US2255408A - Facsimile system - Google Patents

Description

Sept 9, l941 R. w. CARLISLE 2,255,408

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R. W. CARLISLE FACSIMILE SYSTEM Filed Oct. 19, 1939 y 6 Sheets-Sheet 6 f RICH Ro .CA us E @j am! v oRNEY Patented Sept. 9, 1941 UNITED STATES PATENT OFFICE f FACSIMILE SYSTEM 'Richard W. Carlisle, Elmsford, N. lr.` Application' October 19, 1939, Serial No. 300,159 i (cl. irs- 6.6)

19 Claims.

This invention relates to apparatus in which visual matter, which may be characterized as consisting of either half-tones or black-andwhite matter, are scanned by a photoelectric apparatus which yields an electric current whose wave form varies in accordance with variations in the degree of light and shade of the visual mattei'. This electrical current is transmitted over a communication channel such as a telephone or telegraph line, and/or a radio transmitter and receiver, to the recording apparatus. At the recording end, a printing mechanism is caused to traverse a recording surface in synchronism with the spot of light at the scanner, and the printer records each picture element 'with a degree of light or shade which is proportional to the original.

When it is desired to transmit pictures over a long distance, either over a wire line or over a radio channel, certain deleterious influences tend to mutilate the pictures and confuse the print. The principal difficulties encountered are diminution of frequency range, echoes, random variations in signal strength, ground noise and phase distortion.

To facilitate distinguishing real signals from unwanted impulses it is imperative to make the ratio of signal to noise as high as possible at the transmitting end. Similarly, it is imperative to make the minimum change in the ratio of signal to noise as great as possible for each change from light to dark or vice versa in the transmitted image. For instance, if in a halftone picture there is a gradual change from white to grey, a radio transmitter carrier would, by elementary methods of modulation as now practised in the art of broadcasting facsimile pic-I tures, be caused to vary gradually from one strength to another, the total variation being a small proportion of 100% modulation. This lowpercentage type of modulation to' be effective at the recorder must be carried over the various channels normally interposed between the scanner and the recorder without any non-linear dis tortion and without the admixture of static. A great improvement in this system was shown and described in my co-pending application No. 6,993, led Feb. 18, 1935, now Patent No. 2,180,397, granted November 21, 1939, of which this application is a continuation in part, in which the radio transmitter ,is keyed from a normally half -on value alternately to full-01T and full-on values, these alternating pulses being pulses substantially weaker than those causable by a fifty percent change in transmitter modula` tion, and reducing to a common level all pulses greater than expected from this modulation. The signal can be cleared up in this'manner a number of times in the course of passing from the scanner to .the recorder, as for instance when a chain of relay `stations is utilized. Other advantages of the system shown in the abovementioned application are concerned with transmission over telephone lines from the scanner, to

the recorder or to a radio transmitter; an alternatin'g current wave of special form is used in which alternate positive andnegatve pulses are transmitted at` regular intervals, the duration of the individual pulses being regulated to be proportional to the shade of the picture. This signal can also be cleaned up at the point Where it is to be utilized in the same manner as the radio signal mentioned above.

It is the purpose of the present application to provide means for transmitting printed matter and line drawings in addition to half-tones in a system in which advantage is taken of the above-mentioned principles for the transmission of half-tones. Unless the principles hereinafter described are utilized, the above system is not entirely adapted to the transmission of printed matter and line "drawings because the individual pulses have to be so chosen that the recorded dots will be laid down at the recorder like the dots of a common half-tone photoengraving; the characteristics of the system-are such that the nest print which could be transmitted Would be of a relatively coarse nature, such as results from reproducing printed matter by photoengraving through a screen, The theoretical reason that this occurs in the facsimile system as Well as in photoengraving results from the fact that it is generally necessary and always desirable to utilize every part of the frequency spectrum of the various. channels through which a facsimile signal passes to the best advantage and highest eiciency. When transmitting halftones, several shades must be transmissable. In the system as mentioned hereinbefore wherein pulses of varying length are provided for different degrees of shading, as the pulses are made shorter the frequency range of the waves constituting these pulses becomes several times the fundamental frequency of the wave being modulated by the photoelectric output. The fundamental frequency must be so chosen that all the harmonies to be utilized will fall Within the frequency band of the channels available. It will not matter if at the recorder the dots are so far spaced that individual dots are nearly visible to the eye. In print transmission, however, it is not necessary. to show any shades except black and white, and a distinct blurring willl occur unless dots are so closely spaced that each component line within a character is composed of several dots close together. It is therefore necessary to use a higher fundamental modulation frequency for print than for half-tones. These modulation frequencies are herein described as carriers. Line drawings obviously partake of the nature of printed matter, requiring only black and white for complete delineation. This higher carrier may be chosen within the band occupied by the harmonics of the picture carrier, and when modulated by printed matter may be arranged to occupy the same total frequency spectrum as that of the modulated half-tone picture carrier.

It is one object of this invention to provide means for transmitting both half-tones and black-and-white matter in a system in which the frequency spectrum of the available frequency channels is taken advantage of fully and in which static-eliminating methods can be fully employed.-

To accomplish this, ahigher fundamental frequency carrier wave ischosen for black-andwhite transmission than for half -tones, and automatic means are provided for selecting the proper fundamental frequency in accordance with the matter being transmitted.

In recording half-tones which have been transmitted according to the principles of my invention, each pulse is recorded as a separate dot unless special precautions are taken to smooth out thes'e dots, as described in my application 6,993. When recording half-tones as dots, the dots form disagreeable vertical rows unless suitable provision is made for staggering alternate rows, in which case a pleasing appearance is produced as in a photoengraving.

It is one purpose of my invention to provide means for staggering the dots of alternate rows by changing the phase of the transmitted pulses so that no change will be required at the recorder.

Although it is well known in the prior art to change the frequency of a wave in proportion to the degree of light and shade of a picture, such systems were incapable of changing the frequency over a sufficiently wide band to instantaneously accommodate it to either half-tone or printed matter, and due to their effect of averaging the strength of shade over a given area were incapable of transmitting ne print with proper definition.

In the present system, definite selection is made between a plurality of carrier waves by Valves which are caused to depend for their operation upon the rate of change of the scanned matter from light to dark and vice versa.

The novel features that I consider characteristic of my invention are set forth in the appended claims. The invention itself, however, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of certain specific embodiments of my invention, when read in connection with the following claims, in which:

Fig. 1 is a schematic diagram of a scanning and transmitting system in which completely automatic differentiation between half-tone and black-and-white matter is eifected,

Fig. 2 shows the wave forms of electrical waves in various parts of the system of Fig. 1,

Fig; 3 shows the relative frequency characteristics of half-tones, black-and-white matter and the frequency characteristics of the rate of change selector,

Fig. 4 is a schematic diagram of a picture wave generator as indicated by G-I in Fig. 1 and as shown in Fig. 1B of application 6,993, part of Fig. 'I is a diagram of a phase changing control mechanism on the scanner, and a phase changing relay together with one form of the phase retarding network, the latter elements being indicated by P in Fig. 1,

Fig. 8 is a modulator and limiter as indicated by M and L in Fig. 1,

Fig. 9 is a schematic diagram of a phototube amplier and a retarding network as shown at 5| and R respectively in Fig. 1,

- form as shown at W-G2 and is used for print Fig. 10 is a schematic diagram of an alternative scanning and transmitting system in which automatic differentiation between half-tones and black-and-white matter is effected by the use of a special scanning head operating on the pictorial matter being transmitted,

Fig. 11 is a diagrammatic side view of the scanning head used in the system of Fig. l0, and

Fig. 12 is a schematic diagram of a semi-automatic scanning and transmitting system in which differentiation between half-tones and blackand-white matter is effected bythe use of acontrol sheet and separate scanning means therefor,

In the drawings, parts used in both application Ser. No. 6,993 and the present application are indicated by corresponding numerals; and principal elements of the system such as amplifiers having a specific structure as described herein with respect to one embodiment; of my invention are indicated by similar letters when used similarly in connection with other embodiments thereof.

General principles of operation The manner in which the various parts of my preferred system function in order to produce a wave form suitable for transmission is as follows: Referring to Fig. 2, the visual matter which it is desired to transmit is indicated by the letter N and the half-tone region at the side thereof, both being shown greatly enlarged. This matter is affixed to the drum 3 of the scanner A of Fig. l, which is a typical scanner as used inthe art, although any scanning mechanism may be used.

In Fig. 2 thewave forms of the currents emerging from each part of the system is shown, the various waves being designed by W followed by the letter or numeral significant to the particular part of the system.

An alternating current of substantially tri angular wave form, as shown by waveW-Gi of Fig. 2, is generated in the half-tone wave generator G-I of Fig. 1. A second wave generator G-2 is used, which may have a sinusoidal wave and black-and-white matter as will hereinafter be described. The output of each wave generator is passed through its respective valve V-i and V-2, and thence to a modulator M, the phase changer P being intermediate of the valve V-i and the modulator M. The valves control by their action the amount of signal transmitted from each wave generator to the modulator; in

principle, the output of one generator or the other is alternatively transmitted through the valves but both are never transmitted at the same time.- The valves arc in turn controlled by the rectifiers R-I and R-2, which depend for their opera- 5 tionon the rate of change selector S.

Considering the progress of the signal waves forward from the photoelectric device, in Fig.'2 there is shown at W5I the amplified photoelectric output resulting from scanning a shaded portion of a half-tone 42 grading from white t0 black and terminating with a sharp black edge 43. The portion of the letter N which is being scanned is indicated by the band 52, in which one position of the scanning spot is shown at 53.

Principles of automatic carrier selection Two circuits are connected to the photoelectric device through the amplifier 5|, one to the rate of change selector S for controlling the valves VI and V2 andthe other to the retarding device R and the modulator M for purposes of transmission.

The output of the photoelectric device 6 is amplified in the amplifier indicated at 5l to bring 25 the signals up to a convenient level. The rateof-change selector S operates to distinguish between print and half-tone matter by analyzing the frequency spectrum of the wave issuing from the photoelectric device. In printed matter (and of course in black-and-white pictorial matter) all changes from black to white occur suddenly, causing a sudden change in electric current. On the other hand, in half-tone matter changes from black to white and vice-versa are comparatively slow, causing slower changes in electric current. By half-tone matter I mean either ordinary photographic or dot type half-tones. A series of rapid changes in current may be analyzed and will be found to be characterized by the presence 40 of high frequencies, whereas a series of slow changes will be characterized by low frequencies. The relative average frequency ranges for print and half-tones is shown in Fig. 3, in which both may be seen to have direct (zero) current com- 4 ponents due to the necessity for transmitting the background (i. e. white, in the case of print), the half-tones being characterized by a limited frequency range while the print is characterized by a higher frequency range (due to the presence of sudden changes as mentioned above) and a comparative lack of lower frequencies (since there are few gradual changes). An electrical network tuned to high frequencies as found in the print spectrum and shown at` S' will therefore pass currents only when print is being scanned. The -rate of change selector S consists of such a network. The output of it for various conditions of input thereto isl shown in wave W-S. This output is used to control a system of rectifiers R-I and R-2 in such a manner that one gives a high negative potential output while the other has zero output, and viceversa. The output of each part of the rectifier system, as shown in curves AW-RI and W--R2, 65 is connected to a respective valve V-l, V-2. By controlling the grid bias of the valves, or in some equivalent manner, the rectifiers cause one valve or the other to be transmitting. The relative phase of the rectiiiers is opposite, i. e. sepa- 70 rated 180 degrees, in order that when signal is' impressed upon them, one may give an output opposite in polarity to the other so that the valve connected to one rectifier will be caused to trans# mit while the valve connected to the other will 75 be rendered non-transmitting. An averaging actionis introduced into the output of the rectifiers R-I and R2 to cause the valves to be keyed less rapidly than the signal itself varies. For instance, it' is desirable when scanning a letter such as N to have the lvalve V2 held open long enough to cause the generator G2 to be continuously connected to the modulator M while both edges of aA given part ofu the letter such as 55 and 56 are scanned. The Valves are thus deliberately slowed down, which causes a time retardation of their output relative to the original signal. The details of .these circuits will be described hereinafter.

If in a picture sharp changes from black to white occur, the higher frequency wave generator G2 (intended principally for print) is momentarily switched into use and the degree of contrast and definition in the picture is completely preserved.

The amplified photoelectric output is connected to the modulator M through the signal retarder R. This retarder R consists of means for storing the photoelectric signals for an interval of time 5-lv just long enough for the rate-of-change selector S, the rectiers Rl and R2 and the valves VI and V2 to operate. By this arrangement of sequence of timing, half-tone signals will arrive at the modulator M at the same time that properly selected waves from the half-tone wave generator G-i will arrive there, and simi larly black-and-white signals will arrive at the modulator M at the same time as waves from the print wave generator G-2. Curves W-VI and W--V2 show the relative outputs of the halftone and black-and-white wave generators as controlled by valves V-I and V--2 respectively. Curve W-R shows the photoelectric device out put W-5l after transmission through the signal retarder R. It is identical with the original curve W-5I except for the slight lateral shift 54 which is due tothe retardation in time, which is arranged .to be identical to that in the rate-ofchan'ge selector and rectifier systemas hereinbefore mentioned. Curve W-M shows the output of the modulator M. the effective frequency is entirely different for the print and half-tone regions, and the wave form is also different, it being substantially sinusoidal for print and triangular for half-tones. In the modulator M and in the limiter L and radio transmitter T the operation of the system` is the same as described in my aforementioned copending application No. 6,993, wherein the locally generated waves are modulated with variations in the picture being scanned, peaks are limited to a definite value, and currents having less than a predetermined value are eliminated, giving a resultant wave as shown at W-L. Transmission is effected as desired over Wire lines, and modulation of a. radio transmitter is effected inthe same manner and with the same apparatus as ordinarily used for sound broadcasting and communication. In the case of halftones, the output of the limiter L is as shown at the right hand side of W-L an alternating cur-v It may be seen that.

Modulation of radio transmitter put of the radio transmitter when modulated by the wave of the curve W-L. It may be noted that the carrier is modulated from a normal or half-maximum value 64 on zero signal to a twicenormal or 100% modulation value on positive signal as shown at 65 and zero value 66 on negative input signal.

Correction of received signal The appearance of the wave form when received with an appreciable amount of significant disturbances such as static is shown in W--T. When rectified by an audio-frequency detector, the resultant envelope will vappear as shown in curve W-Re0. By limiting maximum values and eliminating weak values of instantaneous signal, this wave W-Rec can be transformed into the wave W-R-L which resembles the wave W-L at the transmitter in every respect, and can therefore ybe used to reproduce the original pictorial matter without any distortion caused by the static introduced during transmission. This is shown in the last line of Fig. 2, in which 61 is the bar'of the letter N bounded by 55a and 56a, corresponding 'to the region 55--56 in the original letter N, and 68 is a. dot constituting part of a half-tone region, halftones being printed as dots similarly to a photoengraving.`

Principle of phase changer The function of the line-by-line phase changer P shown in Fig. 1 is similar to that of the phase changing device shown in Fig. 4 of my application 6,993, filed February 18, 1935, in that it operates to relieve the effect of Vertical striations in the' recorded page by staggering dots of alternate lines.

In my application Serial 6,993, the phase of the transmitted wave was altered in its relation to the transmitted page, and a corresponding shift was made in the recorder. In my present phase shifting device, however, the phase of the half -tone wave generator is shifted in its relation to the recorder, so that no further shift in the recorder is required or desirable. The phase shifting device is operated by the scanner during alternate lines and its construction will be described hereinafter.

My invention in its preferred embodiment has now been described by discussion of the func-y tions and means of coordination of the com- -ponent parts, the more detailed construction of which in one reduction to practice will now be described.

Half-tone wave generator vcathode ray tubes.

@somos should be so chosen that'plcture elements will be vertically aligned in the recorded picture when no phase change is introduced in alternate lines: the phase change mentioned above acts to change this array to a diagonal one which is more pleasing. It is diflicult to choose screen frequencies which will give a diagonal array without such a phase changing system for the reason that various simple harmonics of the frequency of the line 5 give vertical arrays whereas diagonal arrays are producable only by uneven multiples of this frequency.

The harmonic chosen for the screen frequency is segregated in the tuned transformer 40. In one specific embodiment a triode such as the type 31 is used for 39; the line frequency may be 60 cycles, i. e. 300 cycles, the capacitor 94 may be .014 microfarad and the inductance of winding 95, 20 henries.

The plate circuit of 4| may likewise be tuned to the screen frequency by the tuned circuit comprising the inductor 96 and the capacitor 91, which with a 37 tube for 4|, may be 20 henries and .014 microfarad, respectively.

Coupling to the triangular wave generating tube 46 is made through the capacitor 98, which may have a capacitance of .005 microfarad. The synchronizing voltage applied to the grid of the oscillator tube 46 is controlled by the potentiometer 99 which may be 100,000 ohms.

The discharge tube 46 should be of the gridglow or gas triode type such as the RCA 885.

This discharge tube 46 is so termed because its function is to cause the discharge at the proper time of condenser 44 through the resistor 41 in series with its own plate circuit. This condenser 44 is always being charged at a constant rate by the current which passes through the resistor 45 from the B supply. It may be seen that the potential on the grid of the coupling tube 48 will rise steadily as the condenser bank charges, and that when the tube 46 isbiased instantaneously positive, the plate resistance will fall to a Very low value.

In order to secure triangular wave form, resistor 41 is made equal to substantially half of resistor 45. During discharge, a. current twice as great as that through 45 will therefore flow through 41 and the condenser bank will be discharged at approximately twice the rate at which it was charged. This is the criterion for a triangular wave form Voltage. This is impressed on the grid of the coupling tube 48, which is here used as the valve Vl indicated in Fig. 1 by connecting its grid bias line |45 to the rectifier RI.

In one specic embodiment of my invention, the resistor 45 may have a value of 100,000 ohms, capacitor 44 may have a value of .0074 mfd., coupling capacitor |02 may have a value of .005 mfd., and grid resistor |03, 1 megohm.

Other triangular (also known as sawtooth) wave generators may be used, as well known in the art of picture transmission by means of Such circuits are specifically described by F. A. Everest in Communications,

` for August 1939 (Bryan Davis Publishing Co.) in

the article Electron Beam Deflection Methods.

Print wave generator The generator G2 may consist of a standard audio oscillator, as shown in Fig. 5. 'Ihe grid circuit of the vacuum tube 69 is tuned by means of the condenser 10 to a high audio frequency and coupled to the plate circuit by means of the transformer 1|. Any desired portion of the output is taken through the volume control 12 to the valve V2, the grid potential for the tube 13 being supplied from the rectier R2 through the line |46.

The output of this valve may be coupled to the modulator M as shown in Fig. 1 by the transformer 14 and lines ||8a and |29.

Carrier selector The rate of change selector S shown in Fig. 6 is comprised essentially of an electrical network responding only to a predetermined high frequency band, as hereinbefore mentioned. It is coupled to the photoelectric apparatus byl lines |43 and |44. Amplifying means consisting of a two stage amplier using tubes 15 and 16 are incorporated in this embodiment, since considerable amplification is required to operate the` rectiiiers RI and R2. Low frequencies are excluded by making the coupling condensers 11 and 80 and the grid resistors 18 and 8| small, and the plate inductor 19 isA constructed with predetermined low inductance. If it is found desirable to limit high frequency range to secure stable operation this may be done by means of. the capacitor 90. The output of the selector S is coupled to the rectiflers RI and R2 by means of line |33 to the transformers 82 and 83, or a single transformer may be used. The rectiers are shown as push-pull rectifier tubes 88 and 89. The wave form of the output is partially smoothed out .by the condensers 84 and 85, 4as described hereinbefore, and the desired volume output is taken from the volume controls 86 and 81 to valves V| and V2 respectively. It may be seen that rectier Ri is so polarized that there will be no output when there is no input signal, and the output on line |45 will be negative when there is a signal input. On the other hand, rectifier R2 is oppositely polarized, and is moreover connected to ground through a bias battery |09 whereby there will be a high negative bias on the output line |46 when no signal is applied, and when signal is applied the output potential will be raised to a substantially zero value. By these means, valve V| will be keyed on when there is no signal (i. e., the condition of half-tone for which it is desired to utilize generator G|) and valve V2 will be keyed on at other times.

Phase changer structure the Wave output of the generator GI, since each half-cycle is recorded as an individual dot. The various elements may be proportioned in the following manner: Let the resistor |22 have a value approximately equalto the impedance of the circuit ||1, H8; the inductive reactance of reactor |2| and the capacitive reactance of condenser |20 are each made equal to half of this resistance atl the frequency of 'generator GI; and resistor |23 is optionally ,introduced in the case .that it should be desired to make resistorw|22 The phase/changer indicated by P in Fig. 1 and shown in more detail in Fig. 7 is shown in Fig. 1 to be connected with the scanner A. At each revolution of .the drum a projection 1 thereon engages with an extended member 8, the movement of which causes the pawl 9 to push the ratchet I0 one-eighth revolution. This is suicient to cause the switch arm to Ialternately contact with and disconnect from the circuit connected to the group I2 of four switch points in parallel. This circuitv includes the actuating ||3. Arms ||2 and ||4 are thereby pulled from contacts |25 and |21 to contacts |24 and |26 larger than the resistance of the circuitv leading into lines I1, H8. Similarly resistor ||9 is introduced inthe case that circuit ||5, H6 is lower in impedance than resistor |22. This circuit may be characterized as having a phase retardatio proportional to frequency, forv as is well known in the art of electrical networks this is identical in principle to causing all pulses to be delayed an equal interval of time.

Modulator The modulator M receives the output of the wave generators GI and G2, which pass through the paths indicated in Fig. 1, through the input connections |2911 and ||1a. As shown in Fig. 8, a transformer 49 is used to couple the input to the modulators 28 and 28a. Connectionmay alternatively be made to the respective screen grids or suppressor grids of said modulators. The input from the signal retarder R shown in Fig. 9 is directly connected to the grid circuit by lines |30a and |3|a. The output of said modulators is taken through the push pull transjformer 29 to an amplier, one tube of which is indicated at 30.- This may be constructed either as a D. C. or an A. C. amplifier, the specic frequency range requirements being that it pass alternating current from somewhat below the fundamental frequency of the triangular wave up to the highest harmonic utilized.

Limiter The output ofthis amplifier may be transformer coupled to the push-pull limiter L, comprised of vacuum tubes 3|, 32, 33 and 34 and their associated circuits. The tubes 3| and 32 have their grids biased to negative to cutoff, so that only a fairly powerful positivek signal can affect either one; such a signal will, however, cause the grids of either tube 33 or 34 respectively to be driven negative. The latter grids are, however, biased positive, so that the maximum plate current is that with no impressed signal and, since the driving potential will tend to be either negligible or very large, this maximum plate current will be either unaffected or driven to cutoff at the beginning of each pulse. A square-shaped wave will thus result. The action will be polarized, 3 |l and 33 being actuated by one half cycle and32 and 34 by the other half cycle, respectively. The action of these circuits was previously described in a general ,way by reference to Fig. 2, the effect of the limiteriL being to transform the wave output W-M of the modulator M into the wave shown lat W--L. Potentiometers 9| and 92 may be 3500 ohms each. The push-pull transformer 35 couples these circuits to the line |69, |10 which leads toa radio transmitter T or to recording equipment.

A similar limiting may be used in the audio circuits of a radio receiver for reducing the eiect on received signals of significant disturbances such as static, as described hereinbefore in connection with waves W--T', W-Rec and W-R- L of Fig. 2.

Phototube ampler As shown in Fig. 9, ampliiication means may be used between the photoelectric device 6 and the selector S and signal retarder R. I The cathode of the phototube is connected to the grid of the amplifier vacuum tube 25, and the anode is connected to a positive voltage. The grid bias of vacuum tube 25 is established through the resistor 26. Coupling to the modulator vacuum tubes 28 and 28a is accomplished by the resistor 21, variation of which controls the input voltage.

In one specic embodiment of this invention, Dentodes such as the RCA 6C6 are used for 25, 28 and 28a; resistor 26 is one megohm and 21 is 50,000 ohms.

'I'he output of the phototube amplifier 5| is connected to the rate of change selector S by lines |43 and |44 as shown in Figs. 1 and 9 and also tothe retardation device, one form of which is diagramatically represented in Fig. 9.

Signal retardation device (which in this embodiment is an elongated airpassage) for delaying them, re-transformed into electrical waves in a Vibro-electric translating device and rectified to restore the original wave form. The generator G3, which may be identical in principle to generator G2.

shown in Fig. 5, is used to generate an alternating current having at least as high frequency as generator G2 and preferably considerably higher. This alternating current is applied to the grids of vacuum tubes |35 and |36 through the transformer |34. 'The phototube amplifier is directly connected to the grids of tubes |35 and |36. Bias may be controlled by connecting the cathodes of tubes 35 and |36 to the proper potential on the B supply of tube 25. The modulated output of tubes |35 and |36 is applied through the transformer |31 to the electro-acoustic translating device |38. The acoustic waves thereby generated are transmitted through the pipe |38, which has a predetermined length such that the time of travel from one end of said tube to the other, together with the time elapsed in the translating devices, will be equal to the time retardation required as hereinbefore explained in connection with and by reference to Fig. 2. Methods of designing vibrating systems and acoustic pipes are well known in the art, and may be found by reference to Acoustics by Stewart and Lindsey and Applied Acoustics." by Olson and Massa'. The sound Waves are re-transformed into electrical waves in the translating device |40, and coupled to the rectifier tube |42 by the transformer |4 'Ihe rectifier output is filtered of alternating current pulsations by the condenser |32, and is transmitted therefrom to the modulator M by lines |30 and |3|.

Other methods of retarding the signal may al` ternatively be employed, such as a number of network'sections in cascade of the type shown at |08 in Fig. 7. Suitable amplifying means must be employed in that case to supply the loss in intensity in passing through these networks. Elements of the network may of course be employed as coupling elements between tubes, and the internal resistance of the tubes may be utilized for the series resistance Aelements 9 and |23.

The foregoing has been a description of one vcomplete system according to the principles of my invention. Variations on this system will be evident to one skilled in the art, one such embodiment being diagrammatically indicated in Figs. 10 and 11 and another embodiment being indicated in Fig. 12.

Automatic carrier selection by dual scanning` A are' amplied in an amplifier 5| (identicalparts being numbered identically with Fig. 1), and transmitted to a modulating device M and from thence to a limiter L and radio transmitter T. Furthermore, Waves are generated in generators GI and G2, keyed respectively by valves Vl and V2 and controlled in phase by network P. Valves V| and V2 are in turn controlled by rectifiers RI and R2 respectively, and these controlled by the rate of change-selector S. The difference between this system and that of Fig. 1 is that this systemis simpliiied by the removal of the signal retardation network R, instead of which there is used (for controlling the phase of the keying operation) a second optical system and photoelectric device, as shown in Fig. 11, together with amplifying means 5|a as required therefor, said second optical system being connectedto the rate of change selector S for controlling the keying operation of the valves. Re-

ferring to Fig.'11, the dual optical system is shown operated by one light source |50 which is directed onto the drum 3 by means of the lens 5| over a suiiiciently broad region to cover the areas on which the optical pickups are directed. It is necessary to provide picture impulses to the control system comprised of selector S and rectiers RI and R2 before the similar picture impulses are applied to the modulator M, because said control system requires a certain time in which to operate, Whereas the picture signals may be instantaneously transmitted to the modulator as hereinbefore described in connection with Fig. 2. The optical pickup device |48 associated with said control system is therefore disposed advanced towards the direction of approach of'the'drum surface relative to' system.

a manner that the time taken for any point on the drum to travel from one point to the other will be equal to the time lag in the control system hereinbefore described. This time lag is predetermined as a function of the sharpness of detail in the picture, so that it turns out that the displacement of one pickup point from the other will be just equal to the distance over which it is desired to take an average of the pictorial matter for the purpose of selecting the proper generator. Thus if in a picture the minimum width of print is to be .010 inch, the distance between optical pickup points may be of the same order of magnitude.

Semi-automatic carrier selection by cue sheet The second variation embodying the principles of my invention isshown in Fig. 12, in which further simplification is secured by eliminating the rate of change selector S and the rectiers Rl and R2 and keying the valves VI and V2 directly from an auxiliary photoelectric system 6a and 5|b operating from a special cue sheet |6| disposed in the scanner A" in such manner as to be scanned in synchronism with the pictorial matter |60. This cue .sheet IBI is cornprised of a sheet'of the same size as that containing the pictorial matter on which each portion |63, |63a lying in the same relative position as the half-tone regions |62, |6241 on the original sheet |66 are blacked out or otherwise rendered significant to a scanning device. Instead of being the same size, the cue sheet can be smaller or larger as long as relative proportions are preserved. 'I'he simplest way of preparing such a cue sheet is to utilize a transparent or translucent sheet, superimpose it upon the original sheet and black over the half-tone regions. Alternatively, the black-and-white regions may be blacked out, provided suitable precautions be taken to insure correspondence in the electrical The optical system for the pictorial matter is similar to that in Fig. 1 and is indicated by 6 lin Fig. 12. -This is mechanically coupled as indicated at |66'to the optical system 6a used to scan the cue sheet |6|.

The photoelectric device of optical system 6a is connected to amplifier 5|a in the same manner that system 6 is connected to amplifier 5l, and amplifier 5|a is similar to amplifier 5|. The output of amplifier 5| a is connected by line |65 to valve VI and to the grid of the reversing tube |15, bias connection of`,the latter being completed by line |64. Proper bias for valve VI is established by battery |14, such that the bias on VI will be substantially zero when scanning a black region on cue sheet |6 and a high negative value on white. The reversing tube is introduced for the purpose of reversing this effect for controlling valve V2, i. e. the latter bias is arranged by means of the reversing tube and the bias battery |16 to be substantially zero on white and a high negative value on black. In order to facilitate following through the action of tubes a and |15 it may be noted that the grid potential of tube 25a becomes more positive on white and the plate potential thereof becomes less positive.

It is understood that in receiving signals transmitted according to the principles herein described, the recording means will be synchronized to the scanning means by principles well known in the art so that half-tone dots will' fall in line rather than be distributed at random.

While I have shown 'and described particular forms of my invention in the foregoing specification and the accompanying drawings, it is to be understood that they are merely illustrative and that one skilled in the art could effect many 'changes and additions without departing from the spirit and scope thereof.

What I claim is:

1. In a facsimile system, the method of transmitting half-tone matter and printed matter which comprises scanning said matter, selecting a high frequency carrier wave for print transmission and a lower frequency carrier wave for the transmission of half-tone matter, and modulating .the carrier so selected in conformance with the shade of the matter being scanned.

2. In a facsimile system, the method 0f transmitting half-tone matter and' printed matter which comprises automatically selecting a relatively high frequency carrier wave for the print' and a lower frequency carrier wave for the halftone matter, and modulating the carrier so selected in conformance with the shade of the respective matter being scanned,

3. In a facsimile system, the method of transmitting half-tone and printed matter which comprises scanning said matter with a scanning device, distinguishing between said half-tone and said printed matter by analyzing the frequency spectrum of the output of the scanning device, selecting thereby one of a plurality of carrier waves, and modulating said selected Wave in conformance with the shade of the matter being scanned.

4. In a facsimile system, the method of transmitting half-tones and printed matter which comprises scanning said matter with a scanning device, performing a switching operation in accordancewith predetermined frequency components generated by the scanning device, selecting thereby one of a pluralityv of carrier frequencies for purposes of modulation, and retarding said output of said scanning device for an interval of timesubstantially equal to that required for said switching operation.

5. In a facsimile system, inl combination, al

scanning means, a frequency selective means associated therewith, said selective means being adapted to give a distinctive signal when the scanning means is scanning print, and a different distinctive signal when the scanning means is `scanning half-tones, and rectifying means adapted to give an output of one polarity on one signal, and an output of reversed polarity on the other signal, and means responsive to the polarity of saidoutput.

6. In a facsimile system, in combination, selective electrical means for distinguishing between the bands of frequencies significant to half-tones and to printed matter, means for rectifying the output of said selective means, mean's for averaging the output of said rectifying means over a predetermined period of time, two wave generators having different characteristics, a keying device arranged to control the output of each of said generators, and means for operating each keying device alternatively under the control of lsaid rectifying means.

7. In a facsimile system, the method of transmitting half-tone and printed matter which comprises performing a switching operation in response to predetermined frequency components in the output of a scanning device,` selecting thereby for purposes of modulation a relatively high frequency Wave'generator during periods .of transmission of printed matter and a relativening device for an interval of time substantiallyA equal to that required for said switching operation.

8. In a facsimile system,r in combination, a photo-electric scanning device, a wave generator having a frequency predetermined as suitable for use as a carrier for print, another wave generator of lower frequency, means for selecting frequency components in the output of said scanning device characteristic of print and of half-tones, switching means for selecting one of said generators at a time, said switching means being controlled by said frequency selective means and having a predetermined time lag, and time retarding means for introducing a similar time lag into the output of said scanning device, whereby deleterious effects of the time lag in said switching vdevice will be avoided.

9. A facsimile system as described in claim 8 characterized by having the time retarding means comprised of an electro-vibratory translating device, a vibrational transmission. device and a vibro-electric translating device.

10. A facsimile system as described in claim 8 characterized by having the time retarding means comprised of a plurality of sections. of electrical networks in which the phase retardation is proportional to frequency. Y

11. In a facsimile system, the method of transmitting half-tone and printed matter which comprises distinguishing between said half-tone and said printed matter by analyzing the frequency spectrum of the output of a scanning device conquency carrier Wave for half-tones', retarding saidV outp'ut of said scanning device for an interval of time substantially equal to that required for said switching operation, and modulating said scanning output by the carrier wave so selected.

12. .In a facsimile system, the method of transmitting half-tone and printed matter which comprises selecting for print a carrier wave of predetermined frequency and for half-tones a, second carrier wave having lower frequency and triangularl wave form, modulating with the carrier so selected the output of the scanning device controlled by said matter to be transmitted, the amplitude of the wave thereby generated by halftones being substantially proportional to the shade of the picture being scanned, limiting the minimum values of both positive and negative half-cycles of said current, eliminating small in- -stantaneous values of current, whereby to pre- Dare for transmission a current which consists of pulses occurring at intervals corresponding to one frequency or the other, in which the shade of the half-tone regions is denoted by the relative duration of said pulses.

13. A method of preparing a wave for transmission as described in claim 12, and which further comprises modulating a relatively high frequency wave from a nominal value of half-maximum to either maximum or zero in accordance with the polarity of said constant fundamental frequency current.

14. A method of transmitting pictorial matter as described in claim 12, and which further comprises modulating a relatively high frequency carrier Wave from a nominal value of half-maximum to either maximum or zero in accordance with the polarity of said constant fundamental frequency current, propagating said wave through a medium having deleterious disturbances therein, receiving and rectifying said wave, eliminating small instantaneous values of current and limiting the maximum values of pulses, whereby a wave is obtained substantially similar in wave form to that withwhich the transmitting carrier was modulated despite the action of said` disturbances in the propagating medium.

15. In a facsimile System, the method of transmitting half-tone and printed matter which comprises scanning said matter a flrst time, controlling by said first scanning a switching operation, selecting by said switching operation a carrier wave of predetermined frequency for print and a lower frequency carrier wave for half-tones, scanning said matter a second time, and modulating the photoelectric output of said second scanningby the carrier wave so selected.

16. In a facsimile system, the method of transmitting half-tone and printed matter which comprises making a cue sheet for each original sheet of transmitted matter, marking with significant contrast on said cue sheet the regions utilized for half-tones andfor print, scanning said original sheet and said cue sheet simultaneously, selecting by the action of scanning said cue sheet a carrier wave having a predetermined frequency suitable for print when scanning regions containing print and similarly selecting a carrier of lower frequency when scanning half-tones. A

17. In a facsimile system, in combination, an

` original sheet to be scanned, a cue sheet having areas thereon significantly marked to delineate for scanning purposes the regions on the original sheet occupied by half-tones and by print, means println conformance With the marking on said` cue sheet.

18. A systemaccording to claim 16 characterized by scanning means comprising a drum having adequate length on which both the original sheet and the cue sheet are mounted for scanning, dual scanning means and common driving means therefor;

19. In a facsimile system. in combination, means for scanning a picture t0 generate a picture current, means for generating a triangularly shaped carrier wave, means for synchronizing said carrier to said scanning means, means for modulating said carrier with said current, means for limiting the maximum values of both positive and negative half-cycles and eliminating small instantaneous values of current in said modulated wave, means comprised of an electrical network having a phase retardation proportional to frequency for transposing the phase of said limited current by a predetermined angle of substantially degrees relative to said triangular wave, and switching means for causing said transposition to occur during alternate lines, whereby vertical striations in -the recorded half-tone are avoided.

RICHARD W. CARLISLE.

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