US2372876A - Method and apparatus for reducing echo effects in picture transmission systems - Google Patents

Description

2 Sheets-Sheet l RLTE/PNFT/NG. ECHO IFECE/VEA omlr'nwo ///F/?;- BL/IC/f LEVEL BLACK LEVEL /?HY LEVEL 1 ZEI O Ofl/PRIER NO ECHO LMWTIECHOI F. J. BINGLEY IN PICTURE TRANSMISSION SYSTEMS Filgd Match 19,

fcEFLECTl/YQ STRUCTURE April 3, 1945.

METHOD AND APPARATUS FOR REDUCRNG ECHO EFFECTS n fi M J w n n 4% 1 0 n 1. 1| 3% w M 7 v M 5v 1.. M m .J 7 N J 4 E H m M 4 .u 9 P L L H F w 6 m t M w a J m 0 B c 0 5 W W m T carrier echoes of the synchronizing signals may some times be received. This invention is particu- Patented Apr. 3, 1945 'ECHO EFFECTS IN PICTURE SION SYSTEMS TRANSMIS- Frank J. Bingiey, Chestnut Hill, Pa, assignor to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware.

, Application March 19, 1942, Serial No. 435,402

' 28 Claims. (cilia-1.1)

This invention relates to picture transmission systems and the like, "and to a method and means for'substantially reducing or eliminating the eff'ects of echo signals on the received or reconstituted picture. More particularly, the invention relates to television systems of the alternate carrier type, and to a method and means for reducing or substantially-eliminating certain undesirable effects resulting from the arrival, during the picture line periods, of echoes of the horizontal, or line, synchronizing signals.

One of the problems encountered in picture or television transmission, and one that obtains with any type'of modulation, is-that which results from the reception of long-delayed echoes, corresponding to signal path differences of the order of a mile or several miles. Since the speed of propagation of a radio wave is approximately 1,009 feet per microsecond, a path difference of say 3 or 4 miles, as between the direct and the reflected signal path, may-produce echoes delayed by 15 or 20 microseconds. In magnitude these echoes may be relatively faint. due to the greater path length, to the fact that the echo-signal normally sufiers considerable attenuation in reflecticn, and to the fact thatthe indirect path is usually closer to the earth than is the direct path.

n the other. hand, since the blanking, and particularly the synchronizing signals, are of greater amplitude than the line or picture portion of the signal, reflected blanking and synchronizing signals of considerable strength sometimes appear in the picture as a result of these long-delayed echoes. Because of the inherently greater effectiveness of the synchronizing si nals in alternate television systems, unusually strong larly directed,- to' the'elimination or echo efiects in this type of system.

In general, an echo may appear in a television reduction of picture 'by combining or beating with the picture.

carrier. Thus the echo may add to or subtract from the picture carrier depending upon the particular phase. relation existing between the two signals at a given instant. This phase relation depends (except for changes efiected at the transmitter itself) upon the difference in path length between the direct and reflected signals: and

the only way in which this difference can be.

appreciably varied, assuming that the transmitting and receiving antennas are fixed, is by a variation in the placement ofthereiiector causing the echo.

But since reflector is usually a 1 8 1 building, bridge. gas tank, hill, or similar ob Bingley and William E. Bradley, Serial No. 433,-

ject, it too can be regarded as a substantially fixed structure, and hence in any given installation, the echo signal, as it appears upon'the viewing screen at the receiver, is subject only. to such phase relations between the direct and reflected signals as may result from periodic phase changes efl'ected within the transmitter itself.

' In the copending joint application of Frank J.

660, filed March 6, 1942, an echo reduction system was described, by meansof which the deleterious eflects of echo signals could be substantially reduced or eliminated by providing a phasechangingmeans at the transmitter for periodically changing the "polarity" of the echoes seen at the receiver, so that they are of opposite characteristics in successive frames. In one form of the invention, this was accomplished by periodically changing the phase of the synchronizing signal carrier with respect to the video carrier, these changes being'so timed that the successive echoes tend to balance each other out so far as their impression uponfthe observer is concerned.

The phase changes, or reversals, described in the above-mentioned joint application, are effected in a time which is generally short compared to the duration of the'synchronizing or blanking signals. To this end the phase changing or reversing means shown, in the said joint application is adapted to effect substantially instantaneous changes in phase. According to the present inventiomphasereversals are efl'ected gradually; and by quite dif- -ferent meansnamely by selecting video and synchronizing carrier frequencies which bear a numerical relation to each other such that the phase oithe'beat frequency (1. e. the echo) resulting from the mixing .of the video carrier and a delayed synchronizing signal carrier, is substantially opposite (differs by degrees) in succssive frames. The phase changes referred to are eflected at the transmitting station and require no additional equipment at the point of reception. It is a principal object of this invention to provide a method and a means for substantially eliminating certain echo eifects which may be encountered in television -transmission.jsystems of.

the alternate carrier type.

Another object of the invention is to provide a method and means for transmitting a television signal, or the like, which when'received 6 Still another object of the invention will produce a picture whichis free of echoes of the synchronizlng istopro- 'videmeansioreliminatingcertainoithespuriwhich:

progressing at such a rate that echo images or substantially opposite characteristics, arereceived in successive frames.

These and other objects and features oithe invention will be apparent from the following description andthe accompanying drawings, in

Fig. 1 shows a typical disposition of transmitter, receiver, and reflecting structure, which may give rise to objectionable echo signals;

Fig. 2 is an explanatory certain characteristics of typical echo patterns;

Fig. 3 is illustrative of one or the-methods employed to eflect cancellation of echo patterns;

and

Fig. '4 is a partially schematic and partially 7 tie representation or an alternate carrier television system embodying the present inventlon. I

The present invention may best be understood by considering flrst the causes of echo signals;

and their appearance when viewed on the screen of the picture tube. A typical condition which may give rise to objectionable echoes is illustrated in Fig. 1'. In this figure, a television receiver Ris representedasbeing 12 miles dis- *tant from a transmittin station T. At distances or 6 and 10 miles irom the transmitter and receiver, respectively, is a wave reflecting structure such as a a water tower, asteeibridge,orthelike. Takingthespeed or a radio wave as aboutone-flith mile per microsecond, it will be apparent that the time required for the signal to traverse the direct 12-milepath between the transmitter and receiver will be about 60 microseconds, while the thne required for the reflected signal totraverse the indirect l6-mile path will be about 80 microseconds. Consequently, the reflected wave, 1. e., the echo, will arrive at the receiver about 20 behind the direct wave. This may be regarded as illustrative of a simple echo, as diflf from; multiple echoes which arise when a plurality of reflecting structures provide afluralityotindirectsignalpaths or different dia am illustrating rier as corresponding to a very bright signal level, while the synchronizing pulses may be regarded as corresponding to a blacker-than-black "or "infra-l lac signal level. The video signal 8 existing during the 53.3 second line-period has,

for the purpose or this description, been established at 'a'level midway between zero carrier and the black level, and will correspond approximately, therefore, to a gray level.

When a signal oi these characteristics is received without an attendant echo, there will be reproduced, upon the screen oi the picture tube, a line similar tothat shown (with exaggerated width) at A in Fig. 2, and having a uniform grayish appearance, the latter being indicated in the drawing by the closely spaced dots. This uniform appearance will be destroyed, however, if one or. more echo,signals is received along with the desired direct signal. Strictly speaking, under these conditions, the appearance or the entire line will be affected, but in general echoes of the video portion of the signal can, by reason or their lesser amplitude, be ignored, and, consequently, for most practical purposes only those echoes produced by the higher amplitude blanking and synchronizing pulses need be considered. 0! these pulses, the latter are, of course, the more important by reason of their greater-amplitude;

Line B-in Fig. 2 is illustrative oi the normal appearance of a picture line when receiving a strong echo signal. This may be regarded as representing the case of .a common carrier system where the video and synchronizing intelligence is transmitted at the same carrier Irequency. However, as will be explained in greater detail hereinafter, line B (as well as line C) is illustrative of the appearance of an echo in an alternate carrier system where the diiier- 40 ence between the synchronizing signal carrier and the video carrier is very small. The echo -illus-. trated resembles those which are produced by the delayed reception of the blanking and syn-- chronizing signal, and is displaced from the lefthand edge of the screen P by a distance which is proportional to the timeinterval between the arrival of the signal traveling the direct path and the arrival of the signal traveling the indirect or reflected path. It the difference in length between these paths is small, the echo will be reproduced at or near the left-hand edge of the 'screen, whereas greater path diflerences cause the echo to appear further to the right. 0! course, where. the path difference is very great, the attenuation suflered by the reflected signal is usually so considerable that the echo is too weak to be noticeable.

As is indicated in the drawing, the echo shown in line B is produced by the arrival of blanking and synchronizing pulses during the line or video period 8v. Specifically, the arrival oi the pulses 8;, So, by way of an indirect path, approximately 20 microseconds aiter the reception or these pulsesby way of the direct path. The echo signal produces on the screen P an echo pattern whose width is equal to the width or the line, and whose length is equal to the distance traveled by the scanningbeam in 10.2

microseconds, the duration of the combined blanking and synchronizing signal. .The 20- second delay. chosen for this illustration, it will be recalled, is approximately the delay produced 'as a result of a reflected signal traveling an additional 4 miles as illustrated in Fig. 1.

whether the echo,"as it appears upon the picthis echo is produced byture tube screen, will be a dark echoor a light echo, depends upon the phase relation between the videocarrier and the echo carrier at the point of detection. If these carriers arrive more or less in phase, the resultant R. F. signal supplied to the detector will be greater than the amplitude of the video carrier alone, and, 'consequently, the combined signal will tend toward the black level and a dark echo will be produced as shown in line -B. The darkest part of the echo will be that central portion which corresponds to the synchronizing pulse 55', while odd-numbered frames, and frame No. 2 as typi- I cal of all even-numbered frames. The order or sequence of scanning is indicated by the reference numerals l to l8 inclusive.

theouter portions corresponding to the blanking;

signal Sb will be somewhat less dark, but darker than that part of the line which is not distorted by echo. 0n the other hand, if the video and echo carriers arrive in generally'opposite phase, the reverse will be true, and a light echo such as that shown in line C will result.

- Still a difierent type of echo results when television transmission'is carried out in accordance with the system described by A. V. Loughren (Electronics, February 1940, pp. 27-30) or with the system described in the copending application of Frank J. Bingley, Serial No. 401,533, filed July 8, 1941. In the alternate carrier system disclosed by Bingley, the synchronizing pulse is transmitted not only as a variation in carrier amplitude as in Fig. 2), but also as a variation or shift in carrier frequency. In this system, the synchronizing signal carrier may be of the order of l megacycle or more higher than the video signal carrier, I and, consequently, when a reflected (and, hence, delayed) synchronizing signal is received with the video carrier, there will result a beat signal whose frequency is equal to the frequency difference between these signals, e. g.,1 megacycle. Since this frequency lies within the video frequency band, the synchronizing signal portion of the echo Signal will be reproduced on the picture tube screen as an alternating echo, i. e., one which alternates from dark to light to dark, etc. as shown in line D of Fig. 2. The number of these light and dark bars depends upon the number of beat cycles contained within the duration of the synchronizing pulse. to a light bar, and vice versa, is gradual, of course, and not abrupt, as shown in the illustration.

In general, where strong echo signals are received, the echo images reproduced upon the screen of the picture tube are apt to be more objectionable in an alternate carrier system than in the single carrier systems heretofore employed. This is because in the former system the synchronizing signal is transmitted at a frequency for which the receiver's sensitivity is approximately double that in the latter system, and, consequently, the reproduced echo is that much more pronounced.

'Attention is now directed to Fig. 3 which mus trates one of the methods, which may be employed, in accordance with the present invention, to effect substantial cancellation of echo patterns. No attempt has been made in this figureto maintain the identicalscale-employed in Fig. 2. Moreover, for simplicity, only the echo caused by the delayed arrival of the synchronizing signal is illustrated, the lesser echo pro- The change in shading from a dark bar In an alternate carrier television system in which the video'carrier differs substantially from the synchronizing carrier, say by one million cycles, an alternating echo-image will be ,produced on the television receivers viewing screen similar to that already described with particular "reference to line D of.Fig. 2. Thus in line I of Fig. 3 the echo image is seen to consist of alter nately dark and light line-segments, the number of alternations depending upon the number of beats which occur between the video and synchronizing carriers in the time taken for the scanning beam of the picture tube to trace or negotiate the distance between the left-hand side of the echo and the right-hand side thereof.

In accordance with the present invention, and in a manner to be described in detail hereinafter, it is proposed so to relate the frequencies of the video and synchronizing carriers that the phase of the alternating echo images will be displaced by substantially 180 degrees in successive lines.

Thus in line i it will be noted that, scanning from follow the sequence of line 2. "Now since we are duced by the blanking signal being omitted. The

assumption in Fig. 3 is that on is carried out in accordance with the alternatecarrier system; i. e., where video and synchronizins signals are transmitted at. substantially dealing with a raster having an odd number of lines, to wit 9 in this illustration, it will be noted that the spacial sequence of oddand even-numpered lines is'reversed in frame No. 2 with respect to the order existing in frame No. I. This is be-' cause the top line of the odd-numbered frames is an odd-numbered line, whereas the top line in the even-numbered frames is an even-numbered line. The effect of this is to produce alternating echo images in the even-numbered frames which are the reverse of the echoes in the odd-numbered frames. Described in another way, the alternatingechoes in the even-numbered frames may be regarded as the complements or conjugates of the the echoes in the odd-numbered frames.

Alternating echoes of the type illustrated in Fig. 3, consisting of .three black segments and four white segments, or vice versa, are generally illustrative of the appearance of an echo pro-- duced by a television system in which the frequency difference between the video and synchronizing carrier is a large fraction of a megacycle. Obviously as the difijerence between the video and synchronizing carriers is decreased, the number of black and white echo segments will also be decreased. Where the frequency difference issmall there maybe only "one or two black and white segments in the echo, and if the frequency difference is still less the echo in certain odd-numbered lines might be all black and the echo in certain even-numbered lines all white, or vice versa.

In any event, from the standpoint of the observer, the optical eifect produced the rapid alternation of echoes of generally opposite characteristicsupon thescreenofthepicturetubeis substantially that which would obtain if no echoes were reproduced at all. In a conventional television systembased on thirty complete frames per second, there will be fifteen complete echo alternations" per second, each alternation consisting of one frame comparable to frame No. l of Fig. 3 and one frame comparable to frame No. 2

of that figure. Apparently the rapid substitution I of light echoes for dark'echoes. and vice versa,

through the use of separate sources of video and synchronizing-signal carriers wherein the frequency of the synchronizing-signal carrier is made equal to the video carrier frequency plus an odd multiple of half the horizontal line frequency. Conversely the frequency of the video carrier might be made equal to the frequency of the synchronizing carrier plus an odd multiple of half the horizontal line frequency. stated the requirement is that the difference in frequency between the video carrier and the synchronizing carrier be substantially an odd multiple of half the line frequency. As will be indicated at greater-length hereinafter, echo cancellation will also obtain if the diii'erence frequency is made an odd multiple of half the field frequency.

Reference is now made to Fig. 4 in which there is illustrated an alternate carrier television transmitter employing the praent invention. With the exception of the apparatus designated with.

the reference numerals 33, 3|, 3! and 36, the alternate carrier system shown therein has been fully described and illustrated in the above-v named copending application of Frank J. Bingley.

Reference will first be made to the circuit of- Fig. 4 without attention to the principles and .supnlledwithsignalfromthegeneratorslland 2| maybe regardedas an electronic switching responsive toa pairofcontroisignalaforpassingontoflieamplinerandfreqnmcy-multiplier 22, aselected'one ofthecarriersbyflnedcvicesllanda. Bymeansoftheamlflitulbmodulator'flthesig- -nalderlvedfromthedeviee22maybe modulated inaeeordancewiththederlvedfrom the video, and bhnking signal source. Themodulatorflmaybefolloiedby a'sllitablelincaramplifiaILwhich-hragingla llde-bandsystmmaybefollowedbyasesquiside-band fllterflsfinfllythes snllmlybe' sqIlied-toasuitahh antamaorradiatorA.

l'heelectronicswitchingmelnawhichinchide thchlhesvrandvamnybeconfiolledinre mtoapairoteontrolflsmsderlvednm More genera y the source 28 as follows. .By way of a conductor 31, a signal, comprising synchronizing signal components only, may be applied to a pair of networks 21 and 28 The network 21 is adapted to 5 supply, in response to the applied synchronizing signal, a controlsignal which is highly negative I with respect to the cathode of triode V: during the video and blanking intervals (exclusive ofthe synchronizing intervals), but only slightly negative during the synchronizing intervals themselves. Such signal is illustrated within the rectangle 21, the zero level being indicated by the line 0-0. Accordingly when this signal is applied to the grid of triode V: by way of the conductor 29 and the resistor ll, the triode V: is en- 'abled to transfer the synchronizing carrier from source2ll to thestage 22 during the synchronizing intervals, but the triode is blocked during the longer video and blanking intervals by the application to its grid of the highly negative potential supplied by the device 21.

The network 28 supplies a control signal which, as is illustrated within the rectangle, is substan- -tiallythe reverse of that supplied by the network 21. This signal, which is applied to the control grid of the triode V1 by way of the lead 3i and the resistor 32, is only slightly negative during the longer video and blanking intervals, but is highly negative during the synchronizing intervals. As a result of the operation of the devices V1, V2, and 21, 28, there is provided a system wherein the video carrier is switched on during the video and blanking intervals, and 011 during the synchronizing intervals, while the synchronizing carrier is switched 01! during the video and blanking intervals, and on during the synchronizing intervals. Accordingly it will be seen that during the video and blanking intervals the modulator stage 23 amplitude modulates the vichronizing intervals the modulator 23 amplitude chronizing signals derived fromthesourcefl.

Devices of the type represented by the'rectangles21and'2iarewellknownintheartandrequireno detailed illustration.

Inaccordancewithoneembodimentortheinvention itiscontanplatedthatthecarrierfresoquendesbythedevicmllandflbe relatedinsuchamannerthatthetramitted earriergagradiatedfromtheantmmAfdiflq byanoddmultipleofhalfthelinefi'equmcy. Asumethatitisdesiredto-tranmitacarrier nequencyoffi'lzfiflpoflcydsperseconddurht thevideoandblankingintetnk andthltitis desiredtotransmittheslgnlhm acarrierhavingafrequmcyahoutonemb higher. alsofliatthepmicctedm employsarasterot525linq,nth30eompl$e flamespersecmd. Suchamtunlnsaline freqmoflififiocyclespermmlllf thelimtrequgacyk'lmqdespa'sccmd. An

u mama-mammals.

deo carrier with video and blanking de-. rived from the source 26, while during the symmodulates the synchronizing carrier with the gynas'raero carriers directly, If, however, a device 22 is employed which gives a frequency multiplication of, say, five tiines, the devices 19 and 20 should provide carriers of one-fifth of the carrier frequencies to be transmitted, which for the example presently under consideration would be 13,450,000 and 13,646,875 cycles respectively. The video carrier generator 19 may include a crystal controlled oscillator adjusted to oscillate at some predetermined sub-multiple of the desired generator output frequency, in this example 13,450,000 cycles, followed by one or more frequency multiplier stages to bring the frequency up to the desired output frequency. The synchronizing carrier generator 20 may comprise any suitable device for generating a carrier which differs in frequency from the output of the generator 19, by the predetermined, substantially constant difference frequency. Preferably the frequency difference is held constant by means of a positive tie-in, or interlock, between the two generators l9 and 20 in order to prevent drift or slippage as between the two systems. A satisfactory tie-in means is diagrammatically illustrated in Fig. 4. Since, in the example, the frequency difference .between the carriers supplied by the generators I9 and 20 is l96,875,cycles, the 13,646,875 cycle carrier may be developed by first heterodyning the 13,450,000 cycle carrier with a signal having a frequency of 196,875 cycles, and then detecting the combined signal. The 196,875 cycle signal may be derived from the horizontal synchronizing signal in a manner presently to be described. The heterodyne detection of these two signals (13,450,000 cycles and 196,875 cycles) willof course, produce a pair of new signals having frequencies equal respectively to the sum and diiference of the frequencies of the heterodyned signals. The former, or sum-frequency signaLywill have a frequency of 13,646,875 cycles, and since this is the frequency of the desired synchronizing signal carrier it may be selected, to the exclusion of the difference-frequency signal, and appiled to the j input circuit of the signal selector tube V2.

Inasmuch as suitable heterodyne detection circuits are widely used in the art for a great variety of applications, it is not deemed necessary to illustrate the device 20 in detail in the present application.

It will be observed that the required 196,875 cycle signal has a frequency of 25 times one-half the line frequency. Consequently the 196,875 cycle signal may be obtained from the horizontal synchronizing signal source '26 as follows. The 15,750 cycle horizontal synchronizing signal may be applied to a frequency sub-multiplier or"halver 33 to produce a new signal having a frequency of 7,875 cycles, i. e. of half the horizontal synchronizing frequency. Inasmuch as frequency halvers are well known in the art the device 33 is not illustrated in detail; however it may. conveniently consist of a blocking tube oscillator adjusted to oscillate at approximately 7,875 cycles, and arranged to be fired by every second line synchronizing pulse supplied by the device 26. The 7,875 cycle signal from the frequency halver may besupplied to a suitable frequency multiplier 34 for multiplication up to the desired frequency, in the example here under consideration 196,875 cycles, In this case this is accomplished by means of a 25-fold multiplication. The l96,815 cycle output of the frequency multiplier 34, and the 13,450,000 signal from the device 19 may be applied to the heterodyne signal generator 20 by way of the aths 3'5 and 36 respectively.

with reference to Fig. 4 it is to be understood that the frequency values designated thereon are merely by way of example, and for the purpose of simplifying the description, and are not intended to limit the invention to any particular ,mode of operation.

In the foregoing it has beenindicated that the frequency difference between theradiated video and synchronizing carriers should be equal to an odd multiple of half the picture line-frequency. This is not a necessary restriction, since the frequency difference may also be defined as, or set equal to, an odd multiple of half the field frequency. In many, but not. all, instances these definitions will be found equivalent. Thus for the example chosen in connection with the description' of Fig. 4, the frequency difference between the two radiated carriers is also an odd multiple of half the frame frequency, specifically ;the 65,625th multiple of 15, where 15 is half the frame frequency. However, if the frequency difference between the two radiated carriers were only 15 cycles then it is obvious that the firstnamed definition would not apply.

That a system based on the second-named say,-in phase with the echo at time t+ ,0 and out of phase at time t+%0, etc. Now if we establish our video carrier at 60,000,000 cycles,- and our synchronizing carrier at 60,000,030 cycles, there will be thirty beats per second as between the video carrier and the delayed sy ch rier, and each one-thirtieth second the phase of the beat frequency (i. e., of the echo) will be the same. Under these conditions, of course, the echo will not cancel, since if in one frame a dark echo were produced, it would follow that in successive frames similar dark echoes would be produced. But if the video carrier is established at 60,000,000 cycles and the synchronizing carrier at 60,000,015 cycles, i. e., where the frequency difference is an odd multiple of half the frame frequency, echo cancellation will obtain because the hase of the beat frequency is reversed in successive frames. Thesame cancellation of the echo will obtain for carrier frequency differences of 45 cycles, cycles, cycles, etc.

It will be understood, of course, that where the difference frequency is so small that less than one beat cycle occursfor the duration of the echoedsynchronizing pulse, an alternating echo of the type illustratedin line D of Fig. 2 will not obtain. In general, although the invention is notthus limited, it is preferred to have a relatively large difierence in frequency between video and synchronizing carriers sothat the full benefits of the alternate carrier method of television transmisof the requirements, and applicable generally to systems having an odd number of lines (as is conventional in interlaced systems), is that the frequency difference'between the carriers shall be such that the phase of the beats therebetween' shall undergo a change equalto an odd multiple of 180 degrees during each frame period.

So far the description has been directed more specifically to two-phase systems of echo cancellation. However, just as in the above-mentioned copending joint application of I". J. Bingley and Wm. E. Bradley, the present invention may also be employed in three-phase echo cancellation systems, or, in general, in systems wherein the dif- A ferentialphase between the video and synchronizing signal carriers changes at a rate such that complication of circuits, the total number of lines per frame is made equal to an odd number which is not divisible by three. Thus, a 605-line raster would be suitable for use in a three-phase echo cancellation system.

Although the present invention has been described and illustrated with reference to certain preferred embodiments, it should be understood that wide alterations and modifications may be eifected within the scope of this invention as delined in the appended claims.

I claim:

1. In a television transmission system of the alternate carrier type, the method of minimizing thedeleterious effects of echo signals on a desired signal, which comprises generating a first carrier wave, generating a second carrier wave. positively maintaining a substantially constant frequency difference between said carrier waves equal to an odd multiple of one nth the frame frequency, where n is an integer greater than one and smaller than four, utilizing the first of said waves for the transmission of video intelligence, and

utilizing the second of said waves for the transmission of synchronizing intelligence.

2. In a television transmission system of the alternate carrier type, the method of minimizing the deleterious effects of echo signals on a desired signal; which comprises generating a first carrier wave, generating a second carrier wave. positively maintaining asubstantiallyconstant frequency difference between said carrier waves such that the differential carrier phase varies substantially uniformly at a predetermined. rate so that the waves are in phase, as well as in antiphase, n

times 'per second, where n is an odd multiple of 60 half the frame frequency, utilizing the first of said waves for the transmission of video intelligence, and utilizing the second of said waves for the transmission of synchronizing intelligence.

3. In a television transmission system of the alternate carrier type employing an odd number of lines per frame. the method according to claim 2 wherein n is an odd multiple of half the line frequency.

4. In a television transmission system of the alternate carrier type, the method of minimizing the deleterious effects of echo signals on a desired signal, which comprises generating a signal having half the system's frame frequency, frequencymultiplying said signal by an odd factor, genas'za'sve crating a first carrier wave of predetermined and substantially fixed frequency, heterodyning said frequency-multiplied signal with said first carrier wave to produce a second carrier waye differing in frequency from said first carrier wave by an amount equal to the frequency of said frequencymultiplied signal, utilizing one of said carrier waves for the ton of video intelligence,

and utilizing the other of said carrier waves for 0 the transmission of synchronizing intelligence.

5. In an alternate carrier television system, the

method according to claim 4, wherein the frequency of said frequency-multiplied signal is also an odd multiple of half the line frequency of the system.

6. In a television transmission system of the altematecarrler typ the method of minimizing the deleterious effects of. echo signals on a de-' sired signal, which comprises generating a signal whose frequency bears a fixed relation to the system's line frequency. generating a first carrier wave of predetermined and substantially fixed frequency. heterodyning said last-named signal with said first carrier wave to produce a second carrier wave'diifering in frequency from said first carrier wave by an amount equal to the frequency of said last-named signal. frequencymultiplying said first and second carrier waves to produce third and fourth carrier waves respectively so related in frequency that the carrier phase of echoes of the synchronizing signals changes with respect to the phase of the video signal carrier at a rate such that reproduced echo images effectively cancel one another in a small number of successive frames, utilizing one of the two last-mentioned carrier waves for the transmission of video intelligence, and utilizing the other of said waves for the transmission of syn chronizlng intelligence.

7. In an .altemate carrier television system.

a the method according to claim 6, wherein the frequency difference between said third and fourth Y carrier waves is equal to an odd multiple of half the system's frame frequency.

8, In analternate carrier television system, the j method according to claim 6, wherein the frequency difference between said third and fourth carrier waves is equal to an odd multiple'of half the system's line frequency.

9. In a carrier wave television-transmission system of the alternate carrier type, themethod of minimizing the deleterious eifects of synchron-izing signal echoes onthe desired picture signals, which comprises continuously changing the phase of the synchronizing signal carrier relative tothe phase of the video signal carrier to produce echo images of periodically varying and Y contrasting characteristics.

10. In a carrier wave television transmission system of the alternate carrier type, the method of minimizing the deleterious effects of synchronizing signal echoes on picture signals, which comprises progressively varying the phase be tween the video signal-carrier and the synchronizing signal carrier to efiect reversals, in prede- 4 termined time sequence, ofthe diiferential carrier phase between horizontal synchronizing signals and the succeeding line 11. In an alternate carrier television transmission system, the method of minimizing the deleterious effects of echo signals on a desired signal, which comprises generating a video signal carrier of substantially fixed frequency, enerat- 5 ing a synchronizing si n l carrier whose freq ency is substantially equal to the frequency of signal intervals.

the video signal carrier plus, an odd multiple of half the horizontal line frequency, and modulating said carriers respectively with the video and synchronizing signals. v

12. The method according to claim 11, wherein the frequency-difference between the carriers is of the order of a million cycles.

13. In a television transmitter of the alternat'e carrier type, a source of video and synchronizing signals, meansfor generating a first car means for modulating the other of ,said carrier waves with said video signals, means for transmitting said one carrier wave during the syn-, chronizing signal intervals, and means-for trans-'- mitting said other carrier wavedurin'gthe video 14. In a television transmitteroi the alternate carrier type, a source of ,vide'o-and synchronizing signals, means for generatinga-first carrier wave of predetermined frequency, means for general;- ing a second carrier wave of diiferent frequency,

means for establishing and maintaining a substantially fixed frequency-difference between said carrier waves substantially equal to an odd multiple'of half the frame frequency, means for modulating one of said carrier waves with said syn chronizing signals, means for modulating the other of said carrier waves with said video signals, means for transmitting said one carrier vwave during the synchronizing signal intervals, and means for transmitting said other carrier wave during the video signal intervals.

15.A television transmitter of the alternate carrier type as claimed in claim 13, wherein the frequency-difference between said carrier waves isof the order of a million cycles.

16. In a television transmitter of the alternate carrier type, a source of video and synchronizing signals, means for generating a first carrier wave of predetermined frequency, means for generating a second carrier wave. of difierent frequency, means for establishing and maintaining a substantially fixed frequency-difference between said carrier waves substantially equal to an odd -multiple of half the horizontal line synchronizing frequency, wave selector means responsive to a control signal for selecting said carrier waves 5. in predetermined alternating sequence, and

means for amplitude-modulating the selected carquency, heterodyne means for mixing said first carrier wave with said last-named signal to produce sum and difference signals, means for utiliz'ing one of'said last-named signalsas a second carrier wave, means for modulating one of said carrier waves with said synchronizing signals,

.means for modulatingthe other of'said carrier waves with said video signals, means for trans mitting'said one carrier wave during the synchronizing signal intervals, and means for transmitting said other carrier wave. duringthe video Signal intervals.

19. In a television transmission system of the alternate carrier type, means for generating a video signal carrier of predetermined frequency,

- and means for generating a synchronizing signal carrier of different predetermined frequency 7 so related numerically to the-frequency of the v \:id'eo signal carrier that the carrier phase of echoes of the synchronizing signals changes with respect to the phase of the video signal carrier rier wave in accordance with signals from said I video and synchronizing signal source.

17. A television transmitter of the alternate carrier type as claimed in claim 16, including I for generating a signal of half the line frequency,

means responsive to said half-frequency signals for generating a signal whose frequency is an odd multiple of said half-frequency, means for genv crating a first carrier wave of predetermined -fre at a predetermined rate to effect mutual cancellation of reproduced echo images in a small number of successive frames.

- 20. In a television'transmission systsm of the alternate carrier -type, means for generating avideo signal carrier-of predetermined frequency,

and means for generating a synchronizing signal carrier of different;predeterminedfrequency so related numerically 'to the frequency of thevideo signal carrier that the-carrier phase of echoes of the synchronizing signals changes with respect to i" the phase of the video signal carrier at a predetermined rate to impart-substantially opposite,

characteristics to reproduced echo images in consecutive frames, wherebyisaid echo images eflectivcly cancel one another.

21. In a television transmission system of the alternate carrier type, means for generating a video signal carrier of predetermined frequency,

of lines per frame, means for generatlng a video signal carrier of predetermined frequency, and

means for generating a synchronizing signal carrier of different predetermined frequency so related numerically to the frequency of the video signal carrier that the phase of beats between the two carriers changes during each frame period by an amount equal to an odd-multiple of degrees, whereby reproduced echo images are caused effectively to cancel one another. 4

"23. In a television transmission system, the method of minimizing the deleterious effects of a first carrier wave on a second carrier wave, which comprises establishing and maintaining a substantially constant frequency difference between said carrier waves equal to'an odd multiple of one-nth the'frame frequency, where n is an integer greater thanone and smaller than four.

24. In a television transmission system means for generating a first carrier wave, means for gen erating a second carrier wave, and means for establishing and positively maintaining a substantially constant frequency diiference between said carrier waves equal to an odd'multiple of one-nth the frame frequency, where n is an integer greater than one and less than four.

25. In'a television transmission system of thealternate carrier type, means for-generating a 26. In a television on system of the type employingfan odd number of lines per fram means for generating a first carrier wave, means for generating a second carrier wave, and means for establishing and pomtively maintaining a substantially. constant frequency difference between said carrier waves such that the differential carrier phase varies substantially uniformly at. a predetermined rate so that the waves are in phase, as well as-in antiphase, n times per secfor positively maintaining a frequency dilferenee' beaavaavo ond, where n is an odd multiple of half the line frequency.

27. ,In an alternate carrier television transmission system, means for generating a video signal carrier of substantially fixed frequency, means for generating a synchronizing signal carrier: whose frequency is substantially equal to the frequency of the video signal carrier plus an odd multiple of I half the horizontal line frequency, and means for ill modulatin said carriers respectively with the video and synchronizing si nals.

28. In an alternate carrier television transmission system, means for generating a video signal carrier of substantially fixed frequency, means for generating a, synchronizing signal carrier" whoa frequency differs from that of the video signal carrier by approximately a million cycles,

and is substantially equal to the frequency of the video signal carrier plus an odd multiple of half the horizontal line frequency, and means for modulating said carriers respectively with the video and synchronizing signals.

mam J. 'BINGLEYJI

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