, but a plurality of attenuators 5 being used in relation to the single reversing device 4.

In Fig. 8 on the other hand each measuring device 9 is employed to measure and record the residue or the principal of the residue of the echoes composing the trail. A plurality of separators II] is thus necessary.

It is clear that some elementsof each group of separated devices, measuring devices and attenuators may be common to the plurality of the other devices employed for their respective functions. It is also clear that the number of retard paths shown is merely for simplicity of the drawing and may be any other number according to the requirements of each individual application of the invention.

Embodiments of such devices are shown in Figs. 9, 10 and 11 in the particular case in which the measuring devices take a form more completely shown in Fig. 11.

The plurality of measuring devices necessary is conveyed to a cathode ray tube T provided with a plurality of electrodes EC instead of a fluorescent screen, each associated with a circuit of suitable time constant formed of a condenser CL (Fig. 11) and a resistance BL (Fig. 11), outside the tube. Each circuit has an individual, or output terminal BT and all the circuits are connected in parallel to a common terminal BC. The circuit of each electrode is successively closed by the cathode ray FC of the tube T.

The system shown in Fig. 9 comprises the cathode tube T comprising the electrodes EC arranged as indicated in Fig. 11, and the deflecting electrodes of which are governed by a sweeping circuit I4, itself governed by a suitable circuit or separator device In. The cathode ray FC thus sweeps the electrodes E0, the period of this sweeping being such that the end of the period coincides withthe end of the trail which follows the pilot impulse selected by the separator l0. Moreover, as mentioned above, the complete signal is also applied for example, to, the modulation electrode of the tube, each electrode during the time of passage of the ray thereto will consequently collect an electric charge depending upon the amplitude of the trail at the moment under consideration. Such a unit will, consequently, fulfil the function of the whole of the measuring devices of Figs. 7 and 8, the circuits with time constants CL-RL serving to record the charges and the mean voltage of each of them being communicated to its respective one of the variable attenuators 5.

The diagram of Fig. 10 corresponds to that of Fig. 8, the output amplitude only being applied to the modulation grid of the cathode-ray-tube T and the operation of the whole is then similar to that given with respect to Fig. 8 employing the explanation given in the preceding paragraph for the operation of the special measuring device e ployed in this system.

It is clear that the invention is not limited to the embodiments shown and described, but on the contrary, is capable of numerous other applications and modifications without departing from the scope thereof.

What is claimed is:

1. An electric signal transmission system including arrangements, for reducing or substantially eliminating echo parasitic signals produced 15 in. the transmission, medium s'aid arrangements comprising a direct path and'a bye-path inserted in the transmission line and means in the byepath for retarding, inverting and attenuating the distorted signals, and further means for separating the principal signals from the parasitic signals and a device for analysing the parasitic signals as regards intensity and for controlling the attenuating means in accordance with the said intensity.

2. An electric signal transmission system including arrangements for reducing or substantially eliminating echo parasitic signals produced in the transmission medium, according to claim 1, in which the means for analysing the parasitic signals is fed from the input of the said bye-path.

3. An electric signal transmission system including arrangements for reducing or substantially eliminating echo parasitic signals produced in the transmission medium according to claim- 1,'

further comprising means for combining signals from said direct and by-paths, in which the means for analysing the parasitic signals is fed from the output of the combined direct and byepaths.

4. An electric signal transmission system including arrangements for reducing or substantially eliminating echo parasitic signals produced in the transmission medium said arrangements comprising a direct path and a bye-path inserted in the transmission line and means in the bye-path for retarding inverting and attenuating the distorted signal, means for separating the parasitic signal from the principal signal in the bye-path and a measuring device for measuring the intensity of the parasitic signal, and means for applying the measure thus obtained to control the said attenuating means.

5. An electric signal transmission system including corrector arrangements for reducing or substantially eliminating echo parasitic signals produced in the transmission medium said arrangements comprising a direct path and a byepath provided with means for retarding, inverting and attenuating the distorted signal, means for automatically varying the attenuation device in accordance with the intensity of the parasitic signal, means for transmitting periodically a pilot signal of known intensity and form, and means responsive to the pilot signal for automatically adapting the corrector arrangements to changes in the distortion.

6. An electric signal transmission system in: cluding arrangements for reducing or substantially eliminating echo parasitic signals produced in the transmission medium, said arrangements comprising a direct path and bye-paths each byepath being provided with a retard device specific to a particular echo component and an attenuating device, and means for controlling each attenuating device in accordance with the intensity of the particular component.

'7. An electric signal transmission system including arrangements for reducing or substantially eliminating echo parasitic signals produced in the transmission medium, said arrangements comprising a direct path and bye-paths, each bye-path including a retarding, inverting and attenuating device, a separating device for separating the parasitic signal from the principal signal and a measuring device comprising a thermionic valve arranged to be normally blocked, and retard device being such that the principal signal is delayed by a period of time equal to the time lag of the parasitic signal behind the principal signal and means for applying a portion of the principal signal to unblock said thermionic valve at the moment of arrival thereat of the parasitic signal, said measuring device thereby measuring and recording the amplitude of the parasitic signal, means for transmitting the measure of the amplitude to the attenuator and for adjusting this latter accordingly.

8. An electric signal transmission system according to claim 7, in which the attenuator and inverter are combined in the form of a thermionic valve.

9., An electric signal transmission system according to claim '7, in which the retard device comprises an artificial transmission line consisting of a plurality of series connected inductances and shunt connected capacities connected to the junctions of the inductances.

10. An electric signal transmission system including arrangements for reducing or substantially eliminating echo parasitic signals produced in the transmission medium, said arrangements comprising a direct path and bye-paths including an artificial line common to all said bye-paths, and inverting device and attenuating devices in said bye-paths, and a separator device for separating the parasitic echo from the principal signal in said bye-paths and a measuring device to measure the amplitude of the parasitic signal, the connection to the separator device being taken from the artificial line at a point in front of the connection to the attenuator whereby the measuring device is unblocked during the begin ning of the arrival of the parasitic signal,

11. An electric signal transmission system ac- .cording to claim 9, in which the measuring device comprises a screen grid valve, the screen grid of which is fed only during the passage of signal impulses in the form of pilot signals selected by the separating device, and a capacity resistance time circuit of suitable time constant, said circuit supplying the controlling potential for adjusting the attenuator device.

12. An electric signal transmission system including arrangements for reducing or substantially eliminating echo parasitic signals produced in the transmission medium, said arrangements comprising a direct path and a plurality of byepaths each including a device for retarding and inverting and devices for attenuating the principal signal in the respective bye-path, and a measuring device for measuring the intensity of the parasitic signal, said retard device being an artificial line common to all the bye-paths, to which all the measuring devices are connected at different points, and a single separator for separating the parasitic signal from the principal signal, and adapted to control all the measuring devices, said measuring devices being adapted to control the respective attenuators, and a single inverter into which all the attenuators feed.

13. An electric signal transmission system including arrangements for'redu-cing or substantially eliminating echo parasitic signals produced in the transmission medium, said arrangements comprising a direct path and a plurality of byepaths each including a device for retarding and inverting and devices for attenuating the principal signal through said bye-paths, according to a respective echo component of the parasitic signal, a measuring device for measuring the intensity of the echo component, a separator device common to all measuring devices and adapted to control the measuring devices according to the respective echo components, and means for feeding back from the output of the combined direct and bye-paths, a signal for measuring the intensity of the echo components thereof, and means for applying the measurements to control the respective attenuating devices.

14. An electric signal transmission system including arrangements for reducing or substantially eliminating echo parasitic signals produced in the transmission medium, said arrangements comprising a direct path and a plurality of byepaths each including means for retarding, inverting and attenuating the principal signal in the bye-path by amounts depending on the echo component particular to a path, a measuring device in each bye-path for measuring the intensity of the echo component in the respective paths, and means for controlling the attenuating devices in the respective paths according to the measured intensity, said measuring devices being all contained in one instrument consisting in a cathode ray tube provided with a plurality of receiving electrodes and means for applying a sweeping potential to the deflecting plates produced under the control of the parasitic signals from a separator device which separates the parasitic from the principal signals said potentials being applied to sweep the cathode ray across the said receiving plates in succession, and means for applying to the control grid of the tube a voltage derived from the parasitic signals in the output circuit of the combined direct and bye-paths, for controlling the intensity of the said beam, and a circuit associated with each of said receiving plates for measuring the charge received from the passage of the ray thereover, and means for applying a potential proportional to said charge to the respective variable attenuators.

PAUL FRANCOIS MIARIE GLOESS.

CERTIFICATE OF CORRECTION.

Patent No. 2,25 6,15L March 25, 19L 1.

PAUL FRANQOIS MARIE GLOESS.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 5, sec- 0nd column, line 56, claim 11, for the claim reference numeral "9" read --7--; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office. Signed and sealed this 15th day of November, A. D. 1915.

Leslie Frazer (Seal) First Assistant Commissioner of Patents.