US2201309A - Method and system for television communications - Google Patents

Description

. May-21, 1940. T. T. GOLDSMITH. JR

METHOD AND SYSTEM FOR TELEVISION COMMUNICATIONS 3 Sheets-Sheet 1 o a n 3. S o 252 3.5 5.30 5.2 2.2 E.

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twist! INVENTOR worms 1- sows-m1 T. T. GOLDSMITH. JR I 2,201,309

METHOD AND SYSTEM FOR TELEVISION COMMUNICATIONS May 21,1 '0.

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signos' to fiiiien l8. iliu Mont Laboratories, c., Passaic, hi. it, a-corporation oi ilieiawnre (on. it's-tbs),

My invention relates to hnprovements in meth-= ods and systems for television counication.

This application is a division of my application Serial No. 3.955%, died March 12, was.

in the more sucoessiui methods and systems proposed heretofore ior teievision communication, and employing a cathode-ray pick-up tube at the transformer and a cathode-ray viewing tube at the receiver, it has been required that there be ateach station two circuits for deflect ing'the scanning rayat the line and field frequencies, respectively. disc, it has been required that line and field pulses be transmitted on the samecarrier wave as the video signals, and these ps have been used at the receiver for synchronization. ln thme prior methods and systs, not only has"= cu1ty been encountered in holding the deflecting circuits at the receiver.

iocked in step with those at the transmitter, but a substantial portion of the transmitted energy hasbeen expended in transmitting the synchro- .o-Iif 1|; P111888, I I ith the foregoing in mind, it is one of the objects of my invention to provide an improved methodand system for television communication employing cathode-ray tubes at both sta-. tions, and in which no synchronizing pulses are required to be transmitted to the receiver, and in which eitherthe deflection circuit at the line frequency or both deflection circuits at the line and iield frequencies, respectively, can be omitted at the receiver. j

Another object of my invention resides in the provision of an improved method and system for television communication which has advantages over those proposed heretofore in the way of greater simplicity of construction and manner of operation, and higher efflciency.

Another object of my invention resides in the provision of an improvedmethod and system for television communication by which the various necessary signals can be transmitted by wireless over substantially greater. distances than it has been possible with the various methods and systeins proposed heretofora-tho possible range of wireless transmission from the transmitting station directly to a recelvingstation being of the order of hundreds of miles.

Other objects and advantages will hereinafter appear.

1 In accordance with my invention, voltage waves at the respective. line and field frequencies are generated at the transmitter andtransmitted by radioto the receiver. These same waves are applied to cause or at least govern scanning ac- 1 must of necessity be always in step during the scanning action. Because of the impossibility o of this system getting out of synchronization as to both line and frame or field frequencies, a much higher interlace ratio is possible. This allows a given detail picture to be transmitted over a much narrower frequency band. Likewise, the transmission of much higher definition pictures over the same frcouencyband can also be accomplished.

Another advantage, of this system is that it will receive pictures of transmitters operating at 1 any desired line and frame frequencies. With the present systems, it is necessary that all transmittersoperate with exactly the same respective scanning frequencies if all receivers are to be capable of receiving fromazpny one of the trans- 2o mitters.

My invention resides in the improved method and system of the character hereinafter described and claimed.

For the purpose of illustrating my invention, an embodiment thereof is shown in the drawings,

wherein 1 Figures 1 and 2 are simplified, diagrammatic views of a television transmitting station and a television receiving station, respectively, con- 30,

structed and operating in accordance with my invention;

Figs. 3 and 4 are graphical representations of electrical wave forms possible for-use in my improved system for governing the scanning action; and

i Figs. 5, 6 and 7 show circuit details for use in my improved system.

In Fig. 1, the reference numeral l0 designates acathode-ray pick-up tube of a conventional 40 construction, and comprising a' mosaic, photoelectric screen on which a light image of the object is projected, an electron gun for-generating a ray of electrons directed at the screen, and two setsof deflecting plates for deflecting the electron ray at the line and field frequencies respectively so that it is caused to scan the screen. The picture or video signals are thereby developed, and fed by an output connection H to a. modulating amplifier l2. 1

In my improved system, but one ultra-high frequency carrier is used for all four signals, i. e.,-the horizontal and vertical sweep signals,

a and the video and sound signals.

The reference numerals. l1 and I8 designate sweeps and which are, respectively, at the desired field and line frequencies. The lowfrequency sweep, by means of the amplifier l9, modulates the sub-carrier oscillator 20 in its amplifier stage 2|. The high-frequency sweep. by means of the amplifier 22, modulates the subcarrier oscillator 23 in its amplifier stage 24.

The audio or sound signals from the micro phone 25 are passed through an amplifier 26.

The two modulated sub-carriers are combined with the audio signal in the mixer 14 and the combination is'fed through the modulator 15 to modulate the output of the oscillator 16 at the amplifier 11. The composite signal upon the subcarrier from oscillator 16 is mixed with the video signal in the mixer 18, after which, through;

modulator I9, the-single, ultra-high frequency carrier produced by the oscillator 19 is modulated at the amplifier 80 and radiated from the antenna 3|.

Although the two sweep signals are transmitted in the form of sinewaves, in Fig. 1 they are not used as such for direct application to the two respective sets of electrostatic deflecting plates, but are first changed or modified to waves multiplying method for. the purpose of main taining constant, particularly where interlaced scanning is used, the relation of line frequency to field frequency. A system or circuit for such purpose is shown in Fig. 6, in which a low-frequency voltage wave of sine form, such as the 60-cycle power main frequency, is fed through successive full-wave, rectifiers 31, 38, etc., followed by filters, as shown, to allow passage of the double-frequency harmonics. In this way there is produced a final and much higher frequency which can be utilized for the high-frequency sweep coordinated with the original lowfrequency sweep for the two respective deflections for scanning. In Fig. 6 there is shown only two stages of the frequency-multiplication scheme. A'transformer primary 39 is excited by 60 cycles, and the tapped secondary 40 supplies the fullwave rectifier 31 which delivers 120 cycles to the tuned filter comprising the condenser 4| and thenext transformer primary 42. The 120-cycle signal is supplied by the tapped secondary 43 to the full-wave rectifier 38 and thence at the double frequency of 240 cycles to the condenser 44. and the next succeeding transformer primary An alternative and simplified system or circuit,

for the same purpose as that shownin Fig. 6, is shown in Fig. '7. In Fig. '7, the reference numerals 41, 48 and 49 designate transformer wind-' ings, connected as shown with respect to fullwave rectifiers 50 and 5| and condensers 52 and 53.

In Figs. 6 and 7, conventional amplifier stages may be employed when needed. Also, if it is degenerators of voltage waves of sine form for the sired to produce a phase relation of the final frequency, phase-shifting networks may be inserted in one or more stages of the frequencymultiplication system. In cases where it is desired, as in interlaced scanning, that the final,

high frequency be an odd multiple of the original low frequency, rather than an even multiple of the latter, it is proposed to use an excess number of doubler stages and then to employ one or more conventional multivibrator frequency-reduction stages to obtain the desired odd-scanning frequency for the line sweep. i i At the receiver, as shown in Fig. 2, a superheterodyne circuit is employed, preceded by the radio-frequency amplifier stage 55 fed from the,

antenna 54. The detector 51 combines the received carrier with the output of the local oscillator 56. A wide-band, intermediate-frequency amplifier 8| provides sufficient gain, and detector 82' then "delivers signals to the two channels shown. The filter 83 passes only the video signal. The intermediate-frequency stage 84 selects exclusively the sub carrier of 3M0, then detects its modulation at 63, and then delivers the two sweeps and, the audio signal to their respective channels through the filters 64, and 66. and

the detectors 6'! and 70;

The low-frequency sweep signal, still in the form of modulation upon its carrier, is detected at 61 to produce in the output line a voltage wave .of sine form which is changed, by a wav l-form modifying network 69, to a saw-tooth voltage wave which is applied across the corresponding deflecting plates. Likewise, the high-frequency sweep signal, still in the form ofmodulation upon its carrier, is detected at '10 to produce in the output line a voltage wave of sine form whichis changed by a wave-form modifying network 12, to a saw-tooth voltage wave which is applied across the corresponding deflecting plates.

The audio signal, in its original form from th filter B6, is fed to a loudspeaker 13. v

The video signal is applied to ascanning device 62. The device 62 is represented as being in the form of a cathode-ray tube of a common construction, and comprising a fluorescent screen, an

, electron gun for developing a ray of electronsdirected at the screen, and two sets of electrostatic the intensity of the electron rayis made to vary with the picture or video signals.

In Fig. 2, the unit 85 represents an automatic signal-level control circuit which, in the well known manner, utilizesa rectified portion of the received signal to adjust the bias on previous stages to aid in maintaining substantially constantsignal output. In the present case, signal for the ,unit 85 is taken from the high-frequency sweep signal will be steady and independent of background variations of the video signal. As"

shown, the output control signal from the unit 85 is applied to control bias in the first detector stagael51 and in theintermediate-frequency channel Instead of transmitting sinusoidal sweeps as in Fig. 1, the generators I1 and I8 may be made to develop saw-tooth sweep wave forms, as shown in Fig. 3. Such a sweep wave form is characterized by having a relatively wide frequency band necessaryv for faithful transmission. Its fundamental frequency and also its harmonics up to about the tenth should be provided for. This type of scanp a very marked advantage of the present improved ning provides systematic and uniform coverage of the screen, and though high in harmonic contentcan still be used with this system by proper design of the intermediate-frequency filters.

Another wave shape, illustrated in F12. 4, may be transmitted. in which case the generators l1 and I! are designed accordingly. It is characterized'by equal slopes of the forward and return traces, thus containing a much lower harmonic in the selective circuits at the transmitter and receiver. I

It will be understood thatthe actual frequencies indicated in Figs. 1 and 2 are by way of example only, and that these might vary widely to meet particular requirements. In case the other shapes of scanning signals are desired, it will be necessary to choose other frequency bands than here shown, but the general principles will be the same and are believed to i disclosed for those skilled viding a narrow'frequency band and consequent simplified amplification and transmission of the sweeps.

Still. another feature of my improved system resides in the reduced video-frequency band possible with the multiple interlace or high interlace ratio, yet giving high definition pictures. With this reduced video-band width and its relatively narrow associated band containing the audio and the sweeps, it is practical to-utilize other and lower carrier frequencies than those suggested in Figs. 1 and 2, and thus to employ carriers which are not limited to an optical horizon for satisfactory coverage.

i The frequency width of the video. signals illustrated in Figs. 1 and 2 has been shown as those characteristic of a picture of about GOO-line deflnition and the carriers chosen are merely suggestive. One can see. that even with this provision for a video channel of higher definition than provided in a unit with the 441-1ine standards with 2 to 1 interlace,-the ultra-high frequency band still is not increased over that required for the iii-line 2 to l interlace pictures. Now if only 441 lines are. desired, using the system herein disclosed of multiple interlace, and by very stringent selectivety in closely spaced channels, it is possible to reduce the channel band width to approximately 1 megacycle each side of theultrahigh frequency carrier, thus making practical mthod-andsystem.

Another advantage of my improved system is that a receiver of the type illustrated would be. versatile in its ability to accept transmissionsfrom diii'erent stations utilizing diflerent degrees of detail up to a certain limit. Given a specified band pass in each channel. then pictures of any detail up to a certain limit could be accepted by simply tuning to the appropriate R. 1''. carrier of the station in question, without necessity of local achustment to duplicate. their particular sweep system. It will facilitate public held installation of television systems without the fear' of equipment becoming immediately obsolete when definitlon changes are desired.

Another advantage of my improved system is the possibility of the use of automatic volume control, or automatic signal-level control by applicatlon from one or the other fixed signals in the sweep circuits. Automatic control based on this signal will be steady and independent of background variations of the video signal;

It has been foundthat excellent pictures may be obtained when integral ratios between the scanning frequencies are purposely avoided. The television system herein disclosed allows practically any type of interlacing to be employed. For example, assume that there is employed a vertical sweep of sixty fields per second, which is well beyond the flicker limit. Now a horizontal frequency of 3000 cylces per second would give fifty lines with exact progressive scanning. Now an increase to 3030 cycles will provide 101 lines in the complete picture with a two to one interlace ratio. The picture repetition rate is reduced to 30 per second. but there is still no appreciable loss in picture continuity. Now

use some horizontal sweep such as 3002 cycles per second and the scanning will return to a given configuration only after half a second. This produces an effect of linesdrifting by when the pattern is greatly magnified, but on the conventional viewing screen, the use of this scheme employing other than integral ratios between sweep frequencies, makes the line structure practically indiscernable. Use of this peculiar ratio of sweeps is practical with the'herein described television system, as it is necessary to regulate the sweeps only at the transmitter.

A feature of my improved system resides in the utilization of actual scanning wave shape transmission suitable for electrostatic cathoderay' deflection, though with proper'ampliflers. the use of electromagnetic deflection is also pos-' sible. It is obvious that utilization of electrostatic deflection at the receiver would allow the same to follow the changes in scanning frequency or wave-form, whenthese might be varied at the transmitter. It would be more diillcult to design an electromagnetic system having this same flexibility.

Another feature of my improved system is the possibility of further simplification of the receiver equipment if operated on a common power main with the transmitter, for then the power mainsfrequency may be employed for the low sweep, reducing the complexity of the receiver in that locality. However, the transmitter carrier may still ,contain this low sweep component even when exciting such a receiver, for no disturbance will be experienced if units 84 and M in Fig. 2 are omitted and replaced by suitable attachments to the power mains. In the light of this modifica tion, it would be most practical to secure the ment is a limit of the automatic signal-level\ control voltage from the high sweep channel which will be present in all receivers. g V

Still another advantage of my improved system is that its receivers will be versatile enough ning action thereat, the steps in the -method of operation which consist in generating video signals modulating a sub-carrier with the highfrequency sweep wave, modulating a second subcarrier with the modulated first-named subcarrier, mixing the video signal and the modulated second sub-carrier, modulating a main ultra-high frequency carrier with the resulting composite signal, and feeding said modulated main carrier to radio transmission means.

2. In the art of television communication wherein it is required that electrical sweep waves of lowfrequency and highfrequency respectively be supplied to the receiver for governing the scanning action thereat, the steps in the method of operation which consist in generating 'video signals, modulating a first sub-carrier with the low-frequency sweep wave, modulating a segond sub-carrier with the high-frequency sweep wave, mixing the modulated first and second subcarriers with audio signals, modulating a third sub-carrier 'with the resulting composite signal, mixing the modulated third carrier with the video signal, modulating a main ultra-high frequency carrier with the final composite signal, and feedingsaid modulated main carrier to radio transmission means.

THOMAS T. GOLDSMITH, JR.

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