US2258871A - Automatic volume control means for facsimile systems - Google Patents

Description

Oct. 14, 1941. I WEDJG JR 2,258,871

AUTOMATIC VOLUME CONTROL MEAITS FOR FACSIMILE SYSTEMS Filed Oct. 26, 1958 IN VENTOR.

Ffmwc Z. WED/G; BY

ATTORNEYfi Patented Oct. 14, 1941 AUTOMATIC VOLUME CONTROL MEAN FOR FACSIMILE SYSTEMS Frank L. Wedig, .n-L, Cincinnati, Ohio, assignor a,

The Crosley Corporation, Cincinnati, Ohio, a

corporation of Ohio Application October 26, 1938', Serial No. 237,107

10 Claims.

My invention has for its fundamental object the provision of a novel automatic volume control for facsimile systems.

My invention relates to facsimile systems de signed to reproduce pictures or other intelligence transmitted by radio. In an exemplary ar rangement the copy is scanned by a photo-cell at the transmitting station, and the picture signal thus produced is caused to modulate a subcarrier of, say, a frequency of 2000 cycles derived from a local oscillator. The modulated sub-carrier is then used to modulate a radiocarrier which is transmitted. At the receiving end, the carrier is picked up, amplified and detected in any of the usual ways. The resultant modulated sub-carrier is then rectified, preferably by half-wave rectification and is impressed upon a printing device.

My present invention is not limited to any particular printing means, of which there are many, including means for printing upon photosensitized paper by means of "a variable light source. An exemplary printing means comprises a scanning arm which is caused bysuitable means to swing back and forth across a record strip. The armbears a stylus which contacts the record strip. The record strip-rests upona platen. The record strip is of paper, rendered conductive as by being impregnated with carbon, and coated with a substance such as mercuric oxide which will change-color upon the development of sufficient potential-difference between the stylus and the platen.

'The details of such facsimile system 'do not constitute limitations upon the present invention, nor are they necessary toan'- understanding of it. however, set forth in the following-co-pending applications Silver, Ser. No. 237,105, filed Oct. 26, 1938, and

entitled Transmission and detection means and method for facsimile systems. Silver and Wedig, Ser. No. 237,106,- filed Oct. 26, 1938, and entitled Means for improving the operation of facsimileprinters. a

Wedig, Ser. No. 237,108,'filed Oct. 26,1938, and entitled Hum elimination in fac'simile'receivers. It will be understood that volume control, 1.- e;

control of the general signal amplitude, is ex-- of the radio signal will cause the blackness-of- Such systems or portions thereof are,-

" at the trol the volume. Thus it becomes vital to provide means for setting the system so that in spite of fading, the signal will'always be of sufficient strength to print, and for automatically controlling the maximum signal level so that the record is not blackened all over or burned.

Since most facsimile means will be used with radio sets which are also used for broadcast reception and loud speaker reproduction, and since facsimile signals will usually be sent out after the broadcasting schedules for the day are finished so that it is desirable to provide means for properly setting the facsimile system to the Proper signal level before that level can be visually judged by inspection of a record being reproduction, and second that the facsimile receiving system will usually be turned on and off time of facsimile broadcast periods by time clock'mechanism.

Since most facsimile receivers and printers will be used with ordinary radio sets not pro-'- vided with an adequate facsimile volume control, an object of my invention is the provision of a facsimile installation embodying a converter having the features of my invention, which may 'easilyand quickly be coupled to an existing radio set without interfering with its regular operation. 1 To return to a' consideration of the construction of facsimile systems as such, it will be understood that at the transmitting end, before the print to vary, giving an imperfect 'reproduc- 1 tion. 0n large fades the printingmay stop'entirely. Moreover, the printing will usually be done when there is no operatorpresent to conthe start of each scanning stroke a synchronizing signal derived from a local oscillator of, say, a frequency of 500 cycles per second, is sent out. At the receiving end; the scanner aim, at the conclusion of each return stroke, is held orrestrained until the synchronizing signal is received. Thus synchronism' is maintained be-' tween the scanning devices at the sending and receiving stations. a regularly and frequently recurring signal --of constant and relatively high intensity. It is an object of my invention to provide an automatic volume control arrangement which employs the synchronizing signal as the means for generating the A. V. C; voltage for reasons just indicated, and which operates to vary the picture signal level in accordance with variations in the The synchronizing signal is synchronizing signal level, as received, so as to maintain the constancy of both signal levels.

These and other objects of my invention which Will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, I accomplish by that certain construction and arrangement of parts of which I shall now describe an exemplary embodiment. Reference is made to the drawing which is a circuit diagram embodying the features of my invention.

In the drawing, the dotted line box I indicates an ordinary radio receiver, of which I have shown a portion of the output circuits. 2 represents an output tube feeding the primary of an output transformer 3. The secondary of the transformer feeds the voice coil of a loud speakor indicated generally at 4; and ordinarily the speaker circuit is completed by a connection indicated in dotted lines at 5. In installing the facsimile system, this connection is broken and the transformer secondary and voice coil leads, formerly connected by the connection are brought out toterminals 6 and 8. Another terminal 1 is connected into the circuit intermediate the transformer secondary and voice coil as shown.

The other portions of the circuits shown in the drawing may be considered as part of an installation for facsimile to be added to the usual radio set, and may be located in a separate cabinet which also houses the facsimile device; or the facsimile device and the circuits illustrated may be housed in different cabinets. In either event the separate cabinet or cabinets may be placed on or near the radio set. Needless to say, in a radio set especially designed for facsimile reception all of the elements may be built into a single cabinet. A

The terminal 6 is connected bya lead 9 to one of the movable elements It of a triple throw double pole switch indicated generally at H. The terminal 8 is connected by a lead l2 to the terminal l3 of this switch. With the switch in the position shown, the voice coil is connected to the secondary of the transformer 3 justas though the connection 5 had not been broken; and the circuit arrangements hereinafterto be described are open. The position of the switch shown is therefore the position for regular broadcast reception.

The terminal I is connected by a lead I4 to one end of the primary [5 of a transformer indicated generally at IS. The other end of the primary is connected by a lead I! to contacts l8 and IQ of the switch aforesaid, so that in each of the other positions of the switch, excepting the first mentioned one, the loud speaker 4 will be cut out of the output circuit of the radio set I, and the'primary l5 cut into it.

The transformer l6 has a secondary 20, one end of which is connected to an electrode 21 of a tube 22. This electrode, together with the cathode23 of the tube, constitutes a diode combination. The cathode 23 is grounded through the usual combination of resistance 24 and capacity 25 as at 25. The other end of the secondary is connected 'to ground through a parallel combination of resistance 21 and capacity 28, about which'more will be saidghereinafter. The electrode2l is also connected througha capacity 29, and theresistance element of a.

potentiometer, directly to the secondary 23 by a lead 3 I. The lead 3| is connected througha resistor 32 toa lead' 33 which is grounded was at 34 through a capacity 35. The movable contact 36 is connected through a capacity 31 to the grid 38 of an exponential grid audio amplifier tube 39. The cathode 40 of this tube is grounded as at 43 through the usual parallel combination of resistor 4| and capacity 42. The grid 38 is also connected to the lead 33 through a resistor 44. The plate 45 of the tube 39 is connected to the primary 46 of a transformer 41. The secondary 48 of this transformer feeds the facsimile printer, or preferably a power stage for the facsimile printer. It will be understood that the facsimile printer has rectifying means in circuit with it.

The diode combination 2l--23 rectifies the audio signal and generates a D. C. bias across 21 and 28. This bias is put on the grid of the tube 39 through resistances 32 and 44. The unrectified audio signal, flowing through the circuit formed by elements 20, 29, 30, and 3| is impressed on the grid-oftube 39 through the contact 36 of the potentiometer and the capacity 31. The D. C. bias generated by the diode combination 2| to 23 is thus used to' control the gain of the tube 39 so that the gain is inversely proportional to the signal.

i I have-already pointed out that the synchronizing signal as transmitted is a periodic signal of relatively high and constant intensity occurring at regular and frequent intervals. In order to employ this signal to generate the D. C. bias for automatic volume control purposes, I make the rectifying condenser 28 and the diode load resistor21 large enough to have a certain time constant. This time constant is of suflicient value that the D. C. voltage generated by the diode will not change appreciably from one synchronizing pulse to the next. In this way, a bias is developed for the amplifier tube 39 which is Wholly determined by the synchronizing pulses.

It is advantageous to provide for delay on the automatic volume control. By this I mean that I provide means whereby the signal must reach a predetermined value before its intensity is cut down by the automatic volume control means. The utility of this will at once be apparent. The tube-22-also comprises a triode combination including the cathode 23, a grid 49, and a plate 50, which is connected to the positive side of the B source as indicated. The grid 49 is connected to the lead 33 by a-lead 5!. The plate 50 draws current when no bias is applied to the grid. This plate current, passing through the resistor 24, biases thecathode 23 with respect to the electrocle 2| of the diode combination. The signal operates to bias the grid through the connections hereinabove, mentioned. When the signal becomes great enough to overcome the bias produced by the plate circuit, automatic volume control voltageis developed by the diode combination. This bias, applied to the grid 49, biases ofi the triode section of the tube 22, and thus removes the delaybias generated as aforesaid.

In a facsimile-system there is an optimum level at which the receiver should work. This level is highenough to produce a good signal-to-hum ratio, yet mustnot beso high as to overload the radio output tube 2 and produce distortion, nor so high-as to produce sufficient bias from the diode'to cutoffthe amplifier tube. I have already explained the necessity, in'aninst'allation ofthe type I have been describing, for some means for indicating to the operator, when the proper .adljustment has been attained. To this end, Iprovide onthe transformer l6, a tertiary winding 52.

One' end of this winding maybe connected to the other. movable .member ,53. of; the switch ll through a suitable indicating device. Theother end of; the winding 52 is brought out to the middle contact 55 of the switch. The first and third contacts 54 and 56 of the switch are blank. Any suitable indicating means may of course be used, including meters andthe like; but I have found it convenient to employ a small pilot light bulb 51, and to. proportion the windings of the'transformer l6 so that the bulb just lights at the proper audio level. This, as will be clear, enables the operator to adjust the audio level of the radio set prior to the commencement of the facsimile broadcast, as will usually be desired. I V The automatic volume control which forms one aspect of my invention will be in addition to any automatic volume control already possessedby the radio set.

Modifications may be made in my invention without departing fromthe spirit of it. 'Having thus described my invention, what I claim' as new and desire to secure by Letters Patent is: I u I.

1. In a facsimile system employing recurring synchronizing impulses, receiving means includ ing amplifying means, means for generating from a received signal a direct current automatic volume control biasing voltage, means for applying said voltage to said amplifying means, and time delay-means in connection with'said generating means to prevent the biasing'voltage so generated from diminishing substantially betweensaid synchronizing impulses, and means in connection with said generating means for preventing the generation of said biasing voltage until the signal strength reaches a predetermined level, said means for generating the said biasing voltage being a diode rectifier, and said means for preventing the generation of said voltage comprising thermionic means for biasing said diod with a voltage which must be overcome by the signal voltage. a 4 v v 2. In a facsimile system employing recurring synchronizing impulses,-receiving means including amplifying means, means for generating from areceived signal a direct current automatic volume control biasing voltage, means for applying said voltage to said amplifying means, and time delay means in connectionwith said generating means to prevent the biasing voltage so generated from diminishing substantially'between said synchronizing impulses, and means in connection with said generating means for preventing the generation of said biasing voltage until the signal strength reaches a predetermined level, said means for generating the said biasing voltage being a diode rectifier, and said means for preventing the generation of said voltage comprising thermionic means for biasing said diode with .a voltage which must be overcome by the signal voltage, and a connection between said diode and said thermionic means such as to bias ofi said thermionic meanswhen said diode generates said biasing voltage;

3. In a facsimile system employing recurring synchronizing impulses, a receiver, printing means, and automatic volume control means in connection with said receiver for controlling the signal level applied to said printing means in accordance with the intensity of received synchronizing impulses only, said means comprising a circuit having a direct current path including a rectifier, and an alternating current path, n amplifier tube, and means for impressing. on tliegridfof said amplifier tube voltages fromboth of said paths,the voltage from said alternating current path comprising a signal voltage to beamplified, and the voltage from said direct current path comprising a, biasing voltage to cause said: tube .to be responsive in inverse proportion to, the signal strength, and means in said direct current path .to prevent substantial diminution, of the generated biasing voltage thereinr'during'the interval between synchronizingimpulsesa I s f 24. In a facsimile system employing'recurring synchronizing, impulses, .a receiver, printing means, and automatic. volume control means in connection with said receiverfor controlling the signal level applied to said printingmeansin accordance with the intensity of received ynchroni n mpulses only, said meanscomprising a circuit having a direct current path including a rectifier, and an alternating current path, an amplifier. tube, and meansfor impressing on the gridof said amplifier tubevoltages from both of said paths, the voltage from said alternating current path comprising a signal voltage to be amplified-, and ,thevoltage from said direct current path comprisinga biasing voltage to cause said tube, to beresponsive, in inverse proportion to the signal strength, and meansrin said direct current path to prevent substantial diminution of the generated biasing, voltage therein during the interval between synchronizing, impulses, said last mentioned means comprising a parallel 'ree sistanceandcapacity.combination in said direct current path,;having.sufiicientvalue to give the desired time delay; V t i 5. In a facsimile system employing recurring synchronizing impulses, a receiver, printing means, andautomatic volume control means in connection with said receiver for, controlling the signal level applied to said printing means in accordance with the intensity of received syne chronizing impulses only, aid means comprising a circuit having adirect current path including a rectifier, and. an alternating current path, an amplifier tube, and means for-impressing onthe grid of; said ,amplifiertube voltages from both of said paths; the voltage from said alternating current path comprising a signal voltageto be amplified, and the voltage from said direct current path comprising a biasing voltage to cause said tube to be responsive in inverse proportion to the signals rength, means in said direct cure rent path to prevent;substantial diminution of the generated biasing voltage therein during the interval between synchronizing impulses. and means for preventingthe fiow of direct current in said direct current path until the attainment of a signal level of predetermined value. 6-.-In a facsimile system employing recurring synchronizing V impulses, a receiver, printing means, and automatic volume control means in connection with said receiver for controlling the signal level applied to said printing means in accordance with the intensity of received synchronizing impulses only, said means comprising a circuit 'having'a direct current path including a rectifier, and an alternating current path, an amplifier tube, and means for impressing on the grid of said amplifier tube voltages from both of said paths, the voltage from said alternating current path comprising a signal voltage to be amplified, and the voltage from said direct current path comprising a biasing voltage to cause said tube to be responsive in inverse proportion to the signal strength, means in said direct current path to prevent substantial diminutionof the generated biasing voltage therein during the interval between synehronizing impul'ses, and means for preventing the flow-ofdirect current in said direct current path until the attainment of a signal level of predetermined value, said last mentioned means comprising thermionic means for impressing on said rectifier a bias to render-it inoperative in the absence of signal. s

7. In a facsimile. system employing recurring synchronizing impulses, a receiver, printing means, and automatic volume control means in connection with said receiver .for controlling the signal level applied to said printing means in accordance with the intensity of received synchronizing impulses only, said means comprising a circuit having a directcurrent path including a rectifier, and an alternating current path, an amplifier tube, and means for impressing on the grid of said amplifier tube voltages from both of said paths, the voltage from said alternating current path comprising a signal voltage to be amplified, and the voltage from said direct current path comprising a biasing voltage to cause said tube to be responsive in inverse proportion to the signal strength, means in said direct current path to preventsubstantial diminution of the generated biasing voltage therein during the interval between synchronizing impulses, and means for preventing the flow of direct current in said direct current path until the attainment of a signal level of predetermined value, said last mentioned means comprising thermionic means for impressing on said rectifier a bias to render it inoperative in the absence of signal, and a connection between said thermionic means and said rectifier such as to cause direct current in said direct current path to bias said thermionic means to cut off.

8. In a facsimilelinstallation for connection to a radio set, an input transformer having a primary and secondary, means for selectively connecting said primary to said radio set and facsimile means in connection with said secondary, a third winding on said transformer, and means in connection with said third winding for indicating a correct signal level for facsimile reception, and switch means operating in connection with said first mentioned switch means for placing said indicating device in the circuit of said third winding when an adjustment is desired, there being amplifying means between said facsimile means and said secondary, and means for deriving an automatic volume control voltage and for applying it to said amplifying means, located between said secondary and said amplifying means, said means for deriving an automatic volume control voltage comprising a diode rectifier in circuit with said secondary, means for applying direct current generated by said rectifier to said amplifying means, and means in the circuit of said rectifier to cause said rectifier to be responsive substantially only to recurring received synchronizing impulses.

9.'In a facsimile installation for connection to a radio set, an input transformer having a primary and secondary, means for selectively connecting said primary to said radio set and facsimllemeans inconnection with said secondary, a third'winding .on said transformer, and means inconnection with said third winding for indieating a correct signal level for facssimile reception, and switch means operating in connection with said first mentioned swicth means for placing said indicating device in the circuit .of said third winding when an adjustment is desired, there being amplifying means between said facsimile means and said secondary, and means for deriving an automatic volume control voltage and for applying it to said amplifying means, located between said secondary and said amplifying means, said means for deriving an automatic volume control voltage comprising a diode rectifier in circuit with said secondary, means for applying direct current generated by said rectifier to said amplifying means, and meansin the circuit of said rectifier to cause said rectifier to be responsive substantially only to recurring receivedv synchronizing impulses, said diode rectifier having in connection therewith a three element thermionic combination adapted to draw current in the absence of signal and means for applying voltage therefrom to the cathode of said diode to bias it to out off in the absence of signal of predetermined value.

10. In a facsimile installation for connection to a radio set, an input transformer having a primary and secondary, means for selectively connecting said primary to said radio set and facsimile means in connection with said secondary, a third winding on said transformer, and means in connection with said third winding for indicating a correct signal level for facsimile reception, and switch means operating in connection with said first mentioned switch means for placing said'indicating device in the circuit of said third winding when an adjustment is desired, there being amplifying means between said facsimile means and said secondary, and means for deriving an automatic volume control voltage and for applying it to said amplifying means, located between said secondary and said amplifying means, said means for deriving an automatic volume control voltage comprising a diode rectifier in circuit with said secondary, means for applying direct current generated by said rectifier to said amplifying means, and means in the circuit of said rectifier to cause said rectifier to be responsive substantially only to recurring received synchronizing-impulses, said diode rectifier having in connection therewith a three element thermionic combination adapted to draw current in the absence of signal and means for applying voltage therefrom to the cathode of said diode to bias it to cut off in the absence of signal of predetermined value, and a connection between the control element, of said thermionic combination and the circuit of said diode such as to bias said combination to cut off when said diode operates as a rectifier.

FRANK ,L. WEDIG. JR.

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