US2212966A - Light modulation system - Google Patents

Description

' g- 1 v v J. N. WHITAKYER 2,212,966,

LIGHT MODULATION SYSTEM Filed Oct. 2 1957 CARR/ER 5 Y FREQUENCY INPUT INVENTOR. 7 JAMES N. WHITAKER ATTORNEY.

Patented Aug. 27, 1940 2,212,966, LIGHT MODULATION SYSTEM James N. Whitaker, Tuckahoe, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application October 26, 1937, Serial No. 170,997

' IClaim. (01.1'79-1715) This invention relates to an electrical circuit to be used in'connection with a facsimile transmitter.

More specifically, the invention relates to a new and improved system whereby the signals produced by a light responsive device in the scanning head of a facsimile transmitting device may be modulated and amplified for subsequent transmission.

It is well known that photo-electric cells or other light responsive elements may be used in facsimile transmitting systems to produce signals in accordance with conditions of light and shade to which such cells are exposed, and in some instances the modulation of the carrier wave has been effected by means of a light chopper disc, together with the use of associated circuit amplifiers and modulators. In such systems, however, wherein a light chopper is used, it has been 'found that the wave shape developed is not entirely satisfactory to provide advantageous conditions of operation. Furthermore, the use of a light chopper necessitates considerable mechanical apparatusand such apparatus is of course ine herently subject to erratic operation or failure.

Furthermore, in many of the facsimile scanners proposed heretofore, it has been awkward and diflicult to place the photo-electrical cell in the mechanical systems wherein light choppers are.

0 used without sacrificing certain desirable features. In the various systems using direct current amplifiers, difficulties have been encountered due to a pronounced tendency of the amplifier to drift or to depart from its adjustment to such an extent that it materially interferes with the w fidelity of the reproduction.

Another method which has previously been used is that of combining the photo cell output 40 with a carrier tone by means of a mixing tube. Such a system obviates the use of a mechanical chopper disc but introduces certain other disadvantages due to the fact that such systems are generally critical in adjustment and have a slight tendency to vary in response. Such a system wherein amixing tube is used is shown in the patent to J. N. Whitaker #2,055,513, September 25, 1936.

so In the present invention the carrier frequency or oscillations are rrodulated in a new and improved manner with the result that no mechanically moving parts are used and with the further result that the system is more stable and pro- 55 duces signals having a more linear relationship invention will become more apparent to those with respect to the light values to which the cell is exposed.

It is therefore one purpose of the present invention to produce an amplifying and modulating system for use in connection with a facsimile 5 transmitting device wherein no mechanically moving parts are used, and wherein the modulation of the carrier frequency by the photo-cell output is accomplished by entirely electrical means.

A further purpose of the present invention is the provision of an amplifying and modulating system for a facsimile transmitter wherein the output of the amplifier is reduced to a minimum when no light is directed upon the photo-electric cell and wherein the output of the system increases in direct proportion to the amount of light which is directed upon the photo-electric cell.

Further purposes and advantages of the present skilled in the art from a reading of the following specification and claim, wherein:

'The single figure of the' drawing represents, for example, one form of the present invention.

Referring to the drawing a light responsive element or photo-electric cell 5 is provided upon which varying degrees of light are projected in accordance with the light and dark shadings of the particular subject matter to be transmitted by facsimile. This photo-electric cell is, of course, positioned in the scanning head of the facsimile transmitter, but for the purpose of clarity the system is shown schematically and the structure of the scanning head is omitted. as

The source of carrier frequency is not shown, but the carrier current is applied to the terminals l, which are connected to the primary of transformer 2. The secondary of the transformer is shunted by a resistance 3, one terminal of which 40 is connected to ground. An electron discharge tube 4 is provided, which comprises at least the usual elements of an electron emitter, a control electrode and an anode. The control element of the'discharge tube is connected to a point along 5 the resistance 3, in order that the carrier frequency as derived from the carrier oscillator will be impressed upon the tube 4 for amplification thereby and the amplified carrier frequency will, of. course, appear in the anode circuit of the tube. In order that'the anodemay be maintained at a proper potential with respect to the emitter, a source of positive potential is connected to the terminal 6. In this connection is included aresistance and the photo-electric cell 5. At a u correspondsto the carrier frequency impressedupon the control elctrode of the tube 4.

An amplifier tube I0 is provided and is coupled to the anode of tube 4 by means of a condenser ii. The amplifier tube i0 is shown as a pentode in which the suppressor grid is connected internally to the emitter element. The second grid, as well as the anode, are connected to the positive source of potential. In the anode circuit of the amplifier tube I0 is a transformer 68, the secondary Winding of which is connected to the output terminals 20. In order that appropriate potentials may be maintained on the emitter element and control electrode of the amplifier tube 60. a resistance i3 is provided, in parallel with which is connected a condenser M. The resistance and condenser are connected between the emitter and ground as shown in the drawing. To complete the coupling of the amplifier tube It) to the output of the tube 3 another condenser, 52, is

wherein no mechanically moving parts are used tube s, howeve used. A grid leak i6 is connected between the control electrode and ground as indicated in the drawing.

From the above description it may be seen that the conductivity of discharge tube 4 varies in accordance with the potential applied to the control grid of that tube which is afiected solely by the application of the carrier frequency, but the mount of current which is conducted by the' controlled by the resistance of the a de cir it, that is, the conductivity of the photo-elec ric cell 5. When little or no light is directed on the photo-electric cell, the resistance of this element is relatively high and as a result small variations in potentials will appear in the anode circuit of tube 4. As the light which is directed uponthe photo-electric cell is changed, then the corresponding change in the resistance of the anode circuit causes changes in the amount of current which fiows in the anode circuit and as a result greater variations in potentials will appear at the anode of the discharge tube 4. These variations in potential are of course amplified by means of the discharge tube 0 and finally appear at the output terminals 20 of the system.

From the above it may be seen that an amplifying andmodulating system has been devised and wherein amplitude modulation of a carrier frequency is produced. Because of the fact that the light sensitive element 5 is included in the anode circuit of the discharge tube 4-, the potential variations that are impressed upon the amplifier tubev l0 bear a direct and linear relationship to the light which iscaused to fall upon the photo-electric cell 5' over the working range of the system. Furthermore, because of this circuit arrangement and its simplicity the system as a whole is not critical in adjustment and is inherently stable in its operation.

It is to'be understood that, although a simple three-element discharge tube isshown at 4, other discharge tubes may as well be used. Furthermore, it is not necessary that a pentode be used as an amplifier tube since other amplifier tubes may as well be used and also varying degrees of amplification may be employed depending upon the desired results.

It is also to be understood that, although the system is described as useful particularly in connection with facsimile reproduction and transmission, the system as shown and described may also be used in other respects wherein it is desired to modulate a carrier frequency in accordance with changes in light values, the light values being translated into electrical potential variations by any appropriate means.

Other modifications may be made in the present invention without departing from the spirit and scope thereof and it is desired that any and all such modifications be considered within the purview of the present invention except as limited bythe art and the hereinafter appended claim.

Having now described my invention, what is claimed is:

A modulating system comprising a source of oscillations, an electron discharge device including at least an electron emitter, a control electrode and an anode, a photoelectric tube, a circult coupling the control electrode and the electron discharge device is controlled solely in ac-' cordance with changes in impedance of the pho- 'toelectric tube under light activation thereof, an

amplifier tube including at least a cathode, a control electrode and an anode, and a capacity coupling means for coupling the control electrode of said amplifier to one terminal of the photoelectric tube and a second capacity coupling means for coupling the cathode of said amplifier to the other terminal of the photoelectric tube.

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