US2420731A - Facsimile apparatus - Google Patents

Description

May 20, 1947. M. ARTzT FACSIMILE APPARATUS Filed Nov. 18, 1942 NVEN La@ ToR ATTORNEY Patented May 20, 1947 FACSIMILE APPARATUS Maurice Artzt, Princeton,

Corporation of America,

Waffe N. J., assignor to Radio a corporation of Dela- Application November 18, 1942, Serial No. 465,977

7 Claims. (Cl. Z50-41.5)

This invention is directed to facsimile apparatus wherein messages or copies are converted into electrical energy signals for use in reproducing duplicates of the messages or for modulating a transmitter serving to transmit signals over a communication channel at the receiving ends of which the message may be reproduced.

Generally speaking, such apparatus is known as a facsimile scanner and amplifier apparatus wherein a subject copy is scanned by exposing it, element for element, to light and varying the amount of that exposing light which reaches a light translating photo-electric tube in accordance with the varying lights and shadows on the subject.

The present invention is particularly concerned with the amplifying system for amplifying the signals resulting from scanning operations. Such amplifiers are essentially of the type used to raise the signal level from the scanning phototube to a proper level for directly reproducing the subject copy or for modulating a suitable form of transmitter. Usually in such instrumentalities the record subject which is to be scanned is mounted in a fixed position on a support element, such as that disclosed, for instance, in the co-pending application of Charles J. Young, Serial No. 450,855, filed July 14, 1942.

In the application referred to, the record subject' is mounted in fixed position relative to a scanning instrumentality which is moved or rotated relative to the record surface, so that the varying intensities of light and shadow on the record -subject are tested, element for element, to produce signal outputs. In such a device, the scanning system rotates or moves relative to the record subject and usually comprises not only the illuminating light source but also the light translating phototube and usually one stage of amplification.

In other forms of scanning arrangements, the record subject itself may be supported upon a rotary drum and the scanning system arranged in substantially fixed position, except that the scanning system may move laterally of the drum, or the drum moved laterally relative to the scanning system.

In any case, however, in order to scan the record subject, there is provided suitable means for obtaining a relative motion between the scanning instrumentalityand the subject copy.

It is apparent that in such arrangements the frequency range of the signals developed as output energy from the phototube of the scanning instrumentality extends from substantially Zero frequency or D. C. up to' signals of the order of several thousand cycles, and, for instance, where the copies are to be made at relatively high speeds, the frequencies frequently are of the order of 10,000 to 12,000 cycles, or even higher, depending upon the speed desired. Consequently, in amplifying such signals over such a wide frequency range, it is frequently difficult to avoid distortion which would tend to make the reproduced signals inaccurate representations of the scanned subject.

It is known that any distortion in the low frequency and the direct current response of such amplifiers tends to give a variation in the background of the recorded copy, and to avoid such distortion it is frequently desirable to use direct-coupled amplifying devices. However, the circuits previously used for this purpose have been found, generally, to drift gradually from a Zero setting and to require frequent readjustments. This is especially true in cases where the phototube output resistor, across which the signal energy output from the phototube is fed to the first amplifier, is of a relatively low value, which is necessary to carry the entire frequency band at the same amplitude, for, under such circumstances, the amplitude of the phototube sig- .nal is so low at the very lowest frequencies, for example, when a long continuous highlight or lowlight is being scanned, that the drift becomes very troublesome.

In the prior art, capacity-coupled amplifiers have frequently been used to eliminate this drifting effect, and the background whichrit was desired to maintain was then frequently restored by a diode and commutator correction system. Such compensating circuits, however, do not always provide a complete correction becauseV some trailing-out effects of the background are always present.

Accordingly, it becomes an object ofthe present invention to provide an amplifying arrangement wherein provision is made for amplifying uniformly signals of all frequencies between the lowest or practically direct current t0 the highest frequencies utilized in the system.

Other objects of the invention are those of v providing an amplifier arrangement in which no special provisions have to be made for compensating for background setting while amplifying signals through a wide frequency range extending from extremely low frequencies to relatively high frequencies.

Other objects of the invention are those of providing an amplifier arrangement for amplifying the frequencies of the order stated, which is e'icient in its operation, easily and conveniently installed and which requires a minimum number of amplifying tubes and circuit elements.

Other objects of the invention are those of overcoming certain Well known defects of prior art systems in the manner herein to be stated.

Other objects and advantages will become apparent and suggest themselves to those skilled in the art when the following specication is considered in conjunction with the accompanying drawings, wherein the single figure represents in schematic form one suitable form of circuit.

Referring now to the drawings, it will be assumed, for purposes of illustration only, that the subject to be scanned is carried upon a supporting member Iwhich consists of a record drum II adapted to rotate upon a suitable shaft I2 in the direction indicated by the arrow. In order to analyze the varying intensities of light and shadow on the subject carried upon the periphery of the drum member I Iv, light from a source I3 is caused to fall upon the subject strip, which is carried by the drum, at a point I l when accurately focused thereupon by means of an optical system conventionally shown at I5. Light is relected from the subject in accordance with the varying values of light and shade thereof, and the reflected light is then directed by a second optical system I S to influence a photo-electric tube Il which causes the light directed thereupon to be converted into varying electric currents in accordance with the variances in light and shadow on the record subject scanned.

In many instances, the arrangement whereby theA record subject itself is maintained in substantially fixed position while the scanning operation takes place is to vbe preferred, and this was shown, for` instance, in the Young application hereinabove mentioned, but in any case the effect to be obtained isthat of relative motion between the photo-.electric tube and the subject copy, so that any form of scanning system functions substantially equally as well. However, in what is herein tobe stated, it will be assumed that thev arrangement is such that essentially the phototube II and its associated amplifier I9 constituting the first pre-amplifier are arranged on a. moving carriage and in closely adjacent position, so that the output energy from the phototube Il may be fed directly to the amplier tube I9 by way of short connecting leads.

In the arrangement disclosed, the phototube has its positive or anode terminal 29 connected by way of a conductor 2l to ground at 2,2 While the phototube cathode element 23l is maintained at a potential negative relative to ground by way of the connection through the load resistor 24 to the conductor 25 which connects to a negative output terminal conventionally represented at 26. The

output of the phototube for relatively low frequencies and even direct current is obtained across the resistor 24, so that potential drops induced therein due to varying intensities of lights and shadows on the subject carried by the record drum I I causing varying currents to flow through the phototube I'l Will cause varying potential drops across the resistor 24 Which are applied to the grid or control electrode 21 of the amplifier tube I9. However, for high frequency outputs from the phototube Il produced by scanning rapidly changing light values on the record subject, the load resistor 24 is shunted by'a combination of a capacity element 29 and a serially connected resist-or 30, which also connect to the conductor 25.

By suitably proportioning the circuit constants, the circuit arrangement herein described may be so designed that the amplifier is operated in such a manner that the very low frequencies and the direct currents are accentuated and appear at a much higher amplitude than do the higher frequencies, as will be apparent from what is to follow.

If it be assumed, for instance, that the resistor 24 is of a relatively high value, say of the order of ten megohms, and the resistor Sil is 0f a lesser value, say of the order of one megohm, then it becomes apparent that the D. C. output from the phototube will be approximately eleven times the high frequency output.

In one form of the apparatus, the phototube output current can be adjusted so that the voltage appearing between the phototube cathode 23 and the conductor 25 is of the order of 0.25 v-olt at direct current values, but of a value of the order of about 0,024l volt at the higher frequencies. AC- cordingly, it becomes apparent that the D. C. amplier can be readily designed so that it will have negligible drift with a relatively high input such as that hereinabove suggested, but it will `be subject to drift at the lower input assumed for the higher frequencies. Accordingly, at the higher frequencies, the energy to be amplified by the tube I9 is considerably less than is the case for the lower frequencies. Further reference will be made to this point, however, in what is to follow.

In the succeeding stages of the amplier, the output anode or plate electrode 32 of the tube I9, which has itsy cathode 33 biased relatively to the most negative point in the system, represented `by the conductor 245, is connected to supply its output energy to the control electrode 3d of an amplier tube 35, and also to energize the input circuit of a further amplifier tube 3l by way of the connection to the control electrode 38 thereof through the conductor 39. The cathode lll) of the tube 3l is suitably biased by the resistor lll, and the anode i2 thereof is maintained positive relative tov the cathode by way of its connection through the conductor 3 and the resistor Gli to ground by way of conductor 135. Similarly, the plate or anode l? of the amplier 35 connects to ground 2.2 by way of a resistor 38, and the #2 grid 139 of the same tube is held at ground potential by way of the connection through the conductor. 5&3. In this connection the tube 3f! acts as a balanced D. C. amplier to eliminate drift of the D. C. settingV of the voltage applied to the control electrode 3d of tube 35, as was explained in principle by my co-pending application Serial No. 367,899, filed November 29, 1940.

Accordingly, output energy from the amplifier tube I9 uses tube 3l as the plate load resistor and is then amplied by the tube 35 so that. at the output terminals thereof, represented for instance at the points 5I and S5, there appears a voltage which for illustrative purposes may be designated t as eo and eo. Accordingly, output energy from the tube 35 at the lower frequencies Will be obtained across the combination of` the resistors 53 and 54 arranged in series and connected to the conductor 25, which is assumed to operate at the most negative point in the system. However, higher frequency energy outputs from the tube 35 tend to be by-passed relative to the resistor 53 by way of the capacitor 58 which is connected in shunt with the resistor 53. The voltage supply for the tube 37 and tubevv I9 is developed across the serially connected resistors 59 and 80 for which an adjustable contacter 6I is provided, for a purpose later to be explained.

The cathode element 62 of the tube 35 is biased relative to the conductor 25 by means of the'resistor 58, and a suitable condenser 63 shunts the resistor 88 to Joy-pass the higher frequencies.

If now, the amplifier be further considered, it will be seen that by way .of the Vconnection provided, the output energy from the second stage of the amplifier, as represented rby the output from the tube 35, as identied by the voltages represented at eo', will be greatly exaggerated in amplitude for the lower frequencies, and these voltages are also relatively high with regard to the potential in the conductor 25.

In the process of correcting the frequency characteristic by dropping the level of the potential, it can be brought down at the same time almost to the potential of the conductor 25, and

it can be seen that the output is tapped at a point, i

represented at 65, as far las the D. C. in the low frequencies is concerned, but that the higher frequencies are passed directly to the resistor 54 by way of the condenser 58.

In order to obtain this result, it is desirable that a certain proportionality between the varim ous resistors and capacitors be maintained, and, accordingly, it may be -assumed that the ratio of the resistor to that of the resistor 38 be considered as equal to a constant. Under these circumstances, it can be seen that the ratio of the resistor 53 to the resistor 54 shall also be equal to the same constant, and if it new be assumed that the reactance ci the condenser 29 equals some other constant times the value of the resistance at some one frequency, then it will be evident that the reactance of the condenser 58 is equal to the ratio of the first constant selected to that constant increased by one times the product of the second constant chosen times the value of the resistor 54. If these proportions be maintained, then it can be seen that the value of the voltage at the point 68 will be exactly proportional to the output current from the phototube l1 at all frequencies below those at which the tube and circuit capacities enter, and, accordn ingly, the amplification for al1 frequencies between the lowest or practically direct current and the highest of the system, which may be of the order of 15 kilocycles, is substantially uniform.

Considering the arrangement further, it will be seen that for further amplification provision is made whereby the final amplifier tubes 68 and t5 are energized under the control of a driver tube 18 and a reversing tube 1 I. In this arrangement, the output energy from the rst two stages hereinabove described is coupled to the driver tube 18 and the reversing tube 1l, so that the output stages 88 and 55 may be driven from saturation to cutoi in going from black to white in the picture, for instance.

However, it is to be understood that to achieve this type of operation the driver tube cathode 12 should be maintained at a higher voltage on the bleeder than the steady state D. C. voltage represented by the value co at point 58. This objective is achieved in the `disclosed arrangement by utilizing the reversing tube` 1l as .a cathode follower, and also .by providing a cathode resistor 13 between the cathode 12 and the conductor 25.

Where it is desired to provide positive copies in recording with the recorder assumed to be connected at the loa- d connection terminals 15 and 16, or where it is assumed that the modulator of a transmitter is energized by the energy output appearing at these terminals, the voltage eo appearing at the point 68 is supplied to the control electrode 18 of the tube 18 by way of .a switching instrumentality conventionally represented at 19, and the control electrode of the reversing tube 1I is then connected to a suitable point on the potentiometer 8| by way of a conductor 82 connected through the switch instrumentality 19 so that the ycathode of the tube 18 is maintained at a xed and predetermined voltage positive relative to the potential in the conductor 25. This connection is provided in the case of each of the tubes 10 and 1| by virtue of a conductor 83 being provided between the connecting point B6, whereat the voltage co appears, and the contact points and 85 of the switch instrumentality 19. The other contact point 81 for the upper switch arm 88 connects by way of the conductor 82 to the selected point on the potentiometer 8 i, while the lower contact 81', to which the switch arm 88 makes contact, connects by way of a further conductor 89 to a selected point on a second poten tiometer 98 which has its end terminals connected to the tapping point 8l on the one hand. and the conductor 25 on the other hand.

If now it be desired to produce a positive copyv from the copy scanned by the scanning instrumentality and tested by the phototube i1, theswitcharms 88 and 85 (which preferably operate in unison) are moved, as indicated by the arrow, to rest against the upper contact points 81 and 85 respectively. Under these circumstances, the voltage eo which appears at the connection point 65 is supplied by way of the conductor 83, the

contact point 85', the switch arm 8B and the conductor 8i to the control electrode 18 of the tube 18, and, simultaneously, the control electrode of the tube 1| derives its potential from .the connection established on the potentiometer 8l by way of the conductor 82, the upper switch contact 81 and the switch arm 86 connecting to ghe control electrode 88 by way of the conductor If now, however, it is desired to produce nega tive copies representative of the subject carried upon the drum H and scanned and analyzed by the phototube l1, then the switch arms 86 and 88' are moved against the lower contact points 85 and 81' so that the control electro-de 18 of the tube 10 derives its potential by way of its connection to a point on the potentiometer 98 established through the conductor 9i, the switch arm 88 and the contact 81', so that it is maintained at a potential positive'relative to the conductor 25, and simultaneously the control electrode 88 of the tube 1l derives the potential eo, which appeared at the point 68, by way of its connection to the pointl 85 through the conductor 83, the contact 85, the switch arm 88 and the conductor 92'.

It thus becomes evident that the arrangement disclosed is such that there is always a direct connection from the photo-tube to the control electrode of one amplifier tube, and from the output or plate electrode of that amplifier tube to the control electrode of the next succeeding tube in the cascade, so that a direct current amplier has been provided.

In the disclosed arrangement, up to this point, signal outputs have been provided and explained in all tubes through and includingthe driver tube 18 and the reversing tube 1I, and with the connection provided, the driver tube 10 has its plate or anode element 93 connected directly through the conductor 94 and resistors 95and S6 to the control electrodes 91 and 98 of the main output amplifiers 68 and 69, as hereinabove explained. Thus, depending upon the position of the switch arms of the switch 18, depends whether or not the output energy from the tube 10, as it appears at the plate or anode 93, carries the grids or control electrodes 97 and 98 negative or positive, depending upon the position of the positive or negative switch arms.

The output stages 68 and 69 are connected in parallel and function as cathode follower stages with the load connected between the cathodes ||l| and |02 respectively and ground 22 through the terminal connection points 'I5 and 76, to which connection is made by way of the conductor |83. The maximum voltage applied between the terminal points 'i5 and 'I6 is determined by the plate voltage of the driver tube 'I8 and is adjustable over a considerable range by the color control potentiometer |04. In this case, the amount of signal swing toward the cutoff position for the output stage will determine the background, and this is also adjustable by varying the position of the slider contact |05 on the potentiometer |86.

It is evident that the darkest backgrounds will be obtained when the value of resistance of the potentiometer |08 is practically Zero, in which case the resistor element |81 is connected in parallel with the resistor element |88 as the plate resistor of the tube 78. But when the value of resistance of the potentiometer |06 is at the maximum, due to the setting of the contact point |85 for controlling the background level, then it becomes apparent that the plate load for the tube "l0 is supplied almost completely by way of the resistor |08 which will provide the maximum swing in the white direction, that is, towards cutoff for the main amplifier tubes 68 and 69.

In order to provide suitable operating potentials for the tubes of the system, it is evident that a suitable source of voltage shall be connected at the various terminals which have been conventionally indicated on the drawings, and accordingly, since Darts of the system have been indicated as operating above and below ground potential, it will be assumed that a plurality of power supply stages have been provided so that certain parts of the system operate above ground and other parts of the system operate below ground. However, where desired, the entire system may be so connected as to provide the conductor 25 at ground potential and operate other parts thereof above ground.

Obviously, certain modifications may be made in the system without departing from the spirit and scope of what is disclosed. Accordingly, I believe myself to be entitled to those modifications which fall reasonably within the invention as it is hereinafter claimed.

Having described my invention, what IY claim as new and desire to have protected by Letters Patent, is:

l. A direct-coupledv amplifier for amplifying energy extending over a relatively wide frequency spectrum covering a range from substantially direct current or zero frequency to a relatively high frequency value comprising a signal energy source for deriving signal energy having low and high frequency components extending within the wide frequency spectrum, an amplifier for amplifying energy atrthe said frequencies, an input network comprising a. series connected capacity and resistance element and a second 8 resistance element shuntingr the series network for supplying energy from the signal energy source to the amplifier, said input network being adapted to energize the amplifier to accentuate the amplification of the low frequency energy substantially as compared with the higher frequency energy, an output network connected to the amplifier comprising capacity and resistance means for attenuating the amplified low frequency energy to- 1re-establish a substantially uniform output response from the amplifier for all Values of input frequencies within the limits supplied from the source.

2. A direct-coupled amplier for amplifying substantially uniformly signal energy over a wide range of frequency between approximately zero frequency and a relatively high frequency, comprising terminal means for deriving electrical energy extending over the pre-selected wide frequency range, a thermionic amplifier for amplifying the said frequencies selected, a purely resistive means for applying energy of one frequency range from the terminal means to the amplifier, and a series resistance and capacity means for applying energy of another frequency range from the terminal means to the amplifier so that the frequencies of the lower frequency range are exaggerated inv amplitude with respect to signals of the higher frequency range, and a second thermionic amplifier device directly connected to the output of the first thermionic amplifier to receive the energy output therefrom, a load circuit for the second amplifier device including a resistance element and a capacity element shunting a portion thereof with the said capacity element being connected to one terminal ofthe resistance means and to an intermediate point thereon, and an output connection to the said intermediate point so that at the output terminal point signal energy of substantially uniform amplification value for the wide frequency range appears.

3. A direct-coupled amplifier for amplify-ing substantially uniformly signal energy over a wide range of frequency between approximately' Zero frequency and a relatively high frequency', comprising terminal means for deriving electrical energy extending over the pre-selected wide frequency range between the desired limital values, a thermionic amplifier for amplifying the said frequencies selected, a purely resistive means-for applying energy of one frequency range from the terminal means to the amplifier, and a series resistance and capacity means for. applying energy of another frequency range from the terminal means to the amplifier so that the frequencies of the lower frequency range are exag gerated in amplitude with respect to signalsy of the higher frequency range, an output load circuit including a resistance element and a capacity element shunting a portion thereof with the said capacity element being connected to one terminal of the resistance means and `to an intermediate point thereon, and an output connection to the said intermediate point so that at the output terminal point signal energy of substantially uniform amplification value for the wide frequency range appears.

4. A facsimile amplifier comprising light translating means for generating electrical energy of a frequency range varying between a substantially zero frequency value -and'a relatively high frequency value, an amplifier means for amplifying the generated energy, means comprisingan electrical network including the combination of a series connected capacity and resistance element and a second resistance element in parallel put is substantially uniform over the desired frequency range, a pair of amplifier tubes connected to receive the amplified output energy, means for controlling the said amplifier tubes by the previously amplified signal energy so as to vary the said energy output of the said last named tubes between a cutoff value and a saturation value for changes between preestablished limits in the input signal energy, and means for shifting the control of the said last named device so as alternately to provide for changes in sign of the resultant amplifed output energy.

5. A facsimile amplifier comprising light translating means for generating electrical energy of a frequency range varying between a substantially Zero frequency value and a relatively high frequency value, an amplifier means for amplifying ie generated energy, means comprising an electrical network including the combination of a series connected capacity and resistance element and a second resistance element in parallel with the series combination for energizing the amplifier by the generated energy so that the amplifier ampliiies low frequency values in the generated frequency range to a value substantially greater than that of the higher frequency range, an output circuit for receiving the amplified energy from the amplifier, and means `included in the output circuit for attenuating amplified signal output energy in the previously accentuated amplification range for the selected lower frequencies so that the resultant amplified output is substantially uniform over the desired frequency range.

6. A facsimile amplifier comprising light translating means for generating electrical energy of a frequency range varying between a substantially zero frequency value and a relatively high frequency value, an amplifier means for amplifying the generated energy, means comprising an electrical network including the combination of a series connected capacity and resistance element and a second resistance element in parallel with the series combination for energizing the amplifier by the generated energy so that the amplifier amplifies low frequency values in 25 frequencies while 30 fier, said the generated frequency range to a value substantially greater than that of the higher frequency range, an output circuit for receiving the ampliedenergy from the amplifier, means in- 5 cluded in the output circuit for attenuating amplied signal output energy in the previously accentuated amplification range for the selected lower frequencies so that the resultant amplified output is substantially uniform over the desired a pair o-f ampliiier tubes connected to receive the amplified output energy, and means for controlling the said amplifier tubes by the previously amplified signal energy so as to vary the said energy output of the said last named tubes between a cutoff value and a saturation value for changes between pre-established limits in the input signal energy.

7. An amplifier for uniformly amplifying the output from an energy source providing alternating current having components in a wide frequency range along with a direct current component, comprising a filter for attenuating the alternating current components within a range from a low frequency to and including the highest transmitting the direct current component and the components of very low frequency with little attenuation, an amplifier for amplifying the output of said filter, a second filter connected to the output of said amplisecond filter having circuit elements interconnected in a manner to decrease the direct current output of said amplifier and to attenuate the very low. frequency components in the output of said amplifier, the output of said second filter delivering tlie D. C. component and the low and high frequency components at a uniform level but with a substantial gain as compared to the output of the energy source.

MAURICE ARTZT.

The following references are of record in the file of this patent: 1

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