US2037711A - Method and apparatus for television - Google Patents

Description

A ril 21, 1936. P. T. FARNSWORTH METHOD AND APPARATUS FOR TELEVISION 2 Sheets-Sheet .1

P. T. FARNSWORTH HOD AND APPARATUS FOR TELEVISIO Original Filed Nov. 26, 1928 MET 2 Sheets-Sheet 2 FREQUENCY FEEQUE/VCY m? PH/LO 7.-

VENTOZ F E'IVSWOA HA5 ATTOENEK Patents Apr, 23, 2%36 UNETEE anus T1? Fm 2,37,7)lll ME'IIHQD AND APPARATUS F6 TELEWSIION poration oi Qaliiornia Application November 26, 1928, Serial No. 32L$5 Renewed September 21,, 193i 27 Claims. (on. ire-c) My im/eution relates to the transmission of electrical impulses to form pictures. While it is particularly applicable to television-L e., the transmission of successive images, each within the optical period, giving the appearance of mo tion to the object pictured,it also has a general applicability to electrical picture transmission systems.

An object or? my invention is to provide a means of increasing the sensitivity of a television trans== mitter.

Another object of my invention is to provide a means of increasing the detail or" the trans mitted picture.

Still another object of my invention is to decrease the efiect upon the transmitted picture of noise, that is, of stray impulses generated by mechanical shocks to the transmission apparetus, or by induction from adjacent apparatus or lines.

My invention has other objects and valuable features, some of which will he set forth in the following description of my invention which is illustrated in the drawings forming part or the specification. It is to be understood that I do not limit myself to the showing made by the said description and drawings, as I may adopt vary ing forms or" my invention within the scope of the claims.

Referring to the drawings:-

Figure l is an axial sectionalview of a trans mitting photo-electric cell embodying my invention.

Figure 2 is a transverse sectional view of the same cell, the plane of section being indicated by the line 22 of Fig. 1,

Figure 3 is a detail view on a greatly enlarged scale of the target of the cell with its shield, showing the scanning apertures.

Figure 4 is a schematic diagram of the transmitting apparatus.

Figure 5 is a diagram of the transmitting circuits.

Figures 6, 7 and 8 are frequency response curves of the various parts of the apparatus.

. All methods of electrical picture transmission which have thus far been proposed involve the systematic passage of a scanning element over the pictured area to dissect or analyze it into elementary areas, the picture current being proportional at each instant to the mean light in-. tensity from the elementary area covered at that instant by the scanning element or, in indirect processes, to an equivalent quantity. Scanning elements of various types have been devised, such as light beams, conductive feelers, cathode ray pencils, and moving apertures, but since the end product or" the process is a picture, it has been found convenient to adopt optical terms in referring to the process, and therefore the scanning element, whatever its nature, is termed an aper= ture, and its conductivity, permeability to the electron stream, or other equivalent quality is referred to as its transparency.

While this terminology leads to some apparent absurdities, in that an aperture may be a ma.- terial object, it is adopted here as being the most general thus for proposed. It will be apparent that in this sense the aperture is that portion or" the system which determines shape of the elementary picture areas and their size as compared with the picture as a whole.

Since the picture current can represent only the mean illumination from the elementary area embraced by the aperture, it follows that no ole-=- tails smaller than this area can be repfoduoed. It also follows that abrupt transitions from light to shade in the reproduced picture are impcssible, the transition occupying an interval correspondin to the aperture width. Considered in electrical terms, it means that picture current components of frequencies higher than a definite out ofi value are either attenuated or completely suppressed. This is known as the aperture efiect.

It is obvious that from the standpoint of re production of detail, the smaller the aperture used, the better will be the picture. However, a reduction in aperture involves a corresponding reduction in the sensitivity of the device and altho this may in some degree be compensated for by amplification, a point is quickly reached where accidental efiects of induction, mechanical vibration of amplifier tubes, and the like, become so great as to mask the picture currents to a greater or less extent, and the quality of the picture is seriously impaired. These masking influences are all grouped under the term noise, since in genesis and efiect they are directly analagous to noise in a telephone system. The intensity level of the picture currents must be materially higher than the noise level if a satisfactory picture is to result. The interferent noise which disturbs picture transmission has been found by experiment to occur chiefly at relatively low frequencies, its maximum intensity usually occurring at less than 1000 cycles.

In general terms, my invention comprises the generation of picture currents having both high and low frequency components, the component frequencies upon which interference occurs being disproportionately large, and then equalizing or proportionalizing the components after genera-- tion. This may be done in a television transmitter by providing a scanning apparatus having a large and a small aperture, with which the picture area is simultaneously scanned, and by passing the combined currents thru a network which attenuates the low frequencies more than the high, or what is equivalent, amplifies the high frequencies more than the low. The small aperture determines the limit of detail in the picture; the large aperture raises the intensity of the frequencies within the interference range well above the noise level, and greatly increases the sensitivity of the apparatus.

In practice, I have embodied my invention in a television transmitter of the type described in my copending applications, Serial No. 159,540, filed January 7, 1927, now Patent 1,773,980, granted Aug. 26, 1930, and SerialNo. 270,673, filed April 17, 1928. The transmitter comprises a dissector tube having a cylindrical glass envelope 6, having at one end a fiat window 1 and at the other a stem 8 upon which the elements of the tube are supported, and thru which certain of ;he leads pass.

The inner end of the stem carries a short glass )illar 9 terminating in a square button II. The utton supports a glass plate [2 which has a netallized surface on which is deposited a photo- :ensitive film. A metal clip or spider I 3. holds the llate in place and makes contact with the photoensitive film, and a lead I4, secured to the clip, )asses out thru the stem 8.

A band clamp l6 surrounds the stem, and to his are secured wires [1 which carry the anode tructure, which comprises a screen l8 of wire JGSh, parallel and closely adjacent to the photoensitive cathode surface, and a cylindrical screen 9, also of wire mesh, which conforms closely to he inner surface of the envelope, extending from lie transverse screen I 8 to within a short distance f the window 1. A wire frame 2| stiifens the :reens and is secured, preferably by welding, to 1e support wires l1. An anode lead 22 is sealed iru the stem and also connects with the suport wires.

The tip 23, thru which the tube is evacuated,

formed at the seal 24 which joins the stem with 1e cylindrical body of the envelope. The tip protected by the base 26 which is formed of .sulating material and supports the tube when use. The base carries contact pins 21 which rminate the anode and cathode leads.

Joining the envelope at the other end of the e, adjacent the window 1, is a short side tube This tube carries a tubular metal shield 32 rmounted by a cap 33. A wire target 34, pref' ably of nickel or other material which is a od secondary emitter of electrons, extends thin the shield and is spaced and insulated Jm it by the beads 36. Separate leads 31 and are connected to the shield and target respec- 'ely, and are sealed thru the end of the side be 3|. The side tube is protected and shielded ternally by a metal .sleeve 39, to which the ield lead 31 is connected.

Adjacent the end of the shield 3|, facing the thode and in the axis of the tube, is an aper- .e 4|. The aperture is covered by a screen 42 fine wire mesh, and in the center of this screen a. smaller aperture 43 which is materially larger in the mesh of the screen. The aperture 4| preferably rectangular, or at least elongated in form, with the central aperture 43 occupying its full width. Both apertures may be square or round if desired, but this results in a lower transmission of detail in one direction than in the other if the scanning frequencies. are markedly different, as is usually the case.

Associated with the tube is certain auxiliary apparatus, which is diagrammatically indicated in Figures 4 and 5. An optical system, indicated in the figures by the lens 46, focuses an image of the object to be pictured thru the window 1 onto the photo-electric cathode. liberated from the elementary areas of the surface of the cathode in proportion to their illumination, and these electrons are drawn toward,

the anode by a potential supplied by the battery 41, which is connected between the cathode and anode. The major portion of these electrons pass thru the screen I B into the equipotential space within the screen l9, and these continue to travel longitudinally of the tube with practically undiminished velocity.

The tube is surrounded by a coil 48, which is supplied with direct current by a generator 49 to produce a practically uniform magnetic field within the tube and coaxial therewith. This field focuses the electron stream, and is adjusted by means of the rheostat to bring the focus into the plane of the aperture 4|, as is explained in my copending application Serial No. 270,673 above mentioned.

The focused stream of electrons is deflected by coils 52, 52 and 53, 53, which are supplied respectively by the high frequency oscillator 54 and the low frequency oscillator 56, so that all parts of the electrical image in turn sweep over the compound aperture to accomplish the scanning action. The high frequency scanning takes place in the direction of the longer axis of the aperture, the low frequency movement of the image during each highfrequency traverse preferably being equal to the width of the aperture. Those electrons entering the aperture impinge on the target, liberating secondary electrons. The shield 32 is maintained at the anode potential by the battery 41. The target 34 is connected to a point on the same battery which keeps it somewhat negative with respect to the shield, and the secondary electrons are therefore drawn to the shield, constituting a current which flows from the battery to the shield, and thence to the target and back thru a high resistance 51 to the battery. This refers to the conventional current flow, and not to the actual electron flow, which is in the reverse direction.

It will be appreciated that some of those electrons falling on the screened portion of the aperture will be stopped by the wires, which thus serve to reduce the transparency of the aperture. All of the electrons falling on the aperture 43, however, will pass unimpeded to the target, and the central aperture therefore has a higher transparency than the larger aperture which embraces it.

The picture current produced by the tube thus represents two superposed pictures, one. having high contrast and small detail, the other smaller contrast and .greater detail. Electrically this is represented by Figure 6, in which curve 6| represents the frequency characteristic of the tube,

and curve 62 represents the noise level. Pic-- torially we may think of the resultant picture as one in which the large masses are shown in full contrast, while the details are sketched in lightly.

Electrons are The fall of potential across the resistor 51 is amplified and passed on to the line at, which may be either a transmission line or the input circuit of a radio transmitter. The amplifier shown includes the usual vacuum tubestl, S5, with coupling networks comprising the plate resistors M, coupling condensers 68, and grid resistors. 69. The condensers is are of relatively low capacity and the grid resistors of somewhat lower resistance than would ordinarily be used.

This results in the amplifier discriminating against the lower frequencies, its response curve being approximately as shown in curve M, Figure 7. This curve taken in connection with the curve 6! gives an over-all response curve such as is shown'in Figure 8, curve '52. It will be noted that the frequency characteristicis substantially fiat up to the cut off point which is determined by the aperture #33. The low frequencies have been reduced to their normal values, or propor tionalized, the noise level has been correspondingly reduced, and the details, of the pictures resulting from the currents will have their full contrast values. Absolute flatness of the over-all curve is not essential, as the eye has a wide tolerance in this regard. In some cases it has even. been found of advantage to over-attenuate the low frequencies, causing the high frequencies to dominate in the resultant characteristic.

It is obvious that it is immaterial at what point in the system the proportionalizing is done. So called equalizing networks, which attenuate certain component frequencies as compared to others, are well known in telephone practice, and the particular type used in this instance is purely a matter of choice. While I have. here shown an amplifier in which the attenuation of the low frequencies occurs between successive amplifier tubes at the transmitter, it may be advisable to give the transmitting amplifier a flat characteristic, and to place the equalizer at the receiving end of the system, either as a part of an amplifier or as a separate network.

While the system has been shown as adapted to an electrical system of scanning, it is obvious that the method is perfectly general, and that double or multiple aperture scanning, either with concentric or with separately positioned apertures, may be practiced with any scanning system yet proposed.

Other modifications are the use of intermediate apertures, or, carrying this idea to the limit, an aperture which shades by continuous graduations from full transparency at the center to full opacity at the edge, comprising in reality an indefinitely large number of superposed apertures. This arrangement will give a continuously falling frequency characteristic, instead of the stepped characteristic of the double aperture device. Effective transparency refers to transparency" as it affects the intensity of the resultant signal, regardless of apparent transparency as determined by eye.

I claim:

l. A scanning apparatus having an aperture of relatively large dimension in the direction of its traversal of the picture field, and a second aperture of relatively small dimension in said direction, each of said areas efiectively defining a. proportionate area of said field for transmission, and means for efiecting the simultaneous scanning of an object with said apertures.

2. A scanning apparatus having a large aperture of relatively low efiective transparency and a smaller aperture of higher efiective transparency, and means for effecting the simultaneous scanning of an object with said apertures, each of said apertures effectively defining aproportionate elementary area for transmission.

3. A scanning apparatus havinga large aperture of relatively low effective transparency and a smaller aperture of higher efiective transparency centrally positioned with respect thereto, and means for efiecting the simultaneous scanning of an object with said apertures, each of said apertures effectively defining a proportionate elementary area for transmision.

.4. A system for the transmission of electrical impulses to produce pictures comprising means for producing a picture current comprising low frequency alternating components only, additional means for producing a picture current comprising both low and high frequency components, and means for simultaneously transmitting said picture currents.

5. A system for the transmission of electrical impulses to produce pictures comprising means for producing a picture current comprising low frequency alternating components only, additional means for producing a picture current comprising both low and high frequency components, means for simultaneously transmitting said picture currents, and means for proportionalizing the high and low frequency components in the combined currents.

6. A transmitter for producing electrical impulses to form pictures comprising means for analyzing the object to be pictured into elements of relatively large area, synchronously operated means for analyzing the object into elements of smaller area, and means for supplying electrical currents corresponding to the illumination of each of said elementary areas.

7. A transmitter for producing electrical impulses to form pictures comprising means for analyzing the object to be pictured into elements of relatively large area, synchronously operated means for analyzing the object into elements of smaller area, means for supplying electrical currents corresponding tothe illumination of each of said elementary areas, and means for proportionalizing the high and low frequency components in said currents.

8. Ina system for transmitting electrical impulses for the formation of pictures, means for forming an image of the object to be pictured, means for analyzing said image into relatively large elementary areas, means for analyzing said image into smaller elementary areas, a source of electric current, and means for modulating said current in accordance with the intensity of each of said image areas.

9. In a. system for transmitting electrical impulses for the formation of pictures, means for forming an image of the object to be pictured, means for analyzing said image into relatively large elementary areas, means for analyzing said image into smaller elementary areas, a source of electric current, means for modulating said current in accordance with the intensity of each of said image areas, and an equalizer for attenuating the low frequency components in said modulated current as compared to the high frequency components.

10. A transmitter of electrical impulses for the formation of pictures comprising means for forming an electrical image of the object to be pictured, means for deflecting said image in accordance with a predetermined time schedule, a target arranged to receive a portion of the component electrons of said image, a shield for said o electrons; jouv vingzazmore ighl the. p

overfi. ntr'al' larger than the screen mesh.

15. A photo-electric apparatus comprising a surface operative to initiate an electrical dis charge corresponding to the illumination thereof, an electrode positioned to accelerate said discharge, a target in the path of said discharge, a shield for said target having an aperture therein, a screen covering said aperture, said screen being partially transparent to the discharge and having a smaller and more transparent aperture therein, and means for deflecting said discharge. 16. A photo-electric apparatus comprising a surface operative to initiate an electrical discharge corresponding to the illumination thereof, an electrode positioned to accelerate said discharge, a target in the path of said discharge, a shield for said target having anaperture therein, a screen covering said aperture, said screen being partially transparent to the discharge and having a smaller and more transparent aperture therein, and means for forming an optical image on said surface. I

, 17. A photo-electric apparatus comprising 'a surface operative to initiate an electrical discharge corresponding to the illumination thereof, an electrode positioned to accelerate said discharge, a target in the path of said discharge, a shield for said target having an aperture therein, a screen covering said aperture, said screen being partially transparent to the discharge and having a smaller and more transparent aperture therein, means for forming an optical image on said surface, and means for focusing the electriperture therein rig sai literate,

. i etherein,anda screen Q I I I surface operative to initiate an electrical discharge corresponding to the illumination thereof, an electrode positioned to accelerate said discharge, a target in the path of said discharge, a shield for said target having an elongated aperture therein, and a screen covering said aperture,

said screen being partially transparent to the discharge and having a more highly transparent central aperture.

23. The method of electrical picture transmission which comprises the steps of simultaneously dissecting the picture area to be transmitted into superposed elementary areas having diflerent limits, generating picture impulses corresponding to the mean illumination of each of said areas, and transmitting the combined impulses.

24. The method of electrical picture transmission which comprises the steps of simultaneously dissecting the picture area to be transmitted into superposed elementary areas having different limits, generating picture impulses corresponding to the mean illumination of each of said areas and having intensities and frequency limits corresponding thereto, transmitting the combined impulses, and equalizing said impulses to provide proper relative proportions of their component frequencies.

25. The method of electrical picture transmission which comprises the steps of simultaneously dissecting the picture area into a plurality of series of different superposed elementary areas to produce a plurality of trains of picture impulses, and combining said trains of impulses to reproduce the picture.

g forming a cathode, an an:

asparams i mwis i ri a A; photoeelectric apparatus com rising a 26. The method of electrical picture transmission which comprises the steps of scanning the picture area simultaneously with a plurality of apertures of difiering configuration, generating picture impulses having difierent frequency components corresponding to said apertures, and utilizing the combined impulses to reproduce the picture.

27. The method of electrical picture .transmis-: sion which comprises the steps of scanning the picture area simultaneously with a plurality of apertures, transmitting a picture current corresponding to the additive efiect of. said apertures and utilizing said, picture current to reproduce the picture.

PHILO T. FARNSWOR'I'H,

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