US2212971A - Facsimile synchronizing system - Google Patents

Description

Aug. 27, 1940. w. G. H. FINCH FACSIMILE SYNCHRONIZINQ SYSTEM Original FiledIApril 21. 1938 AMPLIFIER MODULATOR RADIO TRANSMITTER 50 CYCLES OSCILLATOR TONE. CAM

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EDOO CYCLE TONE SCANNER YNCHRONIZING CAMS 45 no-vours I i I I SYNCHROBHZ! N3 MAGNET CLUTCH RECTIFIER RADIO RECEIVER INVENTOR ATTORNEY Patented Aug. 27, 1940 UNITED STATES PATENT OFFICE 4 Claims.

This application which is a division of application Serial No. 203,222, filed April 21, 1938, re'- lates to facsimile systems, and more particularly relates to novel methods of and means for maintaining a transmitter and receiver in synchronous relation.

In accordance with my present invention, I contemplate novel means for accurately=predetermining the duration of transmission of a synchronizing signal within very close limits.

In the prior art, a single cam was generally used to both, open and close the sameswitch member to control the beginning and ending points of the transmitted synchronizing current. I have found that for the synchronizing signal to be most effective in facsimile work, it should be of very short duration, lasting on the order of one-fiftieth of the scanning cycle, namely about seven mechanical degrees. Prior art unitary cam switch control has not been reliable and cannot effect such small accurately determined synchronizing signals due to mechanical lagging and other similar difliculties. In accordance with my present invention, I employ two switches in circuit with the alternating current synchronizing signal. The

mine the instant of starting and stopping of the transmission of the synchronizing current. At a the receiver novel means are provided for selectively segregating the synchronizing current from the picture signals for actuating the start-stop synchronizing mechanism and maintaining the receiver scanner in synchronous movement with the transmitter scanner.

Further, in accordance with my present invention, novel circuital arrangements are provided for coupling an individual facsimile unit with an ordinary radio receiver. This coupling is effected in a simple manner. The coupling unit comprises circuits for rectifying the picture signals and means for selectively changing the characteristics of the rectifier unit to provide the predetermined frequency synchronizing impulse for maintaining the recorder in synchronous condition.

It is accordingly an object of my present invention to provide novel methods of and means for transmitting a synchronizing signal.

Another object of my present invention is to provide novel apparatus-for accurately predetermining the duration of a transmitted synchronizing signal. v,

Still another object of my present invention is to provide a novel arrangement for accurately Divided and this application July 2, 1940, Serial No. 343,536

predetermining the cyclic starting and stopping I positions for the transmission of a synchronizing current.

'A further object of my present invention is to provide novel methods of and means for selectively receiving a synchronizing signal for maintaining a recorder in synchronous operation.

Still a further object of my present invention is to provide a novel coupling unit between an individual facsimile recorder and a standard unit is oscillated across an arcuately held 3 record sheet II by a modified heart-shaped cam 12. The transmitter scanner schematically here represented may well be the unit such as disclosed in Patent No. 2,203,461 issued June 4, 1940, entitled Continuous sheet facsimile system," although other well-known scanning units may be used. The transmitter scanner unit consists essentially of a source of light, generating a beam focused upon sheet ll through element I3. I

The beam or pencil of light is reflected from the record sheet and focused through tubular lens element [4 upon aphotoelectric cell l5 contained within the scanner housing l6. -As is well known in the facsimile art, variations in the shading of the recording sheet ll along the scanning region correspondingly variably affect the intensity of light impinged upon photoelectric cell IS. The photoelectric cell is connected in circuit with the modulator stage for producing electrical signals varying in accordance with the elemental shading of the scanned record sheet.

The modulator stage is preferably a screenmodulator stage through a coupling condenser unit I9. The combination of electrical actions in the modulator stage generate a picture modulated carrier frequency current which is coupled to the amplifier stage through coupling impedance elements 20.

The output of the amplifier stage is coupled to a suitable transformer 2| to a schematically indicated transmitter 22 for radio transmission to a remote receiver stage. It is to be understood that different communication channels than radio may be employed. The output impedance of transformer 2| may, for example, be 500 ohms to match the standard coupling impedance of speech input circuits employed in radio broadcast stages. A high impedance level indicator 23 such as a D. B. meter, is connected across the output of transformer 2| the initial adjustments of the transmitter unit.

An electronic oscillator 25 is employed to generate and supply the 2000 cycle carrier frequency or tone to the modulator stage. Oscillator 25 essentially comprises a double triode electronic tube 26 coupled through transformer element 21 to generate the sustained current oscillations at the required frequency. The output winding 28 of the oscillator has one terminal connected to ground potential and the other terminal connected to two potentiometer element units 30 and 3|. Potentiometer 30 is connected to the coupling condenser IQ for the modulator stage through cam-switch members 3233. When switch 3233 is closed the output of oscillator 25 is directly applied to the screen grid I8. This connection occurs preferably during one hundred and eighty mechanical degrees of each rotation cycle of control shaft 35. Actuation of cam switch 36 is through cam 31 secured on control shaft 35.

Scanner l0 generates picture signals in its excursion from left to right corresponding to a 180 rotation of the control shaft 35. The scanner is returned to its left end position during the remaining 180 rotation of shaft during which time no picture carrier signals are transmitted in the preferred arrangement. Carrier tone switch 32-33 is opened by suitable design of cam 31 at this point.

During the return stroke (to the left) of the scanner ID, the frequency of the generatedcurrent of oscillator 25 is changed to that of the synchronizing frequency. In a preferred example, a. 500 cycle synchronizing current is used.

The frequency of oscillator 25 may be changed by' shunting a condenser element 38 between grid element 39 and ground. This switching is effected by elements 4|l4| of cam-switch 3B and occurs alternately with the removal of carrier current from screen grid 8. By properly proportioning the value of the capacitance of condenser 38, an accurate value of the 500 cycle generation is effected. It is to be understood that other known expedients for changing an oscillator frequency may be employed for changing the normal picture carrier frequency of oscillator 25 during the return stroke of scanner I0 and that this frequency might be an increased value rather than adecreased one.

Potentiometer 3|, being in circuit with the output of oscillator 25, is directly connected to the secondary side of the output transformer 2| at point 46 through a novel arrangement for accurately predetermining the duration of the connection of the synchronizing signal across this point. In accordance with my invention,

for assisting in 1 I provide a novel arrangement using two switches 42 and 43 connected in series, respectively actuated by cams 44 and 45. Switch 43 is in normally closed circuit relation; switch 44, in normally open circuit relation. No synchronizing current accordingly flows from potentiometer 3| to point 46 during the return stroke period of scanner l0 until switch 44 is closed.

Cams 44 and 45 are designed with short projecting nibs for actuating the associated switches 42 and 43. Cam 44 is arranged on shaft 35 to close switch 42 at the predetermined instant the synchronizing signal is to be started for transmission during each cycle. A cam can be depended upon to accurately perform this closing function. The closure of switch 42 completes the circuit between potentiometer 3| and point 46. Closure of switch 42 is made to preferably occur when scanner H] has practically completed its return stroke.

It will be recalled that during this interval, switch -4| is closed and the output of oscillator 25 is at the synchronizing signal frequency, namely 500 cycles in the present example. The end point of the synchronizing signal transmis sion is accurately determined by cam which is adjusted to actuate normally-closed switch 43 to open the switch at the proper instant. It will now be evident that an accurate control of the starting and stopping point of the synchronizing signal in each cycle can be accurately prearranged by my novel means.

The advantages of this arrangement by switches 42 and 43 are apparent when synchronizing signals of very small duration are required. I have found through extensive experimentation that synchronizing signals of short duration more accurately effect synchronizing of the receiver scanner. Thus, when the cycle of the scanner is one second long, giving sixty lines of scanning per minute, I have found that a synchronizing signal duration of one fiftieth of this cycle, namely two-hundredths of a second, is most satisfactory. My experiments have further shown that the angular position and duration of the synchronizing signal of this short duration and of even shorter duration can be readily accomplished by the herein disclosed multiple switching means.

The synchronizing signal is ended just before the scanner l0 approaches its extreme left position in readiness for its picture-scanning excursion. At this time, cam 31 alters the position of cam switch 36, opening blades 404| to cause a picture tone of 2000 cycles to be generated, and closing plates 32-33 to connect the 2000 cycle tone to the screen grid l8 of the modulator stage. Control shaft 35 is rotated at a predetermined speed such as by a synchronizing motor through reduction gearing. In the given example, shaft 35 rotates at 60 revolutions per minute, effecting sixty scanning operations and corresponding synchronizing signal transmissions per minute. The picture modulated carrier and synchronizing signals are accordingly alternately transmitted across radiotransmitter channel 22 to the remote receiving stage.

Figure 2 is a diagrammatic representation of a preferred system for recording the transmitted facsimile signals. A standard broadcast radio receiver is indicated at 50. The output stage 5| represents the conventional audio-frequency amplifier stage coupled to loud speaker 52 through step-down transformer 53.

In accordance with my present invention, I

provide a novel coupling unit 55 for coupling the facsimile recorder I to the radio receiver. Coupling unit 55 essentially comprises a step-up transformer 55 and a thermionic rectifier unit 51. Primary 58 of transformer 55 has one terminal connected to ground and the other terminal connected to the radio receiver throug a coupling condenser 59. Switch 50 connects ransf'ormer 55 to terminal 5| of the radio receiver. A preferred coupling point to the radio receiver is to the anode 52 of the output stage 51 thereof. This connection insures the maximum voltage signal output from the radio receiver to the facsimile coupling unit 55.

Connecting-switch 50 may be manually operated when facsimile reception is to commence. However, it is at present usual to connect the facsimile receiver at the earliest morning hours since the broadcasting of facsimile programs occur at that time. Accordingly, switch 5| together with the associated switches for starting both the radio receiver'50 and the facsimile system in opbration may be time-clock controlled in a manner well known in the art. A second switch 53 is shown mechanically ganged with switch to open the circuit connection to the loud speaker 52 to prevent the facsimile signals from causing a sound disturbance. It is entirely feasible to maintain the loud speaker 52 in circuit to aurally determine the quality and nature of the signals.

Coupling unit also contains a step-down power transformer 54 used to energize the heater element of rectifier 5'I. Rectifier 51 is preferably of the duo-diode type, with -the anodes connected to the outer ends of the secondary of coupling transformer 55. The 2000 cycle modulated carrier picture signal currents received by unit 50 are further amplified in magnitude by transgformer 55 and thenrectified at 51. The signal energy output is represented between the cathode 55 and ground. Cathode 55 is connected to the scanner I0 through switch blades 51-58 through the oscillating scanner arm 59. Ann 59 is electrically connected to the pivotally mounted stylus point II which bears against .record sheet I2. Sheet I2 is secured in position against a metallic platen 13. Platen I3 is electrically connected to ,ground to complete the picture signal circuit to rectifier 51. 3

The diagrammatically illustrated recorder is similar to that described in my co-pendlng application S. N. 156,625, filed July 31, 1937 entitled Facsimile recorder. Record sheet I2 is maintained in position to establish contact with pllaten I3 and cooperate with the oscillating stylus I is electrically sensitized in accordance with the picture signal to build up a facsimile reproduction corresponding to that transmitted from scanner l0. Heart-shaped cam I4, in cooperation with spring I5, produces an oscillating movement of arm 59 about pivot I5.

A non-synchronous motor 11 is preferably employed to drive cam I4 through a reduction gear box 18 and a friction clutch I9. Motor 11 may be an A. C. induction motor, or a motor of the .}A. C.-D. C. type. An electrical governor is mechanically connected to the rotor of motor 11, and employed to maintain motor I! at a predetermined speed of rotation. Governor 80 electrically opens and closes the 110 volt power leads to the motor 11 to drive it at proper speed. A spark filter 8| is connected across governor 88 to minimize sparking.

Synchronous scanning movement of oscillat- Record sheet I2 is a conducting paper which ing arm 59 is effected by' means of the governor controlled motor 11 and the start-stop synchronous actuation of the clutch I9. Synchronizing magnet 82 is controlled by the short duration synchronizing signal in a manner to be described.

I have found that by operating the recorder control shaft 85 at a speed about five percent greater than that of control shaft 35 of the transmitter, the scanner arm 59 will arrive at its initial leftend position in readiness for its recording stroke,

in suflicient time before reception of the synchronizing impulse to start it off again for most operating conditions. Driven plate 85 of clutch 19 contains a projection 81 which abuts armature 88 to stop the rotation of shaft 85 until the synstarting position. I have further found that the time lag for the operation of the synchronizing magnet through its armature 88 to release the clutch I9 is, for recorders of-the type described, of the order of about .02 seconds, and that the synchronizing pulse duration should be made with this figure as a minimum for best operation.

In other words, while a synchronizing impulse Signal plate 51 is connected to the synof the shortest duration is preferable, the minimum limit is that of the time lag for the synchronizing mechanism to function. At the instant the synchronizing magnet is energized, cam 92 operates cam switch to the right to open the synchronizing magnet circuit and close signal blade 51 upon contact 58, thereby impressing the rectified picture signals upon the stylus for recording on sheet 12' in a manner already described. The recording stroke occurs during 180 mechanical degrees of rotation of control shaft 85 at which time the cam switch 90 is released to the illustrated position to repeat the cycle of operations described.

' An important feature of my invention resides I in the novel means for selectively controlling the impression. of the synchronizing impulse upon magnet 82 to avoid conflict between the synchronizing signal and picture signal at the output of the rectifier I provide a filtering means embodied by condenser 93 to by-pass essentially all the signal frequencies above the synchronizing frequency of 500 cycles. The 2000 cycle carrier together with its side bands of picture signal modulations are well above the 500 cycle value so that the by-pass condenser 93 connected to one side of the secondary of the coupling transformer 55 is usually sufficient for such cases. Condenser 93 by-passes the higher frequencies from transformer 55 to ground through switch elements 94.95. It is to be understood that my invention contemplates more involved filtering means actuated by the cam switch 98 if more selective ignal separation is required.-

' The preferable signal level for operating the recorder I0 is in the order of 100 to 300 volts.

The expedient of the filtering condenser 93 has been found to readily reduce the 2000 cycle signal level to a value of 15% of the original and thereby insure the prevention of interference with the 500 cycle tone passing through to the rectifier 51 substantially unattenuated. The 500 cycle synchronizing impulse is rectified at 51 and is applied to the synchronizing magnet through switch til-9| as a unidirectional current impulse of short duration.

An alternate method of connecting the coupling transformer 56 of the signal coupling unit 55 to the output stage of a standard radio receiver is illustrated in Figure 3. A push-pull output amplifier stage containing the triodes and 5|" is illustrated as connected to the pushpull transformer 53, normally coupling this stage to the loud speaker unit. 60 is the time clock operated switch corresponding to the switch 6|] in Figure 2, and connects plate 62' of tube 5! to one side of the primary 58 of the coupling unit. A coupling condenser 59 is shown connected to the opposite plate 52" to prevent the passage of direct current to transformer 56. The impedance of the primary winding 58 is preferably of the order of the impedance of the primary winding of transformer 53'.

It is to be understood that my present invention may be practised in various modified forms and arrangements without departing from the spirit and scope thereof and accordingly I do not intend to be limited except as set forth in the following claims.

I claim:

1. A fascimile transmitter comprising means for generating a carrier frequency current and means including circuit connections for transmitting said carrier current for a predetermined short interval as a receiver synchronizing signal comprising a first and second switch, said first switch being in normally closed circuit position during the signalling period, said second switch being in normally open circuit position during the signalling period, and mechanism for cyclically actuating said first and second switches to stop said signalling period and set up a synchronizing period, said mechanism closing said second switch to complete the transmission of the synchronizing signal and opening said first switch to stop the transmission thereof after the predetermined interval.

2. A start-stop facsimile synchronizing system comprising means for receiving picture modulated at one carrier frequency and alternating current synchronizing signals of another frequency, means for rectifying said carrier and synchronizing signals, a synchronizing magnet, switching means including circuit connections to said rectifying means for selectively connecting the output of the rectifying means to a recorder stylus and to said synchronizing magnet, and means actuated by said switching means to electrically segregate said carrier signals frequency from said synchronizing signals frequency at said rectifying means.

3. A start-stop facsimile synchronizing system comprising means for receiving picture modulated carrier and alternating current synchronizing signals, means for rectifying said carrier and synchronizing signals including a coupling transformer and a thermionic rectifier unit connected to said transformer, a synchronizing magnet, switching means including circuit connections to said rectifier unit for selectively connecting the rectifier output to a recorder stylus and said synchronizing magnet, and means actuated by said switching means to electrically segregate said carrier signals from said synchronizing signals at said rectifier.

4. A start-stop facsimile synchronizing system comprising means for receiving picture modulated carrier and alternating current synchronizing signals, means for rectifying said carrier and synchronizing signals including a coupling transformer and a thermionic rectifier unit connected to said transformer, a synchronizing magnet, switching means including circuit connections to said rectifier unit for selectively connecting the rectifier output to said synchronizing magnet and a recorder stylus, and means actuated by said switching means to electrically segregate said carrier signals from said synchronizing signals at said rectifier including a condenser in circuital connection with .said switching means and said coupling transformer whereby the alternating current synchronizing signals are selectively rectified to actuate said synchronizing magnet.

WILLIAM G. H. FINCH.

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