US2347248A - Alternate carrier television system - Google Patents

Description

April 25, 1944. F. J. BI'NGLEY ALTERNATE CARRIER TELEVISION SYSTEM 3 Sheets-Sheet 1 Filed July 8, 1941 April'25, 1944. FQ J. BINGLEY 2,347,248

` 4 ALTERNATE CARRIER TELEVISION SYSTEM F'iied July a, 1941 5 sheets-sheet 2 6.,. l Tim V\j im F. J. BINGLEY 2,347,248

Filed July a7, 1941 s sheets-sheet s ALTERNATE CARRIER TELEVISION SYSTEM April 2s, 1 944.

Patented Apr. 25, 1944 o 2,347,248 ALTERNATE CARRIER TELEVISION SYSTEDI Frank J. Bingley, Chestnut Hill, Pa., assigner to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation `of Delaware Application July s, 1941, serial No; 401,533

(cl. 17a-1.1)v

22 Claims.

' This invention relates to a novel and improved composite modulated-carrier-wave television signal, and to a method of, and means for, generating, transmitting, and receiving such an improved signal.

In the past ithas been customary to transmit the video and synchronizing signal components of the composite television signal by causing them to amplitude-modulate the same carrier. It is customary to do this in such a manner that the two components can readily be separated one from the other in order that they may perform their respective functions at the receiven This has generally been done by dividing the time axis of the carrier wave signal into regularly spaced intervals of alternately long and short duration. During the long intervals the carrier wave is modulated by the video signal, and during the shorter intervals by synchronizing signal. In order to permit amplitude separation of the video and synchronizing signals. it is customary to establish these signals at relatively different amplitude levels. Usually from 20 to 25% of the overall carrier Aamplitude is reserved for the synchronizing component.

Recently there has been `suggested (Electronics, Feb. 1940, pp. 27-30, Article by A. V. Loughren) an improved form of signal for transmitting the video and synchronizing intelligence Lwhich effectively increases the amplit de ofthe synchronizing signal relative to the vi eo signal. and thus improves the relative synchronizing signal-to-noise ratio, without detracting from the amplitude of the video signal. In certain respects the signal employed in the present invention is similar to that described by Loughren. The video and synchronizing intelligence is segregated into alternative long and short time intervals along the time axis ofthe signal as it is transmitted.v During the long intervals the video signal is caused, in the preferred embodiment, to amplitude-modulate the carrier wave in accordance with the aforementioned practice..

During the short interval. however. the synchronizing intelligence is made to shift the carrier wave and simultaneously to increase the amplitude level of the shifted carrier with respect to the blanking level of the video signal. Consequently, the present system may be regarded as one employing two carrier waves, the video carrier and the synchronizing carrier. As `will be pointed out hereinafter, the gain vs. carrier frequency characteristic of the television receive:`

`enables synchronizing signals so transmitted to be received in a highly favorable manner.

A primary object of the present, invention, therefore, is to provide a novel and highly effective form of modulation for a carrier wave television signal comprising both video and synchronizing intelligence. l

A further object of the invention is to provide a television signal of novel character, capable of being favorably received through severe electrical interference.

VAnother object of the invention is to provide a composite television signal of such character that, without requiring y increased transmitter power, and without reducing the transmitted video intelligence with respect to the synchronizing lintelligence, the said composite signal may be so received that the effective` synchronizing signal derived therefrom is very substantially increased over that obtainable heretofore.

Another object of the invention is to provide a system for transmitting a composite television signal in which the transmitter may be driven to an extent closely approaching a full load condition during the video and blanking intervals, but is momentarily overdriven during the alternate frequencyl (synchronizing) intervals, so that the transmitter is operated at its maximum possible usenilness.

Still another object of the invention is to provide an improved composite television signal capable of eecting highly accurate synchronizing in districts of low signal strength, or in districts subject to unusually severe electrical noise conditions.

Other objects and advantages of the inventionv will be apparent from the following description and the accompanying drawings in which:

Fig. 1 is a block diagram of a. conventional television receiver;

Fig. 2 illustrates the improved form of signal above referred to and the advantages thereof;

Fig. 3 is a block diagram of a system conployed, having particular adaptation for separating the video and synchronizing components of the signal, the signal may be efiiciently re- 5e cel'ved by a conventional receiver, such as shown in Fig. 1, coonly employed to receive the purely amplitude-modulated signal used in the past. Not only is such a conventional receiver adapted to receive the new signal, but, when supplied therewith, it will operate more satisfactorily than with the old form of purely amplitude-modulated signal. The rns for this will now be set forth with reference to Figs. i and 2.

Referring rst to Fig. l, the conventional receiver there shown comprises a signal-collecting antenna i, a radio frequently amplifier 2, a rst detector S, an intermediate frequency amplider d, a second detector t, a synchronizing signal separating stage t, a video signal amplifier l, and a visual reconstructing device or picture tube t. In general, the synchronizing signal pick-o stage which derives signal from stage t, may comprise a tube 'whose hiss is so controlled that it selects only that portion of the astuces modulated carrier signal may be amplified in the v radio frequency amplier 2, its frequency may be translated in the rst detector ii. and it may be fed from the output of the said detector or from a point in the intermediate fretiuency amplier d to the audio frequency circuits li. The latter may include an intermediate frequency .in a second detector, and an audio plier from which signal is supplied to the sound reproducing device i2.

. The manner in which the conventional receiver of Fig. 1 operates when supplied with the type of signal may best be understood by reference to Figs. 2-A, 2-B, and 2-C. This signal is shown in Fig. 2-A, only the portion above the zero carrier level being illustrated since the signal is symmetrical with respect thereto. im s ing the intervals c the frequency of the carrier wave is maintained at a substantially concs. value which will hereafter be referred to as the video carrier frequency. During the shorter intervals d, the frequency oi the carrier wave is shifted to a dierent value 'which may he either higher or lower than the carrier frequency. 'This frequency will hereafter be referred to as the synchronizing carrier frequency. In a signal adapted to be received by a conventional television receiver, the synchronizing carrier frequency may, for example, be located appronmately one megacycle above the video carrier frequency. During the intervals a, which ingeneral may be slightly shorter than the intervals c,v the video carrier is amplitude-modulated by means of the signal containing the video intelligence. During the portion of intervals b preceding and succeeding the intervals d, the video carrier is preferably established at a substantially constant amplitude.' This amplitude may correspond approximately to a black picture signal.

According to the invention, the frequency of the carrier wave is not only shifted during the synchronizing interval d, but also the amplitude thereof is increased very appreciably over the blanking level of interval b, as illustrated in Fig. 2-A. This amplitude may correspond to a blacker than black picture signal, i. e., to an infra-black signal. Thus the synchronizing receiving station not only by a variation or shift in the carrier frequency, but also by a variation or change in carrier amplitude. The characteristics and advantages of such a hybrid synchro-- nizing signal will be explained more fully hereinafter.

For purposes of simplifying the explanation of the invention, the exemplary signal shown in Fig. 2-A has been used. However, it should he understood that this is not the only specific form of signal which can be generated. For instance, the intervals a during which amplitude-modulation of the video carrier obtains, the intervals o separating the intervals a, and the intervals d during which the carrier frequency is shifted to the predetermined frequency may all be varied in duration. Furthermore, the respective intervals need not all be of the same duration, nor need the frequency of the carrier be changed to the same extent in every interval d. Under the present practice it has been found desirable, in forming a television signal of this type; to establish the synchronizing carrier 15,75@ times lier second (for 525 line, 30 frame, interlaced television) to eect horizontal synchronization, while further establishing the said carrier Sii times per second to eect vertical synchronization. in ordei` that the horizontal synchronizing intelligence may be separated from the vertical, it is possible to make the intervals during which the 6d cycle signals occur longer than those during which the 15,750 cycle signals occur. To avoid interference between the two, the longer intervals may be serrated to provide for horizontal synchronization during the vertical synchronizing intervals in the same manner as is done when the synchronizing signals are caused merely to vary the plitude of a single carrier wave as has been the practice heretofore.

It will be recognized that in the signal represented in Fig. 2-A, the intervals b may he the horizontal synchronizing intervals. Assuming this to be the case, the respective durations of the intervals a, b, and d (for the case of 525 line,

30 frame, interlaced television) might be approximately as follows:

a=55 microseconds b;- 9 microseconds d= 5 microseconds It will, of course, be understood that Fig. 2-A does not purport to4 show the longer duration vertical synchronizing intervals.

In Fig. 2-B there is represented approximately the standard frequency response charateristlc of those stages preceding the second detector in a standard television receiver adapted for operation with the conventional sesqui-sideband orvestigial sideban'd transmissions. In order to equalize the percentage of modulation of the sdeband components in the vicinity of the carrier, the response is madesubstantially 'uniform down to a point approximately 0.75 megacycle above carrier from which point it decreases substantially uniformly to zero at a point approximately 0.75 megacycle below carrier. Thus. the response at carrier frequency will be approxv imately 50% of maximum. As is well known to signals are enabled to manifest themselves at the 76 sistance in the utilization 'of the new type of sigateniese nal under discussion for reasons which will appear presently.

When the signal of the type shown in Fig. 2-A is applied to a receiver with a response characteristic of the type shown in Fig. 2-B, the re- .sponse to the carrier frequency during the intional to 1/2 E, where E is the maximum response. However', during the intervals d when the syn-v chronizing carrier is established, operation will vbe upon the upper and level portion of the frequency response curve, so that the response to the synchronizing carrier will be proportional to E or twice the response to the video carrier. Of course, in order forthis to be the case, the video carrier frequency must be shifted to such an extent as to cause operation upon the upper part of the response curve. In the case shown in Fig. 2-B, where the response begins -to fall 0H at va point 0.75 megacycle above carrier, it will be sufdcient to shift the carrier frequency upward by approximately lmegacycie.

The advantages to be gained by utilizing this improved form of transmitted signal willl be apparent from a comparison of the detected signals obtained at the second detector when the old and new signals are used. The two signals are shown, respectively, to scale at the left and at the right in Fig. Z-C. Both signals may be thought of as resultingr from the detection of carrier waves having the amplitude modulation envelope of Fig. 2-A. The signal to the left, however, results from the-reception ofA a carrier wave in which both video and synchronizing signals are applied to the isame carrier. The signal to the right, i. e.,

the newl signal, results when the video and synchronizing signals are transmitted'loy means of separate or alternate carriers differing in frequency, In the case of the old signal, it is cus-- tomary that about 25% of the amplitude of the signal be occupied by the synchronizing component. rlhis will leave approximately 60% available for the video signal since, dueto limitations in *the capabilities oi' modulating equipment, it is generally dimcult to obtain more than about that much picture modulation In the slgnaljutilized in the new method, the amplitude of the synchronizing signal modulation may be maintained at its previous value of 25%, while the amplitude of the video modulation may be maintained at its previous value of about 653% of peak carrier, but by the establishment of a new synchronizing signal carrier at a frequency for which the receiver gain is double that of the video carrier (see Fig. 2-B) the detector output during synchronizing signal will-be double that obtained in the case of the oid signal, as illustrated in Fig. 2-C. On the basis o the percent' ages shown for the left-hand signal ot this drawing, the new synchronizing signal amplitude will be 125% of theorig'inal peak carrier. Thus, while the peak detected synchronizing signal for the new system is twice that ofthe old system, the new system yields a synchronizing signal pulse which is ve times as great as that obtained' in .the old system, i. e, 125% as comparedwith 25%.

This voltage gain of five times is equivalent to what could only be obtained, under the old system, by a transmitter power increase vof twentyfive times. Obviously, the new system will enable -the receiver to be maintained in synchronism wwith weak signals, or through strong interference, much more reliably than was the casein the old system. itietual fleld tests which were Vcarried, out to compare the twosystems under various conditions of noise fully bear out these theoretically determined advantages.

Referring now to Fig. 3, which shows in block form a complete system for generating the new signal, it will be lnoted that the controllable frequency oscillator I3 constitutes the prime, source oi? radio frequency energy. This oscillator may operate at the assigned carrier frequency of the station, or at some sub-multiple thereof, in'which latter case a frequency multiplier It may be added to supply a carrier wave of the desired frequency. The output of the multiplier may be amplied in a suitable radio frequency amplifier '-I, whence the carrier wave may be passed to an amplitude modulator i6 for modulation. The video, blanking, and synchronizing signals, in response to which the carrier amplitude is modulated, may be supplied to the modulator it from a suitable composite signal source I1, The modulatedcarrier output of tlie modulator I8 may be further. amplified, if desired, in a suitable linear amplifier I6 before it is suppliedto the sesquiside-band lter I9 andto antenna system A for radiation.

In order that the carrier frequency, as well as the carrier amplitude, 'may be varied in response to the synchronizing signals supplied `by the source il, there may be connected to the oscillator I3 a suitable frequency shifter or frequency control device 20. 'This device, when supplied by way of the connection 2l, with synchronizing signal pulses from the signal source Il, functions `'to vary or shift the frequency of the oscillator I3, and thus also the requencyof the wave radiated bythe antenna system A. Suitable fre-- quency control circuits are now well known in the art, and are described in detail in U. S. Patent No. 2,240,428 of Charles Travis, assigned to the Philco Radio 8i Television Corporation. Other suitable circuits are described in the copending application of David B. Smith, Serial No. 401,494, filed July 8, 1941. Certainspecic circuits and devices particularly adapted i'orv use with the system shown diagrammatically in Fig. 3 are fully illustrated and described in the said eopending application of David B. Smith.

according to one of the features of this invention, the transmitter is driven, during the video and blankng intervals (excluding the synchronizing intervals), to an extent closely approaching full load, but during the brief synchronizing intervals is overloaded to an extent itpproxirrlat-y ing the instantaneous maximum current capabilities of the transmitter. In television transmitters, .as presently designed, the continuous full-load current which. can be delivered by `a given installationV may be of the order of '75% of the maximum instantaneous current supplying capabilities ci the transmitter. The full-load current rating of a transmitter having negative picture modulation is usually based upon the operation oi the transmitter while transmitting a black or an almost black picture, since under theseconditions the system is most heavily loaded, the carrier level corresponding, throughout the intervals o, to approximately the carrier level during the blanking portions of the intervals b.

Within the normal full-load range of operation ci the transmitter the modulation characteristic thereof can be made relatively linear, but between full-load current and maximum current the characteristic may be highly non-linear. According 'to the present invention advantage is taken n! these limiationsby restricting the carrier ampli.`

and signal strength explained while at the seme time the tl'ansrnitter` is driven to its maximum extent to supply a wave having an amplitude corresponding to the 109%.

level reached by peas of the synchronizing pulses 'd of Fig. Z-A. The fact that the linearity of the transmitter may be very poor throughout this upper 25% ronge is oi little or no conscquence, since the synchronizing pulses are similar and of a generally rectangular wave shape, there being accordingly no emplitude relations to maintain between various parts ofthe pulse or pulses.

Since the synchronizing periods d are very short compared to the periods d plus c (e. g. microseconds as compared to 63 microseconds) it will be seen'that if the transmitter is operated only very slightly underloaded during the periods c the momentary overloads corresponding tothe synchronizing pulses of periods d, even though they may he relatively high, will not be substantial when integrated over-a time corresponding, e. g., to one picture line or one picture frame (525 lines) consequently the increased heating of the power amplifiers due to the increased amplitude of the synchronizing pulses will not be substantial. 0n the other'hand, as shown above, the improvement in receiver` operation and synchronization is very substantial, and makes possible a much improved television system.

Reference is now made to Fig. 4 in which there is shown, largely in block diagram, an alternative embodiment of a television transmitter constructed according to the invention. This embodiment, rather than. employing a single master oscillator of controllable frequency, employs two separate oscillators, or carrier generators, of substantially xed frequency. The video carrier generator l2 is adapted to generate e. carrier frequency corresponding to that which is amplitude modulated during the intervnl c (Fig. 2-A) by the video and blanlsing signals. The synchronizing carrier generator d3 is adopted to generate e. carrier frequency diderent from that supplied by the generator 22 and corresponding to that corrier which is amplitude modulated during the intervel d loy the synchronizing signal. Thefoutput of each of these generators, 22 and 2li, muy be supplied respectively to a pair of triodes, it and te, having a common anode loud resistor't, the signal across which may he supplied, by wey of the coupling condenser 2li, to an amplifier und frequency multiplier (optional) d. While triodes have been illustrated in Fig. s in the interests of simplicity, it will he desirable in practice to substitute one or two pentodes (in cascade, if two) for each of the trlodes illustrated in order to maire the keying or switching more positive. The elements 2t to 26 constitute the basic elements of one form ol electron switch by which, in respense to a predetermined signal, the input to the device 28 may he selectively switched from one of the generators @2, 23 to the other'. The

operation-of this electron switch will be described in greater detail hereinafter. v

desnudo Asteige 29 adapted to modulate the amplitude oi the carrier wave in response to e composite video, blanking, and synchronizing signal derived from s suitable source 3d." The modulated output of the modulator stage 2t may, if desired, be emplined further by means of a linear amplifier stage 8l before supplying it to an untenna system A for radiation. If the transmissions are to loe of the vestigial sideband or sesdui-sidehand type, a eesqui-sideband lter 32 muy he connected between the amplier di and the :interina system il.

The electron switching system for establishing the appropriate carriers during the intervisie d and c may be operated in response to asignal derived from the source t@ es follows. By wey of e. conductor 35 o. signal, comprising syncincoq nizing signal components only, may he epplied to a. pair of networks dit and Sii. The network @il isadspted to supply, in response to the spoiled synchronizing'signal, e signal which is negative with respect to the cathode of triode 25 during the interval c, but only slightly neeotive d during the synchronizing interval d. Such signel Following the emplier and multiplier stage l I@ there may be provided an amplitude modulator is illustrated within the rectengie Eil, the zero level being indicated loy the une oo. Accordingly when this signal is applied to the grid of triode 2E by way of the conductor und the resistor El the triode @il is enabled to transfer the synchronizing carrier from source 221 to the emplier stage 28 'during the synchronizing interval d, but is blocked during the longer interval c by the applicationto its grid or the highly negative potential supplied by. the device til.

The network 35 supplies signal winch is sullstantially the reverse oi that supplied hy the network it, as illustrated within the rectangle. 'linie signal, which is applied to the control grid oi the triode 2d hy way of the leed 3d und resistor Sil, is only slightly negative during the longer inter vols c,'but is highly negative during the synchronizlng intervals d. is e, result of the-operation of the devices 2d, 2d and te, it, there is provided a system wherein the video carrier is switched on during the intervals c, and oi during the intervals d, while' the synchronim carrier is switched off during the interveh c, and on during the intervals d. Accordingly it will he seen that during the interval c the modulator Stego 2Q umplitude modulates the video cerrier with the video and blenking signsls derived from the source while during the interval d the moduletor 29 emplitude modulates the synchronizing carrier with the synchronizing signals derived from source til.

Devices of thel type represented by @t und tt are well known und require no detailed illustretion.

An obvious equivslent or this system would he one in which the source il@ supplied only a video and blanking signsi to the moduletor 2i?, and in which the amplitude of the synchroninng cerner supplied by the generator 2li was somewhat greater than that of the video carrier supplied by the source 22. The ratio of the carrier amplitudes supplied by the sources might he such that the radiated corrier would have sin envelope similar however, limited to such asystem, for ii desired the video and blanking portions o1' the composite signal may be imparted tothe carrier by the irea,a4v,a4s

Vvideo carrier, and means operative during the quency modulation thereof. Under these conditions, the carrier amplitude may be set to correspond, for example, to the '75% blanking level of Fig. z-A throughout, with the exception of the short synchronizing periodsd. During the latter periods the carrier is preferably shifted in frequency by an amount somewhat greater than the maximum frequency deviation obta during the video and blanking portions of the line. Simultaneously, during the synchronizing intervals, the amplitude of the carrier may be established at an overload level corresponding to the 100% level illustrated in Fig. Z-A, wherein the '15 level is apprommately full load.

The transmitting systemv illustrated diagrammatically in Fig. 3 may be utilized without substantial change to generate a modulated carrier signal of this character. Assume for example that it is desired to transmit the Video and blanking signals by frequency modulation, and the synchronizing signalsby effecting sudden changes in the amplitude and frequency of the carrier wave as already described. This mode of operation could be effected with the apparatus of 3 by supplying synchronizing, video, and blanking signals from the source Il to the frequency shifter 2li, while supplying only synchronizing signals to the plitude modulator i6. With this inode of operation the frequency shifter 2@ might more correctly be referred to as a frequency deviator."

Under some conditions it may be desirable to employ a combination of frequency and amplitucle modulation in imparting the video intelligence to the carrier wave, and the invention should be understood to contemplate such operation. Generally, however, it seems preferable that the video modulation should be largely one type of modulation or the other, in' contradistinction to the synchronizing signal modulation which should be characterized by substantial shifts or changes in both amplitude yand frequency.

Although the invention has been described with reference to certain specificl embodiments, it will,

foregoing description. `The scope of the invenlmposed by the following claims.

Tclaim:

l. In a system for generating a composite negatively-modulated carrier-wave television signal containing both video and synchronizing intelligence, a transmitter having a full-load current level and an overload current level, a source of signals of video frequency, means included in .said transmitter for generating a first carrier wave for the transmission of said video signals. means for amplitude modulating said carrier wave with said video signals over a predetermined range, the maximum extent of said range being limited to a level which is slightly below said fullload level, a source of synchronizing signals, means for generating a second carrier wave for the transmission of said synchronizing signals. the frequency of said second carrier wave diflering substantially from the frequency of the first carrier wave, means for amplitude modulating said second carrier wave with said synchronizing signals to an extent substantially exceeding the full-load current level of said transtion. ls to be subjected only to those limitations mitter and corresponding to said overload level, y means normally operative for establishing .said

' presence of said synchronizing signals for tem..

porarily suppressing said video carrier and establishing said synchronizing carrier.

2. In a system for generating a composite negatively-modulated carrier-wave televisiorf :signalv containing both video and synchronizing intelli-A sence, a transmitter having a full-load current level, said transmitter having a. relatively linear modulation characteristic below said full load level, a source of signals of video frequency, Va l source of synchronizing signals, means in said transmitter for generating a carrier wave of predetermined frequency for the transmission of said video signals, means for amplitude-modulatine said carrier wave with said video signals over a predetermined range, the maximum extent of said "range being limited to a level which does not exceed said full-load level, and means responsive to said synchronizing signals for amplitude-modulatinz said carrier wave in substantial excess of said full load level and substantially -simultaneously shifting said wave to a diderent odically shifting the frequency of said carrier predetermined frequency. l

v3. In a. s'ystem for generating and transmitting a negatively modulated carrier-wave television signal containing both-video and synchronizing intelligence, a source of video signals, a source of blanklng signals, a source of synchronizing signals, a vacuum tube transmitter having means for generating a carrier 'wave for the transmission of all of said signals, ineans responsive to said blanlsing signals for periodically establishing the amplitude of said carrier wave at a level corresponding approximately to the maximum continuous output llevel of saidv transmitter, means responsive to said video signals for amplitude--modulating said carrier wave between levels corresponding to said-blanking level and a predetermined relatively low level, and means responsive to said synchronizing signals for periwave and substantially simultaneously establishing the amplitude of said carrier wave at a level corresponding approximately to the instantaneous peak output level of said transmitter.

4. In a :modulated carrier-wave television system comprising transmitting and modulating apparatus, the method of transmitting video and synchronizing intelligence, which comprises operating said transmitting apparatus at approximately full-load during the transmission oi the video' intelligence, substantially overloading said transmitting apparatus during the transmission of the synchronizing intelligence, imparting the video intelligence to the carrier wave by varying only one characteristic thereof, and imparting the synchronizing intelligence to the carrier wave by changing both-the amplitude and frequency thereof.

5. In a systemfor generating a composite modulated-carrier-wave television signal containing both video and synchronizing intelligence, a source of signals of video frequency, means for generating a iirst carrier wave for the transmission of said video signals, means for amplitudemodulating said carrier wave with said video signais, a source oi' synchronizing signals, means independent of said drst carrier wave generating means for generating a second carrier Wave for the transmission of said synchronizing signals,.

assigns pendent means for generating a second carrier wave of diiferentfrequeney, a frequency multiplier, means responsive to video and bla signals for varyingthe amplitude of the carrier wave output of said multiplier between levels corresponding substantially, in the reproduced television signal, 'to extremes of white and black, means responsive to synchronizing signals for increasing the carrier wave output of said multiplier to a levelcorresponding to infra-black, and electronic switchingmeans operative in response to an electrical signal for normally connecting the input of said multiplier to said rst carrier wave generating means, and for connecting the input of said multiplier to said second carrier quency-modulating said carrier wave with said.

video signals, and means responsive to said syn- ,chronizi'ng' signals for shiftingl said wave-to a different predetermined frequency while substantially simultaneously establishing the amplitude of said wave at a level approximating said overf load current level.

wave generating means during the synchronizing-signal intervals.

15. In a television transmitter having apparacarrier wave by changing both the amplitude and frequency thereof. driving said transmitter at approximately its maximum continuous output level during the video intervals, and driving said transmitter at approximately its instantaneous peak output level during the synchronizing intervals.v

16. In a television transmitter having apparatus for generating a carrier .wave of controllable amplitude and frequency,I the method of alternately transmitting video and synchronizing intelligence, which comprises imparting the video intelligence to the carrier wave by frequency modulating said wave. imparting the synchronizing intelligence to the carrier wave by changing both the amplitude and frequency thereof, driving said transmitter at approximately its maximum continuous output level during the video intervals. and driving said transmitter at approximaiely its instantaneous peak output level during the synchronizing intervals.

17. In a system for generating a composite modmstednmier-wsvs television4 signal containing both video and synchronizing inteingence, a source of signals of video frequency, a source of synchronizing signals. means for generating a carrier wave for the transmission ofsaid videc signals. means for frequency-modulating said'carrier wave with said video signals during predetermined video intervals. means responsive to said synchronizing signals foramplitude-modulating said carrier wave during predetermined synchronizing intervals, and means responsive to said synchronizing signals for shifting said wave to a different predetermined frequency during the intervals.

, v18. In `a system for generating a composite modulated-wave television Signal containing both video and synchronizing intelligence. a transmit- 1 ter having a and lli-91M'- 19. In a carrier-wave television transmltten' means for generating a carrier wave, said generated wave having predetermined characteristics of amplitude and frequency, a sourceof video signals, a source of synchronizing signals, means for imparting the video intelligence to said carrier wave during predetermined video intervals, said means including apparatus for varying at least one of the said characteristics of said carrier wave, and means for imparting the synchronizing intelligence to said carrier wave during predetermined synchronizing intervals, said -lastnamed means including apparatus for changing both the amplitude and frequency of said carrier wave.-

' 20. A carrier-wave television system as claimed in claim 19, characterized in that said transmit-- shifting means associated with said generating means and adapted to shift the frequency 'of the -carrier wave from one predetermined value to. Aanother predetermined value, an amplitude modulator coupled to said lgenerating means, means for supplying video, blanking and synchronizing signals to said modulator, thereby to amplitudemodulate the carrier wave with the video, blanking and synchronizing signals during different intervals. and means for supplying synchronizing signals to said frequency shifting means. thereby to shift the frequency f the carrier wave during the nizing intervals.

22. In a television transmitter, a video carrier wave generator, a s carrier wave ynchronizing generator, the frequencies of the two carriers dif- 'giammai-minne tering substantially from one another, a common optput circuit for said generators, switching' means for selectively .connecting said generators to` said output circuit,'an amplitude modulator .coupled to said output circuit. means for supplyingvidedblanking andsynchronilingsignalsto said modulator. and for switchingmeanssoastoconnectsaidvideocarrier generator to said output circuit during the video intervals and to. connect saidsynehronix- Vingcarriergeneratortosaidoutput circuit durinternls. l

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