US2435736A

US2435736A - Frequency modulated picture receiver

Info

Publication number: US2435736A
Application number: US378715A
Authority: US
Inventors: Carnahan Chalon Wesley
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1941-02-13
Filing date: 1941-02-13
Publication date: 1948-02-10
Anticipated expiration: 1965-02-10

Description

F b. 10, 1948. CARNAHAN V 2,435,736

FREQUENCY MODULATED PICTURE RECEIVER Filed Feb. 13, 1941 3 Sheets-Sheet 1 IO 1| I I9 20 2: 22 /23 24' PICKUP VIDEO 7 MIXING FREQ-MOD. FREQ. EXCITER PWR- DEVICE AMPL. AMPL. OSC. MULTIPLIER M VERT. 8. HOR- VERT. SYNC. HOR. SYNC. BLANKING IMPULSE IMPULSE IMPULSE GEN. GEN. GEN.

\ F SYNCHRONIZING 7 Go GENERATOR V IllHHI-IIIIHIIHIIIII II II FIG.5

'DISCRIM.

OUTPUT INVENTOR MOD. osc. FREQ.

F G .7 C. 14 5525) CARA/AVIAN ATTQRNEY Feb. 10, 1948.

I c', w. CARNAHAN 2,435,736

FREQUENCY MODULATED PICTURE RECEIVER Filed Feb. 15, 1941 3 Sheets-Sheet 2 I 38 39 /4| 34 4.3 osc,& F.M. VIDEO MIxER I DISCR. AMP. I I 49 5| I H R- HOR. V SYNC! DET. SWEEP FIG. 8 I

VERT. VERT. SYNC. DET. SWEEP PICKUP VIDEO MIXING FREQ.MOD. FREQ. Exc PWR.

DEVICE AMPL. AMPt. 05c. MULT. *AMP.

, M l i j VERT. & HOR. VERT. SYNC. HOR. SYNC. AMP.

BLANKING IMPULSE IMPULSE MOD, IMPULSE GEN. GEN. GEN. I

I W I V F H S'YNCHRONIZING l G GENERATOR I 76 77 I /7a /79 I/sl 80 05c. 8. EM. VIDEO MIXER DISCR. AMF? Ti) as I as I .3 VERT. VERT. SYNC.DET. swEEP a I HOR. HOR. I SYNC. DET. SWEEP INVENTOR I I CWESLEYCAR/VAHAN ATTORNEY Feb. 10, 1948. c. w. CARNAHAN 2,435,736

FREQUENCY MODULATED ncwunnmcmvgn Filed Feb. 13, 1941 3 Sheets-Sheet 3 I us SYN. DET. ami a SWEEP F I I n4 VER'E SWEEP F|cfl3 INVENTOR CWE'SLEYCAR/VAHAN IBYWZQW ATTORNEY Patented Feb. 10, 1948 UNlTED STATES PATENT OFFICE FREQUENCY MODULATED PICTURE RECEIV Chalon Wesley Carnahan, Oak Park, Ill.. assignmto Zenith Radio Corporation, Chicago, 111., a corporation of Illinois ApplicationFebruary 13, 1941, Serial No. 378,715

4 Claims. (Cl. 178-73) rier wave of appreciable intensity, such as required for transmitting stations with power ratings in the order of kilowatts, is accompanied with xtreme difliculties. In order to obtain reasonable efiiciency, it is conventional transmitter practice to employ non-linear amplifiers, socalled class C amplifiers, for power amplification of the carrier waves. Since no non-linear amplification of the carrier wave may occur after modulation thereof, it is necessary to modulate the carrier wave at a high power level. Such a modulation requires considerable modulating power, and since the modulating picture signals extend over a frequency band ranging from 60 cycles to about 4 megacycles, it is extremely diflicult to obtain modulating signals of sufficient power without encountering serious signal distortion. For this reason it has been proposed to efiect carrier wave modulation at a low power level. In that case the power amplifiers for increasing the power of the modulatel carrier waves must have linear frequency response over a frequency band of about 4 megacycles, which is difficult to obtain and inherently makes for very low efliciency of the transmitter.

It is an object of the present invention, therefore, to provide a television transmitting system and a receiving system for cooperation therewith, which overcomes the above defects of conventional apparatus by employing frequency modulation, of carrier waves in accordance with the picture signals to be transmitted.

The advantages of such frequency modulation systems will become evident from the following comparison. Modulation can be effected at a low power level of the signal carrier wave, therefore requiring no appreciable modulating power. The final power amplifier of a transmitter according to the present invention, can be operated for highest power output, that is, so-called class C operation, at the maximum plate dissipation of the amplifier tubes, since no attention need be paid to linearity in the frequency modulation amplifiers. This should result inapproximately four times the average power output of a conventional grid modulated amplitude modulation amplifier stage and considerably greater efficiency. Hence, a transmitter according to the present invention, can produce approximately four times the average power of a conventional transmitter using substantially the-same complement of tubes. A receiving antenna located at a, certain distance from the transmitter according to the present invention, therefore, receives four times the average power than from a conventional transmitter at the same distance. Hence,

the average carrier wave'voltage amplitude at the receiver is doubledandso isthe signal to noise ratio, which considerably enhances the quality of th reproduced television picture.

In order to synchronize the operation of transmitter and receiver, it is necessary to transmit synchronizing signals in addition to the picture signals. It is conventional to transmit both types of signals by way of the same carrier waves. It will become evident from the following that considerable advantages are also obtained if the frequency of the signal carrier waves is also modulated by the synchronizing signals.

In order to appreciate the objects and advantages of the present invention, it is necessary to refer to the typical signals ordinarily employed in television at the present time and for this purpose reference is now made to Figs. 15 and 16 which graphically depict such typical signals and to Fig. 10 which shows diagrammatically in full lines and in dotted lines typical successive scanning patterns for interlaced scanning.

In the usual television system employed at the 7 present time, a single amplitude modulated carrier is transmitted, having an envelope of the general form shown in Figs. 15 and 16. In these figures the video signals are I 02. The blanking pulses between successive video signals are indicated at IIII and during these blanking pulses ocour the horizontal synchronizing pulses :03. It

is preferred to maintain the synchronizing pulses I03 at all times, that is, during the video trans-. mission and also during the vertical return blank ing period.

The vertical return occurs during a blanking pulse I06, equivalent in length to several scan-- ning lines. The vertical synchronizing pulse IN or IOIa occurs during the long blanking pulse,

llgence by which background control may be obtained at the receiver.

The composite signal from the mixing amplifier I9 is supplied to a transmission line 20 capable of transmitting the necessary frequency band width. This transmission line extends from the pickup point, usually a studio, to the broadcast transmitter. The output of the transmission line 20 is, or may be, further amplified, if desired, and applied to a frequency modulator-oscillator 2| which provides a carrier in the usual manner to be phase or frequency modulated by the composite signal received from the transmission line. The output of the modulator-oscillator 2| is supplied to a frequency multiplier 22, which magnifies the phase angle change so as to produce the'desired frequency deviation of the carrier from its nominal or rest frequency value. The frequency modulated carrier produced in the frequency multiplier 22 is fed to an exciter, or driving stage 23, which drives the radio frequency power amplifier 24. The output from this latter amplifier is transmitted to a radiating system indicated generally by the reference character 25. The output of the frequency multiplier 22 may be used, if desired, to energize a monitor system at the transmitting station. The monitor system may comprise a television receiver similar to that shown in Figure 8 of the drawings.

Figures 2 to 5 of the drawings show thewave form of the signal at each point of the system for an assumed horizontal scanning line, which increases in brightness from left to right as viewed by the pickup device IO when it is focused on the field of view. In Figure 2 the output of the pickup device is shown as an increasing potential 2B and is reproduced at higher potentials in a shorter time as the scanning means, which may be an electron beam, retraces the field of view from right to left. This retrace signal 21 is undesirable and, therefore, it is removed or blotted out as indicated at 28 by an impulse from the blanking impulse generator 11, as shown by Figure 3 of the drawings.

The blanking out of the signal during the retrace period may be accomplished by any one of a number of ways known to the art. One such method is to mix the signal of Figure 2 with a large impulse occurring during the retrace period, having an amplitude in the black direction appreciably greater than the maximum signal from the pickup device during the retrace. This combined signal is then fed to an amplifying stage the grid of which is so biased that all of the signal variation during the retrace period lies below the cut off of the amplifier.

The signal level 29 obtained when the blanking impulse is applied may be selected to correspond to the darkest portion or portions of the picture, which is referred to as the black level. Numerous ways of obtaining the black value are known to the art and any of these may be employed. The pedestal level may be adjusted at the transmitter to any point for economical use of the available modulating range. It is preferably as close as possible to the maximum picture signal peaks.

Figure 4 of the drawings shows the addition of the horizontal synchronizing impulses 30, the amplitude of which in the black direction is greater than the value of the potential for the black level 29 of the picture, and which, therefore, do not affect the picture reproduced at the receiver. The horizontal synchronizing impulses 30 cause the scanning means to traverse the field shown in the upper portion of the figure.

of view from right to left in preparation for the derivation of signal 26 from the next following scanning line. The series of signals shown by Figure 4 are repeated until the total area of the field has been scanned once in alternate lines.

Figure 5 of the drawings shows the signal wave form for a complete double scanning of the field. Following present conventional practice, this drawing represents 441 lines, each complete scanning involving 441 horizontal pulses 30. Figure 5 does not disclose the signal which occurs during the main part of the picture scanning, but it will be understood that such full disclosure would merely embody the multiplication of the left-hand and right-hand parts of the figure. The principal purpose of this figure is to show in a comparative relation the form of the signal at two successive vertical return periods.

The upper part of Figure 5 begins with the signals 26 representing scanning lines on the frame separated by blanking signals 29 and horizontal return signals 30.

When the last line is scanned, a prolonged blanking signal 29' occurs for the'vertical return. During this blanking signal 29', the horizontal return pulses 3!] occur at regular rate. It will be understood that the blanking signal 29' causes the signal trace to assume the black level indicated by the reference character 29 and to maintain this level in the absence of additional pulses throughout a period which would be cccupied by the transmission of several horizontal pulses 39. As stated before, the horizontal synchronization pulses 30 are continued at their regular intervals to maintain synchronism of the horizontal oscillator which might otherwise fall out of step during the period of time when the vertical retrace to the top of the field of view is effected.

In order to return the scanning beam to the starting position on the frame for scanning the next successive field, a vertical synchronizing pulse 32 is fed to the mixing amplifier I9 from the vertical synchronizing pulse generator Hi. This vertical synchronizing pulse 32 is preferably less than one half of a line in duration. The horizontal synchronizing pulses 30 are of values beyond the black level 29 of the picture. The vertical synchronizing pulse 32 is still further beyond the level 29, that is, it is greater in magnitude than the pulses 30-, as clearly shown in Fig. 5. The difference in magnitude between the

pulses

30 and 32 provides a basis for separation of these pulses at the receiver in a novel manner.

The effect of the pulse 32 causes energization of the vertical control plates or coils of the reproducer at the proper time with respect to the horizontal synchronization pulses 30 which cause energization of the horizontal control plates or coils of the reproducer, so that the horizontal scanning of the second field will be initiated to provide correct line relationship for interlace.

The pulse 32 is applied at regular intervals. Thus, for pictures in which the scanning occurs sixty times a second, the pulse 32 is supplied each sixtieth of a second. The two portions of Fig. 5 are related so that the form shown below represents the condition of the signal onesixtieth of a second after the signal has the form In other words, if any point on the upper portion is taken, that point shows thecondition of the signal at a particular time. Then the point on the lower portion of the figure immediately below 7 thatpointshows the condition of the signal onesixt-iethof asecond after that time.

Consequently, the two pulses 32 21min vertical alignment. However, in the two portions of the figure, owing to the odd number of lines in a complete doublescanning, the pulses 3d are displacedrelatively half a'line in the two portions ofthe: figure. Therefore,- each of the pulses 32 in-the-twoportions of the figure and'in the corresponding vertical return-periods has a different relation-to the: horizontal pulses 3B. Owing, however, to my. improved method,- of transmission, the horizontal: pulses 30. have no effect upon the result -.of the vertical-pulses 32 and consequently I am able toavoid the necessity of the complex systemlheretofore necessary for the introduction ofequalizing. pulses.

It is to be notedthat the video signal is supplied: to the frequency modulated oscillator at a point-of the transmitter where the power level is stillwlow. -.Consequently,i1;.:ls not necessary to build up the video signal-to a degree of power of thesame orderas the output signal as has heretofore been the case withv amplitude modulated television-transmitters.

The. frequencyexcursions of the transmitter carrier resultingfrom the amplitude values of the synchronization impulses areshown in the formof a curve -36 on Figure 7,.in which the abscissae are values. of modulated oscillator frequencies and the ordinates are the amplitudes of the 'mixing-amplifier output. Point B on the curve representssthe.minimum visual effect and correspondsto the signal level Hand 29 on Figures 3,. 4.. and 5 of the drawings. -Point- W represents themaximum frequency excursion which corresponds-to the. maximum brightnessv obtainable in'the usual types of reproducer. This will represent maximum brilliance of the fluorescent screen or coating-in the electron ray reproducing tube 35. PointsH and-V represent the frequency excursions caused-by the horizontal and

vertical synchronizing impulses

30 and 32 respectively.

Referring to Figure 8=of thedrawings, the receiver may comprise an antenna system 31, an

oscillator and mixer stage 33, and an intermediate frequency amplifying system 39, following conventional superheterodyne practice. The .amplitude variations of the composite intermediate frequency signals are removed in the limiter stage 4|, in the manner usual in-phase or frequency modulated receiving systems. The limiter 4| may be of any conventional type such, for example, as the type disclosed in Frequency modulation," by Ridenonpage 53, or the type disclosed in the copending. application of the present inventor, C. Wesley Carnahan, Serial No. 371,606, filedDecember 26, 1940,.now Patent No. 2,323,880, dated July 6, 1943.

, The composite signal as it comes from the limiter 4| consists of the carrier varying in frepicture tube is provided with the usual horizontal and vertical deflecting coils or. plates. The hurlzontal and vertical deflectingcoils or plates are energized by current or voltage-wavesobsawtooth form, as conventional.

Referring to Figure 9,.adesirable characteristic curve of a discriminator is shown, suitable-for use as the discriminator in the receiverof Figure/8. A point B on this curve may be selected asthe black value corresponding to the black value-29 of Figures 3 to 5 sothat amplitude responses having this value-will produce substantially no'visible luminescence on the fluorescent screenoof the picture tube 35. The synchronizing impulses which attain amplitude-levels having-values-H and V indicated on the curve of Figure 7 havev no effect on the reproduced pi'cture,-as-they have values which are blacker than. black.

The intermediate frequency signal,v derived-in the oscillator mixer stage of'atyplcalspicture receiver from the wave radiated from the antenna system 25, is represented by the frequency band diagram of Figure 6, which shows thetotal-band width through which the frequency of thesintermediate frequency signal varies during the transmission'of the video and synchronizing signals. The frequency variation, or modulation, .of the intermediate frequency signal as 'thejpictureis scanned is confined .to the frequency: range '33. It will be noted that frequency excursionsr beyond the frequency valuevBx will haveno efiectron the quencyinaccordance with the video signal and appearance of the picture, and, therefore; the fre'quencyspectrum beyond the point-1B is' available for: controlling receiver'functions. The horizontalsynchronizationimpulses '30 for controlling the horizontal scanning oscillator: are translated into frequency variations 'in the narrow-band labeledtfif on Figure 6 of the drawings,'these variations being displaced in frequencyfroxn the video band 33 so. as toprovide ready separation therefrom on a' frequency discrimination basis in a manner to be explained in :connectionwith the operation of the receiver. The vertical synchronizing impulses 32 are translated into frequency variations and occupya narrow frequency band'32f which is removed in frequency value from the video signal band 33' and thenarrow band 38 occupied by horizontal'impulses. This displacement in frequency of the horizontal and vertical components of the frequency modulated intermediate signal is obtained by reason of'the larger amplitude of the horizontal andiverti'cal components as supplied by the respective signal generators. The separation and utilization of the set of synchronization signals is done on a frequency discrimination basis and thetotal composite signal demodulated by the discriminator 34 is used to excite the picture tube 35 of the'receiver (Figure 8), the efiects of the set of synchronizing signals being eliminated by biasing the tube in the usual manner to produce no visual response when signals are impressed on the tube which are outside of the amplitude range corresponding 't'o-a black area of the original subject matter at the transmitter.

Intermediate frequency excursions produced by frequency modulation of the transmitter carrier with the horizontal and vertical synchronizing impulses are supplied from the limiter 4| to'the horizontal and vertical impulse separating systems 36 and ll. The separatingsystem 46 comprises a circuit 48, whichis tuned to the interrelationship named by simple means.

amplitude demodulator or detector 4'9. The tuned circuit 48'acts as a filter in'the well-known manner for frequencies other than the intermediate frequency value corresponding to the frequency produced by the horizontal synchronizing impulse. The horizontal synchronizing impulse is obtained by demodulation in the amplitude detector 49 and is applied to control the horizontal sweep generator 5|, which supplies the horizontal deflecting coils or plates of the picture tube 35 with waves having a wave form to cause horizontal scanning at uniform velocity.

The separating system 41 comprises a circuit 52, which is tuned to the frequency value attained when the carrier is modulated with the vertical synchronizing impulse. The tuned circuit 52 causes the intermediate frequency signal produced by the vertical synchronizing signal to be applied to the amplitude detector 53. The vertical synchronizing impulses obtained from the detector 53 are utilized to control the vertical sweep generator 54, which supplies the vertical deflecting coils or deflecting plates of the, picture tube 35 with undulations of a wave form to accomplish scanning in the manner set forth above.

A brief description of the operation of the entire system just described will now be given. Since the invention is especially applicable to a television system in which the entire frame, or field of view, is scanned in alternate lines, a system which is known as interlaced scanning, the following description of the operation of the system will deal with interlaced scanning.

In the conventional television system employinginterlaced scanning, one necessary relation between the several synchronizing signals used to actuate the deflecting means for the scanner is that the ratio between the relatively high frequency of one synchronizing signal and the relatively' low frequency of the other synchronizing signal must be an integer plus a fraction such as one-half, one-third, one-fourth or the like. While the invention is not limited to interlaced scanning, it provides means for readily and clearly separating synchronizing impulses having the For a clearer understanding of the interlaced scanning process and the co-relation between the vertical and horizontal synchronizing signals reference may be had to Figure of the drawings. The first field is scanned beginning with point A at or adjacent to the upper left hand corner of the total field of view, the first horizontal solid line sloping diagonally downward to the right, tracing a resultant of the horizontal and vertical scanning oscillator outputs. At B the video signal is blanked and the beam or other scanning means is returned to the point D. The wave form of the vertical oscillator decreases from its maximum value at such a rate that the second solid scanning line is displaced a distance of two lines on the frame. The first field ends at a point E at or adjacent to the center of the lower edge of the frame from which point it is returned to the point F. From F, it follows the dash line path to the point G, thereby producing the interlaced pattern. If the scanning beam moves substantially horizontally, the horizontal scanning frequency will be considerably higher than the vertical scanning frequency. For good definition, simple interlaced scanning may be employed in which two fields or interlaced sets of lines constitute a frame. There may be 441 lines to a frame and 30 frames per second. This requires,

for interlaced scanning, a vertical scanning wave of 60 pulses per second and a horizontal scanning wave of 13,230 pulses per second. For purposes of illustration the invention will be described with respect to a 441 line system. It will be un-, derstood, however, that the invention is not limited to the particular values givenabove, but is applicable to other systems in which the entire area of the field oi viewlis scanned once in substantially contacting lines.

The pick-up device in is provided in the usual manner with horizontal deflecting means, such as deflecting plates or coils and vertical defiection means, such as vertical deflecting plates or coils, for the scanning ray. These deflecting means are excited from thesynchronizing generator 18. The horizontal deflecting means are excited by an undulating voltage, or current, having what is usually known as a sawtooth wave form. By reason of this special wave form, the

scanning beam will be deflected at a uniform,

speed from one side of the field 01 view to the other. As the scanning waveattains its maximum value and decreases quickly to zero, the scanning beam will return to its starting point on the field of view. This return of the scanning beam, as pointed out above, generates a signal in the pick-up device at a somewhat higher'voltage because of the increased speed of scanning, but this retrace signal is blotted out in the video an;- plifier II by an impulse from the blankingimpulse generator ll,

As the scanning beam sweeps back and forth under control of the horizontal impulse generator l4, it is displaced at a uniform rate by another undulating current or voltage which is applied to the vertical deflecting coils, orplates, which are in space quadrature with the horizontal deflecting coils, or plates. The horizontal and vertical control impulses from the impulse generators I4 and I6 are mixed with the video signal in the amplifier I9 to form a composite wave, which is usedin the modulator 2| to vary the phase or frequency of the carrier by phase or frequency modulation. The instantaneous frequency of the carrier is multiplied in the frequency multiplier 22 and is radiated from the antenna system 25 after further amplification in the usual manner.

The foregoing description in connection with Figure 8 of the drawings contains a description of the operation of the receiver which need not be repeated here. It was pointed out therein that the signal fed to the frequency detector 35 is applied to the picture tube 35 but only the Signals derived from the frequency band 33 in Figure 6 are eilective to operate the picture tube since frequencies lying outside of this band-produce demodulated signalsof an amplitude which is too great, as explained above, to effect the picture tube, since it is rendered insensitive to signals. having amplitudes beyond the picture signal range.

The elimination of the necessity for including equalizing impulses at twice the horizontal line frequency is an important feature of the invention and will be explained more in detail at this point. The equalizing impulses at twice the line frequency are added in known picture transmission systems in the region of the vertical synchronizing impulses to reduce the inequality of the phase relationship, between horizontal and vertical synchronizing impulses for alternate scanning fields when separation of the horizontal ii and vertical impulses is accomplished by wave form discrimination methods. The vertical synchronizing impulses f or odd and even fields occur at "diflerent times with respect to "the last horizontal pulsepreceding the vertical pulse; 'Equalizingpuis s are utilized to correct for 'diSsyinmetry of the horizontal synchronizing impulses with respect to the vertical synchronizing ,impulses. When the conventional methods of synchronizing signal separation are used, the horizontal impulses do produce a transient response of the separating circuit, thus affecting the time of occurrence of the point of the vertical synhro'riizing wave to whieh'the vertical synchronizer oscillator control responds. In the absence of equalizing pulses, the efict is not uniform for odd and even fields. To eliminate this lack of uniformity, it is necessary in prior art systems to substitute for horizontal pulses immediately before "the vertical pulse, a series of equalizing pulses of twice the frequency of the horizontal pulses, alternate equalizing..pulses coinciding in time with the horizontal synchronizing pulses.

There is, therefore, an impulse which is equivalent in'value to the equalizing pulse always occur'ringat the same time interval before a vertical pulse. The end result of this process in knownpicture systems is a complex synchronization signal which, because of the variety of interlocking circuits required 'to produce it, adds greatly to the complexity of the transmitter equipment. A complete separation of vertical and horizontal synchronizing signals on a waveform basis is impossible without the inclusion of equalizing impulses at the transmitter in known television systems.

The system of Figures 1 to 9, whichembodies the present inventions, eliminates the necessity for adding equalization impulses, since complete separation of both signals is obtained on an amplitude and frequency basis and it is not necessary to compensate for residual vertical signals in the horizontal signal separationland utilization circuits. Hence perfect synchronization can be obtainedby transmittingtwo series of simple signals only, namely, ,a series. of 'line synchronizing signals occurring at regular intervals of line scannin'gfrequency only, and a series o'ffield synchronizing signals occurring at regular intervals of 'field scanning frequency only.v Nor is it necessary to separate the picture signals from the synchronizing signals before the former are appl ed to the picture 'rep'roducingitube 35.

In Figure 6, assuming 321 as thirteen and "onehalf m'egacycles at the edge 'ofthe intermediate frequency band separated from 3llfby approximately one-half megacycle. then with a circuit Q of '100f0r the'resonant circuits and "52, the ratio of unwanted to wanted signal is about .93 to 7 M13 per cent inthese circuits. This figure is arrived at by assuming atypical case and assigning arbitrary values to "the chart of'FigureG.

Assuming that the total frequency band in the intermediate frequency amplifier 39extends from 8 megacycles to -14 megacycles and that the sound accompaniment utilizes a'necessary band'at'825 megacycle's, the vertical synchronizing signal may lie at Imegacyclesabove this value or at 13.5 megacycles: "The horizontalv synchronizing signal may be selected to lie at one-halfimegacycle below this or at'13.0megacycles. The ratio of "the response of the tuned circuit 52 at 13.0 mega'cyclesitofitheresponse at 13.5 megacycles 'is' the voltage across the inductance at 13.0 'megacycles divided Tby the 'voltage' across 'the 'ln"-' ductance at 13.5 megacycles which is equal to V 1 we) The above written formula is=to be found in Termans text, Radio Engineering.

In this formula, Q is the reactanceto resistance ratio of the circuit 52- of Fig. '8 and 'isirassigned a numerical valueof 100 in the case assumed-for the purpose-of-illustration; V is lthe ratio of -th'e frequency of the horizontal to the 'frequencyof the verticalsynchronizingpulses, and in-the assumed case has a numerical value or ;965. W-hen the numerical values giventaresubstituted in:the

formula written above the-numerical result-is 0:13. This-means that 13 per cent of the total signal applied tothe wdetector- '53 is an undesired portion of the horizontal synchronizing pulse which is not removed by the tuned circuit- 52'. This is likewise .true .afor the circuit 48 and: the dectector 49. The unwantedv signals. appearing in the outputs of the detectors 249 and 53 may be readily removedby biasing ithesesdeteetors so that thirteen percent of theramplitude'ofrtheapplied signals are not detected.

Equalizing "pulses, as explained. above;.*are-'used in prior. art televisionssystems :to eliminate the eflect of the transient responsewhichfollowscess sation of a horizontal synchronizing :pulse preceding a vertical synchronizing pulsenthi's tramsient response having a .difierent' effect ion "the wave form of the vertical-impulse for odd:and even lines, In thesystem of the present invention, the transient response following cessation of alhorizon-tal pulse .isso small as to ibe-negeligible for reasons now to'be'demonstrated.

The building. up of sinusoidal l currents 'in resonant circuits has .been treated by -.a, number :of writers. One such=treatment iorthe single mesh R, L, C circuit is .found inGuillemin, fCommunication Networks, vol. 1, pp, 118-120. There. it is shown that when a sinusoidal voltage is suddenly impressed on. a resonant circuit containing resistance R, inductance L, .and'capacityC, the envelope of the sinusoidal current flowingdn the circuit builds up in accordance -with .the

quantity and the envelopemfactorv for the building up period becomes In the present case'the building up of theresponse to a suddenly applied sinusoidal synchronizingpulseis not pertinent but rather the rate of subsidence of the response when .the sinusoidal synchronizing signal 'is removed. In this case. the envelope of the response decreases in accordance with the quantity where t is now measured from the end of the synchronizing impulse.

Substituting in this expression, the frequency at resonance, which is 13.5 megacycles, and the time, which is the reciprocalof twice the hori-v zontal scanning impulse frequency of the time interval corresponding to one-half of a line, the result is 10- which is so small compared with unity as to have a negligible effect on the succeeding vertical impulse. If a biased detector is used, so that 13% amplitude of the horizontal impulses is not detected, a clean separation of the vertical pulse is accomplished without the necessity of equalizing pulses. The vertical synchronizing signal may then be of short time duration as shown on Figure of the drawings. Because of its short duration, it need not be of the serrated type to insure continued horizontal synchronism. It is seen, therefore, that perfect synchronization can be obtained with the system according to the present invention without the use of equalizing pulses and serrated frame synchronizing pulses of complicated waveform.

Figure 12 of the drawings discloses a modified receiver for reproducing visual representations of pictures or objects from signals transmitted by a transmitter modified as shown in Figure 11 of the drawings. Referring to Figure 11, it will be seen that the transmitter disclosed thereby is similar to the transmitter of Figure 1 with the addition of an amplitude modulator 58 which amplitude modulates the carrier after it has been frequency modulated in the frequency modulated oscillator 59 by the output of the mixing amplifier which consists of the composite television signal including picture or video signal components and synchronizing signal components. The frequency modulation process has already been described in connection with Figure 1 of the drawings and need not be repeated here. Figure 13 of the drawings discloses the general nature of the wave radiated from the antenna system 62 without showing the frequency variations to any particular scale. The portion 63 of the wave carrying the intelligence or video signal is of substantially constant amplitude but varies in frequency in accordance with the amplitude variations of the video signal from the pick-up device 64. Upon occurrence of blanking in the video amplifier 66, the carrier resumes and maintains its unmodulated frequency until the vertical and horizontal synchronizing impulses are applied to the mixing amplifier 6|. The horizontal and vertical pulses are preferabl of the form shown in Figure 5 of the drawings whereby the frequency excursion of the carrier for the horizontal and vertical impulses is made to difier by some frequency value, for example 0.5 m. 0., so that the signals may be received and translated into visual representations by a receiver embodying features described in connection with Figure 8 of the drawings. Horizontal synchronizing impulses from the horizontal synchronizing impulse generator 68, in addition to being supplied to the mixing amplifier 6!, are also supplied to the amplitude modulator 58 mentioned above. The modulator 58 serves ,to amplitude modulate the carrier in accordance with the horizontal synchronization signals as indicated by reference character ll The percentage of modulation is not critical and need be only sufficient to enable detection by an amplitude demodulator or detector of the usual type. As its effect is to be eliminated by a limiter in the receiver the percentage modulation should not be so great as to necessitate excessive limiting.

Figure 12 of the drawings discloses a modified form of receiver embodying features of the invention which enable it to receive signals from the transmitter disclosed in Figure 11 of the drawings. Referring to this figure, I3 is an antenna system which feeds oscillator-mixer stage 16. The oscillator-mixer stage generates an intermediate frequency which is amplified in the intermediate frequency amplifier TI. The amplitude variations of the composite intermediate frequency signals including the position H are removed in the limiter stage 78 in the manner usual in phase or frequency modulated receiving systems. The limiter 18 may, like the limiter 4| of Figure 8, be of any conventional type. The frequency modulation discriminator or wave detector l9 converts the frequency variations of the carrier to amplitude variations suitable for operating the picture tube 80. A video signal amplifier 8| may be used if desirable or necessary to amplify the video signals. The vertical and horizontal synchronization pulses which are present in the output of the wave detector 19 do not affect the picture tube for reasons pointed above in connection with Figure 8 of the drawings.

The output of the intermediate frequency amplifier 11 is fed to the amplitude demodulator or detector 82 of the usual type which provides horizontal pulses from the envelope of the portion II of the carrier for controlling the horizontal scanning signal generator 82. The generator 83 functions in the same manner as the generator 5| in Figure 8 of the drawings.

' Pulses for controlling the vertical scanning signal generator are derived from the frequency modulated carrier by a separating system 84 which is similar in all respects to the system 41 of Figure 8 and comprises a circuit 85, which is tuned to the frequency of the portion 63 of the carrier. The tuned circuit 85 causes the carrier frequency produced by the vertical synchronizing impulse to be applied to the amplitude detector 86. The output of the detector 86 is applied to the vertical scanning wave generator 88 which corresponds in function to the Wave generator 54 of Figure 12 of the drawings. There is no interference from the horizontal signal in the vertical impulse detector, as it has been completely removed by the limiter 18.

Signals radiated by the transmitter of Figure 11 may be received by a receive-r embodying the features of the receiver of Figure 8 of the drawings when the frequencies representing the horizontal and vertical pulses in the intermediate frequency amplifier differ in frequency. The cirgrating circuit.

thistway; it is advantageous :to provide horizontal and vertical. synchronizing. pulses..ofzthgsame magnitude or. amplitude an'd to :depend :uporndifs ferentiating. between. vertical. and horizontal pulses-by difference in waveform. To :docthis successfully,-iti is advantageous ;to employ :aawave formrof the samegeneral :typeas islnormally employed; with amplitude: irequencytelevision systems. This wave :form is shownin Figs. .15 and 16 and includes the equalizingrpulses m9 and Ill9a.

Figure 14 of .the xlrawings :discloses .:a rmodified form of receiver-for receiving va carrier .wave whose frequency is modulated in; accordance .with picture signals and. whose amplitude I is modulated in. accordance with synchronizing signals; In contrast \withthe systemsdescrib-ed above,..th'e synchronizing. signals: must include. equalizing pulses. .since discrimination. therebetween must nowbe efiected on a wave-form basis. The transmitter which radiates signals suitable. for reception by the receiver of Figure-.314 may :be similar to the transmitter. ofvFigure 11 with the addition, however; of zazmeans to.=amplitude modulate the carrier. in accordanceiwith thevertical synchronizing pulses.- Since it 'is-undesirable to frequency-modulate the carrier .during the synchronizing pulses, the. connectionsbetween'the. horizontal synchronizing pulse-gen.- erator and the ;mixing amplifier and. between the vertical synchronizing pulse generator-and the mixing amplifier, are preferably omitted.

Figures 15and 16 show the;present standard television signal. A brief description :of the standard :signal which modulates the amplitude of' the carrier the portions thereof. representing synchronizing and. equalizing 1.puls'es will be givenin the following paragraphs. The blanking pulses It)! occur'between thevideo signals H32 which correspond .tothevideo signals 26 of Figure 32 of :the drawings. The horizontal synchronizingimpulses H33 occur 'iiuringthe blanking periods. Figure :16 shows the .composite-signal which is derived when scanning of :an: even .field'of view is completed. Atblankingcpulse I06 occurs equivalent ;in: lengthtto several scanning lines 102. The vertical synchronizing pulse I61 occurs during the long blanking .pulse and serves to return-the beam oraother scanning .instrumentality "to the top oiithe' frame orfiel-d of view inpreparation for scanning :of. the next field. It willbe notedcfrom Figure 16 that the next'vertical scanning t.;pulse .Hl'la is displaced one-half line with.,respect to the precedinghorizontal pulse and, th.ere.f.ore,,- equalizing pulses 1 59 and 109a are substituted for the-horizontal pulses in Figurespl and 16 j'respectiv-ely. The

equalizing impulses-occur. at double the frequency of the horizontal. synchronizing pulses and. alternate equalizing impulses coincides in time with the horizontaldmpulses. It :will be seen from'an inspection. of Figures 215 and lb thata horizontal synchronizing. impulse 101- an equalizing 1 impulse; always occurs at :the same=time interval before-the beginningofla vertical synchronizingimpulse. The vertical synchronizing impulses therefore are always :of the :same wave form-:inthe:integratingcircuit of the synchronization detector and separator H0 shown on Figure '14 of thedra'wings. This piece-of. app ratus 1 I0 is of: thesame typelused infknown'television systems and includes a-demodulator or detector, a differentiation circuit and anrlinte- The-last :named .;cir.cuits. serve the purposeof. separating the; horizontal fromithe vertical synchronizing .impulses Dnfthe. :basissoi wave form. The impulses derived from the separating circuits are :applied...to the horizontal sweep generator H2 and the vertical sweep generator I M respectively. These sweepgenerators [I2 and H4 function in the usual manner-to supply currents or-voltages-to-the deflecting 'coils or plates ofthe picture tube I I6 suitable for controlling the scanning -;;beam of the I picture tube H6.

The-signal from the intermediate frequency amplifier, which includes frequency variations representing 'thevideo signal is, passed *to,the limiter which. efiectively removes. the amplitude changes representing the horizontal and vertical synchronizing impulses. The output:v of ',the limiter which contains, only frequency variations is demodulated in the discriminatorand is applied'to control the scanning beam of the picture tube H6 or other scanning instrumentality for reproducing the picture. g

While. the invention has beensdescribedgand explained in detail, in, connection with, several illustrative embodiments thereof,,.-it is to be understood that the invention maybe embodied in other forms and,itheref.ore,-the invention isnot limited ,exceptas indicated lay-the. terms-and scope of the ,appendedclaima- From the foregoing, it. will, be .seen that .there has been provided.v a newt-system of transmitting picture signals byway of a frequency-modulated signal. carrier. .wave and .it has been. showmthat such. transmission has considerable advantages over the-conventional transmission of amplitude modulated waves. Itv has alsobeenshOWnthat the. same signal carrier 'wavezcan alsosbepfrequency-modulated by synchronizing signals, whereby perfect synchronization can be-obtained bymeans-of signals which are much:simpler -than those required for. satisfactory synchronization when. conventional "carrier amplitude modulation is used. Finally, there haveibeenpprovided systems in. which the frequency -of "the signal carrier wave is modulated in accordance with the :picture signals while either-its frequency-or amplitude, or both, are modulated in accordance with the -1ine' or field synchronizing signals or both.

I claim:

1. Ida televisionzreceiver. adapted :to receive" a. signal jcarri-er Wave havinggits .frequency modulated withina-predetermined range in. accord-.- ance with video signals and having its frequency shifted to two different frequencies outsidesaid rangein accordance 'with'line and field synchronizing signals, in combination; means for receivingesa'id signal carrier wave, frequency discrimihating means for deriving video signals from said characteristic of line synchronizing signals; a

second frequency discriminating; ircuit substantially responsive to'a carrier-frequency outside of said range characteristic of said field synchronizing signals saidfirst and second circuits hav- 'ingapplied thereto video signal components and synchronizingsignal components, means for deriving'line synchronizing signals from said-first circuit, ..means for deriving field "synchronizing signals from; said second. circuit; :and means for controlling .zsaidl :pic.t.ure:reproduci-ng l means 1.111,

accordance with said line and field synchronizing signals.

2. In a television receiver adapted to receive a signal carrier wave having its frequency modulated within a predetermined range in accordance with video signals and having its frequency shifted to two different frequencies outside said range in accordance with line and field synchronizing signals, in combination, means for receiving said carrier wave, means for converting said wave into a second frequency-modulated carrier wave of intermediate frequency, frequency discriminating means for deriving video signals from said second carrier wave, picture reproducing means controlled by said video signals, a first frequency discriminating circuit substantially responsive only to the frequency of said second carrier wave characteristic of line synchronizing signals, a second frequency discriminating circuit substantially responsive only to the frequency of said second carrier wave characteristic of field synchronizing signals, means for applying said second carrier wave including video signal components and synchronizing signal components to said first and said second circuits, separate demodulating means coupled to said first and said second circuits for separately deriving therefrom line and field synchronizing signals, and means for controlling said picture reproducing means in accordance with said synchronizing signals.

3. In a television receiver adapted to receive a, signal carrier wave frequency modulated in accordance with video signals, having frequency components within a predetermined range of frequencies, and having its frequencies shifted to two different frequencies outside said range in accordance with line and field synchronizing signals, in combination, means for receiving said signal carrier wave, frequency discriminating means for deriving video'signals from said frequency modulated carrier wave, picture reproducing means controlled by said video signals, a first frequency discriminating circuit substantially responsive only to a carrier frequency characteristic of line synchronizing signals, a second frequency discriminating circuit substantially responsive to a carrier frequency characteristic of said field synchronizing signals, means for applying video signal components and synchronizing signal components to said first and second circuits, demodulating means for deriving line synchronizing signals from said first circuit, demodulating means for deriving field synchronizing signals from said second circuit, and means for controlling said picture reproducing means in accordance with said line and field synchronizing signals.

4. In a television receiver for the reception of a signal modulated carrier wave having its frequency modulated within a predetermined range in accordance with picture signals and having its 18 frequency shifted to two different frequencies outside said range in accordance with line and field synchronizing signals, the combination of a tuner for receiving such carrier wave, a limiter for limiting said received carrier wave to a predetermined amplitude, a frequency discriminator for deriving picture signals from said limited carrier wave, a picture reproducer controlled by said derived picture signals, a first frequency discriminating circuit substantially responsive only to a carrier frequency outside of said range characteristic of line synchronizing signals, a second frequency discriminating circuit substantially responsive only to a carrier frequency outside of said range characteristic of said field synchronizing signal, connections between said limiter and said first and second circuits for impressing thereon said limited carrier waves, and line and field sweep devices connected respectively with said first and second circuits and associated with said picture reproducer for controlling said picture reproducer in accordance with said line and field synchronizing signal.

CHALON WESLEY CARNAHAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,212,968 Finch Aug. 27, 1940 2,086,833 Walton July 13, 1937 2,186,898 DHumy Jan. 9, 1940 1,941,068 Armstrong Dec. 26, 1933 2,075,071 Usselmab Mar. 30, 1937 2,254,435 Loughren Sept. 2, 1941 2,296,919 Goldstine Sept. 29, 1942 2,342,943 Kell Feb. 29, 1944 1,548,895 Mertz Aug. 11, 1925 2,290,229 Finch July 21, 1942 2,289,157 Whitaker Jul 7, 1942 2,326,740 Artzt Aug. 17, 1943 2,290,517 Wilson July 21, 1942 2,084,700 Ogloblinsky June 22, 1937 2,293,233 Wheeler Aug. 18, 1942 FOREIGN PATENTS Number Country Date 433,295 Great Britain Aug. 6, 1935 OTHER REFERENCES "Radio Facsimile by Sub-Carrier Frequency Modulation, by R, E. Mathes and J, N. Whitaker. R. C. A. Review, October, 1939.

The Service Range of Frequency Modulation, by M. G. Crosby, R. C. A. Review, January, 1940.

Electronics for February 1940, pages 26 and 30-32, article by C. W. Carnahan entitled Frequency Modulation. Magazine published by McGraw-Hill Publishing Co., New York, N. Y.

R. C. A. Review, July 1940, vol. 5, No. 1 (pages 31 to 50).