US2310692A

US2310692A - Method of and means for reducing multiple signals

Info

Publication number: US2310692A
Application number: US279402A
Authority: US
Inventors: Clarence W Hansell
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1939-06-16
Filing date: 1939-06-16
Publication date: 1943-02-09
Anticipated expiration: 1960-02-09
Also published as: GB542023A

Description

Feb. 9, 1943. c. w. HANSELL METHOD OF AND MEANS FOR REDUCING MULTIPLE SIGNALS Filed June 16, 1959 7 Sheets-Sheet l RECEIVER MAIN RECEIVED PULSE SECONDARY RECEIVED PULSE MICRO-SECONDS 22000 MICROSECONDS ANTENNA RECEIVER }s/6-AL ourpur RECE/ l/ER ,4 N TENNA OUTPUT OF RECEIVER 1 7 Y ourpur OF RECEIVER #2 com/-50 OUTPUT 0F RECEIVERS AND #21 INVENTOR. CLARENCE W ANSELL 1 ATTORNEY.

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(L/IRENE HAN-SELL BY A H I ATTORNEY.

Feb. 9, L943. 1 w HANSELL 2,310,592

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FINAL RES UL TAN T SIGNAL ORIGINAL SIGNAL AMPLITUDE 0F CARR/ER RECEIVED OVER MAIN PATH W AMPLITUDE 0F CARR/ER RECEIVED OVER SECDNDAQY PATH DETECTWR INPUTCURQENT DUE N4 T0 COMB/NED MA/NPATH AND SECOND/4R Y PA TH CURRENTS A FINAL RE5ULTANT- h SIGNAL INVENTOR. CLARENCE W HANSELL BY WM ATTORNEY.

Fe. 9, 1943. c. w. HANSELL METHOD OF AND MEANS FOR REDUCING MULTIPLE SIGNALS Filed June 16, 1939 '7 Sheets-Sheet 5 G W M a LINE mm w J LU/ SECONDARY PATH CURRENT RESULMNTOF MAIN PATH/1ND TWO SECONDARY OF OPPOS/NG POLAR/TY PAT/1 S/GNALS, BEFORE BALANCING BALANCING CURRENTS FINAL SIGNAL AFTER BALANCING INVENTOR.

HANSELL ATTORNEY.

eh 1943- c. w. HANSELL. 2,316,692

METHOD OF AND MEANS FOR REDUCING MULTIPLE SIGNALS Filed June 16, 1939 'r Sheets-Sheet e RESULTANT OF MAIN PATH AND SECONDARY PATH CURRENTS FINAL OUTPUT CURRENT AFTER BALANCING CURRENT RECEIVED OVER MAIN PATH CURRENT RECEIVED OVER SECONDARY PATH TIME DEL A YED BALANCING CURRENT FINA L OUTPUT CURRENT CURRENT RECEIVED AFTER BALANCING OVER MAIN PATH RES'UL TAN T OF MAIN AND SECONDARY PA TH C URREN T5 TIME DELAYED BALANCING CURRENT CURRENT RECEIVED ave/2 SECONDARY PATH I NV EN TOR. CLARENCE W HANSELL Awe/o ATTORNEY.

Feb. 9, 1943. c, w HANSELL 2,310,692

METHOD OF AND MEANS FOR REDUCING MULTIPLE SIGNALS Filed June 16, 1939 '7 Sheets-Sheet 7 TRANSM/SS/ON LINE IZLII: RECEIVER ANTENNA CONDENSER I i TIME DELAY CABLE CIRCUIT ANTENNA F232;; RECEIVER TRANSMISSION I LINE INDL/CTANCE 3: j

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atented Feb. 9, 1943 i "KEN METHOD OF AND MEANS FOR REDUCING IWULTIPLE SIGNALS Clarence W. Hansel], Port Jefferson, N. K, assignor to Radio Corporation of America, a corporation of Delaware Claims.

The present invention relates to a method of and means for reducing signal distortions in communications systems where transmitted signals arrive at a receiver over a plurality of paths of different lengths, or over paths requiring different transmission time.

In wire and radio communications systems covering great distances and in systems, such as television, requiring large modulation frequency bands, the distortions produced by multiple signaling paths has been found to be very serious. In trans-oceanic radio transmission at frequencies between 3 and 30 megacycles, for example, signals may arrive at the receiver over a considerable number of paths of difierent length. This phenomenon is described in papers published in Proceedings of the Institute of Radio Engineers" for September, 1929, and elsewhere.

A similar multipath problem exists in the case of television transmission in cities. In this case some modulation frequencies are so high that the waves carrying one cycle of modulation may extend over only a short distance. The highest modulation frequency components in television transmission in the United States may range up to 4,000,000 cycles per second or more. For this modulation frequency a whole cycle of modulation will be carried by higher frequency modulated carrier waves extending over a distance of only 75 meters,-or 246 feet. Serious multipath distortion will be met with in this case if there are secondary signaling paths having lengths differing from the main path by about 60 feet or more.

In cities such as New York, for example, reflections from buildings and other obstacles give rise to many multiple signal paths having lengths differing by 60 feet or more and, in consequence, multipath distortions of television signals is a serious problem in the cities. Evidence of this will be found in a paper by Philip S. Carter and G. S. Wickizer, published in the August, 1936, issue of Proceedings of the Institute of Radio Engineers and a paper by Ralph W. George, published in the January, 1939, issue of the same publication.

In long distance transmission, directive antennas at transmitter and receiver are of some aid in reducing the number of multiple signaling paths and have been particularly effective in reducing round the world secondary paths having relatively long time delays. Directivity of the ordinary kind has not been very eiiective against multiple path distortion due to signal reaching the receiver by the shortest paths lying approximately in the plane of the same great circle around the earth. These multiple paths are due to signals reaching the receiver after different numbers of reflections up and down between the ionosphere and the earth and due to reflections, or refractions, from different layers of the ionosph re. In long distance radio circuits they take place within a relatively narrow range of angles of propagation of waves at transmitter and receiver. Efiorts have been made to provide ex-- tremely great directivity for reception and have met with considerable success although very cost- 137 and complicated equipment is required. For a description of such a system, reference may be made to an article by H. T. Friis and C. B. Feldman in Proceedings of the Institute of Radio Engineers for July, 1937.

Of course, receiving antenna directivity will aid in rejecting undesired secondary rays from the transmitter in the case of television transmission, and will undoubtedly be used. In the cities it will also be necessary to choose television receiving antenna locations with care. It will often be found desirable or necessary to serve a large building, a block of buildings or a community by means of a centrally and well located directional receiver for which the location and directivity may be elfeotive in reducing multipath phenomena. Then, from this receiver, modulated radio frequency, modulated intermediate frequency or modulation frequency power may be distributed through suitable equipment, lines or radio relays to a whole group of receivers. In many cases, both in centralized receiving systems and in individual receivers, it will be found that there cannot be obtained sufficient directional eifeot to reduce multipath distortion to unobjectionable proportions, and some additional means will be required to reduce it.

To assist in understanding the multipath problem, reference will now be made to an example, assuming a simple case of one secondary path, in addition to a main path, as illustrated in Fig. 1. This figure illustrates a transmitter T and a receiver R, shown diagrammatically in box form, with two possible paths of transmission therebetween. In this case one path for radiation arriving at the receiver may extend on a straight line from the transmitting antenna to the receiving antenna and is labeled Main signal pat Another path may extend from the transmitting antenna to some point of reflection, which may be the ground, a large building or any other obstacle to the waves, and thence to the receiving antenna, and is labeled Secondary path.

Let us assume that currents set up in the receiver due to the secondary path have a strength which is 20% of the currents due to the main path and that the difference in path length is about 600 meters, resulting in a time delay of about 2 micro-seconds for modulations received over the secondary path. The transmitter carrier frequency might be assumed to be 100 megacycles and the modulation band width might be 60 cycles to 4 megacycles.

If, at the transmitter T, we should set the carrier output normally at zero and then transmit a single pulse with a duration of 1 microsecond, we would, as a result, observe two pulses at the receiver with beginnings separated by 2 microseconds and the second pulse would have a strength 20% of the first. Fig. 2 illustrates graphically the approximate time relations and wave forms of the pulses.

In any communication system which requires only transmission of pulses sufiiciently short and sufficiently separated to prevent overlapping of main path and secondary path pulses at the receiver, as illustrated in Fig. 2, it is only necessary, in accordance with the invention, in order to eliminate the secondary path received pulses in the examples illustrated in Figs. 1 and 2, to take out a correct portion of the main signal pulse, which is time delayed by a correct amount, and then reintroduce it in the receiver circuits in a manner to balance out the secondary path pulse. This may be done in a variety of ways, in accordance with the various embodiments of the invention described hereinafter.

A complete description of the invention follows in conjunction with drawings, wherein:

Fig. 1 illustrates, by way of example only, how signals emanating from a, transmitter may arrive at a remote receiver over paths of different length;

Fig. 2 shows, as an example, how a main and a secondary signal or image may appear at a receiver after traveling over different paths from a remote transmitter;

Figs, 3, 5, 6, 7, 15, 16, 1'7 and 18 show different embodiments of the invention for reducing secondary path signals under different conditions encountered in practice; and

Figs. 4, 8, 9, 10, 11, 12a to 12 inclusive, l3 and 14 are graphs given for purposes of exposition to aid in understanding the different circuit embodiments of the invention.

One way of reducing the secondary path signal is illustrated in Fig. 3. In this case two receiving antennas l and 2 with their respective transmission lines TL and TL and their respective receivers I and 2' are employed to provide a com mon rectified or direct current output in response to the received signals. The locations of the two receiving antennas l and 2 and the lengths of their respective transmission lines TL and TL are so adjusted that signals arrive at one receiver later than at the other by an amount of time equal to the time delay of the secondary path signal which is to be eliminated. If the time delay is 2 microseconds, for example, then one transmission line such as TL may be longer than the other by such a length as to produce the 2 microseconds time delay. If waves on the lines travel with the velocity of light, then, for 2 microseconds time delay, one line must be 600 meters longer than the other. By employing lines with lower wave velocities, the difference in lengths may be proportionately less.

The receivers l and 2' are so designed and connected that their outputs have opposite polarities and the receiver 2 with time delayed output is so adjusted that the output due to the main path signal is of correct magnitude to cancel the secondary path signal in the output of the other receiver. If the secondary path signal has an amplitude equal to 20% of the main signal then receiver 2 of Fig. 3 will be adjusted to give an output which is 20% of the output of receiver l. The signal outputs of the two receivers l and 2' and their resultant combined output is illustrated in Fig. 4.

It will be noted that the main signal output of receiver 2 cancels the secondary path output of receiver I leaving only the secondary path signal of receiver 2 as the final source of distortion. This remaining secondary path signal, in the case assumedis only 20% of 20% or 4% of the main path signal. Even this small remaining secondary path signal can be reduced or eliminated, as will be explained later.

We may accomplish substantially the same results in suppressing secondary path signals with only a single receiver if we provide means for splitting off a correct amount of the received si nal current into a time delay circuit and then combining the time delayed current with the other currents in such a way as to make it cancel secondary path currents.

One way of accomplishing this result is shown in Fig. 5. There is shown a receiving antenna, with its transmission line T'L' and a receiver 3 for delivering signal output current. A portion of the receiver output is taken out through an adjustable resistance R1 into a time delay cable circuit 4. Signal waves of current and potential. enter the cable and travel to the remote end. from which they are reflected back to the input end again and reintroduced into the receiver output circuits with correct polarity, strength and time delay to cancel undesired secondary path signals.

In Fig. 5 there are provided variable or adjustable resistances R1, R2 for governing the strength of potential and current entering the cable 4 and the strength of time delayed potential and current taken from the cable and fed back into the receiver output circuits for balancing out secondary path signals. At the same time. the combination of two adjustable resistances R1 and R2, shown in Fig. 5, makes it possible to control the amount of potential and current reflected back over the cable for a second time and this secondary current may be made to cancel even the small residual secondary path signal indicated in the lower portion of Fig. 4. Therefore, once the cable delay circuit has been cut and terminated for proper time delay, and reversed polarity of returned waves, an operator or installer of th receiver, by varying the two resistances at the end of the cable in Fig. 5 may obtain a minimum of secondary path signal.

To obtain proper timing for signal power returned over the cable, after reflection from the remote end, the length of cable 4 must be so chosen that the length is'equal to half the time delay required multiplied by the velocity of electrical waves upon the cable. If the cable has an electrical velocity half the velocity of light, or 150,000,000 meters per second, which is a reasonable value, then a time delay of 2 microseconds will require a cable meters long. It may be made up of a section of small diameter, insulated, concentric conductor cable which may be Wound up into a coil and the bulk of it placed in any available location in or near the receiver or its output circuits.

To obtain reversed polarity for the waves reflected back over the cable, with the arrangement shown in Fig. 5, the remote end of the cable may be short-circuited by a path of extremely low impedance to energy of the frequency traversing the cable. The short-circuit, which may be in the form of a direct current metallic connection, causes the potential wave to be reflected with reversed polarity from the short circuited point. A single short positive potential pulse entering the receiver end of the cable will be refiected and return to the input end as a negative pulse. It will give rise to currents which are in the same direction as the signal pulses in the receiver output leads and in opposite direction to the signal pulses in the final output terminals. If the receiver output circuits have sufiicient internal impedance, which they usually have, or if sufficient series impedance added, then currents caused by time delayed waves coming back over the cable may cancel secondary path signal currents substantially completely.

If desired, means may be provided for connecting the time delay cable circuits effectively in series with the receiver output circuits, instead of in parallel as in Fig. 5, in which case it is then necessary to have an open circuit at the remote end of the time delay cable in order to obtain a reversal of the polarity of current pulses in the output of the receiver.

In Fig. 6 there is shown another arrangement for obtaining a cancellation of secondary path signals in which a controllable amount of the receiver output energy is passed over a time delay cable circuit and then combined with final output energy in a manner to cancel secondary path signals. In this case it is assumed that the receiver has a balanced, or push-pull output. The cable input energy is then derived from one side a of the circuit and fed back again in the direction of the arrow into the other side I) of the circuit in order to obtain the required reversal of signal polarity. Balancing resistances R" are shown for maintaining approximate balance in the circuits. In this arrangement the cable delay circuit 5 must be twice as long as the cable shown in Fig. 5 because the balancing pulses travel only once over the cable.

In Fig. 7 there is shown still another arrangement for accomplishing the same purpose as the arrangements of Figs. 3, 5 and 6. In this case power is taken from the signal output circuit and fed back in the direction of the arrow to some point P in advance of a signal amplifier SA. In this arrangement the time delayed power sent back may be much less than the output power from the system. In this arrangement the cable circuit 5 must have substantially the same length as the cable used in the arrangement of Fig. 6. It is also necessary to make the feed-back apply between two points in the amplifier system which are normally at opposite signal polarities. If ordinary grid controlled vacuum tube amplifiers are used, this means that an odd number of vacuum tube amplifier stages must be used between the cable circuit terminals.

I have, so far, described operation of the arrangement of my invention for reducing secondary path signals when the main signals and secondary path signals are received without overlapping in their time of arrival at the receiver. If the signals are so long, or the time delay between them so short that they overlap, or if signals are transmitted by partial keying or modulation of the amplitude of a continuous high frequency carrier, then a more complicated situation exists.

If the receiving antenna is so located that high frequency carrier currents over a main path and a secondary path add together in the same phase, then we may assume that resultant currents in the receiver are equal to the instantaneous sum of the amplitudes of the main and secondary path current components; In Fig. 8 there is illustrated a possible signaling condition in which a single rectangular wave irnpulse is transmitted which arrives at the receiver over a main path and also at half amplitude over a secondary path, with a time delay equal to half the duration of the pulse. The two received signals therefore overlap. The overlapping received signals when the radio frequency currents are in adding phase, produce a combined signal, shown in Fig. 8, which is very badly distorted and elongated. It is assumed in Fig. 8 that the main and secondary path received currents have the same radio frequency phase and therefore add amplitudes arithmetically. The amplitude of the secondary path current is 50% of the main path current. In this case we may apply the balancing schemes illustrated in Figs. 3, 5, 6 and 7 to reduce the effect of the secondary path signal in the same manner as has been described for separated received pulse.

If waves are made to travel only once over the time delay transmission line, between its input and output terminals, we may obtain, after suitable adjustment, or final resultant signal such as is illustrated in the lower portion of Fig. 3. It will be noted that distortion of the main path signal has been eliminated and we have left only a following signal with half the relative strength of the original secondary path signal. As previously explained, even this remaining reduced secondary signal may be further reduced or eliminated by multiple reflection over the time delay circuits.

So long as we have chosen receiving antenna locations, best done by trial at each receiving location, where the main path and secondary path currents are in adding phase relation, we may apply the systems of Figs. 5, 6 and 7 to very greatly reduce secondary path distortions of any kind of amplitude modulated signals in all cases where the path length difference remains suiiiciently constant or where means, such as an operator, is provided to make time delay and amplitude adjustments. The scheme is particularly applicable to television reception because in this case the path lengths do remain substantially constant.

Even though the main and secondary path radio frequency currents before detection may not be in perfectly adding phase relation, the balancing arrangements of Figs. 3, 5, 6, and 7 will be at least partially eifective in eliminating secondary path modulations so long as the secondary path current departs from the adding phase relation by approximately less than plus and minus That is, they will be effective so long as the resultant modulations received over the two paths have the same polarity.

In Fig. 9 there is shown the results of balancing secondary path modulations when the secondary path radio frequency currents have 50% amplitude, corresponding to the assumption in Fig. 8, but have a phase relation of 60 with respect to the main path current. In this case also, it will be apparent, the balancing is quite effective in reducing multipath signal distortion.

In Fig. 10 there is shown the results of balancing distortion in the main signal when the main and secondary path high frequency currents are in opposing, or 180, phase relation, it being assumed that the secondary path current is 50% of the main path current. In this and all other cases where the phase angle is within the very approximate limits of 90 and 270, that is, when secondary path modulations are reversed in polarity, it is necessary to reverse the polarity of the balancing currents. In the arrangement of Fig. 3 this is done by reversing the output leads to one of the receivers. In the arrangement of Fig. 5, it is done by removing the short circuit at the remote end of the time delay cable circuit and leaving the end electrically open ended. In the arrangement of Figs. 6 and 7, it may be done by reversing the polarity of connection at one end of the time delay cable circuit.

From Fig. 10 it will be seen that the final resultant signal is no better than the combined main and secondary path signals before limiting. This means that the balancing is of little value when used with transmitted signals such as those assumed. One remedy for the failure of the system is to move the receiving antennas about until the secondary path currents add to the main path currents and the secondary modulations have the same polarity as main path modulations. Another remedy is to employ a constant transmitter carrier which is modulated lightly enough by the signal so that, at the receiver, the carrier is never quite 150% modulated. Still another remedy, for use with a transmitter which radiates a carrier wave at all times, is to employ means for increasing the strength of the carrier wave, with respect to side band frequencies, in the receiver before detection. Various means for exalting or increasing the relative strength of the carrier are already known in the radio communications art.

If we assume that the transmitter modulations never reduce the carrier wave to zero at the detector in the receiver, then the balancing scheme will be as effective in reducing negative secondary signals as it is in reducing positive secondary signals. In Fig. 11 there are illustrated the conditions of the signal at various stages of transmission and reception, when the secondary path current opposes the main path current in phase, but the transmitter has a continuous carrier current which barely prevents the detector input current passing zero and so reversing its direction of amplitude change. In this case the secondary path balancing is as effective as in the cases illustrated in Figs. 8 and 9. Obviously, if the transmitter had been nearly 100% modulated, it would have been necessary to employ carrier exaltation in the receiver to make the secondary path signal distortion small.

The means for balancing out, or reducing, secondary path distortion which have been described may be employed to reduce the effects of more than one secondary path current by employing a balancing arrangement with proper time delay for each secondary path current.

In Figs. 12a to 12 inclusive, there are shown the results of the balancing applied to a case where there are two secondary path currents. These figures illustrate the signal conditions due to scanning one line of a television image made up of dark letters A, B and C on a light background in the United States standard system for television (note Fig. 12a). First, in Fig. 121), I

have illustrated the main path transmitted current and next, in Fig. 120, two secondary path currents with different time delays with respect to the main path current. I have assumed one secondary path current to have an amplitude of one third of the main path current and the other an amplitude of one sixth. The first of these is assumed to have the same polarity and the second a reversed polarity with respect to the main path current.

The combination of main and secondary path currents gives a resultant current, before or after detection and before balancing, which is quite badly distorted and which has very pronounced multiple images of the letters. This is shown in Fig. 12d. I have then indicated in Fig. 12c two balancing currents, each obtained by means of time delay cable circuits from the resultant received signal, as previously described. These two time delayed balancing currents are then combined with the original receiver resultant output current to give a final output current, after balancing, illustrated in Fig. 12f. It may readily be seen by inspection that the balancing has provided a final signal which is far less distorted and accompanied by far less apparent multiple images of the letters than is the resultant before balancing.

In applying my scheme for balancing out the effects of secondary path currents there will be many instances in which the time delay is quite short and comparable with the time required to reach nearly steady state current conditions as determined by the frequency band width of transmitter and receiver. In these instances distinct and separate multiple images may not be apparent but, instead, there will be a loss image detail or an unnatural outlining of the images.

One such possible condition is illustrated in Fig. 13 where I have assumed that main path and secondary path high frequency currents are in adding phase relation and the secondary path current has a strength of half that of the main path current. I have further assumed that the time delay of arrival of secondary path currents is about equal to the time required for transmitter and receiver circuits to respond to current changes. In this case it will be apparent that the secondary path current has substantially doubled the time to reach new current level. This would considerably reduce the apparent detail of a television image. The final output current, after application of time delayed balancing signals, is a much more nearly perfect wave form reproduction of the transmitter current variation and consequently will provide a television image with much better apparent definition.

Fig. 14 illustrates a case similar to that of Fig. 13, exceptthat the secondary path current is assumed to arrive with opposing phase relation with respect to the main path current. In this case, it will be noted, the original resultant current signal change at the receiver momentarily overshoots its steady state value by a ratio of 1.9 to 1. This will result in an unnatural outlining of images making them look something like line drawings with shading. After application of time delayed balancing currents having a strength half that of the initial resultant current the overshooting is reduced from a ratio of 1.9 to about 1.33. The undesired portion of signal response has therefore been reduced in the ratio of .9/.3' =2.7 to 1. In consequence, outlining of the images is greatly reduced.

An inspection of the responses illustrated in Figs. 13 and 14 suggests that still further improvement could be obtained if we added time constant circuits in series with the connections to the time delay cable circuits to bring about a modification of the wave forms of the balancing currents. Fig. 15 illustrates a form of circuit which would improve the final output signal corresponding to Fig. 13, while Fig. 15 illustrates a circuit applicable to the conditions of Fig. 14.

In practice there are cases where a weaker secondary path signal may arrive at the receiver before the stronger or main path signal. In this case it is necessary to introduce a balancing signal with a relative time advance, instead of a time delay. This may be accomplished by passing the main receiver output through a real or artificial time delay transmission line circuit and then combining the time delayed signal with balancing currents which have not been time delayed. One arrangement for doing this is illustrated in Fig. 17. This particular arrangement is suitable for reducing time leading secondary path current distortion when the secondary path high frequency current is in predominately opposing phase to the main path current. If the high frequency currents are predominantly in adding phase, then the polarity U of input or output of one of the parallel output circuit paths should be reversed so as to reverse the polarity of the balancing current with respect to the main current. It should be noted that there is provided in this figure a selector switch SW which can select a capacity C or an inductance L for introducing wave shape correction.

Fig. 13 shows still another arrangement for reducing secondary path signals. In this arrangement there is shown a television receiver which receives both signals from a remote transmitter over a main signal path and over a secondary signal path.

In this system a section of low loss transmission line T'L is shunted across the receiver input terminals through adjustable coupling resistors R. The length of line TL is such that the time taken for radio waves to travel from the input end to the far end and back again is almost exactly equal to the time delay of arrival of secondary space circuit path waves. It may differ from this length in such amounts that the phase relation of the returning reflected currents at the receiver terminals is not changed, provided the difference from the correct length is not too great.

Under these conditions, a portion of the main path received power is time delayed and reintroduced with correct timing and phase relation to minimize the receiver input due to the secondary space path and time delayed modulations received over the secondary path are reduced.

By employing more complicated resistance networks in combination with the compensating line section, improved results may be obtained.

Although I have indicated time delay cable circuits in the figures, they being particularly applicable to television, any other suitable signal delay circuit may be employed. For relatively long time delay, I contemplate converting electrical waves into mechanical waves which will travel through time and distance following which their energy may be converted back into electrical waves. Magnetostriction or the piezoelectric effect may be employed in the energy converting process in a manner well known in the communications art. For extra long time delay, signals may be stored as variable magnetization in a moving steel wheel or steel wire or tape and taken ofi again after suitable time and distance along the path of the record. Such storage devices are known in the art. One such device, employing a rotating, steel disc, was popularly known as a memory wheel because it could store signals and repeat them at a later time with an adjustable time delay or any amount up to a little less than the time for the wheel to turn one revolution.

In applying my invention to television receivers, the installer of a receiver, after having done the best he can with antenna location, antenna directivity, impedance matching and other controllable factors will look at the images received. He may observe one or more secondary images, which may be either positive or negative with respect to the main image. With the images adjusted to standard dimensions, he will measure the displacement of secondary images, horizontally across the screen in the case of horizontal scanning lines. The displacement provides a measure of the time delay of signals received over a secondary path and, by means of a curve or table, enables the installer at once to choose the length of time delay balancing circuit. Observation of the polarity of the secondary image tells him whether the end of the cable must be short circuited or open circuited if he employs the arrangements of Figs. 5, 15 and 16. Having determined the cable length and connection, he then adjusts the variable resistances (also capacity or inductance if used) until he obtains the best overall reproduction of the transmitted images.

The term section of line employed in the appended claims is intended to include, broadly, any type of current carrying line whether it be artificial, concentric, or of the parallel wire type, straight or coiled.

What is claimed is:

1. The combination with a radio receiver, of means for reducing a multi-received signal due to rays received over two difierent paths comprising a section of two-conductor line having solely one end connected to said receiver for withdrawing from said receiver at said one end and reintroducing at said same end a portion of the received signal wave, said section of line having a physical length which is equal substantially to one-half the difierence in effective path length of the two paths of the rays reaching the receiver, said line at its other end having its conductors connected together, and an adjustable impedance element between the receiver and said line for controlling the magnitude of the portion of the signal reintroduced.

2. The combination with a radio television receiver, of means for eliminating a multiple image due to rays received over difierent paths. comprising a section of line of predetermined length having solely one end connected to said receiver, said line having velocity and attenuation characteristics which are substantially constant over the band of modulation frequencies where image signals are an appreciable factor, and resistors in shunt and in series to said line at the points of connection to said receiver for controlling the magnitude of the portion of the signal withdrawn by said line and reintroduced thereby into the receiver at said one end.

3. The combination with a radio television receiver, of means for eliminating a multiple image due to rays received over different paths, comprising a section of two-conductor line of predetermined length connected solely at one end to said receiver, said line being connected to said receiver through shunt and series resistors, said resistors being adjustable to control the effective value of the potential and current in said line.

4. In a radio signalling system wherein there are multiple paths between the transmitter and receiver such that a modulation pulse from the transmitter is received as a first pulse over one path followed at an interval by a pulse of smaller magnitude over vanother path, a transmission line connected solely at one end to one point in the receiver, the other end of said line being reflectively terminated for currents impressed upon said line by said receiver, the length of said line being so chosen that the time of travel from one end of said line to the other for the current impressed on said line is one-half of said interval, the reflective termination on said line being arranged to provide such polarity of the reflected pulse as to neutralize the second pulse, and an adjustable impedance between said line and receiver for controlling the magnitude of the reflected pulse reintroduced to the receiver and to prevent secondary reflections over said line.

5. The combination with a radio receiver, of means for reducing a multi-received signal due to rays received over two difierent paths comprising a section of two-conductor line having solely one end connected to said receiver for withdrawing from said receiver at said one end and reintroducing at said same end a portion of the received signal wave, said section of line having a physical length which is equal substantially to one-half the diiference in effective path length of the two paths of the rays reaching the receiver, said line at its other end having its conductors connected together, and means including an adjustable impedance element connected to said line and in series therewith for controlling the magnitude of the portion of the signal reintroduced.

CLARENCE W. I-IANSELL.