US2254435A

US2254435A - Television broadcast system

Info

Publication number: US2254435A
Application number: US212691A
Authority: US
Inventors: Arthur V Loughren
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1938-06-09
Filing date: 1938-06-09
Publication date: 1941-09-02
Anticipated expiration: 1958-09-02
Also published as: FR855820A; CH217644A; FR51519E; BE434800A; GB546932A; US2293233A; GB540590A; FR869846A; GB529320A

Description

Sept. 2, 1941. A. v. LOUGHREN TELEVISION BROADCAST SYSTEM 3 Sheets-Sheet 1 Filed June 9, 1958 ,INVENTOR ARTHUR V. LOUGHREN W ATTORNEY Sept. 2, 1941. A. v. LOUGHREN TELEVISION BROADCAST SYSTEM `s sheets-sheet 2 Filed June 9, 1938 w. mommw m v.

.NGC

FIG.5.

FIGA.

Sept. 2, 1941. A. v. I OUGHREN TELEVI S ION BROADCAST SYS`TEM Filed June 9, 1958 s sheen-sheet 5 AR HUR V. LOUG REN ATTORNEY Patented Sept. 2, 1941 TELEVISION BROADCAST SYSTEM Arthur V. Loughren, Tuckahoe,r N. Y., assigner to Hazeltine Corporation, a corporation of Dela- Wale Application June 9, 193s, seria1No.21z,s91

(c1. 17a-6.a)

30 Claims.

'I'his invention relates to television systems and is particularly concerned with the provision of an improved type of television signal together with transmitting apparatus and receiving apparatus for genera'ting and utilizing such signal, respectively.

In accordance with present television practice, there is developed and transmitted a signal which comprises a carrier wave modulated during successive intervals or trace periods by videosignal components representative of the light and shade values of an image being transmitted.

During retrace intervals between the trace periods, the carrier wave is modulated by synchronizing impulses or components which correspond to initiations of successive lines and elds in the scanning of the image. At the receiver, a beam is so deected as to scan and illuminate a target in a series of eldsV or parallel lines, the video-signal components being utilized to control the intensity of the beam. The line-scanning and field-scanning synchronizing components are separated from the video-frequency components and from each other and are utilized to synchronize the operation of the receiver linescanning and field-scanning apparatus with similar apparatus utilized at the transmitter in developing the signal. The transmitted program or image is thereby reconstructed on the target of the receiver.

Various types of television signals, as Well as various types of scanning and synchronizing methods and apparatus, have been proposed. For example, in certain systems negative modulation is employed, that is, a decrease in carrier 35 amplitude during the trace periods, when the carrier is amplitude-modulated by the videosignal components, corresponds to an increase in amplitude corresponds to an increase in illumination. The synchronizing-modulation components of the signals used in these systems ordinarily have amplitude values outside the range of video-modulation components. Where negative modulation is employed, they are represented by increase in carrier amplitude while, where positive modulation is employed, they are represented by decreases in carrier amplitude, in either case usually having amplitude values corresponding to blacker-than-black.

In present practice With negative modulation, synchronizing signals represent an increase in transmitted'power of approximately 55% over illumination. In other systems, positive modulation is utilized in which an increase in carrier ulated or trace portions of the signal. This is undesirable not only because of the uneconomical requirement of increased power, but also due to the fact that interference presented by the signal of one program with another is a function of the peak value of the power radiation of the signal of the interfering program.

In 'television receiving systems noise components disturb both the video-signal and the synchronizing-signal compounds. In general, noise impulses are represented by increases in carrier amplitude so that, in positive modulation systems, the noise components appear in the image as white spots while, in negative modulation systems, 'they appear as black spots. The white spots are usually the more annoying to an observer so that, as far as the video signal is concerned, a negative modulation signal is less affected by noise. On the other hand, with negative modulation the synchronizing components are represented by increases in carrier amplitude and are thus most subject to noise eiects, while with positive modulation synchronizing components are represented oy a reduction of carrier amplitude, usually to zero, so that they are substantially less affected by noise. Therefore, as far as the synchronizing components are concerned, a positive modulation signal is less affected by noise.

It is an object of the present invention, therefore, to provide an improved television system utilizing a carrier wave havingl the desirable characteristics incident to negative modulation with respect to the video-signal components and including synchronizing components which are not appreciably affected by the effects of noise disturbances.

It is a further object of the invention to provide a system of the character described wherein the carrier wave is negatively modulated during trace periods by videcsignal components and is modulated by suitable synchronizing components during retrace scanning intervals without any appreciable increase in the transmitted power over that required for the video-modulation components of the wave.

In accordance with a feature of the present invention, there is provided a television transmitting system comprising means for developing a carrier wave, means for developing videofrequency components during trace periods, and means for developing synchronizing-signal ccmponents during retrace periods. The system also includes means for amplitude-modulating the the maximum power required for the video-modcarrier wave in accordance with signal compomeans for transmitting the modulated-carrierwave. The carrier wave may, for example, normally have a predetermined frequency -and be shifted with respect to this frequency during the retrace scanning periods. This frequency modulation, or shifting of the carrier frequency, corresponding to synchronizing impulses may comprise a single shift per impulse, it may be for di'erent durations for line-synchronizing and field-synchronizing impulses,- it may comprise a plurality of successive cyclesof predetermined different periods, or it may comprise any suitable combination of these characteristics. Also in accordance with a feature of the invention, a station is provided for receiving and reproducing a program transmitted by a carrier wave of the type developed by the transmitter described, this station including means for selectively receiving the carrier wave, means for detecting signal modulation of one type for deriving the video-'signal components, and means for detecting the signal modulation of the other type 'for deriving synchronizing-signal components. Means are provided for utilizing the derived components to reproduce the transmitted program.

Also in accordance with a further feature of the invention, a television broadcast system comprises a transmitter of any of the types 4mentioned above and a suitable receiving station.

therefor;

In accordance with another Vfeature of the invention, the `transmitting system -may be ar-` ranged to transmit only the carrier and that retrace periods. also the transmitter, then include a band-pass selector for translating the carrier and its proper 4 sideband, which selector has a response characteristic with a sloping side and a mean frequency so related to Vthe predetermined carrier frequency that the latter is located on the sloping include a cathode-ray signal-generating tube II .having the usual electron gun and photosensitive target and line-scanning and held-scanning elements I Ia and I Ib, respectively. 'Ihere arealso provided a line-scanning wave generator I2 and a field-scanning wave generator I3 with theiroutput circuits connected directly to the linescanning elements IIa and field-scanning elements IIb, respectively. In order to provide pedestal impulses fory blocking out, or for supi pressing undesirable impulses in, and ensuring the proper wave form of, the modulation signal side. By virtue of this arrangement, there is obtained an effective increase in the amplitude of the carrier wave during synchronizing-signal modulation periods with respect to the videomodulation periods, without the requirement of increased power transmission at the transmitter.

For a better understanding of the invention, together with other and further objects thereof, reference is had to thefollowing description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the accompanying drawings. Figs. 1 and 2 are circuit partially schematic, of complete television transmitting and receiving systems, respectively, embodying the present invention; Figs. 2a and 2b are diagrams illustratying modifications of parts of the receiver of Fig.

2; Figs. 3-7, inclusive, are graphs illustrating the wave forms of signals developed by the apparatus of the present invention; and Figs. 8-10, inclusive, are graphs illustrating certain operating characteristics of the systemsv shown-in Figs. 1

developed by the generator III, there is provided a pedestal-impulse generator I4 having its output circuit coupled vto the control grid of the signal-generating tube II. synchronizing-signal-generating apparatus is provided in accordance with the present invention and will be hereinafter described in detail.

In order to synchronize the operations of the generators I2, I3, and I 4 and the synchronizingsignal-generating apparatus, there is provided a timing-impulse generator I 5 having a plurality of output circuits coupled to the generators I2, I3, and Il and to the units of the synchronizingsignal-generating apparatus, as presently to be explained.

A- modulation-frequency amplifier I6 is coupled to outputcircuits of the cathode-ray signalgenerating tube II and the pedestal generator Il. The transmitter also includes means comprising a local or master oscillator I8 for generating a signal-carrier-wave and means includ- .ing a video-frequency amplitude modulator l1 45l for amplitude-modulating the carrier wave generated in local oscillatorl I8 with the video-frequency components generated in unit III and arnplied in unit I6, While maintaining the frequency constant during all of the trace periods. 'I'here is also included a. single sideband filter I9, a frequency changer 20, a heterodyne frequency filter 2|, a power amplifier 22, and an antenna system 23, 24, as shown, the frequency changer 20 comprising means for eiecting a change of frequency of and frequency-modulating, the carrier wave generated in master oscillator I8 in accordance with synchronizing-signal components, while maintaining its amplitude constant during al1 of the retrace periods.

Neglecting for the moment the stages I9, 20, and ZI and the synchronizing-signal-generating apparatus provided in accordance with the present invention, the system just described includes the apparatus of a television transmitting system of conventional design, those parts ofthe system illustrated schematically being of any wellknown suitable construction, so that a detailed description of the `system and its operation is un. necessary herein. Briefly, however, the image of the scene to be transmitted is focused upon the target of the tube II, in which a cathode ray or beam is developed, accelerated, and also focused upon the target. Scanning or deflection currents developed by the generators I2 and I3 are applied to the scanning elements or windings Il and IIb and serve to deflect the ray to scan successive series or fields of parallel lines on the target. Pedestal impulses developed by the generator Il are applied to the control electrode of the tube II to` suppress or block out the beam during certain portions, particularly retrace portions, of the scanning cycles and are applied to the modulation-frequency amplifier I to suppress undesirable impulses developed in the system and to aid in obtaining the required wave form o f the video-modulation signal.

The photosensitive elements of the target being electri-ally affected to an extent depending upon the varying values of light and shade at corresponding incremental areas of the image focused thereon, as the cathode ray scans the target, a voltage of correspondingly varying amplitude is developed in the output circuit of the signal-generating tube and applied tothe amplifier I6. Suitable timing-or synchronizing impulses are applied from the generator I5. to the generators I2, I3, and I4 to maintain these generators in synchronism with either the master frequency, in the case of direct camera shots, or the motion picture camera, in the case where such pictures are being transmitted. -The video-signal com. c

`in accordance with video-signal components during trace periods, while maintaining its frequency constant during all of the trace periods, and means for frequency-modulating the carrier wave in accordance with synchronizing-signal com-- ponents during retrace periods, while maintaining its amplitude constant during all of the retrace periods, as will be presently explained. The

`and' field-synchronizing impulses, respectively.

In a preferred arrangement, for example, the line-scanning and field-scanning frequencies may be 13,230 and 60 cycles per second, respectively, and the frequencies developed by the

generators

25 and 25 may be 1.00548 megacycles and 0.240 megacycle, respectively, that is, the '16th and 4000th harmonics of the line-scanning and fleldscanning frequencies, respectively, as will be hereinafter further explained.

While the generators I2 and I3 develop currents of the saw-tooth wave forms indicated at the left of these generators and at their respective frequencies, they also, in accordance with well-known practice, generate rectangular-impulse waves of the form shown at the right of these generators and of the same frequencies. These impulse waves are timed to fall within, and have durations somewhat less than, the line-retrace and field-retrace periods, respectively, although they are preferably caused to have durations as nearly equal to those of the line-retrace and field-retrace periods as possible. Output circuits of the generators I2 and 25 are connected to a mixing amplifier 21, while output circuits of .the generators I3 and 26 are connected to a mixing amplifier 28, the output circuits of the generators I2 and I3 supplying the impulses of rectangular wave form. The ampliners 21-and 28 may be of conventional design and are ncrresultantmo'dulated-carrier wave is amplified in the power amplierv 22V and is thereupon im pressed upon the antenna system 23, 24 for radiation.

`Corning now to that partv of the system embodying the present invention, it, will first be @noted that the single sideband filter I9 may be of any suitable well-known Vdesgri. preferably having a characteristic such as is shown vby the curve -of Fig. 8,.wherein relative gain is plotted against frequency, `as indicated. that is, a characteristic c such that the band-pass selector or filter I9 translates the carrier signal' and that'one of its modulation`A sidebands on the side of the frequency to which the frequency of the carrier wave is shifted to translate synchronizing signals, as hereinafter more fully described. Specifically the selector is designed to have a response characteristic with a sloping side and a mean frequency so related to the carrier frequency developed by the modulator l1 that this frequency is located on the sloping side thereof, as indicated at X in Fig. 3, so that, although the sideband frequencies corresponding to the lower modulation adjacent the carrier frequencies are passed with relatively less gain, a portion of c ing these frequencies is passed by the selector so that the over-all gain is substantially the same for these lower modulation frequencies as for the higher modulation frequencies. The heterodyne frequency filter 2l is of any suitable conventional type designed to pass'either the difference or sun both sidebands represent- For the purpose of frequencymodulating the' mally biased beyond cutoff, so that the rectangular impulses of the waves supplied from generators I2 and I3 carry these amplifiers above cutoff, rendering them operative for the durations of 1 Therefore, there aptheir respective impulses. pear in the output circuits of the amplifiers 21 and Y2'8 trains of sinusoidal voltage waves of Ylinethe predetermined vportions* thereof corresponding to the durations of the pulses developed by the generators I 2 and I3, respectively.

carrier wave' impresed upon the frequency changer 2 0 from the filter I!l during the retrace periods in accordance with the synchronizingsignal components, there is provided the frequency-modulating or frequency-shifting apparatus indicated generally at 29. This apparatus includes an oscillator indicated generally at 30 and a frequency-adjusting network indicated generally at 3| Preferably the oscillator is of the push-pull

typecomprising vacuum tubes

32 and 33, having their input circuits connected in push-pull relation by way of an inductance 34,

a leak resistor 35 and grid condenser 35a being connected in parallel between a midtapon the inductance 34 and the cathodes of the tubes. 'I'he anode circuits of the

tubes

32 and 33 are also connected in push-pull relation by way of an inductance 38, coupled to the inductance 34 as indicated by bracket' M, a mld-tap on the inductance 36 being connected to the cathodes of the tubes by way ofl anode voltage supply battery 31. The inductance., 36 is normally sharply tuned to a frequency spaced from the carrier frequency developed by the'modulator |1, by means of a condenser 38 or other suitable 'circuit arrangement. An inductance 39 is coupled to` the inductance `Sli and connected to the frequency changer 2|) for the purpose of 4shifting the carrier frequency delivered'fromthe filter I9 in accordance withvfrequcncies developed by the oscillator 30.

The frequency-adjusting network 3| prefer ably comprises a'pair of vacuum tubes 40 and 4| having their control grids connected by way of resistors 42 and 43 to the output circuits of the amplifiers 21 and 28 and by way of by-pass condenser 44 which is effective to by-pass oscillator-frequency currents to ground. The control grids of rthe tubes 40 and 4| are also connected by way of coupling condensers 45,V 46 to their respective anodes which, in turn, are con- 4nected to'the terminals of an inductance 41.

The inductance 41 is connected in push-pull relation in the anode circuits of `the tubes, a. mid-tap thereof being connected to the tube cathodes by way of a suitable anode voltage supply source, indicated by the battery 48. The inductance 41 is inductively coupled to the inductance 36 of the oscillator frequency-determining circuit.

In the operation of the parts of the transmitter embodying thev present invention, referring first to thefrequency-adjusting network 3|, which it will be assumed is excited from oscillator 30, it will be seen'that, since the impedances of the condensers 45 and 46 are high compared with those of resistors 42 and 43, the voltages across the resistors lead the voltage across the inductance 41 by a large angle and that the space currents of the tubes, being in phase with the tube input voltages, also lead the voltage across theinductance 41 in corresponding degree. Hence, the system simulates an impedance having capacitive reactance and resistance components, the values of which vary in accordance with the grid-bias voltage applied to the tubes. Adjustment of the grid-bias voltage on the tubes 40 and 4|, therefore, varies the effective reactance oi' the circuit 3| and, hence, the natural frequency of the oscillation circuit 36, 38 to which it is coupled.

Normally the tubes 40 and 4| are biased beyond cutoff' and remain so biased during the video-signal or trace periods, being rendered operative only Vduring synchronizing or retrace periods when impulses from one or both of the generators |2 and I3 are applied thereto. Since the tubes 40 and 4| are cut 0E during the videosignal periods, the oscillation circuit 36, 38 has its own natural frequency during these periods, while its frequency is shifted in the desired manner during the occurrences of the .synchronizing impulses. During the latter periods the impedance reflectedinto the

oscillation circuit

38, 38 by thefrequency-adjusting network 3| harmonic and field-harmonic voltages applied to the grids of the tubes 4|) and 4| from theA generators and 26 as controlled in ampliflers 21 and` 28 by the impulses from generators |2 and I3, respectively, so that the frequency of the oscillation circuit is intermittently varied is varied in accordance with `the line-v cyclically in a manner that will presently be further explained.

The oscillator 30 being-coupled as above described to the frequency changer 20,' its frequency is mixed with the carrier frequency of the video-modulated signal delivered from the filter I9 and the sum and difference heterodyne frequencies are developed iny the output circuit of the modulator in the well-known manner. Either the sum frequencies or the difference frequencies are selected by means -of the filter 2| and delivered to the powerf'ampliiier 22 to be broadcast. Similarly, the frequency of the selected heterodyne carrier output of frequency changer 20 is cyclically varied or shifted in correspondence with the frequency variations of oscillator 30, as described above.

In Fig. 3, there is illustrated half'of the envelope of the resultant modulated-carrier wave developed by vthe transmitting system. The video-frequency signal component is represented at the portions of the wave indicated at V and is developed during line-trace periods, the por? tions representing line-retrace and field-retrace periods, during which the frequency of the carrier is shifted or varied, being indicated at L and F, respectively. In Figs. 4 and 5 there are shown exploded curves illustrating the cyclic frequency variations of the carrier wave during the line-retrace and held-retrace periods L and F, respectively. In these figures, the abscissae represent time and the ordinates frequency shift or deviation of the signal carrier.

Thus, the carrier wave developed in the output circuit of the modulater |1 is amplitudemodulated bythe video-signal components only during the trace scanning periods, and during these periods the frequency developed by the generator 30 and applied to the frequency changer 20 is constant so that the carrier frequency as developed in the output circuit of the frequency changer 20 is not varied. During a portion of the line-retrace periods the impulses from the generator 2 unblock the amplifier 21 and permit the line-harmonic voltages developed by the generator 25 to be passed by the amplifier 21 and impressed on the frequencyadjusting network 3|, thereby to effect variations in the frequency developed by the generator 30 and, hence, in that of the carrier frequency developed in the frequency changer 26, as indicated by the curve of Fig. 4. -During one or more' portions of the field-retrace periods, on the other hand, the impulses from the generator I3 unblock the amplifier 28 and permit the fieldharmonic voltages developed by the generator 26 to be passed by this amplifier and impressed on the network 3|, thereby to effect variations in the frequency developed by the generator 30 in different predetermined manners for the lineand field-synchronizing signals, specifically in different intermittent cyclic manners and, hence, in that of the carrier-frequency output circuit of the frequency changer 28, as indicated by the curve of Fig. `5. Thus, the carrier wave developed in the output circuit of modulater |1 is frequency-modulated in frequency changer 2l by the synchronizing-signal components during all of the retrace periods. The amplitude of the carrier-frequency output circuit of frequency changer 20 is maintained constant during all of the retrace periods and the carrier signal comprising both amplitude-modulation and frequency fmodulation components is directly radiated after translation in units 2| and- 22.

the ,synchronizing-signal' components in accord- Since the output frequencies of the generators 25 and 26 are harmonics of the line-scanning and eld-scanning frequencies, the successive pulse groups are all of the same phase and wave form, and vare not subjected to progressive changes in wave form from'one group to the next, as would be the case if a nonharmonlc cyclic variation were employed. The cycles bf variation which correspond to the line-synchronizing impulse are thus of one predetermined frequency: that is, the carrier frequency is varied .in one manner and for a first predetermined duration. A line synchronizing period immediately precedes each field-synchronizing period, as shown in Figs. 3 and 5, so that line synchronization is not interrupted during eld-retrace periods.

Referring now to Fig. 2, the system there illustrated comprises a television receiver of the superheterodyne type for receiving and reproducing a program transmitted by a carrier wave of the type developed by the transmitter -de scribed above. This receiver includes means for selectively receiving the carrier wave and specifically includes an antenna system 50, 5| to which there are connected 'in cascade, in the order named, a radio-frequency amplifier and frequency changer 52, an intermediate-frequency amplifier 53, a detector 54 comprising means for detecting the amplitude modulation to derive the video-signal components, va video-frequency amplifier 55, and a cathode-ray signal-reproducing tube 56 comprising means for utilizing the detected video-signal components to reproduce the transmitted program. Preferably, as explained below, the

stages

52 and 53 are designed for single sideband reception. A linefrequency scanning generator 51 and field-frequency scanning generator 58 are also coupled to the output of the amplifier 53 by way of synchronizing-signal-deriving apparatus 59 embodying the present invention and .hereinafter described in more detail. The generators 51 and `58 are coupled to the scanning elements of the mediate-frequency signals being, in turn, selec tively amplified in the intermediate-frequency amplifier 53' and delivered to the detector 54.

The video-modulation components of the signal y are developed by the detector 54 and are supplied to the video-frequency amplifier 55, wherein they are amplified and. from which they are supplied in the usual manner to' a brilliancy .control electrode of the cathode-ray tube 56. The intensity of the" electron beam of the tube '56 1s thus modified or controlled in accordancewith the video-frequency voltages` impressed upon the control electrode of the tube in the usual manner.

The intermediate-frequency signal is also supplied from the amplifier 53 to the apparatus 59 comprising means responsive to the frequency modulation of the received carrier for detecting ance with the present invention, the line-'synchronizing and field-synchronizing components being effectively separated from the video-freque y signals and from each other and applied to e control circuits of the

generators

51 and 58 as will be hereinafter further explained. Saw-tooth current/or voltage scanningA waves are\ generated 'in the line-scanning and

fieldscanning generators

51 and 58 and applied to e scanning elements of the cathode-'ray tbe 56 to produce electric scannin fields, thereby to deflect the intensity modula cathode ray vertically vand horizontally so as to trace successive fields of parallel lines on the target of the tube to reconstruct'the transmitted image.

For the purpose of more effectively receiving and reproducing a signal of the single sideband type transmitted, fofI example, from a transmitter such as described in connection with Fig. 1, the frequency changer 52 is arranged to n develop such an intermediate-frequency signal that the intermediate-frequency carrier is normally located on one of the sloping sides of the band-pass characteristic of the selector 53, which is similar to that of the filter I9 of the system of Fig. 1 illustrated in Fig. 8. Referring now more particularly to the synchronizing-signal-deriving apparatus 59, there is here provided an amplifier '60 having its input circuit connected tothe output circuit of the intermediate-frequency amplier 53 and its output circuit connected in turn to a broad single sideband selector 6|. The selector 6| also preferably has a band-pass characteristic similar to that of the filter I9 of Fig. 1, that is, so designed that theintermediate-frequency carrier is located on one of its sloping sides. The amplifier 60 and selector 6| serve to provide a signal of sufdcient amplitude to ensure proper operation of the succeeding parts of the apparatus 59. Due to the characteristics of the amplifier 53 and selector v6|, the shift in the intermediate-frequency carrier from the sloping side of the band-pass characteristic to the uniform higher response portion thereof during the retrace periods effectively increases the amplitude of the carrier output of selector 6| so that the form of the wave developed in the output circuit ofthe selector 6| is as'shown in Fig. 6.

Connected in cascade to the output circuit of the selector 6|, in the order named, are a vacuu'm-tube limiter 52, a band-pass selector 63, a vacuum-tube amplifier 64,.a modulation .converter or selector 65, a double-diode rectifier circuit,66, and parallel-connected band-pass filters 61 and 63. The limiter 62 preferably comprises a f resistor 6212, as-shown, and has an operating .characteristic as illustratedv in Fig. 9, the points X-X indicating generally the range of operation of the tube beyond both its upper and lower cutoff limits for any useful received signal. Y Due to the action of the limiter, only a small portion of the wave received from selector 6|, such as that indicated at s in Fig. 6, is'passed by the limiter 62. The condenser 62a and leak resistor 62h provide a self-biasing or stabilizing action forthe limiterso that the interval s is alwaysimmediately below the peaks of the signal and independent of the signal amplitude. The half-envelope of the voltage developed in the output circuit of the limiter, therefore, is as illus- `trated by the curve ofFig. 7. Y

.The selector 63 may comprise primary and secondary windings u and itfeach broadly 'output Pulau oflimiter l2. The tube u, to-

gethen'with selector il, serves to couplethe limiter l! to the modulation converter or` selector 6i and to prevent back coupling between the circuits. Asuitablebiasingbattery aispreferablypmvldedinthegridcircuitofthetube .5.4. as shown. '.lheselector' may be 'constructed similarly to the selector l2 and comprise transformer `windings 12 and 14, each tunedhyaccndenserllshuntedbyaresistor 16. VThis selector, however, is preferably so tuned as tohave a transmission characteristic such las is illustrated by .the curve of Fig. 10, wherein relative gain' in volts is plotted against frequency, in mealycb, relative to the intermediate-can'ier frequency developed by the oscillater-modulatorz l2. Selector Si is thus a bandpassselectorfortranslatingthecarersignal having a response characteristic with a sloping -side and of a meaufrequency so related to the normal frequency of the carrier that the latter is located on the sloping side. Thus,l thisselector comprises means responsive to periodic frequency variations of the received carrier wave and serves the purpose of converting the frequency shifts or modulation into amplitude variations; that is, the amplitude of its signal output varies in accordance with its input v frequency shifm or variations.

Therectifiermaycompriseapairofdiode rectiiim I'l and 1l connectedacross the secondary circuit of the selector Il for full-wave reciicaticn and provided with a load resistor 1l. The volh'ge acrossload resistor 18 is applied `negatively to the input circuits of the filters 61 and Il. The-filters Il and Il are preferably timed so that they are moet responsive at frequencies corresponding to the cyclic carrierfrequency variations occurring during the line retrace and field-retrace periods, respectively,

- that is, to frequenci of the oscillations generated by circuiis 2i and 26, respectively, at the transmitter of Pig. 1. In the present case, for example,thefilte1si1andlmaybetunedto 1.0 and 0.24 megacycles, respectively'. In order that these filters will have a reasonably rapid response, they are preferably designed with relatively wide hand-pass characteristics, for example, of the order of :50% of their respective mean resonant frequencies. 'Ihe output circuits of the filters l1 and Il may be connected to the line-scanning and field-scanning generators 51 and il, respectively, for delivering synchronizing impulses thereto.

, In the operation of the apparatus 59, 4the inimmediate-frequency carrier and its single side- 'band of modulation frequencies are delivered increase Yin the carrier amplitude output of selector il during each line-scanning and fieldscanning period.

The tube 62 serves, as described above, to limit the signal, therebyeiiectively to separate the synchronizing-signal components thereof from the video-signal components and to provide a signal in its output circuit, the half-envelope of which is illustrated in Fig. 7. This signal is delivered by way of the selector il and amplifier 64 to'the modulation converter or selector l5 and therein eectively converted into a signal, the amplitude of which variesor is modulated in accordance with .shifts or modulation of the frequency of the intermediate-frequency carrier. This signal is thereupon impressed upon the diode rectifier 6I. The rctier functions to develop across its load resistor 19 a periodicimpulse signal comprising two components each including recurring impulses. The frequency and duration ,of vthe impulses of one component correspond to the line-scanning impulses developedfby the transmitter generator I2, while the frequency and duration of the impulses of the other component correspond to the field-scanning impulses developed by transmitter gener-- determined by the transmitter generators 25 and 26, respectively. The lters 61 arid 88 are designed to be most responsive to such line-har- .monic and held-harmonic frequencies, respectively, and thus comprise means responsive to the variations ,of the frequency of the carrier wave in a first predetermined manner for deriving line-synchronizing components and means responsive to variations of the frequency of the carrier wave in a different predetermined manner for deriving held-synchronizing components. It will be noted that the envelopes of the waves passed by the filters I'Land 6I constitute periodic waves comprising impulses occurring at the linescanning and held-scanning frequencies, respectively, these impulses being impressed upon the generators 5l and 58, respectively, and serving to synchronize their operation in accordance with the corresponding generators l2 and I3 at the transmitter.

In the event that it is desired not to utilize the cyclic or harmonic variation feature of the invention, the generators 25 and 26 of the transmitter may be omitted so that the generators i2 and I3 and the apparatus 29 simply eifect a single shift of the signal-carrier frequency into a portion of the frequency spectrum occupied by a sideband of the amplitude-modulated signal during each line-synchronizing and field-synchronizing impulse, which shift has a duration equal tothat of its respective impulse. In this case, the receiver lters 6l and B8 may be omitted and any suitable signal duration responsive device may be substituted therefor. Such a device functions in a conventional manner to separate the line-synchronizing and field-synchronizing impulses in response to their relative durations and to deliver the derived linevsynchronizing and field-synchronizing impulses to the generators 51 and'58, respectively. For this purpose, idlerefore, the input and output terminals of the filters "and 68 may be disconnected at the points indicated at A, B, and C, and a suitable duration responsive separator 80, such as indicated in Fig. 2a, may have its corresponding terminals, also indicated at A, B, and C, connected to these points. The device may be of any suitable well-known design, such as that shown in applicantfs copending application, Serial No. 177,679, filed December 2, 1937, for Television scanning systems.v

Moreover, if desired, the frequencies to which the carrier is shifted during the line-scanning 'and field-scanning pulses may be of different predetermined values, in which case Ithe devices Ulf-8l, inclusive, may be replaced by two selectors 8| and 82, as shown in Fig. 2b, sharply tuned to these respective frequencies and having individual rectifiers 83 and 84,' suitably connected to the

scanning generators

51 and 58, respectively.

' Similarly to Fig. 2a, the several points of connection for the circuits of Fig. l2b in the system of Fig. 2 are indicated at B, C, and D. With this `or similar arrangements, if desired, a section of the modulation envelope below that represented at s may be selected by the limiter 62. For this purpose it is .necessary only to adjust the value of the bias to an appropriate more negative value vandto omit the self-biasing elements 62a and.

B2b. r

It is to be noted that, in both arrangements described above, the advantage of an increase in synchronizing-signal amplitude, without any corresponding increase in power output at the transmitter is obtained by means of single sideband transmission and the special band-pass filter characteristic which has been described, employed at either or both the transmitter and receiver. It will, however, be appreciated that that single sideband feature may, if desired, be omittetdfk from a system embodying the other y While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modications may be made therein without departing from the invention, and it is, therefore, aimed in the lappended claims to cover all such changes and modications as fall within the, true spirit and scope of the invention.

1. A television broadcast system comprising a station for developing and transmitting a modulated-carrieriwave including means for developing video-signal components during `trace periods and synchronizing-signal components during retransmitted wave including means responsive to amplitude modulation of the received carrier for detecting and utilizing said signal components -of said one type, and means responsive to frequency'modulation of the received carrier for detecting and utilizing said components of said other type.

2. A television broadcast system comprising a station for developing and radiating va high'- frequency modulated-carrier vwave including means for developing video-signal components during trace periods and synchronizing-signal components during retrace periods, means for amplitude-modulating said carrier wave in accordance with said video-signal components, means for frequency-modulating said carrier wave in accordance with said synchronizingsignal components, means foi' directly radiating said amplitudeand frequency-modulated carrier wave, and a station for receiving and reproducing said transmitted wave including means responsive to amplitude modulation of the received l carrier for detecting and utilizing saidvideosignal components, and means responsive to frequency modulation of the received carrier for detecting Aand utilizing said synchronizing-signal components.

3. A television broadcast system comprising a station for developing and transmitting a modulated-carrier wave representing successive trace and retrace periods including means for developing a carrier sinvave normally of a predetermined frequency, means for developing video-signal components and synchronizing-signal ponents, means for amplitude-modulating said carrier wave during trace periods in accordance with said video-signal components, means for shifting the frequency of saidl carrier wave during retrace periods into a portion of the frequency spectrum occupied by a sideband of said amplitude-modulated signal and in accordance with said synchronizing-signal components, and a stationfor receiving and reproducing said transmitted wave includingmeans responsive to amplitude modulation of the received carrier for' detecting and utilizing said video-signal components and `means 'responsive' to frequency variations of the received carrier for developing and utilizing said synchronizing-signal components.

4. A television transmitting system comprising means for developing a carrier wave, means'for developing video-signal components during trace periods, means for developing synchronizingsignal components during retrace periods, means for amplitude-modulating said carrier wave in accordance `with signal components of one of said types and formaintaining its frequency constant during all of said periods corresponding to said one of said types, means for frequency-modulating said carrier wave in accordance with signal components of the other of said types and for maintaining its amplitude constant during all of said periods of said other of said types, 'and means for transmitting said modulated-carrier-wavel 5. A television transmitting system comprising means for developing a carrier wave, means for developing-video-signal components, means for developing synchronizing-signal components, means for amplitude-modulating saidl carrier wave during trace periods in accordance with said video-signal components, means for frequency-modulating said carrier wave during rea radiating said modulated-carrierv wave. f

6. A television transmitting system comprising means for developing 'a carrier wave normally of a predetermined frequency, means for developing video-signal components, means for develop- COming synchronizing-signal components, means for amplitude-modulating said carrier wave\during' tace periods in accordance with said videosignal-components. means for shifting the frequency of said carrier wave during retrace periods into`a portion of the frequency spectrum occupied 4by a sideband of said amplitude-modulation compon`ents and in accordance with said synchronizing-signal components, and means for transmitting said modulated and frequency-varied carrier wave.

7. A television transmitting system comprising means for developing a carrier wave normally of a `predetermined frequency, means for developing video-signal components, means for developing line-frequency synchronizing impulses. means for developing field-frequency synchronizing impulses, means for amplitude-modulating said carrier wave during4 trace periods in accordance with said video-signal components, means for varying the frequency of said carrier wave for a first predetermined duration during at least a portion of said line-synchronizing impulses, means for varying the frequency of said carrier wave fora different predetermined duration during at least a portion of said field-synchronizing impulses, and means for directly radiating said modulated, frequency-varied carrier wave.

v8. A television transmitting system comprising meansfor developing a carrier wave normally of a predetermined frequency, means for developing video-signal components, means for developing line-frequency synchronizing impulses, means for developing field-frequency synchronzing impulses, means for anplitude-modulating said carrier wave during trace periods in accordance with said video-signal components and for maintaining its frequency constant during all of said trace periods, means yfor varying the frequency of said carrier in a iirst predetermined manner during line-retrace periods, in accordance with said line-synchronizing impulses and for maintaining its amplitude constant during allof said line-retrace periods, means for varying the frequency of said carrierin a different predetermined manner during field-retrace periods in accordance with said field-synchronizing impulses and Afor maintaining its amplitude constant during all of said field-retrace periods, and means for transmitting said modulated, frequency-varied carrier Wave.`

9. A television transmitting system comprising means for developing a `carrier wave normally of a predetermined frequency, means for developing` video-signal components, means for developing line-frequency and field-frequency waves having trace and retrace periods, means for arn plitude-modulating said carrier wave during trace periods in accordance with said video-signal components, means for varying the frequency of said carrier in a first predetermined manner and for a first predetermined duration during 1ineretrace periods, means for varying the frequency of said carrier in a diiferent predetermined man. ner and for a diiferent predetermined duration during field-retrace periods, and means for transmitting said modulated, frequency-varied carrier wave.

10. A television transmitting system comprising means for developing a carrier wave normally of a predetermined frequency, means for developing video-signal components, means for developing line-frequency andl field-frequency scanning waves having trace and retrace periods, means for amplitude-modulating said carrier assises wave during successive trace-scanning periods in accordance with said video-signal components, means for varying the frequency of said carrier in one or more successive cycles of predetermined frequency during successive line-retrace-scanning periods, means for varying the frequency of said carrier wave in one or more successive cycles of a different predeterminedJ frequency during successive field-retrace-scann'ing periods, and means for transmitting said modulated, frequency-varied carrier wave.

11. A television transmittiig system comprising means for developing a carrier wave normally of a predetermined frequency, means for developing video-signal components, means for developing line-frequency and field-frequency scanning waves having trace and retrace periods, means for amplitude-modulating said carrier wave in accordance with video-signal components during trace periods, means for shifting the frequency of said carrier wave with respect to said predetermined frequency and in accordance with synchronizing-signal components during retrace periods, a band-pass selector for translating said carrier and that one of its modulation sidebands on the side of said predetermined frequency to which said carrier frequency is shifted, said selector having a response characteristic with a sloping side and having a mean frequency so related to said predetermined carrier frequency that the latter is located on said sloping side, and means f or transmitting the frequencies translated by said selector.

12. A television receiving system for receiving and reproducing a program transmitted by a car' rier wave having amplitude modulation and frequency modulation, one of said types of modulation representing video-signal components during trace periods and the other of said types of modulation representing synchronizing-signal components during retrace periods and said carrier wave being of constant frequency during all of said periods corresponding to said amplitude modulation and of constant amplitude during all of said periods corresponding to said frequency modulation, comprising means for selectively receiving said carrier wave, means for detecting signal modulation of said one of said types to derive said video-signal components, means for detecting signal modulation of said other of said types to derive said synchronizing-signal components, and means for utilizing said derived components to reproduce theA transmitted4 program.

13. A television receiving system for receiving and reproducing a program transmitted by a di rectly radiated carrier wave having amplitude modulationrepresenting video-signal compolation representing synchronizing-signal components during retrace periods, comprising means for selectively receiving said signal, means for amplitude-detecting said carrier to derive said video-signal components, means for frequencydetecting said carrier to derive said synchronizing-signal components, and'means for utilizing saidr derived components for reproducing the transmitted program.

14. A television receiving system for receiving and reproducing a program transmitted by a carrier wave normally of a predetermined frequency and amplitude-modulated in accordance with video-signal componenisduring Y, trace periods but having its frequency periodically shifted into a portion of the frequency spectrum oclated signal and in accordance with synchromining-signal components during retrace periods, comprising means for selectively receiving said carrier wave, means responsive to periodic frequency variations of said carrier wave for deriving said synchronizing-signal components, detecting amplitude-modulation means for deriving said video-signal components, and means for utilizing said derived components for reproducing the transmitted program.

15. A television receiving system for receiving and reproducing a program transmitted by a carrier wave normally of a predetermined frequency but having its frequency,1 shifted for a rst predetermined duration during line-retrace periods while its amplitude is maintained constant during all of the line-retrace periods and shifted for a different predetermined duration during eldretrace periods while its amplitude is maintained constant during all of the field-retrace periods and amplitude-modulated during trace periods in accordance with video-signal components while its frequency ismaintained constant during all of said trace periods, comprising means for selectively receiving said carrier wave, amplitude-modulation detecting means for deriving said video-signal components, means responsive to frequency variations of said carrier Wave of said first predetermined duration for deriving said line-synchronizing components, means responsive to frequency variations of said carrier wave of said different predetermined duration for deriving said field-synchronizing components, and means for utilizing said derived components for reproducing the transmitted program.

16. A television receiving system for receiving and reproducing a program transmitted by a carrier wave normally of a predetermined frequency and amplitude-modulated during trace periods in accordance with video-signal com-ponents but having its frequency shifted in a first predetermined manner during line-retrace periods in accordance with line-synchronizing components and having its frequency shifted in a second predetermined manner during eldretrace periods in accordance with said fieldsynchronizing components into a portion of the frequency spectrum occupied by a sideband of said amplitude-modulated signal, comprising means for selectively receiving said carrier wave, amplitude-modulation detecting means for de-l riving said video-signal components, means responsive to variations in the frequency of said carrier wave in said first predetermined manner for deriving said line-synchronizing components, means responsive to variations in the frequency of said carrier wave in said different predetermined manner for deriving said held-synchronizing components, and means for utilizing said derived components for reproducing the transmitted program.

17. A television receiving system for receiving and reproducing a program transmitted by a carrier wave normally of a predetermined frequency but having its frequency shifted in a iirst predetermined manner for a first predeterminedA duration during line-retrace periods in accordance with line-frequency synchronizing components while its amplitude is maintained constant during all of the line-retrace periods and having field-frequency? synchronizing components while 'its amplitude is maintained constant during all of the held-retrace periods, said 'carrier wave being amplitude-modulated during trace periods in accordance with videosignal components while its frequency is maintained constant during all of said 4trace periods, comprising means for selectively receiving said carrier wave, amplitude-modulation detecting means for deriving said video-signal components, means responsive to variations in the lfrequency of said carrier Wave in said rst predetermined manner and of said first predetermined duration for deriving` said line-synchronizing components, means responsive to variations in the frequency of said 4 carrier wave in said diierent predetermined lation frequencies manner and of said different predetermined duration for deriving said field-synchronizing components, and means for utilizing said derived components for reproducing the transmitted program.

18. A television receiving system for receiving and reproducing a program transmitted by a carrier wave and a sideband of amplitude-modurepresenting video-signal components during successive trace periods, said carrier being normally of a predetermined frequency but shifted with'respect to said predetermined frequency during successive retrace periods in accordance with synchronizing-signal components, comprising means for selectively receiving said carrier wave, amplitude-modulation detecting means for deriving said video-signal components, a band-pass selector for translating said carrier having a response characteristic with a sloping side and of a mean frequency so related to said predetermined frequency that the latter is located on said sloping side, means responsive to the signal output of said selector for deriving said synchronizing-signal components,

and means for utilizing said derived components 'manner for a different predetermined duration during field-retrace periods in accordance with to reproduce the transmitted program.

19. A television receiving system for receiving and reproducing a program transmitted by a carrier normally of a predetermined frequency, but having its frequency varied Athrough one or more cycles of one frequency during line-retrace periods and varied through one or more different cycles of a diiferent frequency during fieldretrace periods and amplitude-modulated during trace periods in accordance with video-signal components, comprising means for selectively receiving said carrier wave, amplitude-modulation detecting means for deriving said videosignal components from said received carrier wave, frequency-modulation detecting means for deriving from said received carrier wave said cyclic frequency modulation, resonant circuit means tuned to the respective frequencies of said one of said different cyclic variations for deriving synchronizing impulses, and means for utilizing said derived video-.signal components and said synchronizing impulses for reproducing the transmitted program.

20. The method of transmitteing and receiving a television program which comprises, developing video-signal components during trace periods and 'synchronizing-signal components during retrace periods, amplitude-modulating said carrier wave in accordance with said signal components of one of said types while inaintaining its frequency constant during all of said periods corresponding to said one of said types, frequency-modulating said carrier wave in accordance with said signal components of the other of said types while maintaining its amplitude constant during all of said periods corresponding to said otherofsaid types, transmitting said modulated-carrier wave, selectively receiving said transmitted carrier wave, detecting said amplitude modulation of the received carrier to derive said signal components of said one type, detecting said frequency modulation of the received carrier to derive said signal components of said other type, and utilizing said derived components to reproduce the program.

21. The method'oftransmitting and receiving a television program, comprising developing video-signal components during trace periods and synchronizing-signal components 'during retrace periods, amplitude-modulating said carrier wave in accordance with said video-signal components, frequency-modulating said carrier wave in accordance with said synchronizingsignal components, directly radiating said modulated-carrier wave, selectively receiving said transmitted wave, detecting said amplitude modulation of the received carrier to derive said video-signal components, detecting said frequency modulation of the received carrier to derive said synchronizing-signal components, and utilizing said derived components to reproduce said program. A

22. The method of transmitting and receiving a television program, comprising developing a carrier wave normally of a predetermined frequency and representing successive trace and retrace periods, developing video-signal components and synchronizing-signal components, amplitude-modulating said carrier wave during said trace periods in accordance with said video-signal components, shifting the frequency of said carrier wave during said retrace periods in accordance with said synchronizing-signal components into a portion of the frequency spectrum occupied by a sideband of said amplitude-modulated signal, transmitting `said modulated, frequency-varied carrier wave, selectively receiving said transmitted wave, detecting said amplitude modulation of the received carrier to derive said video-signal components, detecting said frequency variations of the received carrier to derive said synchronizing-signal components, and utilizing said derived components to reproduce `said program.

23. The method of transmitting a television program, comprising developing a carrier wave, developing video-signal components during trace periods, developing synchronizing-signal components during retrace periods, amplitude-modulating said carrier wave in accordance with signal components of one of said types while maintaining its frequency constant during all of said periods corresponding to said one oi' said types, frequency-modulating said carrier wave in accordance with signal components of the other of said types while maintaining its amplitude constant during all of said periods corresponding to said other of said types, and transmitting said modulated-carrier wave.

24. The method of transmitting a television program, comprising developing a carrier wave. developing video-signal components, developing synchronizing-signal components, amplitudemodulating said carrier wave in accordance with said video-signal components during trace-scanning periods, frequency-modulating said carrier wave during retrace-scanning periods in accordance with said synchronizing-signal components,

vand directly radiating said modulated-carrier ,wave. 25. The method of a television developing video-signal components;

program, comprising developing a carrier wave,

line-frequency and nem-frequency scanning waves having trace and retrace periods, amplitude-modulating said carrier wave in accordance with said video-signal components during one .or more successive cycles -of one predetermined frequency during lineretrace scanning periods, varying the frequency of said carrier wave in one or more successive cycles of a different predetermined frequency during successive field-retrace scanning periods, and transmitting said amplitude-modulated and y frequency-varied carrier wave.

' said signal, detecting amplitude modulation of the received carrier to derive said wideo-frequency components, detecting frequency modulation of lthe received'carrier to derive sai synchronizing-signal components, and utilizing said derived components for reproducing the transmitted program.

27. The method of receiving and reproducing a television program transmitted by a carrier wave normally of a predetermined frequency and amplitude-modulated in accordance with videosignal components during trace periods but having its frequency shifted in accordance with synchronizing-signal components during retrace periods to a. portion of the frequency spectrum occupied by a sideband of said amplitude-modulated video componen-ts, comprising selectively receiving said carrier Wave, detecting said frequency variations of said carrier wave to derive said synchronizing components, detecting said amplitude modulation of said carrier to derive said video-signal components, and utilizing said derived components to reproduce the transmitted program.

28. 'Ihe method of receiving and reproducing a television program transmitted by a. carrier wave normally of a predetermined frequency but having its frequency varied in one or more successive cycles of one predetermined frequency during line-retrace-scanning periods and varied in one or more successive cycles of a different predetermined frequency during successive fieldretrace scanning periods and amplitude-modulated by video-signal componentsduring tracescanning periods, comprising selectively receiving said carrier wave, detecting said amplitude modulation of the received carrier to derive said video-signal components, detecting the frequency modulation of the received carrier to derive said cyclic frequency modulation, selecting said cyclic variations of said one predetermined frequency to derive line-synchronizing impulses, selecting said cyclic frequency modulation of said different predetermined frequency to derive field-synchronizing impulses, and utilizing said derived video-signal components and synchronizing impulses to reproduce the transmitted program.

29. A television signal receiver adapted to receive a modulated-carrier wave amplitude-modulated by vided-signal components during trace periods while its frequency is maintained constant during all of said trace periods and frequency-modulated by synchronizing-signal components to a predetermined frequency deviation during retrace periods while its amplitude is maintained constant during all of said retrace periods comprising, means selectively responsive to said predetermined carrier-frequency deviation for deriving synchronizing-signal components, means for amplitude-detecting said carrier wave to derive said video-signal components, and means for utilizing said derived video-signal components for reproducing the transmitted program.

30. Atelevislon signal receiver adapted to receive a directly radiated modulated-carrier wave amplitude-modulated by video-signal components during trace periods and frequency-modulated by synchronizing-signal components to a predetermined frequency deviation during retrace periods comprising, means frequency-selectively responsive to said predetermined carrierfrequency deviation for deriving synchronizingsignal components, amplitude-selective means coupled -to said frequency-selective means for selecting only amplitude values above a predetermined value ior synchronizing an operation of said receiver, means for amplitude-detecting said carrier wave to derive said video-signal components, and means for utilizing said derived video-signal components for reproducing the transmitted program.

ARTHUR V. LOUGHREN.

CERTIFICATE 0F CORRECTION.

Patent No. 2,2 ,11.55; September 2, lSLLl.

ARTHUR V. LOUGHREN.

It is hereby certified that error appears in the printed specification ofthe above numbered patent requiring correction asfollows: Page 1, first column, line 14.7, for "increase" read --increases-; and second column, line l0, for "compounds" read -components; page Lp, first column, line 20, for "later" read--lation; page 5, first column, line 8, beginning with the words I'The cycles? strike out all to and including "duration." in line l5; page?, first column, 11n@ 52, for "that" read tn1s; line 6o, claimi, strike out and"; page 9, second column, line 65, claim 20for "transmitteing" read --trananitting--i and that the sam Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 5rd 'day of March, A. D. 19142.

. Henry van Arsdale, (Seal) Acting Commissioner of Patente.