US2350902A - Television system - Google Patents

Description

June 6, 1944. H. E. KALLMANN TELEVIS ION SYSTEM Filed Aug. 2'7, 1941 2 Sheets-Sheet l TIME FREQUENCY- MC 7:)

Y AQ M m .R mEWWm V A 2 w E m Patented June 6, 1944 UNITED STATES" PATENT OFFICE TELEVISION SYSTEM Heinz E. Kallmann, New York, N. Y., as'signor to Radio Corporation of America, New York, N. Y a corporation of Delaware Application August 27, 1941, Serial No. 408,579

7 Claims.

This invention relates to a, television system and more specifically to a system of television transmission and reception in which only a single carrier is used for the trion of both the video signal and the accompanying audio signal.

The transmission of video and accompanying audio signals in television systems on a single transmitter has been suggested in the past, but all of these so suggested systems have suffered from drawbacks which have rendered them impracticable. for one or another reason. My invention provides a television transmitting and receiving system which makes use of onlya single transmitter for transmitting the video and sound signals and receiving these signals on a single receiver, while overcoming all of the drawbacks of previous systems, and introducing, in addition, a number of advantages not present in television systems known to the prior art. In accordance with my invention, the carrier of the single transmitter is amplitudeor frequencymodulated with video signals for a predetermined length of time. Thereafter the carrier is shifted in frequency to provide the line synchronizing impulses. The shifted carrier providing line frequency impulses is frequency-modulated by the sound signals. Periodically at the end of each frame the carrier is shifted to a third frequency to provide the frame synchronizing impulses, The predetermined time of transmission of the video signal has a time duration equal to transmitting one line of the picture, while the predetermined time during which the line synchronizing impulse is transmitted is made equal in time duration to that allowed for the return line or flyback time. It will be noted that'the sound signals are transmitted only during the line synchronizing pulses. As is well known in the prior art, where the line frequency is high and is above the maximum sound frequency sought to be reproduced, highly satisfactory sound reproduction can be obtained; that is to say, that it is not necessary that the 7 sound transmission be continuous, but may be interrupted at a relatively high rate without introducing objectionable distortion, While it is known toltransmit sound intermittently durthis is especially true of subcarrier modulation.

By my invention, where the line synchronizing impulses are wide-band frequency-modulated by. the sound, the signal-to-noise ratio becomes very high.

The advantage accruing from my new, improved and novel system of transmitting and receiving television images and sound are: (1) only a single transmitter is required; (2) in a given channel a wider band of frequencies is available for television proper to provide better definition; (3) better signal-to-noise ratio is provided for line and frame synchronizing signals, resulting in better synchronization; (4) improved signal-to-noise ratio is obtained for sound transmission; (5) cross modulation, or

any other interaction, between the sight and sound signals is completely avoided; (6) cross modulation or interference between the line and frame synchronizing signals is also eliminatel.

By utilizing only a single transmitter, in accordance with my invention, the entire frequency band of a given television channel is available during the picture transmission time for picture signals. This results from the fact that no fre-v quency band needs to be provided for a separate audio carrier with intervening guard bands between this carrier and the maximum frequency of the video channel, so that efiectively substantially another megacycle of band width is added to the video channel.

By shifting the carrier to a new frequency to provide the line synchronizing signal and shifting the carrier to yet another frequency to provide the frame synchronizing signal, and providing these shifts at different times, better signal-tonoise ratio is obtained, since both amplitude and frequency selection can be utilized for the synchronizing signals. As is well known, the noise is a function of the band width, so that if at the receiver the band width for the synchronizing signals is effectively reduced, the noise is correspondinglyreducedwhich automatlcally'improves different time intervals and on different shifted ingthe return line time in television systems,

the signal-to-noise ratio. Moreover, the. fact that the synchronizing signals are transmitted at carrier frequencies means that there can be no interaction between these signals in my improved system of'television, since no cross modulation effects can take place unless there be a simultaneous transmission of the two signals. Since the transmission of the sound signals is restricted to the fiyback or return line time, there can be no cross modulation or other interference between the video and audio signals, since during the return line time there is nov transmission of the video signals.

aBy frequency-modulating the shifted carrier for the line synchronizing signals over a very wide band improved signal-to-noise ratio for the sound signal is obtained. As is well known in the art, the signal-to-noise ratio in frequency modulation is improved as the ratio of the frequency shift tothe maximum rate of frequency shift is made greater. Since I provide a ratio on the order of 200, the signal-to-noise-ratio in the disclosed system is high so that a consequential improvement in sound reproduction results.

Accordingly, it is the main objective of my invention to provide a newand improved method,

7 system, and apparatus for television transmission.

Another objective of my invention is to provide a television transmitting system utilizing only a single transmitter and antenna.

A further objective of my invention is to provide a television system requiring only a single antenna and receiver 'for receiving both sight and sound signals.

Yet another objective of my invention is to provide method and apparatus for transmitting from a single antenna amplitudeor frequency-modulated signals for pictures, frequency shifted carriers for line and frame synchronizing signals, and frequency modulated shifted carrier signals for the accompanying sound.

A still further objective of my invention is to provide a new method and apparatus for transmitting and receiving video and audio si nals requiring only a single transmitter and receiver and having improved signal-to-noise ratio for synchronizing and sound signals, greater definition, and complete avoidance of cross modulation between sight and sound signals, and line and frame synchronizing signals.

Other objects of my invention will become evident upon reading the detailed description of my invention taken together with the drawings.

In the drawings, Figure 1 is a graphical representation of television signals interspersed with synchronizing signals. Figure 2 is a graphical representation of the utilization of the frequency spectrum compared with the antenna transmission characteristic. Figure 3 shows a blocked diagram of a transmitter embodying my invention, while Figure 4 shows a blocked diagram of a receiver embodying the principles of my invention.

Turning now to Figure 1, there is shown the emitted signal of a transmitter incorporating my invention and amuming negative modulation, as understood in the art, in which the peak value I represents the line synchronizing interval, during which time no video signals are transmitted, while the portion 3 of the curve represents the video signal. The interval during which the video or picture signals are transmitted isshown by the. letter P, while the interval of the synchronizing signal is shown by the letter S. The synchronizing signal amplitude extends from the black level 5 on the order of 75% of the maximum amplitude to the maximum amplitude shown on the scale as 100%. The audio signals are transmitted only during the retrace or flyback intervals s and are transmitted as frequency modulation of the line synchronizing pulses. It will be appreciated that the frequency of the line synchronizing pulses has been, in accordance with my invention, shifted inofrequency from the frequency, Io, of the video signals to a different frequency,

f1. This is indicated in Figure 1 where the line synchronizing pulses have a frequency. f1, while the frequency of the carrier for the video signals is indicated as 10. At the end of each frame a frame synchronizing signal having a similar amplitude characteristic of that of the line synchronizing signal, but a different frequency. ft, is transmitted.

The line and frame synchronizing pulses are preferably (although not necessarily) of constant height, extend above the black level, 5, and are separated from the video signals by amplitudeselection at the receiver. Since the line and frame synchronizing signals occupy different frequency ranges, frequency selection after the amplitude selection will differentiate between the line and frame signals. The frequency selection serves also to exclude the frame signal impulses from the discriminator circuit, which serves to extract the audio modulation from the sound frequency modulated line synchronizing signals. No loss in signal-to-noise ratio for the sound signal is experienced, even though the sound signal is transmitted for only 10 to 15% of the time because a very wide frequency swing is provided for the sound signals. Moreover, during the picture interval (-90%) in which no sound signal is received no noise can be received in the sound channel because the noise is suppressed together with the picture signals in the amplitude selector,

In Figure 2 there is shown a frequency spectrum in which the solid line 9 represents the over- 1 all response of the vestigial side band'transmitter and receiver for the video signals during the periods P of a 525-line 30-frame system, for example. The broken line II shows the over-all response of the transmitter and receiver during the fiyback time interval 8 for the line synchronizing pulses extending, for example, from fo minus 1 megacycle to f0 plus 2 mc., while in is the carrier frequency of the video modulation. This band of 3 mo. width yields line synchronizing impulses, of a frequency of the order of 15 kc. for the assumed system, of ample steepness and permits frequency modulation of the sound signals over a frequency range of :1 mc. The line synchronizing pulse is shifted during its transmission time to a frequency of {a plus 0.5 mc. The dotted line l3 represents the over-all response curve for the frame timing impulses in which the carrier is shifted to a different frequency 1: equal to, as shown in the diagram, for example, In plus 3 mo. A band width of *;0.5 mc. is provided to insure ample steepness of the frame signal wave front. The frame ,impulse may, for example, be sent out during the picture period having a duration equal to that of the line synchronizing impulses, or even as long as 25% of a line scanning period, and commencing 5% of a line scanning period after the end of the line synchronizing signal for even fields, and 55% of a line scanning period after the end of a line synchronizing signal for the odd fields in an interlaced picture system.

More specifically, my method may embody apparatus shown schematically in Figure 3. Since my method utilizes conventional apparatus such as is well known in the art, a block diagram has been used in order to simplify the complete understanding of the operation 'of my system. Wave energy from a carrier oscillator 21 is amplified by a suitable amplifier 35. The amplified carrier is then modulated in the modulator ll by signals from the camera amplifier 31, which .is keyed by the retrace blanking unit 39. The retrace blanking unit is fed from the line pulse generator 3| so that during the time of transmission of the line synchronizing impulses no signals are fed from the amplifier 31 to the modulator 4|. 7

to control both the line pulse generator 3| and the frame pulse generator 33. A portion of the frame pulse generator output is also fed to the retrace blanking unit 39 to prevent transmission of the picture signal during the framing synchronizing interval. The retrace blanking unit and line and frame pulse generator may be of the type shown in Smith, U. S. Patent No. 2,- 132,655. A microphone amplifier 2| feeds the am-' plified sound signals to a frequency modulator 25 through a keying tube 23. The keying tube 23 is actuated by the line pulse generator 3| to pass the audio signals from the amplifier 2| to the frequency modulator; that is to say, normally the keying tube is in a blocked condition so that no sound signals get through. The occurence of a line synchronizing signal, however, unblocks the keying tube 23 and permits the sound signal to pass through the frequency modulator to frequency modulate the carrier. The keying tube 23 may be of the form corresponding to the keyed amplifier of the Zworykin U. S. Patent No. 2,- 146,376. The line pulse generator 3| also passes signals directly to the frequency modulator 25, and its amplitude is so chosen to shift the frequency of the carrier to a value of f0 plus 0.5 me. The frame pulse generator 33 also feeds signals to the frequency modulator 25, which may be of the type shown in Finch U. S. Patent 2,- 225,691, which signals have an amplitude sufficient to shift the frequency of the carrier from f0 to jo plus 3 mc. The carrier amplifier 35 serves both to amplify the modulated energy from the carrier oscillator 21 and to limit the amplitude of the carrier so as to suppress all spurious amplitude modulation. Theoutput of the amplifier 35 is then fed to the amplitude modulator 4|. The output of the modulator 4| is then fed, if a vestigial sideband system is used, to the vestigial side band filter 41. The output of the filter 41 feeds through the synchronizing or timing pulse keying unit 45 to the power stage of the transmitter 43 and is thence radiated by the antenna 49 The timing pulse keying unit 45 is actuated by signals from both the generator 3| and the generator 33 to increase the carrier level from the black level 5 of Figure 1 to maximum level I In Figure 4 there is shown a blocked diagram for a receiver embodying principles of my invention and adapted to reproduce both picture and sound from the signals transmitted from the antenna 49, in which the units may be of conventional design known in the art. In Figure 4 the antenna 5| receives the transmitted signals and the received signals, fed to a conventional television receiver, are amplified by the radio frequency amplifier 53. The amplified signals are passed to a mixer 51 where they are heterodyned by energy from the oscillator 55, detected and passed to an intermediate frequency amplifier- 59. The intermediate frequency amplifier may be of conventional design and passing frequencies from in minus 1 me. to in plus 3 mc. with substantially constant gain, dropping off slowly to it plus 5 mc. The output of the intermediate frequency amplifier 59 is then split up as follows:

A portion of the output is fed to the video rectifier 83 and thence to a video amplifier 85 and to the control electrode of a cathode ray tube.

A master pulse generator 29 serves- The direct current component of the rectifier output is fed to the AVG system 6| in conventional manner. The AVC, in turn, serves to control the gain of the amplifiers 53 and 59. Another portion of the output of the amplifier 581s fed to an amplitude separator 63, known in the art as the zero bias detector type or of the conventional type in which an amplifiertube is biased beyond cut-off so that it responds only to amplitudes extending above the black level, that is to say, to the frame and line synchronizing signals. This serves to suppress the picture signals in the output of the separator 63, and the output containing the line andframe synchronizing signals is then passed through a pair of filters 65 and 1|. The filter 65 has a mid-frequency of f0 plus 3 mc. and has a band width of $0.5 mc. This filter, therefore,

permits only the frame signals to pass through,

which signals are then utilized to control a frame sweeposcillator 61, the output of which produces vertical deflection of the cathode ray beam of the kinesqope. The filter H has a mid-frequency of fo plus 0.5 mo. and a band width of :1.5. As a result, only the line synchronizing frequency modulated pulses can pass through the filter ll. The'output of the filter, on one hand, is rectified and fed to the line sweep oscillator 13 to provide a synchronized horizontal deflection of the cathode ray beam.

A portion of the output ofthe filter H is also fed to a limiter H and thence to a frequency discriminator, of the kind shown in U. S. Patent No. 2,229,640 to Crosby, and rectifier 19, which includes a low-pass filter having a cut-off frequency substantially equal to one-half the line synchronizing frequency. The output of the lowpass filter feeds the audio amplifier 8|, which, in turn, feeds the loud speaker 82, to provide a reproduced sound. It will be appreciated that the audio amplifier 8| may include a suitable integrating circuit, such as is well known in the art, for reproducing the sound. It will be further appreciated-that energy from the vertical and horizontal deflection circuits can be supplied to the cathode ray tube control 81 to provide suitable blanking out during return line time of the cathode ray tube.

It is helpful to remember that no two signals can interfere with each other unless they'are simultaneous. Thus there can be no interaction during the picture periods P (Figure 1), since during these the video modulation alone is transmitted.

During the transmission of the synchronizing pulses, the video rectifier receives signals of constant height and varying carrier frequency. All these pulses are rectified equally well, yieldin blacker-than-black pulses suitable for blacking out the retrace of the beam. As stated above, there is no trace of picture signals left in the anode circuit of the amplitude selector tube, only line and frame synchronizing pulses. Any interaction between them, such as cross modulation in the non-linear cut-off region of the amplitude "selector tube, is ruled out since these pulses never These units may all be of conventional design.

occur at the same time; overlap of transient phenomena is excluded by ample intervals between line and frame synchronizin pulses of at least 0.05 of the horizontal scanning period.

The sideband frequencies of the frame synchronizing pulse are restricted to about +0.5 me. and therefore cannot penetrate into the frequency range reserved for the linesynchronizing pulses. Yet this bandwidth permits the frame synchronizing pulse to rise from 0.1 to 0.9 of its final height in Just over .01 of the horizontal scanning period, though a rise one-tenth as fast would suffice for accurate interlace.

The line timing or synchronizing pulses. during moments of silence, when the carrier is at In plus 0.5 mc., may occupy a band of 3 megacycles width. Thus the front of the rectified line timing pulses may rise from 0.1 to 0.9 flnal height in about 0.30 microsecond. This steepness of front compares well with the present R. M. A. standard requiring the pulses to rise to their final level in 0.005 ofthe horizontal scanning period.

However, the carrier of the line timing pulse will seldombe at the center of the allotted band, swinging up to +0.5 megacycle with the audio modulation. It may be suspected that the front of the pulses may depend, in steepness and shape, on the momentary position of the swinging carrier in the band. Such an effect theoretically exists, but it is fortunately very small and experience shows that it may be neglected.

Change of the carrier frequency during the time of a pulse can be neglected, since the pulses are short compared with the period of the highest sound frequency.

The effect of the line synchronizingpulse on the sound modulation, that is, the fact that the sound tranlnitted only during discrete short bursts of a frequency-modulated carrier, does not impair the quality of sound reproduction. The pulses, limited to equal height, will produce in the frequency discriminator circuit bursts of oscillations of a height varying in linear proportion to the modulated frequency. The output of the rectifier thus contains, apart from the i/ carrier frequency and sidebands, not only the envelope of these pulses of the sound modulation, but also the pulses themselves. These pulses represent a subcarrier frequency amplitude-modulated by the audio modulation, but so long as the low-pass filter in the audio detector has a frequency cut-off substantially equal to one-half of the line frequency, there will be no overlap between the audio band and the eflective lower sideband frequencies of the line synchronizing impulses.

It will be appreciated that while concrete examples of the magnitude of the frequency shift of the carrier and the bandwidths have been given, these. values were merely mentioned to make applicant's invention clear. It will'also be appreciated that where it is desired to achieve greater steepness of the line synchronizing pulses that a wider bandwidth than 3 me. may be provided with a corresponding suitable shift of the carrier. Moreover, it will be appreciated that in the event that the television channel is broadened out that the over-all response curve shown in Figure 2 may. correspondingly be increased, that is to say, in general, the values are merely indicated for descriptive purposes and are not to be construed as specific values which must be followed in order to derive the benefits of my invention.

It will be further appreciated that while a number of patents have been cited to show conventional embodiments, these citations were merely by way of example and that it is not necessary to utilize the specific embodiments disclosed by these patentees. The citations were made by way of example and, of course, any other forms of conventional apparatus known in the art may be substituted or utilized without departing from the scope of my invention.

Various alterations and modification of the present invention may become apparent to those skilled in the art and it is desirable that any and all such modifications and alterations be considered within the purview of the present invention except as limited by the hereinafter appended claims.

Having described my invention, what I claim is:

1. The method of reproducing images and sound accompaniments which includes the steps of producing video signals representative of the image to be reproduced, generating carrier oscillations of a predetermined frequency, modulating the produced carrier oscillations by said video signals, generating line synchronizing pulses and frame synchronizing pulses, generating signal energy representative of the sound accompanying the video signals, interrupting the video signal modulation of the carrier frequency under the control of the line synchronizing pulses, producing a shift in the frequency of the generated carrier to a differentvalue during the intervalsof interruption of video signal modulation, frequency-modulating the shifted frequency carrier "by the sound signal energy, periodically shifting the predetermined frequency to a second different value under the control of the frame synchronizing signals, transmitting all of said modulated and frequency shifted carrier frequency oscillations, receiving the transmitted carriers, demodulating the received carriers, segregating the video signals from the line synchronizing, frame synchronizing, and audio signals, segregating the line synchronozing signals from the frame synchronizing signals, frequency-demodulating the limited signals, producing sound under the control of the frequency demodulated signals, and synthesizing the image under the control of the segregated video signals and both the line synchronizing and frame synchronizing pulses.

2. The method of transmitting signals representative of images and sound accompaniments which includes the steps of producing video sigi nals representative of the image to be reproduced, generating carrier oscillations of a predetermined frequency, modulating the produced carrier oscillations by said video signals, generating line synchronizing pulses and frame synchronizing pulses, generating signal energy representative of the sound accompanying the video signals, interrupting the video signal modulation of the carrier; frequency under the control of the line synchronizing pulses, producing'a shift in the frequency of the generated carrier to a different value during the intervals of interruption of video signal modulation, frequency-modulating the shifted frequency carrier by the sound signal energy, periodically shifting the predetermined frequency to a second different value under the control of the frame synchronizing signals, and transmitting all of said modulated and frequency shifted carrier frequency oscillations.

3. The method of reproducing images and sound accompaniments which includes the steps of producing video signals representative of the image to be reproduced, generating carrier oscillati ons of a predetermined frequency, amplitudemodulating the produced carrier oscillations by said video signals, generating line synchronizing pulses and frame synchronizing pulses, generating signal energy representative of the sound accompanying the video signals, interrupting the video 7.6 signal modulation of the carrier frequency under the control of the frame synchronizing signals,

the control of the line synchronizing pulses, shifting the generated carrier frequency to a different value during the intervals of interruption of video signal modulation, frequency-modulating the shifted frequency carrier by the sound signal energy, periodically shifting the predetermined frequency to a second different value under the control of the frame synchronizing signals, transmitting all of said modulated and frequency shifted carrier frequency oscillations, receiving the transmitted carriers, demodulating the received carriers, segregating the video signals from the line synchronizing, frame synchronizing, and audio signals, segregating the line synchronizing signals from the frame synchronizing signals, limiting the separated line synchronizing signals, frequency-demodulating the limited signals, producing sound under the control of the frequency demodulated signals, and synthesizing the image field under the control of the segregated video signals and both the line synchronizing and frame synchronizing pulses.

4. The method of reproducing images and sound accompaniments which includes the steps of producing video signals representative of the image to be reproduced, generating carrier oscillations of a predetermined frequency, frequencymodulating the produced carrier oscillations by said video signals, generating line synchronizing pulses and frame synchronizing pulses, generating signal energy representative bf the sound accompanying the video signals, interrupting the video signal modulation of the carrier frequency under the control of the line synchronizing pulses, controlling the carrier frequency generation to produce a carrier frequency of a difierent frequency value during the intervals of interruption of video signal modulation, frequency-modulating the shifted frequency carrier by the sound signal energy, periodically shifting the predetermined frequency to a second different value under the control of the frame synchronizing signals, transmitting all of said modulated and frequency shifted carrier frequency oscillations, receiving the transmitted carriers, demodulating the received carriers, segregating the video signals from the line synchronizing, frame synchronizing, and audio signals, segregating the line synchronizing signals from the frame synchronizing signals, limiting the separated linesynchronizing signals, frequency-demodulating the limited Sig-- nals, producing sound under the control of the frequency demodulated signals, and synthesizing the image field under the control of the segregated video signals and both the line synchronizing and frame synchronizing pulses.

5.' The method of reproducing images and sound accompaniments which includes the steps of producing video signals representative of the image to be reproduced, generating carrier oscillations of a predetermined frequency, modulating the produced carrier oscillations by said video signals, generating line synchronizing pulses and frame synchronizing pulses, generating signal energy representative of the sound accompanying the video signals, interrupting the video signal modulation of the carrier frequency under the control of the line synchronizing pulses, controlling the generation of the carrier frequency oscil-- lations to produce an output .of a different frequency value during the intervals of interruption of video signal modulation, frequency-modulating the shifted frequency carrier by the sound signal energy, periodically shifting the predetermined frequency to a second difierent value under transmitting all of said modulated and frequency shifted carrier frequency oscillations, receiving the transmitted carriers, demodulating the received carriers, segregating the video signals from the line synchronizing, frame synchronizing, and

audio signals, detecting the segregated video 513- nals, segregating the line synchronizing signals from the frame synchronizing signals, limiting the separated line synchronizing signals, frequency-demodulating the limited signals, producing sound under the control of the frequency demodulated signals, and synthesizing the image field under the control of the detected video signals and both the line synchronizing and frame synchronizing pulses.

6. A television transmitting system comprising a television camera, a master pulse generator for controlling the scanning of said camera, a line pulse generator, a frame pulse generator, means to control said line pulse and said frame pulse generators from said master pulse generator, a carrier wave oscillator, an amplitude modulator for controlling the amplitude of the energy from said carrier wave oscillator, means to actuate the said modulator by energy resulting from the scanning of said camera, means to periodically interrupt said actuation by the line pulse and frame pulse generators, means to transmit the amplitude modulated carrier wave energy, a source of sound energy, a frequency modulator, means to energize the frequencymodulator by said source of sound energy, means to connect the frequency modulator to the carrier wave oscillator to vary the frequency thereof, means to interrupt the 'energization ofthe frequency modulator by the said line pulse and frame pulse and amplitude modulated by said signals from the camera,

'7. A television system comprising a television camera, a master pulse generator for controlling the scanning of said camera, a line pulse gen-.

erator, a frame pulse generator, means to control said line pulse and said frame pulse generators from said master pulse generator, a carrier wave oscillator, an amplitude modulator for controlling the amplitude of the energy from said carrier wave oscillator, means to actuate the said modulator by energy resulting from the scanning of said camera, means to periodically interrupt said actuation by the line pulse and frame pulse generators, means to transmit the amplitude modulated carrier wave energy, a source of sound energy, a frequency modulator, means to ener-' gize the frequency modulator by said source of soundenergy, means to connect the frequency modulator to the carrier wave oscillator to vary the frequency thereof, means to interrupt the energization of the frequency modulator by the said line pulse and frame pulse generators during the time interval in which the amplitude modulator is actuated, means to shiftthe frequency receive the transmitted energy representative of the synchronizing sight and sound signals, means to detect the received signals, amplitude separator means to segregate the line and frame synchronizing pulses from the camera signals, filter means to separate the line synchronizing pulses from the frame synchronizing pulses, means to control vertical deflection oi a cathode ray tube by the separated frame synchronizing pulses,

means to control the horizontal deflection of said 10

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