US2465265A - Photoelectric controlled carrier modulator - Google Patents

Description

Patented Mar. 22, 1949 UNITED ES- FA'IENTA PHOTOEIJECTR'IC" C'ONTRODIED CARRIER MODULATOR Hughv C. Ressler, Bayside,

Wireless,.-Inc., NewYork, NnY., a. corporation of Delaware N. Y., assigner to Press In certainkinds of signalling systems, such as'A facsimile transmission, it is' desirable totransmit thefacsimile' signals inthe'form of a modulated tone which, for example, may be of vaudio frequency. It is also desirable tobeabletocontrol the modulations directly by means `ofthe photo electric cell which responds to the shadecharacteristics of the subject matter to be transmitted. Various kinds of' these directcell-.modulators have .been proposed- Infgeneral, such prior. modulators are based upon the use of the cellina` balanced Wheatstone bridge network, whereinthe varying impedances of the cell are converted into bridge unbalanced currents.. The tone signal or carrier is applied to the bridge across one set of conjugate points, and the-modu lated tone-isv taken offy froml the remaining two coniugate points.

However, with these prior arrangements', there are a-number of disadvantages which it isthe purpose of the present inventionfto overcome. Since the tone signal is in the nature ofan. alternating current. of. audio frequency, itis necessary toemploy specially designed and expensive coupling transformers of the balanced` winding type. Secondly, in order to balance. out the residualimpedance of the photo-electriccell, it.. is necessary to'employ'a separatesimilar cell.: or to provide a specially'designed duplex` photoelectric cell, and care must bel taken to make sure that the capacity of the various leads.- toV theduplex cell are fully balanced.. Thirdly, .with the prior Wheatstone bridge type of. tone modulatorit is not easy to-balance outthe inherent. cell impedance over av wide range of incident light values. For example, if with the. prior arrangement the carrier. is balanced out for one given value of incident light, it is` not. easy to4 balance out the carrier for other widely different. light values.

Accordingly, it is a principal object. of this. invention to provide an improved tonemodulator controlled. by a photo-electric cell wherein. the amplitude of the tone appearing at the output.. of the modulator can be variedaccordingtothe level of incident light and balancedV out for any, desired datum or reference levelof incident light normally encountered.v

Another object is to provide a photo-electric cell carrier. modulatingnetwork of the parallel-1'. resistance-capacitance type, wherein. the tone source can be .directly connected=to the network.

2 without. requiring special input coupling transformersand the like.

A` feature of the invention relates' toia modu-l lating network of the parallel-T type for use' in producing aimodulated tone or carrier signal,

and wherein the input tone signal or carrier can;

bereadily balanced out.

Another feature relates to an improved' carrier.. modulation control network employing a `photo-electric cell, wherein the input tone signal or carrien can be suppressed by a relativelysimple: combmatlon of resistors and. condensers, thusv enabling. the standard two-terminal photo-elem tric. cell: to be used for a wide variety of workingrequirements.

Another. feature relates to a photo-electric cellv carrier modulator employing a "parallel-T networkwhereby part of the network together with the..cellcanbeassembled as a separate unit for n lounti'ng4 on. the facsimile machine chassis, and

the remainder of the network can be located ata-.remote .point convenient to the tone source andconnectable to the facsimile machine yby a'y shielded. cable or the like, which forms a com- I mon.return for. the network.

Other features and advantages not particu-- larly enumerated: will be apparent after a` consideration of the following detailed descriptions:

and.the1appended. claims.v In the drawing whichA tion,

Fig. lis a simplified schematic diagram of a. modulatmg network embodying features of the invention.

35.y Fig..2` isaschematic diagram of a facsimile transmitter systemembodying the network simila-rto-that of Fig. 1.

Fig. 3 is a schematic diagram of a modication-ofFig. 1..

iication` of Fig. 1.

Thephotocell modulator herein described cani bey generally classified as a three terminal network` containing two kinds ofr impedance ele- Y. mentsonly, namely resistances and capacities.

The generator or tone source is connected to'v one of the terminals and to the common terminalfreferred to as ground; the loador utilization circuit is connected to a second terminal andl tothe. common or ground terminal.

-rent from the generator is transmitted to the shows. one preferred embodiment of the inven-A Fig..4 is .a schematicdiagram of another modi--Y pedance elements in one of the current branches so that the currents arriving at the load over the two separate paths are equal in magnitude but opposite in phase at the carrier frequency, when the cell is illuminated at the reference level. The circuit can be designed to bring about a condition of carrier suppression at a second reference level of illumination by a further adjustment of the impedance element usually called the balance control. This type of modulator in contrast to the conventional Wheatstone bridge, balances at a single frequency only but can be made to balance at any desired reference level of cell illumination normally encountered.

The network can take the form of a parallel- T network shown in Figs. 1 and 2 in which the two current paths are through the T sections, or it may take a more general form, a ladder network with a bridging element shown in Figs. 3

and 4, wherein the current paths are through the ladder itself and through the bridging element.

Referring to Fig. 1, the numeral I represents any suitable source of alternating current, for example a tone or carrier current generator of audio frequency, and numeral 2 represents any suitable utilization circuit to be worked with modulated carrier. The units I and 2 are connected together through a network 3 according to the invention. This network may be referred to generally as a parallel-T network. One T- section comprises the condenser 4, and photoelectric cell 5 in series, and with their `junction point P connected through resistor 6 to the common return conductor I0. The secondT-sec tion comprises the two resistors 1 and 8 in series, and with their junction point P1 connected through variable condenser 9 to the common return I. The return II) may be a common ground return or it may be a separate conductor such for example as the sheath of a sheathed cable.

From the foregoing, it will be seen that there are no D. C. potentials applied to the cell 5, and the maximum swing of the carrier is preferably such as not to subject the cell to any A. C. voltages above 2 volts. With such an arrangement the cell operates substantially by reason of its apparent changes in impedance in response to incident light signals. However, even in the absence of light, there is always a small inherent impedance between the electrodes, the effect of which must be balanced in order that zero carrier may exist at the terminals II, I2, when the cell is completely dark.

I have found that by suitable adjustment and selection of the values for resistors 6, 1 and 8, and condensers 4 and 9, it is possible directly to modulate the carrier from source I, merely by con--y trolling the light on the cell 5. Furthermore, the f balancing of the inherent impedance of the cell 5 and the suppression of the carrier at terminals I I and I2 can be effected for no light incident on the cell. Thus, in certain cases, it may be required to have zero tone or carrier corresponding to a black shade of a picture. In that event, the network is initially adjusted by means of condenser 9 to the desired balance with the cell 5 completely screened, resulting in substantially en-`v tire balancing-out of the carrier applied to the utilization circuit 2. Therefore, when light signals of increasing intensity from black to white are impressed on cell 5, the level of the carrier is correspondingly increased. In other cases, it may be desirable to work with a suppressed carrier amplitude corresponding for example to a picture shade of gray. In that event, the network must be initially balanced by having a mean light shade e. g., gray incident on cell 5. Therefore, values of light above and below this mean light shade value will result in corresponding variations in the amplitude modulation of the carrier from source I. In order to effect this result, the twin T network of Fig. 1 is modified as shown in Fig. 2. The modification consists in adding a. section composed of a condenser in series with, and a resistance in shunt to, the T section containing the cell. Thus, as shown in Fig. 2, the twin T section comprising the elements 4, 5, 6, together with the modifying elements is in parallel with the T section comprising the elements l, 8 and 9. An additional section composed of .adjustable resistor I8 and condenser I 9, is connected in cascade with the T section 4, 5, 6, and this combination is connected in parallel with the T section 'I, 8 and 9. Suppression of the carrier at any given value of illumination is then accomplished as follows. Variable resistor I8 is set to its maximum value. The cell 5 is screened from light and the carrier is balanced out by adjusting condenser 9. When the correct setting of condenser 9 is found, it is left in its adjusted position. For suppression of the carrier at anyother desired reference level of illumination, resistor I8 is reduced in value until the carrier is again reduced to minimum or zero with this reference level of illumination on the cell.

Heretofore, where Wheatstone bridge modulating networks have been employed for this purpose, it has not been easy to balance the bridge to zero setting to correspond to widely different values of light on the cell. In such cases, it has been necessary either to use different cells or a critical adjustment of the potentials applied to the bridge is necessary. Furthermore, in these prior systems great care must be taken that the input transformer connecting the carrier source to the bridge is properly balanced and the lead wires connecting the network to the photo cell must likewise be critically balanced. These prior arrangements therefore, have necessitated either the use of two separate photo cells, one corresponding to cell 5 which is the main or signal responsive cell, and the other an auxiliary cell for balancing-out the capacity of the main cell. In other cases, it has been necessary to employ duplex photo-electric cells having three separate lead-in wires, the balance between which must be accurately adjusted. With the arrangement described in connection with Fig. 1, the cell 5 is capable of operation with complete elimination of vthe inherent impedance of the cell, and zero tone signal can be produced corresponding to any desired datum level of light incident on the cell 5.

` One of the important advantages of the modulated network above described is the fact that the cell 5 and the balancing condenser 9 can be assembled as a unit for convenient mounting anywhere on the framework of .a facsimile machine. Thus, as shown in Fig. 2, the cell 5 together with the resistors 1, 8, and the balancing condenser 9, are assembled inside the usual photoelectric cell box or housing I3, which can be mounted so that the cell 5 receives the light reflected from the subject matter on the scanning drum I4 in the usual way. The input tothe box I3 can be supplied by means of a sheathed twowire cable, Athe 4conductors Iof which are designated I5, I6, and -the sheath of which is desig-y nated I'I. Elements I8, I9, 4 and yIi shown in Fig. 2 can be placed on the stationary part of the facsimile machine, remote from the travelling photocell box. The circuit elements shown can be so lproportioned that 'the capacity to ground introduced by the cable, as well as coupling capacity between leads I5 and I6 either have a negligible effect on the circuit performance, or their effects can easily be accommodated by means of the balancing elements. Element I8, the carriersuppression control, is usually placed on the control panel of the unit. Carrier tone is supplied to the modulator through the cable leads I5 and IB and through the comm-on return and cable sheath Il. A concentric cable is Iprovided to connect the output of the modulator to the utilization circuit. This consists of the concentric Vconductor II and the sheath I2.

As one typical example of an arrangement found to produce the desired results, the various components of Fig. 2 had the following values.

Ra= megohms, R1=3 megohms, Rc=80,000 ohms, R1s=110,000 ohms Variable, C9=180 mmfd. Variable, 04:.002 mfd., C19=.03 mid., frequency of source 1 1800 C. P. S. Ph-otocell 5 is a type RCA934.

Fig. 3 shows a -modic-ation of Fig. 1, wherein instead `of employing .parallel-T networks, a ladder network is employed .composed of series resistors 25, 26, 2l, 28, with shunt condensers 29, 30, 3I, the latter one of which is adjustable and corresponds in function to the balancing condenser 9 of Figs. 1 and 2. The photo-electric cell 5 is bridged across the entire network. The remaining parts of Fig. 3 which correspond functionally to those of Fig. 1, bear the same designation numerals. By lproper choice of the elements constituting the network and by appropriate adjustment of the condenser 3 I, for a given tone e. g., 1800 C. P. S. applied to the input terminals and with no light on cell 5, the -currents arriving at the output terminal II Iover the two parallel paths represented respectively by the network land the cell 5, will be equal in magnirtude but opposite in phase. If it should be desired to balance out the carrier at the terminal II for any reference level lof light other than black, a similar arrangement is employed such as shown in Fig. 4. This arrangement is substantially the same .as that of Fig. 3 except that an additional section comprising condenser 32 and variable resistor 33 is connected vacross the input end of the network. When it is desired to balance out the tone or carrier at the terminal I I corresponding for example to a gray light on cell '5, the condenser 34 is adjusted with the cell 5 completely screened against light so as to produce substantially zero or minimum carrier at terminal II. During this adjustment, the resistor 33 has been set at its maximum Value. Cell 5 is then exposed to the gray reference light, and resistor 33 is lowered in value until the carrier at the terminal I I is again reduced to minimum or zero.

Fig. 4 shows one particular set of values for the various resistors Iand condensers that were found to achieve the desired result at an input tone frequency of 1800 C. P. S. It will be understood of course, that if the tone signal is a different frequency from 1800 C. P. S., the various elements will lbe accordingly designed to produce 'the desired balancing-out of the carrier at the load circuit.

While certain embodiments of the invention have been disclosed herein, it will be understood that various changes and modifications may be made without departing from the spirit and scope of the invention.

What is claimed is:

1. A modulator arrangement for telegraph systems and the like, comprising a carrier source, a carrier modulating network comprising two parallel paths, one path including in series a photo-electric cell and a capacitance, the other path including in series a pair of resistances, a common return point from said two paths to said source, a resistor connecting the first of said two paths to said common path and a condenser connecting the other of said pair of paths to said common point, said resistor and condenser serving to suppress the carrier for any given value of light on the cell.

2. A modulator arrangement for telegraph systems and the like, comprising a carrier source, a utilization circuit to be energized by a signalmodulated carrier, a line connecting said source and said circuit, said line including a phase-shifting network, one arm of which comprises in series a photo-electric cell and a capacitor, the other arm of which comprises in series a pair of resistors, means connecting said line to one pair of points of said network, and a resistor and condenser connected to another pair of points of said network; and means to adjust said network to suppress the carrier to correspond with any predetermined light level on said cell.

3. A modulator arrangement for telegraph systems and the like, comprising a carrier source, a. utilization circuit to be excited by signal-modulator carrier, a line connecting said source and circuit, said lne connecting a phase-shifting network, one arm of which includes in series a photoelectric cell and a capacitance, the other arm of which includes in series a pair of resistors, means to adjust said network to balance-out the carrier, means connecting said line to one pair of points of said network, and a resistor and condenser connected to another pair of points of said network, the last-mentioned condenser serving to suppress the carrier for any desired light condition on said cell.

4. In a modulating system of the character described, a source of carrier current, a utilization circuit to be worked with modulated carrier, means to modulate said carrier in accordance with light variations and comprising a connection between said source and circuit, which connection includes two parallel paths, one of said paths being substantially entirely capacitative and including a light-sensitive cell and a series capacitor, the other of said paths being substantially entirely resistive, a common return for said source and circuit, and adjustable impedance means between said paths and said common return for suppressing the carrier at the input of said circuit when said cell is subjected to a predetermined light intensity.

HUGH C. RESSLER.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 1,992,055 Cockrell Feb. 19, 1935 2,054,836 Shepard Sept. 22, 1936 2,177,366 Iams Oct. 24, 1939 2,191,795 Kolb Feb. 27, 1940I 2,298,466 Cooley Oct. 13, 1942 2,407,293 Shepherd Sept. 10, 1946

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