US2299937A - Wave signaling system - Google Patents

Description

Oct. 27, 1942. R. M. sPRAGuE l 2,299,937

` WAVE SIGNALING SYSTEM i Filed oct. 27, 1959 SSheQtS-Sht 1 LOW'- PASS FILTER' SIGNHL. GENERATOR 4 Oct. 27, l942 R. M. SPRAGUE WAVE SIGALING SYSTEM 5 Sheets--SheeilI 2 Filed oct. 27, 1939 ril .wrLl

INV'ENTOR ,M..

ATTORNE oct; 27, 1942.

R:A M. PRAGUE WAVE SIGNALING SYSTEM- `Filed Oct. 274I 1939 5 Slleells-Shee'l'l 4 fi w . www

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oct- 27, 1942- R. M. SPRAGUE WAVE SIGNALING SYSTEM Filed oct. 27, 19:59v 5 sheets-sheet 5 wfg-:N R

BY ATTORNEY Patented Oct. 27, 1942 WAVE SIGNALING SYSTEM Robert M. Sprague, Garden City, N. Y., assgnor to Press Wireless, Inc., Chicago, Ill., a corporation of Delaware 'Application October 27, 1939, Serial No. 301,563

14 Claims. (Cl. TIS- 6.7)

This invention relates to wave signaling systems and more especially to systems of carrier wave transmission wherein the effects of parasitics, noise, harmonic distortion fading and undesirable level variations are to be avoided.

A principal object of the invention relates to a system for transmitting signals whether oral, visual or control, through the intermediary of a transmission link, for example a high-frequency channel or radio channel, wherein the signal reproduction is rendered substantially independent of undesirable level changes.

AAnother principal object is to provide an improved r'adio signaling system which is singularly free from the effects of fading, echo, harmonic distortion, parasitic noises'and the like.

Another principal object is to provide an improved system of secrecy signaling` over a carrier channel wherein parasitic disturbances including level distortion or fading are materially reduced.

A further object is to provide an improved system of transmitting message signals over a carrier or high frequency channel by frequency modulation.

A feature of the invention relates to a novel A further feature relates to a frequency modulating system employing a photo-electric cell or similar device as the source of modulating voltage, and wherein a frequency modulated oscillator is employed and so arranged that the oscillator output is substantially free from amplitude variations of frequency components of the said voltage source.

, A further feature relates to a receiver or reproducer of facsimile signals and the like, employing a specially designed band-pass filter for eliminating harmonic distortion in the transmitting medium or link and also for eliminating noise components below the actual frequency spectrum employed for signaling.

A further feature relates to a facsimile receiver or the like for use in a frequency modulation system and having an automatic level control 'determined by the received side-bands as dismethod of converting amplitude variations into corresponding frequency variations vof substantially uniform amplitude.

Another feature relates to a frequency modulation system employing a pair of oscillators for f producing a variable beat frequency wherein the beat frequency is varied in accordance with capacitychanges controlled by a grid-controlled electron tube. As a result of this feature oscillators can be employed whose frequency is independent of voltages on their elements thus giving a beat frequency oscillator that is substantially independent of power supply voltage variations on the oscillators.

of the band-pass type. Preferably, in accordance y with the invention, the saidl lower limit is also chosen so that unattenuated frequency shifts are l obtained up to the limits of the filter.

A further feature relates to an improved frequency modulator wherein a linear variationI of output frequency with relation to input voltage is obtained over a Very wide range.

tinguished from the carrier.

A further feature relates to a receiver for frequency modulation systems employing the combination of a specially designed filter and power limiter whereby all undesirable level variations which are not fully compensated for by the automatic volume -control are ironed out in'order to supply an undistorted signal of unvarying level to succeeding circuits in the receiver.

A further feature relates to a frequency demodujlator or converter arrangement for a receiver operated by a received frequency modulated wave, whereby the frequency modulations are converted to amplitude variations with a linear ratio between the frequency and amplitude and with a high ratio of signal-to-noise.

A further feature relates to a transmitter of the frequency-modulated wave type wherein novel means are provided for adjusting the frequency bands in accordance with the minimum and maximum amplitudes of the signals to be transmitted. Thus in the' case of a facsimile transmitter, it is possible conveniently and expeditiously to obtain the zero or minimum frequency-modulating Voltage which is correlated to -l zero output voltage from the facsimile generator. A still further feature relates to facsimile transmitting and receiving systems wherein the picture signals are transmitted by frequency modenumerated will be apparent after a considera` While the invention will be illustrated in connection with facsimile transmitting and receiving systems, it will be understood that this is merely `for explanatory purposes and that various quency-to-amplitude conversion section of Fig. 4.

Fig. 5 is a layout of Figs. l to 4 of the drawings. Fig. 6 is a modication of the amplitude-tofrequency conversion section of Fig. 2.

The transmitter Referring to Fig. 1, block T represents any well-known form of signal generator, for example a facsimile converter of the type vshown in application Serial No. 213,584, filed June 14, 1938. Preferably the device T is of a known type whereby the signals to be transmitted are generated in the form of a carrier of relatively low or audio frequency, for example 1800 C. P. S. and having amplitude modulations or variations corresponding to the original signals to be transmitted. It will be understood that the term carrier is used in its broad aspect as including either a sustained alternating current, a pulsating current or a chopped current such as that produced for example by a light chopper photo-electric cell combination as is well-known in the facsimile art. f

The signal from device T is applied across the input terminals A, B, and thence through the network comprising resistor elements I to 5 inclusive and thencethrough input transformer 6. The output of transformer 6 is fed to any wellthrough potentiometer 1. Merely for purposes of illustration, this amplifier comprises a three element tube 8 connected into circuit inA the usual way and including the grid-bias resistor 9; the

plate resistors II, I6, through which plate voltage is fed over the conductor C from the A. C.

' power supply, the latter consisting for example switch I3 to the control-grid 28a of tube 28.v

Tube 28 may be of any well-known type, for example, a type 6C8-G comprising two sets of triode elements with the cathodes 28h and 28e connected together and biassed with respect to the grids 28a and 28d by the bias resistor 29. The plates 28e and 28j are connected to the plate supply conductor C through the limiting resistor 33 and the respective coupling resistors 38, 3|, so as to provide a balanced input to the succeeding amplifier stage comprising amplifier tubes 4| and 42. to tube 28 through the condensers 34 and 35 in balanced or push-pull relation, the tubes 4| and 42 being preferably of the pentode type. The

The tubes 4I and 42 are coupled'y Aknown form of linear amplifier preferably tion of the following detailed descriptions and the appended claims.

plates of tubes 4| and 42 are connected to the D. C. supply through the primary winding of coupling transformer v48. Preferably, although not necessarily, transformer 48 is provided with an auxiliary secondary or feed-back winding D which is connected through resistor I1 to the cathode of tube 8 to reduce distortion in the amplifier. The main secondary of transformer o43 is applied to a rectifier 44 and thence through the low-pass nlter 45 to eliminate the 1800 cycle carrier component. Consequently at the output terminals of lter 45 there is present only the amplitude demodulated signal corresponding to the photo-electric current delivered by the photoelectric cell of machine T.

The frequency inverter or modulator The demodulated and amplified signal voltages developed across resistor 41 are then fed through the isolating resistor 48 to the control grid 50a o a pentode tube 58 which, in 'accordance with the invention, is employed as a modulator tube to control the beat frequency developed by the oscillator tubes and 85. Tube 85 is provided with a 'variable frequency control circuit while tube 86 'is of a fixed frequency which is determined -by the tuned oscillatory circuit 15, 16, connected between the control grid 88a and the cathodes 86h, 86C, through respective bias resistors |28, |21, and through by-pass condensers |30, |3I. The plate 86d is coupled back to the control grid 86a through the feed-back condenser 89 and a grid leak condenser combination 81, 88. Likewise plate 88e is coupled to control grid 86f through condenser 85. As a result of these connections, oscillations cf xed frequency, for example 600 kilocycles are developed across resistor 82. The tube 85 is similarly provided with circuit connections including elements 62, 63, 64, 66, 81, |24, |25, |28 and |29, and a tunable oscillatory circuit comprising coil 8| and shuntcondensers 59, 68, |23, whereby sustained oscillations of the desired range, lfor example 601.8 to 603 kilocycles are developed across resistor 12. Oscillator tube 65 is adjusted to the correct frequency, for example the lower limit of 601.8 kilocycles by variable condensers 58 and 60 and the frequency of this oscillator tube is automatically varied in accordance with a change in the plate resistance or plate-to-cathode capacity of tube 50.

This frequency modulation is accomplished in the following manner. The plate circuit of tube 50 is connected across the oscillatory circuit 59, 60, 6|, through series condenser 54. Tube 50 when connected as shown, is capable of varying its plate resistance and to a certain extenty its plate-to-cathode capacitance in accordance with the control grid voltage impressed thereon, and since this plate resistance is in series with con denser 54 any change in said plate resistance has the effect of changing the effective capacity of condenser 54 so far as affecting the frequency of the oscillatory circuit is concerned and hence the frequency of the voltages developed across resistor 12. Preferably tube 58 is of the pentode type, for example a type GF6, which after numerous tests I have found appears to change the output frequency of tube 65 substantially linearly over the widest frequency range. The suppressor-grid 5817 of tube 50 is connected to cathode 50c which latter is biassed negatively with respect to control-grid 58a by the bias resistor 5l and its b-y-pass condenser 52. Preferably, plate 50d and screen-grid 50e are connected totube 65, very little change modulation frequencies applied to .transmitter gether and to the plate supply conductor E' through resistors 53 and 51 and the filter choke |22 which latter is by-passed by the condenser 58.

I have also found 'that by using the varied plate resistance or the varied plate-to-cathode capacitance of tube 50 to vary the frequency of in the amplitude of the resultant beat frequency between tubes 65 and 86 is produced. Consequently the beat frequency of tubes 65 and 86 is substantially undisturbed by any frequency component that may exist in the photo-electric cell output voltage ol machine T. f A

.The beat -frequency from tubes 65 and 86 is obtained by applying their respective outputs to respective signal grids of the mixer tube 60 condenserl |32 is also provided, and by means of condenser 93 and high frequency choke coil 94,

the output currents flowing through coupling resistor 99 are the desired audio frequency signals which vary in frequency for example between 1800 C. P. S. and 3000C. P. S. in accordance with the corresponding shade values of the particular area of the subject matter in' machine T which is being momentarily scanned. The D. C. supply for tube 90 is applied over conductor F and thence through resistors and 91, the voltage for screen-grid 90e also being applied over conductor F in series with resistor 90, it being understoodthat the resistors 08 and |00 are by-passed by respective condensers 96, |0| and |02.

The beat or audio frequencies flowing in the plate circuit of tube 90 are impressed upon the ,control-grid of a suitable power amplifier tube |03, the grid of which is biassed by resistor |04 and by-pass condenser |05. The amplied output of tube |03 is applied through audio frequency transformer |06 to a low-pass lter |01 RT, there is provided a tube which is a monitor mixing tube to combine apart ofthe 1800 cycle sub-carrier signal from machine T with the output frequency from tube |03. `If as above assumed, the picture shades are represented by a frequency range between 1800 and 3000 C. P. S., with 1800 C. P. S. representing black andv 3000 C. P. S. representing white, it is necessary to adjust the output frequency from tube |03 to 1800 cycles when zero or a predetermined minimum-voltage is impressed on the grid of tube 50.. The, 1800 cycle signal which is appliedto the tube 20 directly from the output of tube 8 is used as a standard of comparison. For this purpose, tube 20 is preferably of the GFS-G type comprising two sets of triode elements. The grid 20a of lonetriode is excited by a part of the 1800 cycle signal from the output of tube 8. The grid 20h of the other triode is excited by part of the output of tube |03. The plates 20c and 20d are supplied with D. C. plate potential respectively over conductors C and E; Likewise plates 20c and 20d, are connected through a single-pole double-throw switch 23 to amonitoring jack 24 into which a frequency meter or a pair of head phones may be plugged. Switch 23 is preferably of the type such that in its extreme end positions it makes contact with the respective xed in order to eliminate any harmonic distortion.

Filter |01 is designed to Acut-off frequencies above the first octave of the lowest signal frequency.

- The filtered signals are then fed through the resistor network or pad |06 to ||2 to any suitable form of radio transmitter represented schematically by the block RT.

Preferably, and in accordance with the invention, the system is so adjusted that for the maximum range of shade values in the subject matter being scanned by the machine T, for example that corresponding to black and white respectively, the signal frequenciesl are confined to one octave or less. -Thus, if a black area isv represented by a frequency of 1800 C. P. S., then a white area should be represented by a frequency of less than 3600 C. P. S., for example` 3000 C. P. S.` since during transmission the second harmonic often appears with an amplitude many times that of the desired fundamental. If the signal frequencies are restricted below the second harmonic of the lowest signal frequency as will be described hereinbelow, then by` means of lter |01 the second harmonic distortion during transmission is effectively eliminated so far as any effect on the reproduced signal is concerned.

In order to be able to correlate the upper and lower shade values of the subject matter with the corresponding upper and lower limits of the contacts, but in an -intermediate position it makes contact with both xed contacts. When the blade of switch 23 is in its intermediate position as shown in the drawing, the 1800 C. P. S. signal from machine T and the signal from tube |03 are both applied to jack 24 for comparison purposes. When the blade of switch 23 is in its right-hand position, only the signal from tube |03 is applied to the jack. A single-pole double-throw switch I3 is provided between the tubes 8 and 28, and when the blade of this switch is in its uppermost position, direct ground is applied to the control grid of tube 28. When the blade is in its lowermost position, the switch I3 connects the output of tube 8 through potentiometer I4 to tube 28. By throwing the blade of switch I3 to its uppermost position, no

voltage exists at grid a of modulator tube 50.

A normal white signal is then sent; from machine T and is heard at jack 24 with switch 25 in its intermediate position which combines input and output frequencies. The beat frequency of tubes and 66 is there adjusted, for example by condenser 59 until the output from tube |03 appearing at jack 24 is the same as that derived from machine T through tube 8. The switch |3 is then thrown to its lowermost position whereby vtheamplified picture signal voltages from ma- The receiver Referringv to Figs. 3, 4, a description will now be given of a receiving arrangement that may be used. The signal from the radio transmitter RT of Fig. 1 is applied to any well-known form of radio receiver represented `by the block RR, wherein the modulations of from 1800 to 3000 cycles are detected or demodulated. These demodula-ted signals are then applied to the resistor pad comprising resistor elements to 205 inclusive and thence through the band-pass filter 206. Filter 206 eliminates all harmonics of the signal frequencies of the 1800-3000 cycle band that may have appeared as a result of selective fading or other causes during transmission, and the said filter passes only the signal as actually passed by the filter' |01 (Fig. l). Filter 206 also eliminates any noise voltage outside its band-pass. Since there may be side-band frequencies present because of the variation of the carrier during transmission, the lower cut-off frequency of filter 206 must not be chosen too close to the lower end of the actual signal band, that is, too close to 1800 C. P. S. nor to the upper limit of this band, -that is, 3000 C. P. S. Preferably, the lower cut-off of filter 206 is spaced from the lower limit of the signal band the same distance as the spacing between the upper cut-off of filter 206 and the upper limit of the signal frequency band. Thus the pass-band is symmetrical and is suiiicient to pass all desired side-band frequencies. tween 1500 and 3300 cycles.

The signal from filter 206 may vary in amplitude and must therefore be ironed out to a uni' form level. This is effected by the A. V. C. circuit of amplier tubes 236 and and by the power limiter tube 21|. Tube 2|1 is a straight amplifier tube preferably o'f the double triode type having two separate output circuits, one of these output circuits including plate 2|1a and cathode 2|1b leads to the monitoring meter 226 and monitoring jack 245, whereby the averagelevel of the signal can be determined and adjusted by potentiometer 2|3. The other output circuit including plate 2|1c and cathode 2|1d feeds into amplifier tubes 236 and 25|. Tubes 236 and 25| are preferably high gain pentodes having a remote cut-off. The D. C. plate supply for tube 2|1 is derived from a suitable A. C. source 342 over power transformer 333, full-wave rectifier 334, D. C. smoothing lter 335, 336, 331, conductor G, thence through the low `frequency choke 290, conductor Gi and resistor 233 to plate 2|1c. Plate supply conductor GI is also connected by way of resistors 221 `and 222 to plate 2|1a the various resistors being provided with suitable bypass condensers as shown. The signal developed across coupling resistor 229 is impressed through condenser 230 on the control-grid 236a of tube 236, the cathode 23617 of which is connected to the suppressor- grid 236,0. The plate 236d and screen-grid 236e are supplied with appropriate D.A C. potentials from conductors G, GI. The outputfof tube 236 is fed to a similar amplifier pentode 25|. The output of tube 25| is coupled through resistor 255 and condenser 256 to the connected control grids 264a, 264b of the double triode tube 264. The signal current in the circuit of plate 264C is applied to the rectifier tube 252 and this rectied signal voltage is ltered and smoothed by the resistance- condenser lter 248, 249, 250. over A. V. C. conductor'H through resistors 241 `and 235 to the respective control grids of tubes .25| and 236. Thus when a drop in level occurs,

.the voltage rectified by tube 252 decreases, with a similar decrease in negative grid bias of tubes 236 and 25| and the gain is thereby increased to bring the voltage back approximately to its former magnitude. In this A. V. C. arrangement, the A. V. C. signal is therefore controlled not by The filter used in practice passes bethe level of the received radio carrier voltage but [by the dem-odulated signal voltage. "I'hus, even though the carrier remains constant and the sideband signals fade, the demodulated signal will remain constant.

The other plate circuit of -tube 264 feeds the power limiter tube 21|. Tube 21| perfonms several functions. First the action of the A. V. C. circuit above described is not instantaneous because of the resistance-capacity filter and therefore on a sucl'den fade of the received radio signal, the latter will drop for a short period. Secondly, the A. V. C. is not perfect and should the signal fade to the order of db., the A. V. C. will hold the fade down to only 3 or 4 dfb. I have found that this level variation is still too great for the best results in facsimile or picture. transmission. Tube 21| takes care of both these conditions by amplifying the signal impressed thereon by transformer 269 up to a predetermined value and then as the signal increases further the output remains constant over a very wide range. The tube 21| is preferably connected in balanced or push-pull relation to the secondary winding of transformer 269, and the controlgrids 21|a, 21|b are at zero bias with respect to the associated cathodes '21|c, 21|d. Consequently, as the grids swing positively, grid current flows through the resistance 210. This automatically applies a bias to the tube 21| which bias builds up substantially in proportion to the The rectied voltage is then applied l input voltage, thus holding the output-substantially constant. Preferably, a resistance 213 is provided in the common plate lead to make the characteristics of the tube 21| linear up to a certain point and then to cause it suddenly to flatten out. This is probably due to the fact that as the grid voltage increases positively, the bias built up, in resistor 210 does not increase by quite the same amount, but resistor 213 serves to lower the plate voltage as the plate current increases. The loutput of tube 21| is a substantially square wave wherein substantially only odd harmonics Iare present as a result of .the balanced or pushpull connection. These odd harmonics must be eliminated before passing the signal to the converter circuit, the elimination being effected by filter 214 which is a low-pass or preferably a The signal is then applied to the converter circuit wherein the frequency changes in the signal are converted to amplitude changes. The control elements of this converter consist of resistor 28|, condenser 282 and inductance 283'connected to the plate circuit of amplifier tube 216. The condenser 282 and inductance 283 are designed so asto be series resonant at some frequency below the lowest signal frequency, which has been assumed to be 1800 cycles. In order to make the resultant voltage applied to the control grid of amplifier tube 281 linear with respect to frequency, the impedance of condenser 282-inductance 283 combination must be low compared to resistance 28| even at the highest frequency passed. The series resonant frequency of v282 and 283 is placed at the lower frequency attenuation peak of bandpass filter 214, for example at approximately 1380 C. P. S. This is done because at frequencies less than the said series resonant frequencies, the voltage will rising to too greatl a value.

again increase and will reach a maximum vat very low frequencies, with the result that any noise being received at frequencies less than the said l lowest signal frequency transmitted, e. g. 1800 C. P. S., the greater will be the change of voltage with change of frequency, thus giving a better response ratio of signal-to-noise. Another resistor 2'84 is provided having about onetenth of the value of resistor 28`i and it serves to prevent low frequency noise that the filter -214 may not have completely eliminated, from This resistor 284 must also be high compared to the maximum impedance of 282-283, otherwise an undesirable phase shift will result.

The converted signal is'then applied to an amplifier tube 281 which may be of any suitable type preferably a type 6C8 upon whose main control grid 281e the converted signal is impressed. The output of tube 281 is further coupled in balanced relation to another ampli- In order that the proper range of tone values may-be employed at the receiver R, means areprovided for obtaining zero output of the locali ly generated 1800 cycle carrier from tube 301 to correspond with zero voltage on the controlgrid 50a of tube 50 at the transmitter. This is accomplished by applying a fixed Anegative bias on the plates 3I8c, 3|8d of the modulator tube 3|8 as indicated schematically by the bias source 350. By-means of thepotentiometer 215, the gain of tubes 216, 281, 30| and 302, are then increased until the voltage developed across reer stage comprising tubes 38| and 302 connected ,in push-pull relation whereby the signal level is raised to the desired value. It will be understood of course that any well-known linear amplifier may be used for amplifying the converted signal.

The amplified output of tubes H, 302, is then rectified in a suitable rectifier 224 and the rectified 'output is passed through a low-pass filter 304. This'fllter attenuates all frequencies above the lowest transmission frequency, e. g. 1800 C. P. S. leaving only the demodulated varying amplitude signal corresponding to the original photo-electric cellsignal as delivered at the machine T (Fig. 1). The variable amplitude signal from filter A304 can `be applied directly to any jwell-known picture reproducing machine. However, in the event that the machine R is of a type which is operated on a modulated audio frequency, for example an 1800 cycle tone, the

signal from filter 304 is used to modulate a locally generated 1800 cycle source. For this `purpose, there is an oscillator tube 3 01 which is provided with circuit connections 308, 309, 310, 3II, whereby a sustained 1800 cycle tone is generated. This 1800 cycle tone or current is then impressed in balanced relation on the control-grids 3I8a, 3|8b of a modulator tube 3|8. The plates 3I8c, 3l8d, are supplied with plate potential from the output of filter 304 which is connected in balanced relation to the primary of coupling transformer 320. As a result, there is impressed upon the balanced amplifier Atube 322, an 1800 cycle current modulated in accordance with the signals from filter` 304. If desired,l the outputof the amplifier of tube 322 may be passedthrough a filter 325 having a band-pass width suitable for passing the 1800 cyclel wave with the necessary sidebands. The modulated 1800 cycle signal may then be passed through a suitable resistorv pad 321 to 33! and thence to the facsimile reproducing machine R. It will be understood of course that the facsimile transmitting machine T and the facsimile reproducing machine R are synchronized by any method well-known in the facsimile art.

sistor `305 is equal to the negative bias from the source 350. Thus, a net voltage of zero is applied to the plates of tube 3I8 which is the threshold point or zero voltage point.v Under the above assumption of a minimum signal frequency of 1800 cycles, this zero voltage corresponds therefore to this lower signal frequency, consequently any change in the frequency applied to tube 216 will give a corresponding 1800 cycle output from the modulator 3I8.

. In adjusting the receiver of Figs. 3 and 4, the average input is adjusted to normal by the meter 226- and the level of the received signal Ais increased by potentiometer 215 until the voltvoltage to be impressed on the plate of tube M0 resulting in a certain level of the local ,1800 cycle oscillations applied to tube 322. This level is adjusted to normal by increasing the gain by means of potentiometer 32|. After these preliminary adjustments at the transmitter and receiver, the entire system is in readiness for the transmission of the subject matter which may be a picture, written matter or any visual display consisting variations of tone or shade values.

While in the foregoing description reference has been made to the transmission of a picture by means of frequency modulations between 1800 C. P. S. and 3000 C. P. S., it will be understood that the invention is not necessarily limited thereto. However, I have foundv from actual tests and demonstrations that the above lshift of from 1800 to 3000 C. P. S. is more than sumcient to transmit all the frequencies present in modern picture or facsimile transmitters. On the other hand, I have transmitted with good results with the equipment described, using a frequency shift of from 1200 to 1800 cycles. Furthermore, while particular apparatus and parts have been described, it will be understood that -T by a voice frequency generator and by replacing the reproducer R by a voice frequency reproducer. With a higher range of frequencies such as between 5000 and 7500 C. P. S., or even .the receiver RR. may be of the diversity type.

r5000 and'6500 C. P. S., frequencies up to 4500 C. P. S. can be passed. The invention is not limited to facsimile transmission or to ordinary voice 'frequency telephony. The system is well suited for secret radio transmission because the original voice signals that may be impressed by the transmitter T are radiated from the radio transmitter RT in an unintelligible form. and in order that they may be reproduced, a receiver such as shown in Fig. 2 and adjusted and correlatedwith the transmitter of Fig. 1, must be employed. I have also found that a system as described when used in facsimile or voice communication eliminates to a substantial extent. well-known echo effects. As a result of using the system as disclosed, the following among other results have been obtained. Y

1. Unattenuated frequency shifts up to the limit of the low-pass filter 45.

2. Constant amplitude of the oscillator 86 with resultant constant amplitude ofthe beat frequency thereby introducing no frequency component of the picture machine photo-electric cell voltage.

3. Linear variation of frequency with voltage over a very wide range.

4. Independency of the frequency with respect to the power supply voltages for tubes 65 and 86.

5. The elimination of all harmonic distortion by using the various low-pass and band-pass filters such as |01, 206 and 214 and the elimination of all noise below the actual frequency range used for` signaling.

6. The automatic volume control is rendered more effective by being controlled by the demodulated signal in tube 25| ygiving constant amplitude to the converted signal applied to tube 216.

7. The supplementing of the automatic volume control by the limiter tube 21| and filter 214 whereby all changes in level which escape the automatic volume control circuit are ironed out, thus supplying an undistorted signal at perfectly constant level to the succeeding circuit.

8. The provision of a converter circuit asso- 45 ciated with tube 216 whereby frequency changes are made to amplitude changes with perfect linearity and with high ratio of signal-to-ncise.

It will be understood of course that while one particular form of level control has been dis- -5o closed in connection with the tubes 236 and 25|, any other well-known form may be employed and in addition if desired, yanother A. V. C. signal may be derived from the radio carrier and if desired,-

The A.'V. C. voltage derived as shown may be applied to control the gain of the receiver itself,

being applied to all controlled tubes in the re# ceiver, or to some of them, leaving the radiocarrier-derived A. V. C. on the others.

While in the foregoing, the frequency conversion at the transmitter is effected by a fixed frequency oscillator 86 and a variable frequency oscillator 65, a pair of variable frequency oscillators may be employed. Suchan arrangement 6 is shown in Fig. 6 wherein the parts shown between the dot-dash lines may be substituted for the part of Fig. 2 between the dot-dash lines. In Fig. 6 the left-hand rectangle represents diagrammatically the transmitting equipment shown in detail in Fig. l; while the right-hand rectangle of Fig.Y 6 represents diagrammatically the part of Fig. 2 to the right of the dot-dash line. In the embodiment of Eig. 6 the signal conductors, J, J I are connected not only to the tube\ 6 6 as described in connection with Fig. 2, but are also connected to a similar tube 402 so that the amplitude variations on conductors J, J l. result in simultaneous shifting inopposite directions of the frequency of the oscillators and 88. For this purpose the resistance 41 is connected in balanced relation across the control-grid of tube I0 and the control-grid of tube 462. the connection to the latter grid including a potential source 40| poled as shown and having a potential equal tc the peak voltage swing applied to conductors J. JI. The plate oi tube 60 is coupled through condenser 84 to a tunable oscillatory circuit such as that represented by |28, Il, 68, 6| `(Flg. 2). Likewise the plate of tube 402 is coupled through a similar condenser 84a to a tunable oscillatory circuit 16, 16, associated with the oscillator tube 86. If desired, the tuned circuit 18, 16, may be replaced by a circuit similar to that of Fig. 2 including elements |28, 68, 60 and 6|. 'Ihe arrangement of Fig. 6 has the advantage that the frequency shift for a given amplitude variation on conductors J, J is doubled as compared with the arrangement of Fig. 2. It also has the advantage kthat any non-linearity cf frequency shift on one oscillator tube, e. g..tube 65 is balanced by an equal and opposite non-linearity on the other tube 86.

Instead of employing a series condenserinductance combination 282, 283 (Fig. 4) for inverting the frequency modulations to amplitude variations, an arrangement such as shown in Fig. 4A may be employed wherein the parts corresponding to those of Fig. 4 bear the same nu`- merals. In this modification, the condenser 218 is designed to be of high impedance compared to resistance 284. The voltages developed across resistance 284 as a result of the frequencies impressed onv tube 216, are used to control the amplitude in the output of tube 281. Instead of using a series resistance-condenser, a series resistance-inductance combination may be employed, this modification being shown in Fig. 4B wherein the impedance of inductanee 283 is low compared to resistance 28|. While the embodiment of Figs. 4A and 4B each give zero voltage at zero frequency when operating between 1800 and 3000 C. P. S., only approximately 67 percent voltage change is produced; whereas with the series resonant circuit 282, 283 of Fig. 4, when this combination is resonant at 1400 C. P. S., over the same frequency range of 1800 to 3000 C. P. S., there is obtained a 300 percent voltage change. Various changes and modifications may be made in the disclosure without departing from the spirit and scope of the invention. Thus, if under certain conditionsof operation, for example at very high speeds of transmission, undesirable phase shifts occur in the various filters described, these phase shifts may be compensated for by connecting in circuit with one or more of the filters suitable phase equalizers. However, it has been found that at ordinary commercial transmission speeds, such equalizers are not necessary.

What I claim is:

l. The method of signaling which includes the steps of generating signals of different amplitudes representing different signal conditions to .be transmitted, said signals being generated within a frequency range which is lower than that required for radiation through space, converting said signals into corresponding variableto a frequency spectrum of substantially the same order as said first-mentioned signals, and further limiting thee band of said converted signals so that the uppermost frequency is less than the second harmonic: ofthe lowermost frequency.

2. The methodiof signaling which includes generating variable amplitude signals in the audio frequency range. impressing said signals on a frequency' modulator to produce7 a frequency4 range, adiusting said modulator so that the frequencyl range thereof corresponding to the 'lower and upperf limits ofthe said signal amplitude is such; that: the uppermost converted audio frequency' is; less than the second harmonic of the lowermost converted audio frequency, passing said converted frequencies through a bandpass, filter to cut-off second harmonics of any transmitted frequency, and modulating a high frequency carrier wave by the output of said filter. .i

3. The method of reducing the effects of selective fading in transmission over a radio channel and the like which comprises, converting variable amplitude signals in the audio frequency range into corresponding variable frequency signais also in the audio frequency range, limiting the frequency range of the converted signals so that the uppermost audio frequency is always less than the second harmonic of the lowermost audio frequency, and applying the converted frequencies to modulate a radio carrier wave.

4.'In a signaling system, means to generate variable amplitude audio frequency signals, means to convert said signals into variable frequency signals of substantially uniform amplitude but confined within the audio frequency range, means to limit the converted signals to an audio frequency range such that the uppermost. frequency most frequency, and means to modulate a high frequency carrier wave by said converted frequencies.

5. A signal transmission system wherein the signals to be transmitted are of different amplitudes corresponding to different signal conditions, means to convert said signals into corresponding beat frequency signals with the beat frequencies confined to an audio frequency range, and lter means upon whichA said beat frequencies are impressed, said filter passing only frequenciesl below the second harmonic of the lowermost beat frequencies and means to modulate a high frequency carrier wave by said passed frequencies.

6. A signal transmission system comprising means to generate signals in the form of a modulated alternating current of audio frequency with auido frequency signal modulations, means to detect said modulations, means to impress only said detected modulations on the grid of a gridcontrolled electron tube to vary thereby the effective shunt capacitance of the oscillatory circuit of an oscillator generator to vary the frequency of said oscillator without substantially affecting the amplitude of the generated oscillations, means to transmit said variable frequencies over a high frequency carrier channel by amplitude modulation of a high frequency carrier by said variable frequencies, said grid-controlled tube and said oscillator being provided with adjustable means to correlate the amplitude of the said impressed detected modulations with the frequency variations of the oscillator so that the upper limit of the converted frequency range is less than the second harmonic of the lower limit of said range.

7. A signal transmitter comprising `means'to generate signals' in the form of a modulated alternating current, means to detect said modulaltions.v means to convert the amplitude of said modulations into. corresponding frequencies, means to determine the lower frequency linut of said converted frequencies including a mixing network, and means to impress on said network a part of the said modulated alternating current and a part of said converted frequencies for comparison.

8. The method of transmitting pictures or the like over a transmission channel which tends to introduce harmonic distortion which method comprises, generating picturesignals in the form of a frequency-modulated audio frequency carrier, limiting the frequency spectrum of the modulated carrier.l so that the uppermost frequency is less than the second harmonic of the lowermost frequency, transmitting said frequency modulated carrier over said channel, receiving and detecting said frequency modulations, filtering out all frequencies outside the range corresponding to the said spectrum as limited at the transmitter, and applying said spectrum to control a picture reproducer.

9. The method of transmitting pictures or the like over a transmission channel which tends to introduce harmonic `distortion which comprises, frequency-modulating an audio frequency carrier in accordance with picture signals to produce a frequency spectrum wherein the lowermost frequency represents one picture shade and the uppermost frequency represents substantially the opposite shade, receiving and frequency-demodulating said spectrum to convert it into corresponding amplitude modulated signals and limiting the spectrum both at the transmitter and at the reeciver so that prior to conversion into said amplitude modulations at the receiver the uppermost frequency is less than the second harmonic of the lowermost frequency.

10. The method of transmitting pictures or the like over an existing radio transmission system of the amplitude-modulated high frequency carrier wave type which method comprises, scanning the picture to produce signals in the form of an audio frequency carrier amplitude-modulated by audio frequency signals, amplifying the amplitude-modulated carrier and rectifying it to derive the said modulations, converting said modulations into a frequency-modulated carrier within the audio frequency range, limiting the frequency band of the frequency spectrum of the frequency-modulated carrier to one wherein the vuppermost frequency is less than the second harcarrier wave having amplitude modulations corresponding tc the picture shades, amplifying the frequency carrier, limiting the spectrum of the converted frequencies so that the uppermost frequency represents one degree of shade and the lowermost frequency represents substantially the opposite degree of shade and with the uppermost frequency lower. than the second harmonic of the lowermost frequency, amplitude modulating 'a radio carrier wave by said converted frequencies, receiving and detecting from said radio wave the said converted frequencies, filtering out from said converted frequencies all frequencies exceeding those within the said spectrum.- andapplying the passed frequencies to control a plcture reproducer.

12. A signal transmitter comprising means to generate signals in the form of a modulated alternating current of audio frequency with-audio frequency modulations, means' to detect said modulations, means to impress only said detected modulations on the grid of a grid-controlled elec tron tube to vary thereby theeffective shuntcapacitance of the oscillatory circuit of an oscillator generator to vary the frequency of said oscillator without substantially aecting the amplitude of the generated oscillations, means to transmit said variable frequency over a high frequency carrier channel by amplitude modulation of the high frequency carrier by said variable frequencies, and manual adjusting means for adjusting the lower limit of the frequency from said oscillator to a predetermined value -when no detected modulations are impressed on said tube and for adjusting the maximum level ofthe detected modulations so that the upper limit of the frequencies ,from said oscillator is less than the second harmonic of the lower limit of said fre- I quencies. a

13. In a facsimile system, afacsimile transmittervhaving means to convert the tone values of a picture or the like into a frequency-modulated audio frequency carrler wherein the frequency spectrum is limited so that the uppermost frequency corresponding to one extreme range of tone value is less than the second harmonic of the lowermost frequency corresponding to the opposite range of tone value, means to transmit said carrier over a transmission channel which vis subject to harmonic distortion, means to receive said carrier and to eliminate therefrom all l.parasitic frequencies which are a second or higher "harmonic of the lowermost frequency of said spectrum. and a facsimile reproducer controlled by CERTIFICATE oFlcoRREcTIoN,

'Patent No" 2"`2 99,957 October 27,v

ROBERT n. sPBAGu-E.-

`It'.lsfhe'aby certified than: error appears in the prntedfspeeiifiotion of the abovelnumbered patent requiring oyrreotion'as follows: Page",' first co1m,",11.nef9, clamZ, Vafter "a" second 'occt'arence5-l insert frequency mogulated'ucarier also -in the audio-F; and that the .said 'Lette'rs-.Pstent should'be read'A with this' correction therein that the samefm'ay'confong to theA v mme 'o'f ,the case 'chepate'nt office.

Signed'end seeled.' this `'163:11 'day Cif-November, A. D. 1.9145..

Henry Van Arsdale, (Seal) Acting Commissioner'- of Patents.

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