US2290229A - Frequency modulation facsimile transmission - Google Patents

Description

July 21, 1942. w'. G1 H; INCH 2,290,229

FREQUENCY MODULAT ION FACS IMILE `TRANSMI S S ION Filed April 5, 1940 5 sheets-sheet 1 'igl Pulse 'csmie l orrecan Blmd Gener-afar 'Scanner Neiwork adulwor wys Zo n mp lfm iiar 5ide=mad hfw amzer www Frequency Power Wulplirs mplgjer Pulse Ampli ier INVENTOR wliiam Qlmb.

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ATTO RN E YS July 21, 1942. w. G. H. FlNcH l 2,290,229

' FREQUENCY MODULATION FACSIMILE TRANSMISSION scllalor Broadand Rftuge .Sieges Lumier .Deecor 'E of AM. fzler 5 Pulse l :l Peecor Discriminqar "g of EN.

C 2 1? a s: Pulse Audie Ampli ier E Amplifier .Synchrmnizng Magne? '5,1 las A T INVENTOR ATTURNEYS July 21, i942. w. G. H. FlNcH 2,290,229

FREQUENCY MODULATION FACS IMILE TRANSMI S S ION Filed April 5, 1940 -2 Sheets-Sheet 3 wllimgYG. Slflcb July 21, 1942 w. G. H. FlNcH 2,290,229

FREQUENCY MODULATION FACSIMILE TRANSMISSION Filed April 5, 1940 -5 Sheets-Sheet 4 f @/zg scllafor Braad-3am I /f L" i RE Stage age zml er gig Z2 50 n n Deecor 0f EM.

f3 :Li/54 Picture Amplifier .5i lus \`53 y 7 .Synchronzng 59 Mayne' Il I: I INVENTOR 'Belam @.lincb.

ATTDRNEYS I w. *G. H. EINCH. A 2,290,229l

July 2l, 1942.

I FREQUENCY moDULATIoN FACSIMILE TRANSMISSION B sheets-shears Filed April 5, 1940 Picture Curreni' @I w f1 lgzueny Madulaled Wave wh Synchraniznq Pulses n ggf/lbf Ampliude Deecfor iime Ulli/ggg Pulse Discriminaor H H f H H (d) Y time INVENTOR ATTORNEYS I Patented .lnly 21,1942

r OFFICE l FREQUENCY MonULA'rIoN FACSIMILE TRANSMISSION 'willinn'i d.' H. Finch, Newtown, conn. Application April s, 1'94o,\serlal No. 328,131 s claims. (cl. 17a-6.6)

My invention relates in general to the eld of facsimile transmission and reception but more specifically concerns the application of frequency modulation transmission methods toy picture transmission.

Frequency modulation offers several distinct advantages over the more common amplitude modulation methods when transmitting intelligence in the form of speech or` pictures. The most notable advantage 'obtained in the transmission of speech by broad band frequency modulation is the low noise level and the minimizof generaldisturbances, such as'man-made or natural static. This may briey be explained by the fact that both man-made orvstatic noises generally are of a nature which affect the ampliresults in considerable distortion of thehigh frequency components of the wave because of the broad band of frequencieswhich must necessarily be amplified. Withlow level modulation of the carrier, the band width becomes relatively small,

and a more linear amplification characteristic is obtained. .I accordingly contemplate low level picture modulation in my system.

In facsimile transmission systems, it is necessary to periodically transmit a synchronizing pulse in order to maintain synchronism between the scanning mechanisms at the transmitter and receiver. The most practical method heretoforeemployed involves an arrangement for switching tude of the signals, but cannot produceuctuations of frequency..

vWhen applied to quency modulation offers additional advantages, not only in the reductionl of the faults, streaks, and spots due to interference and noise, butin improvements in the transmission channels and' simplification of the problem of synchronization.

The quality of a received frequency modulation signal is dependent upon the maximum frequency deviation or band width. For a given signal the quality at the receiver will ,increase picture transmission, fre- I the receiver when the receiver scanning mechanism has reached a predetermined point for receiving a synchronizing signal in a special synchronizing circuit which lby-passes the picture receiving circuit. The moving .part of the receiver is operated start-stop i. e. it is caused to rotate just enough faster than the transmitter as to insure its return ,to` the starting point Just 'ahead of the transmitter.

The receiver then stops and awaits a synchronizing pulse which chronizingfpulse the receiver starts into movement again, in synchronism with the transmitter.

rapidly as the band width is extended,sand for high delity transmission, broad band modulation is utilized, the lfrequency'deviation sometimes being as great as 100 kilocycles.

Low band frequency modulation may be employed with band widths not exceeding-that of the picturesignal itself. Although the reduction of faults will not be the maximum attainable', such a system would be justified by the economy and the saving of space in the frequency spectrum.

Sincethey common facsimile image is transmitted with a band width of 'a few thousand cycles, I contemplate as one imm of my invention to employ the ordinary broadcast frequencies with a 10 kilocycle band width for picture transmission by frequency modulation.

The superposition of intelligence such as a picture signal upon the carrier wave in a frequency modulation transmitter is accomplished by 'lowlevel modulation of the oscillator after which the modulated wave is successively amplified up to the power amplifier. Thisis alternative to high level modulation in which the picture signal is first amplied to relatively high p'ower level and then superimposed on the carrier. K picture level required for high level modulation the transmitter sends out as soon at it reaches the starting position. (Jn-receipt of the syn- This has some disadvantages. Foremost it is based on deliberately driving the receiver slightly out ofsynchronism with thel transmitteri Moreover, it materially reduces speed limits and operation determined by the speed with which The high the receiver can be stopped and started again i without mechanical strains.

Frequency modulation, when applied to facsimile transmission, offers several new possibilities insofar as synchronization is concerned.

synchronizing lsignals may be transmitted on. the frequency modulatedfcarrier by means of 'a distinct frequency component removed from the' picture band by some ten or twenty per cent. .Atl

the receiver, ahighselectivity filter is paralleled with the input to separate the synchronizing signal from the picture for the purpose of applying t it to the synchronizing means, which may be a magnet controlled friction clutch orv the like.

In` television, where two types of synchronizing pulses are required, one-for scanning and one for framing, two distinct frequencies associated with two sharply tuned circuits to accomplish separation, effect proper synchronization. 'I'lie objectionto this synchronizing method is that the maximum band width allowable can not befgainfully applied to the picture to secure a f'eiful reproduction; but must be reduced by` andamplitudemodulatlon of a single `carrier the aforementioned ten or twenty per cent in may be carried one step further in which thenic`"` order to transmit the synchronizing signal.

I have discovered that I may employ the full width of this signalling channel for simultane-l ous transmission of picture and synchronizing pulses which may be applied to facsimile transmission and to-television transmissions. More broadly, I have discovered that I may secure the benefits of two channel transmission over what has heretofore been regarded as a single channel by employing a composite system of frequency modulation and amplitude modulation.

Moreover in accordance with my invention I can transmit by radio a synchronizing signal which can be used for driving synchronous motive transmitter and receiver equipment lfor maintaining synchronism instead of a start-stop system.

In the conventional system for picture transmission, the receiver employs a switch operated by a cam on the scanning unit, which is timed to interrupt the current to the receiver stylus and complete the circuit to the synchronizing means for the duration of the synchronizing pulse. Such switching is undesirable because of constant wear and other electrical problems but is a necessary precaution in the amplitude modulation system to prevent the synchronizing pulse from recording as a black streak along the edge of a picture. Also, if amplitude pulses are used for synchronization in a facsimile recorder using the discoloration of dry paper to produce the image, a large current pulse passing through a point in the paper constitutes a fire hazard.

In accordance with my invention the synchronizing signals are transmitted by variations in the amplitude of the carrier. Inasmuch as the amplitude of the frequency modulated picture signal is constant and the frequency of the amn plitude modulation synchronizing signal is constant discrimination between the picture and the synchronizing pulse at the receiver is made upon an amplitude basis in accordance with the novel circuit I have devised and I may therefore eliminate the switch and its operations.

Such discrimination is effected by my novel circuit arrangement for discriminating between the frequency modulated signals and the amplitude modulated signals, in which the frequency detector in the frequency modulated circuit operates only in response to variations in frequency, to produce corresponding variations in ture signal may be employed to modulate the frcquency of the carrier, as in the foregoing, while a constant frequency continuously modulates the amplitude of the carrier. This constant frequency may be employed to effect synchronization between the transmitting andreceiving ends of a. facsimile system avoiding the necessity of the start-stop system with its inherent mechanical limitations. If the amplitude modulating signal is controlled at the transmitter, it may be employed to maintain synchronism-between devices at both transmitter and receiver by the expedient of locking an oscillating circuit at the receiver into synchronism with the original source of alternating currents. These two currents of identical frequency may then be employed to drive their respective facsimile systems in synchronism with each other.

Another application of my novel combined amplitude and frequency modulation scheme would be in television systems in which I transmit two distinct amplitude variations each of constant frequency on the frequency modulated carrier, which after detection at the receiver operate both the scanning and framing means, namely, the cathode ray tube deflecting plate oscillators. Such a system eliminates the need for the synchronizing pulses of an amplitude nature transmitted upon the amplitude modulated carrier, which pulses are quite often a source of interferencedue to the relatively high frequency at which they occur.

It is therefore an object of my invention tc provide for a novel method of transmitting images by frequency modulation.

Another object of my invention is to combine frequency and amplitude modulation in one transmitter and employ a single carrier.

Still another object of my invention is to provide means for the transmission of synchronizing pulses by amplitude variations on a carrier which is frequency modulated in accordance with the picture signals.

A further object of my invention is to provide means for a system of transmitting television amplitude in its output and will therefore pass through only the frequency modulated signals which are of constant amplitude but vary in frequency, and in which the amplitude detector in the amplitude modulated circuit operates only in response to variations in amplitude to produce corresponding variations in amplitude in its output and will therefore pass through only the amplitude modulated synchronizing signals.

I thus employ two circuits, through one of which I pass the frequency modulated signals and through the other the amplitude modulated signals.

Thus, the synchronizing circuit and the picture circuit may be in parallel without any switching means between them. There can be no interference between the two signals mentioned because a frequency modulation receiver l responds only to the rate of change of frequency deviation and the amplitude modulating frequency and its associated side bands are constant. The combination of the frequency modulation signals by frequency modulation along with synchronizing signals of an amplitude nature, which cannot be 'a source of interference.

Still a further object of my invention is provide a novel synchronizing means.

These objects and others will become evident from the following specication taken in conjunction with the accompanying drawings in which:

Figure l is. a block diagram of a frequency modulation transmitter coupled with a facsimile scanning unit.

Figure 2 is a block diagram illustrating one embodiment of a receiver designed to receive and to discriminate between amplitude and frequency modulation signals.

Figure 3 is a schematic wiring diagram of the transmitter of Figure 1.

Figure 4 is a schematic diagram of the receiver indicating the various circuit details.

Figure 5 is a diagramillustrating the wave form of the current in successive stages of the facsimile system.

Referring now more particularly to Figures 1 and 3 there is shown one possible embodiment of a frequency modulation transmitter coupled to a facsimile scanner. Figure 1 illustrates genylating current.

@scopesvv erally the various. scanning`,andfrequency moc-luf' lation circuits, grouped into a block diagram. i

The facsimile scanner mayfbe, of Aany well known form asthe oscillatingparm, the endless' erator thereto.

. 4 For purposesof illustrating myf-invention,` 1V have hereinrdescribed and .illustrated the fre.4 'quency modulation system a disclosed in Patent No. 1,941,068-, although Ait willvbe understood that I am not limited to this type otfrequency moduformer I'I the plate volta-ge ofone tube will be Adriven to a'higherJvalue than the other, and

the plate current of one willincrease while that of the other decreases proportionately. Under vthese 'conditions afvoltage willbe induced in the secondary 2l which'will be lapplie'dtc the grid ofthe amplifier 20.5 i

yThe vcltage induced in the transformer secondary 2l will bean amplitudev modulated wave,

l Therefore, the resultant` current contains only the side band frequencies generatedlin the modulator.. Furthermore, it should be .noted` that since the plete eueuitser thezva'euum Ytubes la and I, 'have been tuned to resonance, the impedance of the circuits is purely resistive. Therelation. transmitter or receiver and may useany otheripractical form.

'Tracing through the block. diagram of Figure 1 1 the facsimilescanner convertsA thelight fluctuations impinging upon the photoelectric' cell into corresponding amplitude variations "of current in' trol-grids.

vforaine current in tneplete cireuitis in phase with the oscillator voltage applied to the con- The potential drop across .the inductive branch of the circuitgnamely, the'- transfonner primary the output circuit. These amplitude'iiuctuay tionsfin'the manner WeilI known in the art as shownin theaforementioned patents,are of an` alternating 'current nature as shown in Figure 5a and are applied to the input of the frequency Y l modulation transmitter for producing variations in frequency varying from the carrier in accordance with amplitude fluctuations as will now be described.

Referring to Figure 3, the frequency modulator circuit includes a low frequency crystal oscillator 2| for generating the carrier wave. 'I'he natural frequency of the crystal is some submultiple of the finally radiated 'carrier frequency and,I must be multiplied in Ia circuitwhich will i hereinafter be described, in order to raise the frequency to that required for transmission.

The output of the crystal oscillator 2|, as shown,- is fed to two distinct channels, one of which is thef balanced modulator, comprising'the` vacuum tubes I8 and laandtheir associated circuits.

Thebalanced modulator issn-amplitude modulator which suppresses the carrier from the output circuit.

The grids of the modulator tubes Il and II are driven by the oscillator 2l which supplies the carrier ,.wave. Modulation is eifected by varying the plate voltages applied to tubes" Il and i9 through the source vof electromotive force I0, by means of the' modulating currents-that flow in the transformer I'l, coupled into the'plate circuits of the tubes.-

The plate circuits of each of these tubes, which may be triodes or of multigrid construction, each inclues one-half of the primary winding of the output transformer 22-22.v

This output circuit is made non-reactive by placing the variable condensers 24--2l in-series with the transformer primaries 22-'22 and in parallel with the secondary of the modulating transformer l1, and tuning the condensers to make the net reactance zero for frequencies in the order ofthe oscillatornatural frequency.

The sections 22-22=of Vthe output transformer primary are dierentially coupled to the secondary 25 and since the voltages applied to the control grid elements of tubes i8 and I8 are in phase the net voltage produced in the secondary v25 will be zero, for the conditionsof no moduthe secondary 25 of the output -transformer 23..

22, is 90 ahead of the currenttherein, and de- ,pending upontheA sense ofthe coupling between the primaryand secondary ofthis transformer the -voltage induced in L 25 will .be either 90 leading or lagginginfphase with respect to the oscillator voltage'applied to the `controlgrid.

'The Vbalanced modulator comprising the vacuum tubes Il and I9, and the associated transformers I1 and 22, have therefore served to generate the side band frequencies displaced 90 in phase with respect to' the oscillator voltage. In Flsure`1 this is schematically represented in the block diagram by the series circuit of the balanced modulator and side band shifter.

The shifted side bands are amplied by the vacuum tube 26 which employs resistor 21 for a plate circuit load and battery 28 for a source of E. M. F.

At this point another-circuit comprising the oscillator'I 2| andthe vacuum tube amplifier 29,

may be traced to the-resistor 21 and the battery 2l, which `functions merely to amplify .thecurrent at oscillator frequency. load resistance 21 is common to both the yc scil'- lator amplifier and the side .band amplifier, the

. power When modulating current-flows inthe trans- 75 Paragraph voltage impressed lacross it will at all times be tlieiesultant of two Wavesyrs't', the. constant amplitude carrier wave and second,l the side bandsot the amplitude modulator, the phase displacement between the two waves being 90.

In accordance with the principles described in Patent No. 1,941,068 the resultant wave will be the frequency modulated carrier, in .which the frequency deviations from the carrier are in accordance with the amplitude variations.

The frequency modulated voltage developed acrossresistor 21 is coupled to the amplifier circuit'll by means o f the condenser 33 and the resistor I2 andthey amplified outputvis coupled to the current limiter 44, to smooth any amplitude fluctuations which may be present in the frequency modulated wave. 'I'he current limiter may be a saturatlng transformer, vacuum tube, or the like, and any harmonics generated therein are removed by the band pass filter 45.

The frequency multiplier 34V into which the output is then passed, functions to raise the frequency ofthe frequency modulated wave up to the desired carrier, and comprises several frequency doublers and triplets. At this stage, the

frequency modulated wave is in its final form, but at a low power level. The discussion of the amplifier will be completed in a later Inasm'uch as the The signal or modulating current employed to drive the balanced modulator is derived from the circuit comprising the photoelectric cell in the facsimile scanner, the correction network and an amplifier. In order to produce a change in carrier frequency whichis constant over all ranges of modulating frequency, it is necessary l to introduce a correction network which varies the modulating voltage inversely with thesignal frequency. The correction network comprises the vacuum tube Il and the network composed of the resistor I2 and condenser I3. The modulation input is connected into this circuit by the transformer I I and the voltage that actuates the control grid of the tube is the voltage which is developed across the condenser I3. In a series circuit composed of resistance and capacitance the voltage across the capacitance will vary inversely with the frequency of the voltage injected into the circuit, if the resistance is high compared to the impedance of the condenser for the frequencies of modulation.

Therefore the output of the vacuum tube I4 will have the desired characteristic and this modulating voltage is coupled to the vacuum tube amplifier I6 by means of the coupling transformer I5. The output of the amplifier contains the primary of the modulating transformer l1, and the modulating voltage 'is amplified sufficiently to drive the balanced modulator to its full capacity.

summarizing the above, picture signals produced in the facsimile scanner are rst impressed across a correction network which varies the modulating voltage inversely with the signal frequency. These signals then modulate a low frequency carrier generated by the crystal oscillator in the balanced modulator which suppresses the carrier. The output, consisting of the side bands of the amplitude modulated signals are shifted 90 and applied to the original carrier to produce frequency modulated signals of constant amplitude. Any fluctuations in amplitude are eliminated by the limiter. Any harmonics in these signals are eliminated by the band pass lter. The resulting frequency modulated picture signals are `then passed through a series of multiplications and raised to the carrier frequency, then passed through a power amplifier and transmitted, as by radiation from an antenna. As will now be clear, I employ the full band width of my carrier channel for these pictures. v

In accordance with my invention, I contemplate transmitting a synchronizing frequency over this same carrier channel without requiring the need of a switching operation and attendant contact trouble at the receiver.v I

The synchronizing signal generated in a second circuit originating at the facsimile scanner will be traced to the power amplifier through the pulse generator and pulse amplifier. This circuit comprises the usual mechanical switching arrangement or any other means for generating synchronizing pulses at the facsimile scanner, such as shown in my Patent No. 2,047,863. The synchronizing pulses may be generated by switching a direct current circuit which may comprise a series circuit of batteries, switches, and resistors as disclosed in my above referred to patent.

Switching such a circuit will generate a pulse of rectangular wave form for a predetermined interval. The pulses are amplified by conventional amplifiers if the magnitude thereof is not sufficient to drive the modulator. The amfor a period equal to the duration of the pulse.

The energy of the pulse may be supplied to the transmission circuit by the high level modulation scheme illustrated in Figure 3, wherein the synchronizing circuit is coupled through transformer 38 to the plate circuit of the power amplifier 35.

However, the synchronizing pulse circuit may Ibe coupled to the grid of the power tube 35 so that the power requirement will not be too great, or may be coupled to any of the preceding voltage-amplifiers, if the point at which coupling is accomplished follows the current limiter 44.

The output circuit of the power amplifier 35 comprises two parallel circuits, as illustrated in theA embodiment of Figure 3, namely the pulse modulating circuit and the resonant tank circuit.

The modulating circuit comprises the secondary of the modulating transformer 38 and inductance 39 which restricts the flow of radio frequency energy to the pulse generator and to the battery 42. The tank circuit is composed of the coil 31 and the variable tuning condenser 36, the

distinction between this and tank circuits used v in conventional amplitude modulation transmitters being that the coil 3l must have a relatively low Q toprovide for a broad band characteristic. The condenserdl is placed in series with the tank coil to preclude the short-circuiting of the battery 42 by the low resistance tank coil 3l.

The coil is coupled to the antenna 43 whereat the wave is radiated into space. Theradiated wave is of the form indicated by Figure 5b, and may be analyzed in the following manner: Between points A and B on the generated wave the carrier frequency is alternately varied above and below the mean value in accordance with the fluctuations of modulating picture currents, illustrated in Figure 5a. At point B the scanning cycle has been completed and the frequency remains constant until point C Where the next cycle chronizing pulse occurs and that the amplitude varies as shown at D. It should be emphasized that the synchronizing pulse occurs at carrier frequency immediately after which the succeeding scanning stroke` begins and proceeds to modulate the frequency. This process is repeated once for each cycle of the scanning arm.

Since the synchronizing pulse occurs at carrier frequency there is little danger of interference between this frequency and communicatin circuitsl of neighboring frequencies. Therefore, the magnitude of the pulse may be considerable to facilitatediscrimination at the receiver between the amplitude pulse and any fluctuations in the amplitude of the wave due to disturbances such as lightning, `ignition interference and the like.

An important characteristic of the transmitting circuit described, and of all other frequency modulation transmitters, is that-the signal is converted to variations in frequency, and that any constant frequency which may be superimposed thereon will not affect or interfere with the signal. Therefore the circuit employed to superimpose the synchronizing signals upon the carrier to continuously modulate the may be modified amplitude ofthe carrier with a constant frequency which is Vlsome multiple of the scanning frequency. Inasmuch as the synchronizing signal comprises a single predetermined frequencyv amplication the wave may be passed to the discrlminating circuits or the wave may be passed .into the ilrst detector 49 of a conventional superl`vheterodyrie receiver.- Here the radio Vfrequency of constant amplitude, 4the amplitude modulasignal islieterodyned tothe desired intermediate frequency by means of the local oscillator 48, and

the output of this circuit 1s the voltage which may l'be passed tothe selective circuits.

constant, as may easily. be. effected by the well known frequency control circuits.V

This amplitude modulation maybe effected by the high level circuit illustrated by Figure 3 or by low level modulation of the output of the frequency multiplier.

The facsimile scanner may be used directly to generate the synchronizing frequency or it may be generated by a crystal controlled oscillator *The receiver illustrated has two channels con-` nected in parallel in the output circuit of the intermediate frequency amplifier 49. These -cir- `cuits comprise the frequency modulation and amplltude modulation detectors andV have the synchronizingmeans and picture recording means connected into the corresponding circuits.

The frequency modulation-detector may be of the type designed to convert the frequency variations to'corresponding vuctuations in amplitude.' Before reaching the actual detector, however, the

incoming wave is firstv passed .through a current limiter '60 which functionsl to'smooth the wave and exclude any variations in amplitude as its and employed at both the transmitter and the receiver to operate thesynchronizing means as will be described hereinafter,

The frequency modulation circuits herein illustrated are not limited to facsimile transmission but may be also employedin other communications systems as for high speed picture transmission of the type encountered in television. Of course, the signal frequencies of a television modulator comprise va much greater band than the facsimile signals but this simply necessitates changes in the design of the tuned circuits, which is easily performed by those skilled in the art.

When this frequency modulation circuity is yemployed for television Vthe amplitude modulator shown may be again modified to apply two distinct frequencies to the carrier wave, one of which may be coupled to the scanning and the other to the framing circuit.

'I'he receiver of course will require a channel A tuned to the amplitude side band frequencies in` order tor discriminate between the signalling waves and the synchronizing frequency signals tooth oscillators to maintain synchronism between the cathode ray tube beam and the similar beam in the television camera. At the receiver, as at the transmitter, discrimination between the different wavesmay be simply accomplished bewill represent zero current in the output circuit..

Furthermore interference between the amplitude fluctuations and the picture signal may be avoided because the current limiter connected in series with the frequency modulation detector excludes any variations in amplitude.

One possible embodiment of the receiving circuit is illustrated herein by Figures 2 and 4. The receivers illustrated are multi-channel circuits, each channel responding to a predetermined wave form.

The voltage'induced inthe antenna circuit will have a wave form similar to that illustrated by Figure 5b, if the amplitude synchronizing pulses described are employed with the facsimile transmitter. The first stage o f the receiver may be a broad band radio frequency lamplifier l1, the band width being determined by the maximum frequency deviation allowable.` After suitable counterpart 44 in the transmitter; 'I'he current limiter 60 may be a saturable element such as a transformer or a saturable Vvacuum tube device.

The numerous harmonics generated in theY limiting circuit may be excluded by means of the band pass lter 8| connected in seriesl therewith.

Although the limiter serves primarily to reduce the amplitude variations encountered in atmospheric and other disturbances it also serves to block the increased amplitude synchronizing pulses.

As a result the stylus current will be zero at y the time of the synchronizing pulse.- vThe frequency modulation detector S2 illustrated'schematically in the block diagram is again not limited to any. particular construction but may be composed of any practical, operable circuit` elements.

v The output of the detector is a current varying in accordance with the photoelectric cell light variations in the transmitter. This current which are then applied to their respective saw may be amplified by an audio amplifier 63 and' v supplied to the stylus` 58 of the lrecorder if the recorder is of the dry paper, type, or the picture current variations may be translated by any other of the well known facsimile means,'as shown in my' Patents'2,047,863 and 2,136,789.

Ihe second channel of my novel combination of amplitude and frequency modulation detectors is a detector which may be of the type commonly employed in amplitude modulation receivers and may comprise a grid bias detector or a rectifier detector or the like. If the broad band system of modulation is employed, the tuned circuits of the amplifiers and detectors will have to be designed accordingly.

The circuit illustrated in Figure 4 is a full wave rectier detector 50 where the incoming wave is coupled to the plates through the coupling transformer 5|, which is tuned to the proper and it may be seen that the amplitude remains substantially constant throughout the picture cycle except for the amplitude' pulse of rectangular wave form employed for synchronization.

The radio frequency by-pass condenser 54 will serve to eliminate the radio frequency components of the incoming signal even though they are varying considerably as a result of the frequency modulation.

This voltage is then applied to the grid of the pulse discriminator 55, suiliciently biased below cut-off to prevent current from flowing in 55 until the grid potential exceeds the constant amplitude lof the frequency modulated wave. Therefore the potential appearing across the discriminator load resistance 55 will contain only the amplitude pulses as illustrated in Figure d.

After suitable amplication in the vacuum tube amplifier 59, the pulses are directly connected to the synchronizing means 51 which may be in the form of a magnet operating a friction clutch or the like. Y

Discrimination between the frequency modulation signal and the amplitude pulse could also be effected in this circuit by suitably biasing the cathode of the rectier 50, that is, the' cathode could be maintained at a positive potential, high enough to prevent current flow until the large amplitude synchronizing pulse occurs.

Also, it is possible.to dispense with the discriminator circuit by adjusting the synchronizing means to be inoperative for values of current less than the amplitude of the frequency modulated wave and operative only for currents exceeding this, such as the synchronizing pulse.

'I'he output of the synchronizing circuit may therefore be connected directly to the synchronizing means without the need for interposed switches or other circuit interruptors. The synchronizing current will operate the magnet once following each scanning line for a time controlled by the switching operation at the transmitter.

Therefore both the picture and the synchronizing circuits have beenconnected to the input circuit of the receiver without means for selecting the individual signals other than the receiver itself, and as a result the need for a switch operated by a cam on the scanning mechanism has been eliminated.

In another form of my invention I employ frequency modulation of the carrier both for the transmission of picture signals and of the synchronizing signals.. In this case I eliminate the circuit from the pulse generator to the pulse amplifier and power amplifier and instead conto await the receipt of the synchronizing pulse from the transmitter which is received when the transmitter has reached the corresponding starting position of the scanning cycle. In response to this synchronizing pulse the receiver scanner then starts up for the next cycle. l

This has the disadvantage, where large facsimile units are employed, of mechanically limiting the speed of operation since such speed is dependent upon the mechanical limitations in stopping and starting the receiver in each cycle.

It has long been recognized as preferable that the transmitter and receiver be rotated continuously and at' synchronous speed. This is possible where the transmitter and receiver are connected to the same source of alternating currents which may then be used to dn've synchronous motors at the transmitters and receivers. Such is the case, for example, where the driving power line and Where the transmitter is also driven from power from the same local power line.

In broadcasting operations however itis rarely possible to have the transmitter on the same power supply with the receivers which are to be operated by the transmitters. In such cases, the transmission of alternating currents for supplying the power for driving the transmitter and receiver has been suggested, but this requires a separate synchronizing channel or as is the case of television where the same source of alternating current is employed for providing the sweep circuits of the cathode ray tubes a. part of the nect the pulse generator of the facsimile mech- 4anismby means of switching mechanisms, as is now commonly well known and as is illustrated in my Patent No. 2,047,863, to the correction network. At the end of each scanning cycle a synchronous signal is then transmitted by my pulse generator through the correction network to the balanced modulator and thence on as described above for the transmission of a synchronous signal by frequency modulation.

In this case I do not employ any separate discriminatingamplitude modulating channel at the receiver but instead merely pass the synchronizing signal through the frequency modulation receiver circuit Where it is switched after selection as described in my above referred to patent, to maintain the receiver in synchronism with the transmitter. y

In the above I have described two forms of my invention in one of which I have eliminated the need of switching equipment. In these synchronizing systems the receiver is usually rotated slightly faster than the transmitter and brought to a stop at the beginning of each scanning cycle signals channel must be devoted for this purpose which correspondingly reduces the accuracy and eiiiciency of transmission of the signals.

In 4accordance with my invention I may, without sacrificing any of the signalling channel, simultaneously transmit a synchronizing irequency for driving the transmitter and receiver continuously in synchronism with each other.

In this form, an alternating current of predetermined frequency is employed to drive the facsimile scanner at the transmitter. This alternating current is then impressed upon the frequency modulated carrier through an amplitude modulator after the picture carrier has passed through the current limiter, or the carrier frequency may be amplitude modulated by the synchronizing frequency independently of the frequency modulated wave. Y This wave may then be transmitted along with the frequency modulated picture wave. By this means, an amplitude modulation of the carrier by the synchronizing pulse is prcfduced.

Dispensing with the discriminating circuit 55, the constant frequency wave may be ampliied sufciently and may be employed to maintain synchronism between a device at the receiver and a similar unit at the transmitter.

In order that suicient power may be provided, the synchronizing signal at the receiving end may be amplified through thyratrons which may operate in response to the synchronizing frequency to generate amplied signals at the frequency of the synchronizing signals for directly operating a motor; or the energy of the synchronizing signal may be employed to act as a trigger for controlling the synchronous motion of a tuning fork driven from a local source of power, the incoming synchronizing signal thus maintaining the tuning fork at synchronous speed. The output of the fork may be amplified to supply sufficient power to drive a motor for operating the receiver. .The power method of amplifying the signal at the receiver is not a part of my invention, as any well known means may be employed. Under these conditions of operation, the scanning mechanism at the receiver will lock into step with that of the transmitter. However, it is also necessaryl to determine that the phase relationship between the two scanning mechanisms is identical, that is, it is necessary for both scanning arms to start and complete their strokes simultaneously. To accomplish this phasing operation the friction clutch of the start-stop system maybe utilized, and at the beginning of each transmission, and if necessary during the period of transmission, an amplitude pulse may be transmitted to operate the synchronizing magnet and bring both scanning devices into step.

It will thus be seen that I have provided a system in which a single carrier may be frequency modulated for the transmission of one form of communication signal and may be amplitude modulated for the transmission of another form of signal, each without interfering with the other and proper discriminating circuits at theA receiving end may be employed for separating or filtering each of these signals from each other for operation of the receiver.

I claim:

1. In a frequency modulation facsimile system, scanning means for producing signals varying in amplitude in accordance with the lights and shades of a picture to be transmitted, a source of carrier current, means for frequency modulating said carrier current at constant amplitude in accordance with the amplitude variations of said picture currents, means coacting with said scanning means for generating synchronizing pulses at the termination of each picture scanning cycle, means for increasing the amplitude of said carrier in accordance with said synchronizing pulses, means for transmitting said frequency and amplitude modulated carrier, facsimile receiver means for receiving said transmitted wave comprising a frequency modulation detector in circuit with the picture recording means, and an amplitude modulation detector in circuit with the recorder synchronizing means, said frequency modulation detector being nonresponsive to amplitude variations and said amplitude modulation detector being responsive only to signals of magnitude greater than said constant amplitudefrequency modulated wave, said synchronizing means coating with the output signals of said amplitude modulation detector for maintaining said picture recording means in syn- 2. In a frequency modulation facsimile system, scanning means for producing signals varying in amplitude in accordance with the lights and shades of a picture to be transmitted, a source of carrier current, means for frequency modulating said carrier current at constant amplitude in accordance with the amplitude variations of said picture currents, means for generating synchronizing pulses at the termination of each picture scanning cycle, means for increasing the amplitude of said carrier in accordance with said synchronizing pulses, means for transmitting said frequency and amplitude modulated carrier, said frequency modulation detector being non-responsive to amplitude variations and said amplitude modulation detector only being responsive to signals of magnitude greater than said constant amplitude frequency modulated wave, means for periodically arresting the motion of said recording mechanism, and means in the output circuit of said amplitude modulation detector for starting said recording mechanism on receipt of said synchronizing signal.

3. In a frequency modulation facsimile system, scanning means for producing signals varying in amplitude in accordance with the lights and shades of a picture to be transmitted, a source of carrier current, means for frequency modulating said carrier current at constant amplitude in accordance with the amplitude variations of said picture currents, means for generating a constant frequency synchronizing signal and a rst synchronous motor energized by said constant frequency signal for driving the transmitting scanning mechanism in synchronism with said synchronizing signal, means for continuously modulating the amplitude of said frequency modulated wave in accordance with said synchronizing signal, means for transmitting said frequency and amplitude modulated carrier, facsimile receiver means for receiving and recording said transmitted signals, including a frequency modulation detector in circuit with said recording means, an amplitude modulation detector, means for amplifying the output frequency of said amplitude modulation detector and means for operating said recording means in synchronism with said output frequency, said last mentioned means comprising a second synchronous motor energized by said amplified output frequency, said synchronous motor being similar to said first transmitting scanning synchronous motor, said synchronous motor driving said recording means.

WILLIAM G. H. FINCH.

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