US2369206A - Picture transmission system - Google Patents

Description

Feb. 13, 1945. J. BARNES 2,369,206

' PICTURE TRANSMISSION SYSTEM Filed Jan. 28, 1943 :2 Sheets-Sheet l W ii i I; H, /4

1 1 5! I =5 INVENTOR.

James C'- Barnes Feb. 13, 1945. BARNES 2,369,206 I PICTURE TRANSMISSION S YSTEM Filed Jan. 2a, 1943 2 Sheets-Sheet 2 I Ilummafion INVENTOR. James C. Barnes.

BY d Q- J- Patented Feb. 13,1945

PICTURE TRANSMISSION SYSTEM James C. Barnes, White Plains, N, Y., assignor to Press Association, Inc., New York, N.

poration of New York Application January 28, 1943, Serial No. 473,789

22 Claims. (01. ire-171.5)

This invention is directed to improvements in that portion of a picture transmission system,

which is employed for converting the elemental areas of a scene such as a photographic negative or print into corresponding electrical variations for transmission to a receiving point.

The broad object of the invention is to provide an assemblage of devices by means of which a carrier signal modulated in accordance with the representative light produced by the usual scanning devices is simply and efiiciently produced.

A specific advantage of the invention herein disclosed lies in producing the modulated carrier substantially free of any component which is not related to the carrier. In circuits which produce a carrier modulated by the light obtained by scanning the scene there may be present in the output carrier, side bands, and a component re'- lated to the illumination from the scanned picture but not related to the carrier. The latter will be referred to as the signal component. The sig- Y., a cor- January 15, 1942, and likewise'entitled Picture transmission system.

This invention resides substantially in the com-- bination, construction, arrangement and relative location of parts, as will bedescribed in detail below.

In the accompanying drawings, Figure 1 is a diagrammatic illustration of that portion of a picture transmission system to which nal component occupies a frequency band from nearly zero frequency to a frequency depending upon the transmission speed and definition. It

represents a distortion of the modulated carrier. It is desirable, of course, to eliminate the signal component and the most common procedure has been that of making the carrier frequency sufficiently high so that a band pass filter or a high pass filter or other frequency selective circuitcan be used to pass the carrier and side bands Y while attenuating the signal component. However, when the speed or definition is high the carrier becomes so high to permit filtering of the signal component that it cannot be transmitted over the ordinary telephone talking circuit commonly used in picture transmission systerns. This requires a second modulation to.produce a lower carrier frequency for transmission. Accordingly, it is an advantage where high speed orhigh definition is required to use a modulator whose output does not contain the signal component.

Another advantage of the invention is found in the method of applying the carrier signal voltage to the electron multiplier, whereby the current saturation characteristics of its output cirdisclosed will be best appreciated by a brief reference to several solutions heretofore proposed.

v This application is .a continuation in part of my copending application Serial No. 426,813, filed trated at I.

the invention herein disclosed has been applied;

Figure 2 are curves of current and voltage relationship for the output circuit of the electron multiplier;

Figures 3, 4, 5 and 6 comprisecharacteristic curves representing light and current variations with respect to time involved in the operation of the system; and

Figure 7 is a diagrammatic illustration of an arrangement like that of Figure 1 involving certain modifications of this invention as will hereinafter be fully explained.

As illustrated, the circuit of this invention employs an electron multiplier of the type having preferably a photo-electric cathode and having more than one stage of secondary electron emission amplification. In Figure 1 of the drawings such a device has been diagrammatically illus- Within a properly evacuated vessel is disposed the photo-electric cathode 2 which is the primary source of electrons released by the scanning light beam when impinged thereon. In accordance with the usual practice this light signal is produced from a suitable light source 23, the light from which is focused by a suitable optical system 22 to a point upon the rotatable cylinder 2 I. As is well known, this cylinder serves practice is effected between the picture surface and the focused scanning light so that the lightspot progressively scans the entire picture, and therefore successively traverses the entire area thereof by tracing a helical path from one end to the other.

Light is reflected from each point or elemental area illuminated by the focused light point as it scans the picture to the photo-electric cathode 2 in varying intensity in accordance with the variations in the light reflecting ability of the surface being scanned, which ability, of course, varies with the lvariations in the tonal value of the picture. The electrons released from the cathode 2 are focused in accordance with established principles upon the secondary emitter 3 where they in turn release electrons which arefoeused upon the secondary emitter l, and'so on to the.

I properly proportioned'resistors 8, 9, I0,

' final, emitter 6; thecuantities of electrons released being progressively increased in accordance with one function of such a device.,, Electrons reincreasing positive potential. These connections,

of course, may be varied to determine the proper relative operating voltages for these electrodes. The junction point of resistors I I and I2, both of which are variable, as illustrated, is connected by a grounded wire to the negative terminal of a suitable high voltage current supply source 62. The primary ll of the transformer I6 is connected to a suitable alternating carrier current source I5 such as a vacuum tube oscillator, with the result that the carrier voltage is applied in series with the anode l and the voltage supply source I3.

A potentiometer I8 is connected across one-half of the primary winding I1 and the mid-point of the secondary is grounded by wire 28, as shown. The movable contact of the potentiometer is connected through a variable resistor 60 and a variable capacitor I9,to the cathode of a triode 55.- Thecathode of the triode is also connected to ground through the resistor 59. The grid electrode of this triode is connected as shown to the electrode 6 of the multiplier. The anode of the triode is connected in series with the primary of the output transformer 6 I and the current source 62. From a study of this circuit it will be seen that the electrode 6 is coupled to the grid cathode circuit of the triode 55 by means of resistor 5| which acts as a coupling resistance. It may be noted that suitable combinations of resistance, inductance and capacity may be substituted for the resistor 5I to give the desired frequency characteristics to the circuit. The output of the circuit thus described may be supplied through the transformer 6| to any suitable work circuit such as a transmission line.

The operation of this circuit and the manner in which the ignal component previously referred to is eliminated from the output current will be described by reference to Figures 2 to 6 inclusive. In Figure 2.the anode current Ia is plotted on the ordinate of the graph while the voltage E9. between last secondary emitter 6, and the anode I I is plotted along the abscissa of the graph. The

curves marked I, 2 and 3 in Figure 2 are those resulting from different light values employed to energize the multiplier I, and are of increasing intensity in that order. It is to be noted that the corresponding curve for the condition of no i llumination is a straight line coincident with the abscissa Ea. The current flowing in the anode circuit and the multiplier is proportional to the light impinging upon the photo-electric cathode 2 for a constant value of Ea. It will be seen that for any particular light value as Ea becomes negative, the anode current falls to zero at the point E82. As Ea becomesincreasingly positive the anode current increases'r'apidly at first but ultimately flattens off to an almost constant value. The carrier voltage impre sed upon the anode 1 causes a periodic variation in Ea about an average'value determined by the direct. current voltage drop across the resistor I2. {This average value maybe adjusted by varying the value of the resistor I2, but upon adjustment fora particular working condition this adjustment may be madepermanent if desired. This periodic variation'in Ea seen from Figure 4. that Is. includes a signal component as Well as carrier and side bands.

The current in electrode 6 to which the output is coupled may be regarded as the algebraic sum of the anode current consisting of secondary emission electrons going to the anode plus the current consisting of electrons coming from the preceding electrode 5. These currents are opposing. The current from the-preceding electrode is proportional to the illumination and is not modulated and therefore consists entirely of signal component. The ratio of the secondary emission current to the current from the preceding electrode 5 may be controlled by the polarizing voltage across resistor I I. By adjusting the value of the resistor II so that the current from the preceding electrode 5 equals the signal component of the anode current the signal components cancel so that only the carrier andside band currents flow in the coupling resistor 5|, in series with the electrode 6. In the example of Figure 5, Ib is the current flowing from electrode 5 to electrode 6 and is proportional to the illumination reaching the electron multiplier tube I. The total current in electrode 6 is shown as Ic in Figure 6 and is a modulated carrier without a signal component. This is the type of'output desired and secured by the circuit of Figure 1.

The current in electrode 6 flows through the resistor-5| and therefore produces a corresponding voltage drop across it. This voltagedrop is applied to the grid of the triode 55, and of course appears in the anode circuit thereof suitably amplified for application to the work circuit through the coupling transformer BI. It will at once be apparent that the amplifier may comprise more than one vacuum device depending upon the requirements and the conditions under which the apparatus must operate. It is, of course, apparent that the source 82 provides the operating voltage to the anode of the triode 55 and the resistor 59 in the cathode circuit of the triode provides a proper grid bias. This resistor is also included in a circuit for introducing a balancing voltage, as will be described.

Thus, in addition to the carrier voltage drop across resistor 5I produced by the emission current as described above, there will be an undesired carrier voltage component which will result from current flowing in the circuit comprisin the secondary of transformer I 6, the inherent capacity between the anode 1 and electrode 6, conductor 26, resistor 5|, resistor I2, and conductor 21. This voltage component can be balanced out or opposed by introducing a part of the carrier voltage properly phased across the resistor 59 through the variable capacitor I9 and variable resistor 60 which by the proper, choice of values thereof will substantially neutralize this changes in the scanning light intensity actuat the cathode 2.

The effect of fluctuations of the carrier voltage am litude may be minimized by having the carrier voltage cause a change in Ea from avalue less than EaZ where the current is zero, to a value greater than Eal where the current has reached.

an almost constant value. The average voltage E30 is preferably selected so as to correspond to for the sake of simplicity.

The electron multiplier I is similar t that previously described as well as the primary operatingcircuits therefor. In this circuit, however, the secondaryof transformer I 6 is connected between electrode 6 and the high voltage current supply source I 3, as appears in Figure 7. The primary of this transformer is connected to the carrier current source l as before, and the potentiometer I8 employed as before has its contact connected through the variable resistor capacitor combination 60-|9 to a movable contact which age applied to the grid of the amplifier". As a result no signal component will appear .in the secondary transformer 6|. Thus by means of this circuit both the undesired signal components and the undesired carrier voltage component due to the inherent capacity between the electrode 6' and anode l of the multiplier are eliminated.

From the above description it will be apparent to those skilled in the art that the subject matter of this invention may be embodied in other circuit arrangements, and I do not, therefore, desire to be strictly limited to the disclosure as given therein in an illustrative sense, but rather by the scope of the claims granted me. What is claimed is: I f 1. In a system for converting light variations to a modulated alternating current, the combinadivides a resistor into the parts I4 and 20. The

resistor combination |420 is connected in series between the anode I and the high voltage source tion including an electron multiplier having a cathode and a plurality of secondary emitters and an anode arranged so that electrons travel from the cathode to the secondary emitters and anode in succession, a circuit including said anode and the secondary emitter from which it receives electrons, means. for impressing an alternating carrier voltage on said circuit, .a circuit including the secondary emitter cooperating with'the anode and the secondary emitter from which it receives electrons, means for impressing operating potentials upon said secondary emittersand anode, said two circuits having .a

common connection, an impedance in said connection and an output circuit connected across Hi. The grid of the triode 55 is connected to the movable contact of a resistor 54 connected between the anode 1 and ground through a capacitor 51, cathode of the triode 55 to ground as shown. The grid of another triode 56 is connected to the electrode 4 through the conventional resistancecapacity network consisting of the resistors 53 and 5| and the capacitor 52. The resistor 58 connects the cathode of the triode 56 to ground. It will be seen that the voltage source 62 supplies the operating potentials for the anode circuits of both triodes 55 and 56. Theprimary of the coupling transformer 6| is connected in the output circuits of both triodes 55 and 56. As a result of the connection through the variable resistor 60 and the variable capacitor l9 a properly phased voltage may be made to appear across the resistor 21) which will oppose the voltage drop across resistors l4 and 20, resulting from the cur. rent flowing due to the inherent capacity between the electrode 6 and the anode I.

In the operation of this. circuit the current in the anode circuit, as previously shown, will have the form illustrated by the curve in Figure, 1. The current in electrode 4 will have the form illustrated by the curve. in Figure 5, but of a lower amplitude than the current in the circuit of anode I. Its, direction is such that the voltage across the resistor 5| will be in the opposite di- The-resistor 59 as before connects the rection tothe signal component of the voltage across resistors I4 and 20. Therefore, by applying a, portion of the voltage across resistors I4 and 20 to the grid of the vacuum tube of the triode the signal component in the plate circuit of triode 55 will be cancelled by the current in the plate circuit of triode 56. The current in electrode 4 is of too low a valuefor direct use as a neutralizing current, and therefore, the amplifier 5G is employed to amplify it to the proper value. Adjustment of potentiometer 54 also aids in this effect by controlling the value of the voltsaid impedance. 6

2. In thecombination of claim l, the means for impressing operating potentials upon said secondary emitters and anode including adjustable resistors for varying the potentials applied to the two secondary emitters specifically mentioned.

3. In the combination of claim 1, said impedance comprising a resistor.

4. In the combination of claim 1, said means for applying operating potentials to the secondary emitters specifically mentioned comprising a pair of adjustable resistors and said impedance comprising a resistor.

5. In the combination of claim 17, said im-' pedance comprising a'resistor.

6. In the combination of claim 17, said means including adjustable resistors.

7. In the combination of claim 17, said impedance comprising a resistor and said means including a pair of resistors.

8. A system which converts light variations to a modulated alternating current including an undesired signal component comprising a photo-electric multiplier having a photo-electric cathode, a plurality of secondary emitters and an anode associated with one of said emitters, means for supplying operating potentials to said emitters and anode, means for impressing an alternating carrier voltage upon the emitter as y sociated with the anode, an output circuit for said multiplier including its anode, a vacuum tube amplifier having an input and an output circuit, means for coupling said input circuit to the output circuit of said multiplier, and means connected to an emitter preceding the emitter associated with the anode of the multiplier for applying a properly phased neutralizing current to the output circuit of said vacuum tube amplifier to cancel the signal component therein.

- 9. In the combination of'claim 8, said last means comprising a vacuum tube amplifier having an input circuit and an output circuit, the

.- anode'associatedwith one of said emitters, means for supplying operating potentials to said emitters and anode, means for impressing an alternating carrier voltage uponv the emitter associated with the anode, a vacuum tube amplifier having an input and an outputcircuit, means for coupling said input circuit to the output circuit of said multiplier, means connectedto an. I emitter preceding the emitter associated with the anodeof the multiplier 'fo-rfapplying a, properly phased neutralizing current to the'output circuit of said vacuum tube amplifier to cancel the signal component therein, and means including a-variable impedance ior'applying a, portion of the carrier voltage properly phased to the input cir cuit of the vacuum tube amplifier coupled to the output circuit of said multiplier.

11. In a system for converting light variations,

to a modulated alternating current, the combination including an electron multiplier having a cathode and a plurality of electrodes and ananode arranged so that electrons travel from the cathode to the electrodes andanode in succession, a circuit including said anode and the electrode from which it receives electrons, means for impressing an alternating carrier voltage on said circuit, a circuit including the electrode cooperating with the anode and the electrode from .which it receives electrons, means for impressing operating potentials upon'said electrodes and anode, said two circuits having a common con nection, an impedance in said connection, an output circuit connected acro-ss,said impedance, andan impedance connection from said carrier phasedior neutralizing the carrier voltage component therein due to the flow of current through the inherent capacity between the electrode cooperating with the anode and the anode of the multiplier. I

12. In the combination of claim 11, said last impedance comprising an adjustable series re-' sistor-capacitor combination.

13. In a systemfor converting light variations to a modulated alternatin current, the combination including an electron multiplier having a cathode and a plurality of electrodes and an anode arranged so that electrons travel from the cathode to the electrodes and anode in succession, a circuit including said anode and the electrode from which it receives electrons, means for impressing an alternating carrier voltage on said circuit, a circuit including the electrode cooperating with the anode and the electrode from which it receives electrons, means for impressing operating potentials upon said electrodes and anode, said two circuits having a common concathode properly phased for neutralizing the carrier voltage component therein due to the flow of current through the inherent capacity between the electrode cooperating with the anode and the anode of the multiplier;

14, In the combination of claim is, said last voltage means to said output circuit properly ing with the anode trons, means for'impressing an alternating carrier voltage on said circuit, a circuit including the emitter electrode cooperating with the anode and. the 'emitter electrode from which itre-- ceives electrons, means 'for impressing operating potentials upon said emitter electrodes and anode, said twocircuits having a common connection, an

impedance in said connection, a vacuum tube hav ing at leasta cathode and grid across said impedance, a resistor in series with said cathode,

and an impedance connection from said carrier voltage means to said cathode properly phased for neutralizing the carrier voltage component therein due to the flow of current through the inherentcapacity between the emitter electrode cooperatand the anode of the multiplier.

16. In the combination of claim 15, said last impedance comprising an adjustable series resistor-capacitor combination.

17. A system for converting light variations into a modulated alternating] current including an electron multiplier having a cathode and a secondary emitter positioned to receive electrons therefrom and a plurality of additional electrodes for receiving electrons in succession from said emitter, means for applying operating potentials to all of said electrodes, means for applying an alternating'ca'rrier voltage to one of said additional electrodes, both of said means including a common connection'to another of saidadditional electrodes, an impedance in said common connection, a vacuum tube amplifier having input and output circuits, and means for connecting the amplifier across said impedance.

18. In the combination of claim 1'7, means for applying a neutralizing voltage component of the' carrier voltages to said amplifier.

19. In the combination of claim 17, means for applying a neutralizing voltage component of the carrier voltage to the input circuit of said amplifier.

20. In the combination of claim 17, means for applying a neutralizing voltage component of the carrier voltage to the output circuit of said am-, plifier. r

21. A system for converting light variations into a modulated alternating current including an electron multiplier having a cathode and a secondary emitter positioned, to receive electrons therefrom and a plurality'of additional electrodes for receiving electrons from saidemitter, means for applying operating potentials to all of said electrodes, a circuit including a source of carrier ,current connected to a pairof said additional electrodes in which circuit both the signal modulated carrier and signal currents flow, a second circuit connected to. another pair of said additional electrodes in which the signal current alone flows, and a work circuit interconnecting said two circuits so as to substantially neutralize the sig-

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