US2299471A - Method and system for television communication - Google Patents

Description

Oct. 20, 1942.

B. DUMONT ETAL METHOD AND SYSTEM FOR TELEVISION COMMUNICATION Filed June 25, I940 3 Sheets-Sheet 1 v (Ari/141111144351711591111.7115;Fir 1 5 WW v W 5 92...... 5:35.; g H. mui avi Q. 1 W iuFagv 055. m2 A own; 1 .3 5 1.535%

Oct. 20, 1942. A. B. DU MONT EIAL METHOD AND SYSTEM FOR TELEVISION COMMUNICATION Fi led June 25. 1940 3 Sheets-Sheet 2 INVENTORS Allez .B- .1 14 Haze? Oct. 20, 1942- A. B. nu MONT arm,

METHOD AND SYSTEM FOR TELEVISION COMMUNICATION Filed June 25, 1940 3 Sheets-Sheet 3 INVENTORS Allen .3. Da IIotzf &

Patented Oct. 20, 1942 UlTE STATE METHOD AND SYSTEM FOR TELEVISION COMMUNICATION Allen B. Du Mont and Thomas '1.

Upper Montelair, N. J.,

Goldsmith, In, assignors to Allen B.

Du Mont Laboratories, Inc., Passaic, N. 1., a

corporation of Delaware Application June 25, 1940, Serial No. 342,196

11 Claims.

The necessity, in television communication, of

keeping the respective scanning devices at the transmitter and receiver in synchronism, has' given rise to many problems. For this purpose, it has been proposed-to generate at the transmitter the required line, field or frame, and blanking pulses, and to transmit these on the same carrier with the video signals. The different pulses are selected out at the receiver and used to serve their respective purposes. A typical system belonging to this general class is disclosed in Patent No. 2,132,655, issued October 11, 1938, to John P. Smith.

Another proposed system or method of keepin in step the cathode-ray tube scanning devices at the transmitter and receivers, resides in transmission to the receivers of the actual sweeps for horizontal and vertical deflection respectively of the scanning electron rays. Such a system and method is disclosed in Patent No. 2,186,634 issued January 9, 1940, to Allen B. Du Mont, and Patents Nos. 2,164,176 and 2,201,309 issued June 27, 1939, and May 21, 1940, respectively, to Thomas T. Goldsmith, Jr. I

While the various systems and methods above referred to have been eflective to produce satisfactory results, there still is the disadvantage in each that the control or synchronizing signals or wave forms must be transmitted to and separated out at the receivers. This not only requires use of a greater band width than would be the case if it were not necessary for these signals or wave forms to be transmitted with the video signals, but also gives cause for distortion and interference generally, due to undesirable and extraneous surges or signals which occur and which must be reckoned with.

With the foregoing in mind, it is one of the objects of our invention to provide an improved method and system for television communication which is simple and reliable in operation, and by which the various disadvantages of the synchronizing means and methods proposed heretofore can be avoided.

Other objects and advantages will hereinafter appear.

The basic principle involved in our invention resides in the utilization of a single electrical control eiit'ect, which may be a 30-cyc1e sine wave, for example, to maintain synchronism of the respective scanning devices at the transmitting and receiving stations. In one form of our invention,

where cathode-ray tubes are used for generating the picture signals at the transmitting station and for reproducing the picture at the receiving stations, there is provided at each station a local control device in the form of a cathode-ray tube having a screen whose surface has a special, circular pattern, the different portions of which have diflerent degrees of secondary emission. The pattern is such that if the beam of electrons is deflected circularly at a relatively low frequency such as 30 cycles, to scan the pattern at this rate, there is developed in the output circuit of these local, control devices, synchronizing pulses at the desired line frequency, synchronizing pulses at the desired field frequency, and blanking pulses at both the line and field frequencies. These pulses, obtained locally at each station from the respective, single cathode-ray tube which supplies the same, are used to drive the local horizontal and vertical sweep circuits and to out oif the electron scanning beam at the end of each line and field. The common, 30-cycle sine wave is generated at the transmitting station and transmitted to the receiving stations for the purpose explained.

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

For the purpose of illustrating our invention, an embodiment thereof is shown in the drawings, wherein Figures 1 and 2 are simplified, diagrammatic views of television transmitting and receiving stations, respectively, which are constructed and which operate in accordance with our invention;

Fig. 3 is a detail, elevaticnal view, looking toward the right in Figs. 1 and 2, of the screen in the local, control cathode-ray tube devices;

Figs. 4 and 5 are fragmentary, lay-out views of parts of the special pattern of the screen in Fig. 3; and

Figs, 4a and 5a are graphical illustrations of the operating effect produced during the scanning of the pattern parts shown in Figs. 4 and 5, respectively.

With reference to Fig. 1, the numeral Hi designates a cathode-ray tube of a conventional construction for generating video or picture signals, and in which a ray of electrons is caused to scan a photosensitive screen on which is pr0- jected an image of the view for transmission. The scanning ray or beam is deflected by coils or plates horizontally at a suitable line frequency and vertically at a suitable field frequency. For this purpose, a horizontal sweep circuit H applies a saw-tooth wave at line frequency across the horizontal-deflection coils or plates, and a vertical sweep circuit i2 applies a saw-tooth wave at field frequency across the vertical-deflection coils or plates. The sweep circuits are controlled and kept locked in step by synchronizing pulses at the desired line and field frequencies. These pulses are generated by the control device, designated generally by the reference numeral l3. and supplied by the output connection It to the sweep circuits l l and l2.-

The video signals are amplified in the video amplifier IB-and in the line amplifier l8, and are then fed to the transmitter ll.

The horizontal blanking pulses and the vertical blanking pulses are also generated by the control device 13, and are supplied by the output connection It and line it to a blanking amplifier I9, and thence at negative polarity to the intensity-control grid of the electron gun in tube 10.

A 30-cycle sine wave from a local source represented at 20, is supplied to a transmitter 2i and sent to the receiving stations. However, the transmitter ll may be used for this purpose, in which case the separate transmitter 2i would be omitted.

The control device I3 is shown as being in the form of a cathode-ray tube comprising an evacuated tube 22 having in the neck portion thereof an electron gun of a conventional construction for developing a ray or beam 23 of electrons directed at and focused on a screen or target it at the other end of the tube.

The screen 24 comprises a base plate 25 of aluminum foil to which a supporting contact 26 is electrically connected. The aluminum plate may be backed by a suitable plate to impart s'uflicient rigidity, as will be well understood.

n the surface of the screen 26 which is scanned by the electron beam 213, is a pattern designated generally by the reference numeral 21 in Fig. 3, and which is circular and concentric with respect to the center of the screen 24.

By passing the 30-cycle sine wave through a phase splitter 28, the beam 23 is deflected circularly and caused to scan the pattern 2'6 at the rate of 30 times a second.

The diametrically-opposite sections or parts 4 and of the pattern 2i are shown laid out on an enlarged scale in Figs. 4 and 5, respectively. On the aluminum surface 25 there are printed 441 silver marks or lines 3|, equally spaced circumferentially with respect to each other. At the diametrically-opposite points, and over a small ,arc embracing only a few of the silver marks 3|, there are printed on the aluminum surface a large number of very fine and uniformly spaced silver lines 32. The surface between the several silver marks 3! on either side of the two, diametrically-opposite groups of the fine, silver lines 32, is that of the aluminum foil 25,

. but beyond this there is carbon printing 33 on The desired operating action in our improved method and system depends upon the fact that different materials have different degrees of secondary emission. In other words, certain substances or materials yield a greater number of electrons of secondary emission when bombarded by primary electrons, than certain other substances. For example, carbon has a relatively low degree of secondary emission, aluminum has a relatively high degree of secondary emission, and silver has a degree of secondary emission even higher than aluminum. The focusof the beam 23 on the pattern 21, and the spacing of the silver marks or lines 3| and 32 and the carbon printing 33, are such that the electron beam 23, as itscans the pattern 21, has a predominating eifect on only one of the silver lines at a time.

From the foregoing, it will be seen that if the electron beam 23 is caused to scan the pattern 21 at the rate of thirty times a second, there will appear in the output line It a control effect or signal such as is graphically illustrated in Figs. ea and 50,. As the beam traverses the carbon surface, the signal or pulses in the output line M will be at the white level. As the beam traverses the aluminum surface, the signal or pulses in the output line it will be at the black level. As the beam traverses the silver surface, the signal or pulses in the output line M will be at a level corresponding electrically to the condition of blacker than black. With the particular pattern and operating action as described, it will be seen that in the output connection It from the control device Hi, there will appear horizontal synchronizing pulses 34 occurring at the rate of 13,230 per second, and pulses 35 of a radio 'frequency wave, these R. F. pulses occurring at the rate of sixty per second. That is, the silver lines 32 are so hue and. closely spaced that a 'radio-frequency wave is generated every time will occur at the desired field frequency of sixty.

It is to be noted that the horizontal synchronizing pulses at occur continuously, and are not interrupted by the R. F. pulses 32 for vertical synchronization.

The sweep circuits M and I2 are aperiodic, and may be constructed and operate, and be driven by the line pulses 3 5 and the vertical R. F. pulses 35, in much the same manner as is disclosed in the pending application by Richard L. Campbell, Serial No. 310,591, filed December 22, 1939.

As the beam 23 traverses each of the carbon surface portions 33, the scanning beam of the transmitter tube or camera ID will be at full intensity, and video or picture signals will be generated and transmitted. At all other times, this beam will be cut off by the blanking pulses at and 3?.

With the horizontal line pulses 34 occurring at the rate of 13,230, and the vertical R. F. pulses 35 occurring at a field frequency of sixty, it will be seen that the frame frequency will be thirty, and the tube Hi will operate at an interlace ratio of two.

Instead of taking the control signals directly from the target or screen 24 by the output line M, the tube 22 may be provided with a collector ring 38 adjacent the screen, and from which the synchronizing and blanking pulses or other control effects may be taken by the output connection 39. The signals taken from the collector ring 38 will be of polarity opposite to that of the signals taken directly from the target itself. Having either polarityv available is an advantage.

By the application of bias voltages between the target 24 and the collector ring 38 of the tube 22, it is possible to increase materially the amount of output signal from either of the output lines l4 and 39. For example, by making the potential of electrode 38 one hundred volts positive with respect to the target 24, the amount of output signal is materially increased.

At each receiving station, as shown in Fig. 2, there is a cathode-ray tube control device l3, identical in construction and manner of operation with the control device I 3 at the transmitter. In Fig. 2, the various parts or units have been designated by the same reference numerals,

primed, as the respective and corresponding parts or units in Fig. 1.

In operation, the 30-cycle sine wave, transmitted by the transmitter I! in Fig. 1, is intercepted, along with the transmitted video signals, by the receiver designated generally by the reference numeral 2|, and detected in the well known manner to give a source of supply 20' of a 30- cycle sine wave. This sine wave, as at the transmitter, is passed through a phase splitter 28' and used to cause the beam 23' to scan the circular pattern of the screen 24' at the rate of thirty times a second and in phase with the scanning beam 23 at the transmitter. The same signals or control effects, as in Figs. 4a and Set, will therefore appear in the output line H, and these are used to drive the aperiodic sweep circuits II and I25. The scanning beam of the cathode-ray tube scanning apparatus It) will therefore always be in synchronism with the scanning beam of the transmitter tube or camera l0. By the connection IS, the blanking pulses 38 and 31 are supplied to the blanking amplifier l9, and thence at negative polarity to the intensity-control grid of the electron gun in tube It) to cut off the beam thereof during the horizontal and vertical return periods. The video signals "are applied to the tube III by the connection 40.

The design of the pattern 21 may be varied to suit particular requirements. For example, materials different from those suggested may be used for the screens 24 and 24, and the number and character of the lines or printings may be varied to obtain different scanning frequencies, or to obtain straight scanning instead of interlaced scanning. Several forms of cathode-ray tube control devices which may be used in other embodiments of our invention, are disclosed in our Patent 2,229,556, Jan. 21, 1941. Other modifications, within the conception of those skilled in the art, are possible without departing from the spirit of our invention or the scope of the claims.

We claim as our invention:

1. In the art of television communication, scanning apparatus in the form of a cathode-ray tube having a multiple series of surfaces arranged in a circle, each series differing from the other as to secondary emission, and the surfaces of each series being equally spaced from each other and differently spaced from the spacing of the other surfaces from each other, and means to cause the cathode-ray of said tube to scan said surfaces in a circle.

2. In the art of television communication, scanning apparatus in the form of a cathoderay tube having a multiple series of surfaces arranged in a circle, each series differing from the other as to secondary emission, and the surfaces of each series being equally spaced from each other and differently spaced from the spacing of the other surfaces from each other, and means to cause the cathode-ray of said tube to scan said surfaces in a circle, the spacing of the surfaces of the respective series being such as to produce pulses of a radio frequency wave, horizontal synehronizing'pulses and picture blanking pulses, respectively.

3. In a television system, apparatus for transmitting and receiving television pictures comprising a simple sine wave control voltage, a cathode-ray tube to be scanned by said control voltage having a pattern printed on thescreen of said tube to yield by secondary emission horizontal and vertical synchronizing signals and blanking signals to control scanning at the transmitter,

and means comprising a like cathode-ray tube to a be scanned by said sine wave control voltage to produce control signals at the receiver togovern scanning action of the picture receiving tube.'

4. In a television communication system, means for scanning a photoelectric mosaic screen to produce a picture signal, a 'cathode ray tube having a circular row of electron emitting surfaces of different degrees of secondary emission spaced at' predetermined intervals to produce'control impulses, and means for applying said impulses to said first named scanning means.

5. The system according to claim'4, in which the predetermined spacing is, respectively, for line scanning frequency, field scanning frequency and blanking impulses.

6. The system according to claim 4, in which the predetermined spacing is, respectively, for

line-scanning frequency, field scanning frequency and blanking impulses for horizontal and vertical blanking, respectively.

7. The system according to claim 4, in which the predetermined spacing is, respectively, for line scanning frequency, field scanning frequency and blanking impulses with the surfaces for producing each vertical synchronizing impulse spaced closely enough together to produce radio frequency.

8. In the process of television communication, the steps which comprise scanning with an electron beam the photoelectrlc mosaic screen of a television transmitter upon which an image is focussed, to generate picture signals, generating control signals by means of a cathode ray tube having a circular screen with at least three sets of concentric surfaces thereon each having a different degree of secondary emission from the others, and applying said control signals to said first named electron beam.

9. The process of claim 8, in which one set of surfaces are spaced for line scanning frequency,

. one set for field frequency and one set for synchronizing impulses.

10. The process of claim 8, in which one set of surfaces are spaced for line scanning frequency, and one set for field frequency, said last named set being in part for vertical synchronizing and in part for horizontal synchronizing.

11. The process of claim 8, in which one set of surfaces are spaced for line scanning frequency, one set for field frequency and one set for synchronizing impulses, the part for vertical synchronizing being generated at radio frequency but at vertical synchronizing intervals.

ALLEN B. DU MONT. THOMAS T. GOLDSMITH, Ja.

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