US 2527835 A
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K. w. MILLER 2,527,835
TELAUTOGRAPH SYSTEM Oct. 31, 19 50 2 Sheets-Sheet 1 Filed Nov. 4, 1947 lA/VENTOR By k. w MILLER ATTORNEY Oct. 31, 1950 K. w. MILLER 2,527,835
TELAUTOGRAPH SYSTEM Filed Nov. 4, 1947 2 Sheets-Sheet 2 H IH 9, INVENTOR fii-H-Mdk ATTORNEY K W MILLER l- JZWIII Patented Oct. 31, 1950 UNITED STATES PATENT". oFF eg TELAUTOGRAPH SYSTEM- Kenneth W. Miller, New York, N. Y., assignor' to Bell Telephone Laboratories, Incorporated; New York, N. Y., a corporationof'New York- Application November 4, l94-7v,; Serial No. 783,902
has the unique properties of low resistivity together with uniformity of resistance over the coated area and good characteristics for the transmission of visible light.
The present invention is concerned with'the utilization of such conductively coated glass sheet as a means for deriving electrical signals representative of the position of a stylus or other writing such as is necessary in telautograph and similar systems. The invention is also concerned with the utilization in such systems of cathode ray tubes of the type having a crystalline target which is normally transparent to visible light but which, when struck by an electron beam, form a deposit which isopaque to visible light. Cathode ray tubes employing targets of that type are particularly adapted for projecting the images formed upon the target upon an external screen and are also useful in applications where it is necessary to retain the images formed for extended periods of time. This latter characteristic is of interest in the art of telautography where artificial signal storage means have been utilized in the past in order to produce a recurring trace upon the screen of the cathode ray display tube. Finally, the invention is concerned with a novel combination of a coated glass sheet employed as a transmission device With a cathode ray tube of the type described above wherein the image received is projected upon the underside of the transparent writing surface. Such' an arrangement has the advantage of providing a means whereby an operator at either of two such stations may add to or modify the graphic material being transmitted by'the other operator.
Accordingly. it is an object of the invention to provide an improved system for the transmission of graphic information;
It is an object of the invention to provide a system for the reception-of graphic images which will retain the received images for relatively extended periods of time;
It is an object of the invention to provide a novelmeans-of deriving potentials representative 2 of" the position of a writingstylus on a writing surface;
It isan object of the inventionto provide a tela'utograph system suitable. for use in confer ence systems. f V
Theseand'otherobjects, features and aspects of the invention will be more readily apparent from a' consideration of the following. detailed specification and appended claim'takenin connection' with the accompanying drawings, 'in" which:
Fig. 1 shows a schematic diagram of one S153! tion of a telautograph system illustrating one embodiment of the invention; 7
Fig. 2 shows a schematic diagram ofa second station of av telautograph system, illustrating alternative embodiments of the invention.
Referring now" to Fig. 1, there is shown a telautograph station embodying a transmitting apparatus [0 for deriving signals representative of the position of a writing pencil or stylus into electrical signals for transmission to a remote station, and" a receiving apparatus Ijl for converting electrical signals received from the remote station into images representative of those being traced at 'theremote station. Such a remote station i'sjshown in Fig. 2, wherein a transmitting apparatus I2 and a receiving apparatus l3 are essentially similar to the corresponding apparatus of Fig. 1. I
The transmitting apparatus [0 of Fig. 1 is provided with a'sheet I4 of transparent material having a thin, transparent, andielectric'ally'conductive coating on one side thereof which will be designated as a writing surface [5. Thesheet [4 may be of plate glass while the conductive co'atingmay be that knownas the NE'SA coating which has a resistance in the order of" to ohms per unit area. Such a coated glass is described in the Transactionsof the American Institute of Electrical Engineers, at page 789 of volume 65. j v
The sheet I4 is provided along two of itsisides with a pair of electrodes l6 and I1 which serve as terminals whereby"'alt'ernating potentials of a frequency fl from a source l8 may be. applied across the conductive coating in one coordinate direction. Similarly, there is provided along'the' remaining two sides'a pairof electrodes l9 and 20 whereby alternating potentials of afrequency )2' from asource 2| may be applied across the conductive coating of the writing surface inthe remaining coordinate direction. The voltage gradients thus produced across the writing surface 15 are essentially mutually perpendicular;
at least within an area such as that enclosed by the dashed line 22, and a writing device 23 in contact with the writing surface will detect voltages of frequencies fl and f2 whose amplitudes are proportional to the two-dimensional coordinate position of the writing device as it moves across the Writing surface. The writing device 23 may be a pencil or stylus adapted to provide an electrically conductive path as well as to produce visible marks upon the writing surface l5.
The voltages detected by the writing device 23 are applied to a band pass filter 24 which is designed to pass onl the alternating potentials of frequency H. The transmitted potentials are amplified by an amplifier 25, rectified by a diode 26, and smoothed by a filter 21. The variable amplitude unidirectional potential thus obtained is utilized to modulate a carrier wave of frequency f3 supplied by a source 28 to a modulator 29 and the modulated carrier transmitted over a wire or radio transmission channel 30 to a remote receiving station such as the receiving apparatus l3 of'Fig. 2. Similarly, the potentials of frequency f2 detected by the writing device 23 will bepassed by a band pass filter 3| and amplified by an amplifier 32. The amplified potentials are converted into a variable amplitude unidirectional potential by a rectifier 33 and a filter 34, and'the resultant wave utilized to modulate a carrier wave of frequency f4 supplied by a source 35150 a modulator 36. The modulated carrier is then transmitted over a wire or radio transmission channel 31 to the remdte receiving station. It will be realized that, since the only information transmitted is that representative of the instantaneous position of the writing stylus 23, the transmission channels 30 and 37 may be of relatively narrow bandwidth.
The receiving apparatus includes a cathode ray tube 38 having means to generate an electron beanrsuch as a conventional electron gun 39, means for deflecting the electron beam, such as a pair of horizontal deflection coils 40 and a pair of vertical deflection coils 4|, and a target 42, The target 42 consists of a crystalline layer 43 provided on each side with electrodes 44 and 45 in the form of thin, transparent, sputtered metallic layers or fine meshes. The target 42 is maintained at a positive potential with respect to the electron gun 39 by a source 46 while the electrode 45 is maintained at a potential positive with respect to the electrode 44 by a source 41.
The crystalline layer 43 of the target 42 is composed of crystals of the class known as ionic crystals which are normally transparent to visible light but, upon being struck by an incident beam of electrons, form an opaque deposit of a densit proportional to the instantaneous density cf'the beam. This opaque deposit persists after the beam leaves the area and moves transversely through the crystal toward the positive electrode 45 at a speed determined by the temperature of the crystal and the potential difference between the electrodes 44 and 45. These effects occur primarily in crystals of the alkali-halides, such as potassium chloride. For a further description of the phenomena, reference may be made particularly to an article entitled A system of Large-Screen Television Reception Based on Certain Electron Phenomena in Crystals, by A. H. Rosenthal and published in the'Proceedings of the Institute of Radio Engineers for May, 1940, at page 203.
' The electron beam from the electron gun 38 is deflected over the surface of the target 42 to produce an opaque image of the graphic material being drawn at the remote transmitting station upon the normally transparent target. Carrier waves modulated in accordance with signals representative of the horizontal coordinate position of the writing device at the transmitting station are received over a transmission channel 48 whence they are demodulated by a detector 49. The potentials so derived are amplified by a direct-coupled amplifier 50 and applied to the defiection coils 4|! to produce a horizontal deflection of the electron beam proportional'to the horizontal movement of the distant writing device. Similarly, carrier waves modulated in accordance with signals representative of the vertical coordinate position of the writing device at the transmitting station are received over a transmission channel 5| and demodulated by a detector 52. The potentials so derived are amplified by an amplifier 53 and applied to the deflection coils 4| to produce a vertical deflection of the electron beam proportional to vertical movement of the distant writing device. It is to be understood that the intensity of the moving electronbeam is to be maintained at a constant level such that an opaque image of the proper density is formed upon the target 42.
In accordance with one feature of the invention, the image formed upon the target 42 is projected upon the under surface 54 of the sheet M by means of light from a source 55 which passes through the target to produce a reproduction of the target image upon the surface 54. It is to be understood that the projection system illustrated is symbolic only, since it will usually be necessary for the rays of light from the source 55 to pass through the target in a direction normal to the surfaces thereof over the entire target.
It is an important aspect of the invention that the target 42 has a high storage ratio, that is, the image formed upon the screen is retained for a considerable period after formation by the moving electron beam. This effect is due to the fact that the opaque deposits produced by the electron beam move transversely through the crystal at a relatively slow rate determined by the potential normally applied between the target electrodes 44 and 45. The necessity for the use of artificial signal storage means such as have been necessary in the past in telautograph systems utilizing the usual type of cathode ray tube for display purposes is thus obviated. It is, however desirable that a means he provided whereby the operator can erase the image at will. Accordingly, there is provided a switch 55 whereby a relatively high potential may be applied across the electrodes 44 and 45, thereby temporarily hastening the movement of the opaque deposit toward the positive electrode and causing the rapid dissipation of the recorded image. Alternatively, other known means, such'as increasing the temperature of the crystal 42, may be utilized to perform the same function.
Referring now to Fig. 2, the remote station illustrated includes a transmitting apparatus l2 connected by means of the transmission channels 48 and 5| to the receiving apparatus of Fig; 1. Similarly, the receiving apparatus l3 of Fig. 2 is connected by means of the transmission channels 30 and 31 to the transmitting apparatus ll! of Fig. 1. While the receiving and transmitting apparatus 2 and 13 of Fig. 2 is essentially the same as the corresponding apparatus of Fig. 1, alternative embodiments of certain aspects of the respective circuits are provided and will be described in some detail.
The transmitting apparatus [2 includes a sheet 5'! of transparent material having a, writing surface 58 which includes a thin, transparent, electrically conductive coating similar to that of the sheet I of Fig. v 1. Alternating potentials of frequency f5 are applied to the conductive coating along two opposite sides of the sheet 51 by means of a pair of electrodes 66 and SI while alternating potentials of frequency f6 from a generator 62 are applied to the conductive coating along the remaining two sides of the sheet 5'! by means of a second pair of electrodes 63 and 64. A po tentiometer 65 connected across the generator 59 together with the conductive coating of the sheet 51 combine to form a bridge circuit which becomes more or less unbalanced in accordance with the horizontal position of a writing stylus 66, thus impressing upon a band pass filter 61 potentials of frequency f5 whose amplitude varies in accordance with the horizontal position of the writing stylus 66 on the writing surface 58. Similarly, a potentiometer 68 connected across the generator 62 together with the conductive coating of the sheet 51 combine to form a bridge circuit which impresses potentials of frequency f6 and amplitude proportional to the vertical position of the writing stylus upon a band pass filter 69. The variable amplitude alternating potentials of frequency f5 passed by the filter 61 and the variable amplitude alternating potentials of frequency f6 passed by the filter '69 are utilized to modulate carriers for transmission over the transmission channels 48 and 5| after the same fashion as is employed in the transmitting apparatus In of Fig. l.
The alternative circuit just described is advantageous in that the writing stylus '66 may be maintained at ground potential, thus obviating any possibility of even a slight discomfort to the user due to potentials existing between the pencil and the writing surface. However, the bridge circuits including the potentiometers 65 and 68 should preferably be adjusted for balance condition when the writing instrument 66 is in contact with some extreme point in both coordinate directions, such as the point designated as 10, in order that difficulties due to phase reversals of potential are not encountered at the balance point.
The receiving apparatus l3 utilizes the principle that a target of the nature described, that is of alkali halide crystals, when scanned by an electron beam of high intensity so as to produce an opaque deposit of high density and subsequently scanned by a second beam of lesser intensity, will have the opaque deposit produced by the first scanning beam reduced in density to a level corresponding to the intensity of the second scanning beam. There is accordingly provided in the receiving apparatus IS a cathode ray tube H similar to the tube 38 of Fig. I, having an electron gun 12, and a target 13 composed of a layer of alkali halide crystals 14 and a pair of supporting electrodes 15 and 16. A control electrode 11 of the electron 12 is supplied through a switch 18 by a potential source 19 so that, when the switch 18 is in a first position A, the electron beam is of such an intensity as will produce an opaque deposit in the crystal 14 of a density suificient to preclude the transmission of light therethrough. When the switch 18 is in a position B the electron beam is of an intensity such as will reduce the density of the opaque deposit to allow the free transmission of light. An electron beam deflection system comprising the usual deflection coils B9 and BI is also connected through the switch 78 so as to be made responsive either to incoming signals from the transmission channels 30 and 31 or to signals from a scanning wave generator 82. The scanning wave generator 82 may be of a well-known type which-will produce deflection currents for deflecting the electron beam in a raster of closely-spaced scanning lines so as to produce a uniform opaque deposit over the entire surface of the crystal 14 when the switch 18 is placed in position A. Thus, when the apparatus I3 is made responsive to incoming signals by the adjustment of the switch 18 to position B, the electron beam will form an image upon the target 13 which is transparent to visible light. The image then projected upon the sheet 51 by light from a source 83 will be bright upon a dark background rather than dark upon a bright background as in the case of the receiving apparatus of Fig. 1.
In the operation of the invention in the intended fashion, the graphic material drawn on the writing surface of the transmitting apparatus of one station is reproduced as an image upon the underside of the writing surface at the receiving station. The person at the receiving station may therefore, by making appropriate markings upon his writing surface, effectively add to or modify the original sketch. It will thus be seen that the system of the invention is particularly adaptable to use in conference systems wherein it may be desired to exchange ideas or information involving graphic or other symbolic data.
What is claimed is:
In combination, a writing surface composed of a sheet ofmaterial having a uniform resistance over an exposed area, a first generator for alternating potentials of a first frequency, means for applying potentials from said first generator across said writing surface in one coordinate direction, a potentiometer connected across said first generator said potentiometer having a tap, a band pass filter having an input circuit connected to said tap, a second generator for alternating potentials of a second frequency, means for applying potentials from said second generator across said writing surf-ace in a second coordinate direction, a potentiometer connected across said second generator, said potentiometer having a tap, a band pass filter having an input circuit connected to said last-named tap, an electrically conductive writing stylus in contact with said writing surface and connected to input circuits of the two said band pass filters whereby the single frequency potentials passed by each filter have amplitudes representative of the coordinate positions of the said writing stylus.
KENNETH W. MILLER.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 2,000,014 Du Mont 1 May 7, 1935 2,168,049 Skellett Aug. 1, 1939 2,227,083 Handrick Dec. 31, 1940 2,338,949 Kupfmuller et a1. J an. 11, 1949