US2386088A - Method and apparatus for reducing echo effects in picture transmission systems - Google Patents

Description

Oct. 2, 1945. F. J. BINGLEY ETAL 2,386,083 METHOD AND- AIPARATUS FOR REDUCING ECHO EFFECTS l IN PICTURE TRANSMISSION SYSTEMS Original Filed March 6, 1942 4 Sheets-Sheet 1 DHRK'EcHo l'L/qm" EcHo furba-amar mz FHME. N2 Hem N2- WMW? I .Oct.2, 1945. lr-'..LILaIICYLEY ETAL S '2,386,088

METHOD AND APPARATUS FOR REDUCING ECHO EFFECTS y f IN'PICTURE TRANSMISSION SYSTEMS Original Filed March 6, 1942 4 Sheets-SheetZ @IDMIVLLy `e Q VIDEO,

SYNC/mN/zmg@ /27 miga@ @MHWJ @bmw/62m F. J. BINGLEY Er'Al.

Oct. 2, 1945.

` .METHOD AND APPARATUS FOR REDUCING ECHO EFFECTS IN PICTURE TRANSMISSION SYSTEMS 4 Sheets-Sheet 3 Original Filed March, 1942 NOROMEMQ 4 Wwubl lll Il.

QNUSS boum lill QSMQQU ,OCL 2, l945. F. J. NGLEY, ETAL 2,386,088

METHOD AND APP TUS FOR REDUCING ECHO EFFECTS v IN PICTURE TRANSMISSI SYST S Original Filed Marc 19 4 Sheets-Sheet' 4 U V1 HV1-'Wm'.- 4 1 1. A

v@ V W/ MM Athe drawings by the closely spaced dots.

,tain of the details of the system of Fig. 6; and

Fig. 8 is an explanatory diagram illustrating the Aoperation of the system of Fig. 7.

The present'invention may best be understood of the video portion of the signal camby reason assaose of their lesser amplitude, be ignored, and, consequently, formost practical purposes only those `echoes produced by the higher amplitude blankby considering first the causes of echo signals, I

and their appearance when viewed on the-screen of the picture tube. A typical condition, which may give riseto objectionable echoes is illustrated in Fig. 1 In this figure, a television receiver R is represented as being 12'mi1es distant from a transmittingstation T. At distances of 6 and 10 miles from the transmitter and receiver, respectively, is a wave reflecting structure such as a tall building, a. water tower, a steel bridge,l or the like. Taking the speed of a radio waveas about one-fth mile per microsecond, it will be apparent that tl'ietime required for the signal to traverse the direct 12-mile path between the transmitter and receiver will be about 60` microseconds, while the time required for the rellected signal to traverse the indirect 16-mi1e path will be about 80 microseconds. Consequently, the reected wave, i. e., the echo, will arrive at the receiver about 20 microseconds behind vthe direct wave. This may be regarded as illustrative of a'simple echo, as differentiated from multiple echoes which arise when a pluralityof reecting structures provide a plurality of indirect signal paths of different length.

Fig. 2 is an explanatory diagram in which the time and amplitude characteristics of a typical television signal S are related to the screen. P of a conventional television picture tube. 'I'he television signal, in accordance with the present practice, may comprise synchronizing pulses Ss of approximately 5.1 microseconds duration, blanking pulses Sb ofapproximately 10.2 microseconds, and a line or video period Sv of approximately 53.3 microseconds,. These are substantiallythe specications employed in a conventional 525-line interlaced television system, with 30 frames (60 elds) per'second. Since the line 4the screen P has, for convenience, been made equal in length tothe line 'portion of the televing and synchronizing pulses need be considered.

Of these` pulses, the latter are, of course, the more important by reason of their greater amplitude.

Line B in Fig. 2 is illustrative of the normal appearance of a picture line when receiving a strong echo signal. 'The echo illustrated resembles those which are produced by the delayed reception of the blanking and synchronizing signal, and is displaced -from the left-hand edge of the screen P by a distance which is proportional to the time interval between the arrival of the signal traveling the direct path and the arrival of the signal traveling the indirect or reflected path. If 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 differences cause the echo to appear further tothe right. Of course, where the path' difference is very great, the attenuation suffered by the reflected signal is usually so considerable that the echo is too weak to be noticeable.

As is indicated in the drawings, the echo shown in line B is produced by the arrival of blanking and synchronizing pulses during the line or videc period Sv. Specifically, this echo is produced by the arrival of the pulses S, Sb, by way of an indirect path, approximately 20 microseconds after the reception of these pulses by way of the direct path. AThe echo signal produces on the screen P an echo pattern Whose width is equal to the width of the line, and whose length is equal to l the distance traveled by 'the scanning beam 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 reected signal traveling an additional 4 miles as illustrated in Fig. 1.,

. Whether the echo, as it appears upon the picture tube screen, will be a dark echo or a light echo, depends upon the phase relation vision signal S. As is conventional, the ampli-` tude of the blanking pulses Sb may be regarded as corresponding to a black signal level, zero` carrier as corresponding to avery bright signal level, while the synchronizing pulses may be regarded as corresponding to a blacker-than-black or infra-black signal level. The video signal existing during the 53.3 second line'period has, for the purpose of this description, been established at a level midway between zero carrier and the black level, and will correspond approximately, therefore, to a gray level.

vWhen a signal of these characteristics is received without an attendant echo, there will b e reproduced, upon thescreen'of the picture tube, a line similar to that shown (with exaggeratedy width) at A inY Fig. 2, and having a uniform the desired direct signal. `Strictly speaking, un-

der these conditions, the appearance of the entire line will be aiected, but in general echoes between the video carrier 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. Thev darkest part of-th'e echo will be that central portion which vcorresponds to the synchronizing pulse Ss, while `the outer portions correspond to the blanking signal d will be somewhat less dark, but darker than that part of the line which is not distorted by echo; On the vother hand, if the video and echo carriers arrive in generally opposite phase, the reverse rwill be true, and a light echo such as that shown in line C will result.

Attention is now directed to Figs. 3 and 4 which illustrate certain of the methods which may be employed, in accordance with the present inventionfto elect substantial cancellation of echo patterns. No attempt has been made in these gures to maintain the identical scale employed -ln Fig. 2. Moreover, for simplicity, only the echo caused by the delayed arrival of thesynchronizing signal vis illustrated, the lesser echo produced by the blanking signal being omitted. Of the numerousecho cancellation schemes that we have times.

gaseosa developed, with the present invention as a basis,

Vperhaps one of the simplest is illustrated in Fig. 3.

The assumption in Fig. 3 is that transmission is carried out in accordance with the single-carrier system, i. e., where video and synchronizing signals are transmitted on acommon carrier frequency. Assume that a dark echo, such as is shown in frame No. 1 of Fig. 3, is obtained. This echo extends from top to bottom of the`picture as shown, for each individual line of the frame includes an echo of the type shown in detail in line B of Fig. 2, and the echo in each of the lines will be, of course, in substantially identical positions relative to the edge of the picture.

manner, frame No. 2, and all succeeding frames,

would present the same appearance, so far 4as echo is concerned, as frame No. 1. According' to the present invention, however, transmission is carried out in such a manner that a periodicl reversal of` phase relation takes place between the echo carrier" and the video carrier, causby a frame having a light echo, such as that shown in frame No. 2 of Fig. 3. This may be accom- .plished by the transmitting station by changing If ,transmission were carried out in the conventional ing each frame having 'a dark echo' to be followed` carrier phases or polarities at predetermined .Y

For example, all odd-numbered frames might be transmitted in the conventionalmanner with no carrier phase changes, while all even-` numbered frames mightbe so transmitted that the carrier phase obtaining during the synchronizing (or blanking and synchronizing) lintervals is reversed with respect to the carrier phase obtaining during the video intervals.

This may bestbe understood in connection with Vthe composite signal representation of Fig. 5 (a) which is intended to illustrate, in reduced scale, a television signal of the type shown in the lower portion of Fig. 2. The\signals to the left of the dashed line maybe considered as those producing the scan shown as frame No.V 1 in Fig. 3, whilethe signals to the right of the dashed line vare those producing the scan denoted frame No.

2 in Fig. 3. In `each frame the echo is produced,

` as hereinbefore described, bythe delayed, ar-

rival of echoes of the synchronizing and blanking signals'. In Fig'. 5(a) the absence of crosshatching in the signals to theleft of the dashed lines-s: is intended to indicate that no phase shift is produced in the carrier asbetween the of dark and light echoes upon the screen of the picture tube is substantially that which would obtain if no -echoes were `being reproduced at all.4 In a conventional television system based on 30 complete frames per second, there will be iifteen complete echo alternations" per second, each alternation consisting of one frame having a dark echo followed by a frame with a light echo. Apparently the rapid substitution of light echoesffor dark echoes, and vice versa. causes the e eye to average the echoeffects and to substan-v tially ignore the individual echo" images themselves. i We have found that cancellation of echoes can be made even more veiective if, in each frame,

the echo is broken up into a series of alternate light andgdark areas as illustrated in .Fi-g. 4. This figure shows a pattern that may advantageously be employed in a conventional system. A pattern of this character may be `obtained by using a signal inwhich carrier phase reversal is" effected at the transmitter at approximately the beginning and the end of each of the crosshatched intervals shownin thesignal represen-l tations (b) to(e) of Fig. 5. With asignal of this type',the character of the echo (i. e., whether dark orlight) changes for successive lines in time sequence. In a conventional interlaced scanning system, theeiectproduced will be similar to that illustrated in Fig.` 4 in whichthe lines are numbered from l to I B intime sequence for two complete frames, i; e., four complete iields. relation between lthe synchronizing signal echo In line l, it is assumed that the phase and the video carrier is such that a darkv echo results. In line 2 (which in an interlaced system ris spaced from line l by the width of one line),

' this phase relation is reversed to produce a light echo,reversed again in line 3 to produce a dark field. y'Ihe second field, comprising lines-G to 9,

i. is transmitted without change in -sequence so that the scanning pattern, or raster as itzis called.V

consists of a plurality of pairs of lines with alterecho, and soon for live lines to produce the first nately dark and light echoes, as illustrated in Fig. 4, frameNo. 1. In'frame No. 2, this process is continued'without change in sequence, the rst eld of frame 2,compris'ing lineslll to Mr, while the second eldcomprises lines i5 to I8.

video and the synchronizing and blanking intervals, whereas in the signals shown to the'right' of :1r-m, the cross-hatched blanking and synchronizing signals are intended to indicate that the phase of the carrier wave, during the blanking and synchronizing intervals,'is reversed with respect to the pha'seof the carrier during the video intervals.

frame No.` 1, as to' produce a dark echo, it follows that during frame No. 2 (with relatively reversed phase relationships) light echoes will be produced. `The characteristic .of ythe echo (whether dark or light) during the video intervals of frame No. 1 is indicated arbitrarily in Consequently; if the delay synchro-` nizing signal echoes arrive in such phase, during Fig. 5 (a) by the circled plus signs; whereas, lduring frame No. 2, circled minus signs are employed to indicate that the echo would have a different characteristic as a result of the reversal in phaseA the observer, the ,Jop-

It will be seen, however, that because each frame includes an odd number of lines, the dark echoesL in frame No. 2 occupy those` parts of the raster which in frame No. 1 'were occupied by the light echoes. and conversely. Since these frames are eii'ectively superimposed at short intervals in transmission, the echoes tend to cancel as far as the observer is concerned. 'Ihe system illus'- trated in Fig. 4 has the advantage over that in Fig. 3 thatit breaks up the echo signal so completely-as to substantially eliminate. all trace of ickering in even thoselocations where echo sigvnals are very strong. Eiectively, the'system of Fig. 4 interlaces the ec'hoes in both time and .i

- space relation.

It has already been explained, in general, howV echo cancellation maybe obtained by successive- 1y reversing the phase ofthe echo with respect to the picture carrier in time relation. Specic examples showing just when. these phase reversals may be made, to secure echo cancellation of the type described with reference `to Fig. 4, are

illustrated in Fig. 5 (b) to (e) inclusive. In these illustrations, the'cross-hatched portions of the signals may beregarded as representing one-ar-l bitrary phaserelation, the open portions o f the signals representing a lsubstantially' opposite phase relation.

In Fig. (b), at time t1, the carrier phase is,

end of the manning fblanking and synchronizing interval so as 'to avoid anypossibilityoi impairment of interlacing. Preferably, the phase changes are suspended for the nine lines following the beginning of the vertical blanking period.

It is not necessary that the desired phase changes be eiected precisely at the beginning and end of the selected blanking pulses as shown in' Fig. 5 (b). If desired, these chan-ges may be made to occur at the-beginning and end of the synchronizing pulses', or at some time within the blanking signal intervals preceding and succeeding the synchronizing-signals themselves. The latter system oi timing is illustrated in Fig. 5 (e). Where phase changes are effected to coincide in time with the synchronizing signals, rather than with blanking signals, it follows that echo cancellationv will be secured only for thesynchronizing signal echoes, but since echoes of the synchronizing signals are by far the most important,

- the choice between the various timing sequences may befound to be largely one of convenience. Referring generally to the signals represented in Fig. 5 at (b) and (e), Iit will be seen that every other blanking and synchronizing signal istransmitted with its carrier phase reversedrelative Ato the phase of the carrier during the rest of'the television signal. Consequently, ifV the synchronizing pulse transmitted in the interval ti -.t2 is received as an echo during the immediately following video line interval, an echo image will appear on the television screen for that particular line.` Ii' this echo be a light one, indicated by the circled minus sign, the echo in thenfollowing line (intime sequence) will be a-dark one, as is indicated by the circled'plus sign. That the sign" of the echowill be'dierent in the two-cases will be seen from the fact that in one case an echo of one phase will beat with a video line of opposite phase, whereas in the other case, the echo and video line are of like-phase.' Thus, the echo image alternates from dark to light to dark, and

. the synchronizing pulses themselves.

5 (c). Here the phase oi the carrier is reversed after alternate synchronizing or blanking pulses. i. e., afterevery second pulse. Thus, at t1, the end of the first blanking interval, the carrier phase is reverse, but no further reversal in relative phase occurs until time tz. which corresponds to thev end of the third blanking intervaLand again at ta, the y end of the fifth blanking interval, and so on. Here againthe echo image will alternate from light to dark, etc.. as indicated by the circled plus and minus signs.

In Fig. 5 (d) the carrier phase is reversed for --alternate video (line) periods, the phase reversals taking piace at times t1, tata. etc., as indicated. It will be seen that this procedure will also produce an alternating echo similar to those produced by the signals of Fig. 5 (b) to (e) inclusive.

In all of thesevariations. it should be understood that the phase changes arenot necessarily made precisely at the beginning and/or Aend of the blanking periods, but may be eilectedI within the blanking periods. as illustrated .in Fig. 5 (e), or may coincide with the beginning and/or end of Similarly, it shouldbe understood that the invention is not limited to the speciiic methods of echo cancellation illustrated in Fig. 5, since other suitable sequences of phase reversal may be utilized by those skilled in the art without departing from the methods and teachings of this invention.

From the illustrations of Fig. 5, it will be seen that the carrier phase is preferably reversed at least `time-s per second, where L is the number of picture lilies per frame and F is the number of complete frames transmitted per second.

O ur invention may be put into effect by means of the transmitting system 'shown diagrammatically in'Fig. 6. Ant-oscillator I 9 serves as the primary source of can'ien signal. If desired, this so on, from line to line in time sequence, this being indicated in Fig. 5 (b)y the alternating plus and `minus signs.n m y In vthe roregolng, cases have been described where the echo arrives'alternately in-phase and out-of-phase with the videovsignal. Obviously, of course, there will be instances wherein the echo will arrive alternately, leading and'laggingthe video' signal, forexample, by 90 degrees.- Where this occurs, the echo is of little importance, since the resultant of a strong signal (the direct signal).

.and a weak signal (the echo) differing l90 in phase is not substantially different in magmtue from the strong-signal.

Another sphase changing sequence capable o! ,l me.

ent invention may be produced in the transmitproducing-an echo image which alternatesfrom line to line in time` sequence, is illustrated in Fig.

source may operate at a 45 sired carrier frequency,

f' sesqui-side-band illter 2l,

Amplitude modulation of submultiple of the dethe desired carrier trequency` being obtained by passing the wave from the source I9 through a suitable frequency multiplier circuit l2|). .v unit'20 may then reversing means 2| whose reversing operation is controlled in response to a keying signal from a source 22. The operation and construction of the units 2| and 22 will be described in detail hereinafter.k The carrier signal output o1- the unit 2| may then be passed through a suitable modulated amplier stage 23, thence through the antenna or radiating system ,25.

the `carrier wave may of a conventional modsupplied with video, synchronizand blanking signals from the source 21. `The 'phase reversals contemplated in the presbe accomplished bymeans ulator stage 26 ting system of Fig. `6 by means ofi-the phase-reversing device 2|- which is capable of producing a pair oi' waves of carrier frequency, but whose relative phases differ by 'I'he device 2l is further characterized by th? Provision of means for selecting either of `these waves tothe exciusionof ,the other, -in response to a control or keying, signal. .This keying signal may be derived from the unit 22 which comprises the circuit means necessary for generating the proper. keying signal in response to signals de- The carrier derived from the v be supplied to a suitable phaseand-finally to a suitable' substantially degrees.

i tube V1 is operative, the signal supplied Vto the connected by way of the lead 33 to the cathode v impulses whose duration and timing correspond to the duration and timing of every second blanking" signal, i. e., to the cross-hatched signals of liigf Reference is now made to Fig. 'I in which there is shown a schematic diagram of a, circuit adapted for use inthe controllable phase-reversing means 2| and the keying signal source 22 of Figr.

-The controllable phase-reversing means 2l il- .l5 lustrated in Fig. 7 comprises a pair of vacuum tube amplifiers V1 and V2 having their input grids connected in push-pull relation to the balanced secondary winding of the carn'er input transformer 30, and their anodes connected in parallel to the interstage transformer 3l. In the operai tion of this circuit the tubes V1 and Vs are diertransformer I8 will be in relatively reversed phase to the signal supplied in the event that tube V2 is operative and V1 inoperative. This follows from the fact that the grids ofVi and V2 are connected 3 to opposite ends of the vsplit secondary of transformer 30, whereas the anodes of Vi and V2 are connected together and to the upper or high potential end of the primary winding of transformer 3|. f

-The keying signal unit 22 is a device for con; trolling `the bias of tubes Vi and 'V2 of unit 2| A in such a manner that a selected one of the said tubes is rendered operative while the other is rendered inoperative, or .vice versa. The bias of .4 the tubes V1 and V2 are controlled by the tubes Vs and Vs respectively, the cathode'of V1 being load 34 of the tube V5. Similarly, the cathode of V: is connected by way ofthe lead 35 to the cath- 45 ode load' 36 of tle tube Vs. Neither of the-tubes Vo and Va is here provided with a plate circuit load, their screens and anodes being connected directly to the positive high potential `supply terminal B+.

The control grids of tubes V5 and Ve are condenser-coupled to the anode and cathode loads 31 and 39, respectively, of the` signal inverting ldriver tube V4. Preferably, the anode and cathode loads 31 and 38 are substantially equal in 55 v f. magnitude so that the signals applied to the control grids-of Vs and Ve will be not only opposite in phase, butequal in* magnitude as well.,l The operation of tubeV4 may be controlled by a suitable source oi control pulses represented by the rectangle. To carry on the preceding description of a system "for supplying-a signal of the type described wlthreference to Fig. 5 (b),` the device 29 may comprisea circuit adapted tobe energized by the lbianking signal and capable of 35` the tube V2 inoperative during the periods ti--ta ta-t4, etc., but operative during the periods tze-ja..

supplying to the control 'grid of V4 a signal similar to the blanking signal, but having only half the number of pulses per second. In Fig. 7 the rectangle 39 has, accordingly, been referred to as an "alternate pulse rejectcr. Circuits capable of longer`intervals-t2-ts, t4 -ts. etc. of Fig. `5 b);

vurotion during the shorter intervenir-a, t` e Under these conditions there wuibe' estati nais.

'signal occurring at the rate of sixty per second,

scanuanyto plate-currenteut-off (i. eoring the but `is driven substantially to plate-currents t etc., by vthe pulses received fromthede across the cathodeload 3B a signal havin wave shape and polarity (phase) of Atires applied to the grid of'V-i, while across the cathode, f load 34` there will appear asignal havingfflike wave shape, but opposite polarity. These'signals f.

' (i. e. the signals appearing at the, cathodeslgof'Vs and Vo) may then be employed as fkeying: signais to differentially render operative` and i'nopjerative tubesVi and V2 in the phase reversingunit 2|, for the purpose hereinbefore described,`r

It has already been stated that matarme?? H .phase reversals are preferably suspended during a portion of the vertical synchronizing interval This is readily accomplished through the :agenc of one of the signals supplied;A by conventiona generators of standard RMA synchronizing sig- ,i

This signal is a substantially :rectangular and having a duration of approximately nine line periods. vThe signal in questionstarts"v in syn- Accordingly, this 9 -line signal (which can be" derived from unit 21 of Fig. 6) may be` supplied 35 to the keying signal source 22 along with the modified blanking signal supplied by the alter` nate 4pulse reject-or" 39 of Fig. 7. The signals may be added together-by means of a conventional signal combining circuit, of which many varieties are well knownin the art. 1

The relation between the blaiking and keying signals, the operation of tubes V1 and V2, and

the phase changes involved in the operation of the specific embodiment illustrated anddescribed f,

with reference to Figs. 5 (b), 6, and '7, may best lbe understood by referring to the explanatory` drawings of Fig. 8. The various functions here illustrated are alldrawn against, a common time axis whichfemploys thenotations t1, t2, etc. alreadyusedinFig.5 (12)'. Thefirst Isignal illustrated is the blanking'sig--` nal which is transferred from `the source 21 to the alternate pulse rejector 39 of Fig. 7. `The output` of `unit 39 is the second signal illustrated in Fig. y8. I'he keying 'signal derived from cathode load ze of Vois denoted S- as in Fig. a. The keying ysignal K. S, 34, derived from cathode load 34 of V5,` is'seento `be of similar wave form,- but of opposite polarity.` Signal S.A 3l, which is applied to the cathode of Vi. renders tube Vi inoperative during the periods ta--ta` tri-rta, etc. but operative during the periods tiftz, ts--ta V36 which is applied to the cathode of V2, renders tl-f-t's, etc., as represented by thecrosslhatched areas in lineVa of Fig. 8.

`The phase changes undergone byjthecarrier i w'aveare illustrated in the diagram designated r Relative phase.`

During the longer intervals tz-,fta tee-t5, etc., Y the relative phase of thev carrier maybe regarded as substantially ilxed at some arbitrary value 1. -l

between a directly received carried signal. and an `The phase is periodically reversed, however, and,

accordingly, during the shorter intervals t1-t2,

ts-t4, eto., the relative phase of the carrier is j advanced to a value c+ 180. Of'course, it might just asv well be retarded toa value -180. These phase shifts are effected during alternate blanking signal impulses in accordance with the echov cancellation method described with reference to Fig, 5 (b). l o Strictly speaking, the phase of an alternating wave varies continuously at .the rate of 360 del grecs per cycle. In the present invention, however, we are not concerned with this kind of phase variation, but rather-with thel phase difference indirectly received carrier (echo) signal. Consequently, in the foregoing description, and in the claims, the term phase `that concerns us in the description of our invention. Thus, in the function designated Relative phase in Fig. 8, the regular phase progression from cycle to cycle of the R. F.- carrier is ignored, as are also the less regular phase changes which may occur as a result of carrier frequency drift or variation, and only the phase reversals produced by the phase reversing means 2i of Figs. 6 'and'7 are represented.

While the operation of the apparatus of Figs. 6 and 'I has been described in detail .only with'respect to the echo cancellation method of Fig.

5 (b), the foregoing example will enable one skilled in the art to put into practice other echo cancellation methods, such, for example, as those described with reference to Fig.v 5 (a), (c), (d) and (e). Since methods for generating the necessary keying signals in response to the blanking and/on synchronizing signals are well known, it

isnot deemed necessary to refer to the details of this process.- i

Thus far our invention has been described Vwith Particular reference to television transmission system in which the video signal is transmitted v Y by amplitude modulating the carrier. I'he invention-is also adapted for use, however, in systems wherein the vide'o`signal is transmitted by fre- Quency modulating the carrier. In fact, because of the particularly obnoxious appearance of the echo images in FM' television transmission systems, some means for eliminating or cancelling echoes is especially tobe desired.

In FM systems the'widths of the echo bars are not xed as they are in an AM system. Instead they vary in width from time to time in accordance with the changing illumination 'of the picl ture in the screen area affected bythe echo. This is because in 'an FM television system the inystantaneous carrier frequency is a function oi' illumination, and,-.consequently, the number oi' 'beats (which produce-the echo bars) between thev delayed synchronizing signal (i. e'. the echo) andV the received picture signal varies with .picture illumination. Thus, as the illumination of the picture varies in a given area-e. g. as caused by the movement of actors or vehicles, or by movement of the television camerathe echo will present an ever-varying-and moving image, which tbaclluse of its motion is much more objectionable e echoes encountered in AM-'systems of television.,

'Ihe present invention can greatly reducethe ei'- 70 rect of these echoes in spite of their movement, because in` general "this movement (of the echo over the screen) takes place slowly enough .so that between identical lines in successive frames there i l is suiilcient similarity as to echo image position 75 afasaoas observer than the Imotionless or fixed to enable an alternating dark-and-light echo in one frame to be replaced by a substantially correspondingly-placed alternating light-and-dark echo in the immediately, following frame. Thus, it will be seen that our invention is adapted not only to systems wherein the echo produces a substantially fixed pattern upon the picture tube screen, but also to systems in which the echo pattern. while varying substantially from minute tominute, or second to second, changes only slightly, or to a negligible degree, from frame to frame.

Although our invention has been described and illustrated with reference to certain preferred embodiments, it should be understood that wide alterations and modiilcations may be-made with` in the scopefof this invention as deiined in the appended claims.

We claim: l 1. In a constant-carrier-frequency television deleterious eiects of echo signals on the desired `signal, which comprises periodically altering the relative carrier phase of selectedcarrier intervals to produce echo images of contrasting characteristics in successive frames.

2. In a television system, the method of generating a television signal which will ensure substantially echo-free.reception, whichcomprises' generating a carrier wave of constant frequency,

periodically altering the relative phase of said wave in a predetermined time sequence, and vmodulating said wave in vaccordance with thefintellif gence to be transmitted.

3. In afIconstant-carrier-frequencyl television transmission system, the method of reducing the deleterious effects of echo signals on the desired signal, which comprises phaseat least l 2 times per second in predetermined sequence prior toy transmission. thereby to cause echo images ofcomplementary characteristics to appear upon a receivers picture viewing screen in alternating sequence. where Lis the number of picture lines per' frame, and F is the numberl of complete 'frames transmitted per second.

4. In a television transmitting system including a source of constant-frequency carrier wave oscillations, apparatus ,for reducing the deleterious effects of echo signals on the desired signal, comprising controllable means for periodically shifting the relative phaseof saidl oscillations in response vto a control signal, the magnitude of each 'phase shift being substantially electrical degrees, where n is ajsmall integer other l. than l. and-a source of control signals connected to said phase'shifting means tocontrol the periodicity of said phase shifts.

5. in a television transmitting system including a source of constant-frequency carrier wave os cillations. apparatus for reducing'the deleterious ee'cts of echo signals on the desired signal, com'- prising controllable means for reversing the relative phaseof said. oscillations in response to a control signal, means for modulating the oscillations i' derived-.from said phase reversingmeans in accordance with anintelligence signal, a source of .synchronizing and blanking signals, a control signal source connected to and deriving signals from said second-named source, and a connection bereversing the. carrier asaoss tween said control signal source and said phase reversing means for eecting carrier phase reversals in accordance with a'predetermined function of signals derived from said second-named source. A

6. In a constant-carrier-frequency television system, avsource of carrier wave oscillations, a source of video signals. a source of synchronizing signals, modulating means for imparting to said carrier wave the video and synchronizing intelligence from said secondand third-named sources, means operative during odd frame intervals for effecting the transmission of a given line at a differ#v ential carrier phase of 0 degrees with respect to the preceding synchronizing pulse, and means operative during even frame intervals for effecting the transmission of the corresponding line at a dif' ferenti-al carrier phase of @+180 degrees with respect to its preceding synchronizing pulse, where o is an arbitrary and substantially fixed phase angle. l

7. In a constant-carx'ier-frequency television system, a source of carrierl wave oscillations, a source of video signals, a source of synchronizing signals,` modulating means -for imparting to said carrier wave thevideo and synchronizing intelligence from said secondand third-named sources,

FRANK J. BINGLEY. l WILLIAM E. BRADLEY.

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