# Патент USA US2410275

код для вставки4 Oct. 29, 1946. s. D. EILENBERGER " ' 2,410,275 ‘ SIGNALING BY SUB-MODULATION Filed Dec. 13, 1941 AUDIO AMI? MICRO A_ 5 Sheets-Sheet 1 moouLATOR' 03c. BUFFER 8 ac RF t>__;i|;A'm F‘lGul , ,PoWER SUPPLY L.‘ ~ CbNVERTL-‘R I I’? /.a N’ 15AM? O dilf'fwi'l ‘FILTER D/Sc.: : V F’ Fi<5¥2 - tum; ' E c lLJ<—-T R5 on. AJEc. REPRO- ‘P- 31%‘ gm L.P. MIXER FILTER . A5 A M F.’ AME ___'__y F l Ga 3 E M (gal M00. a 0%. ‘ 7‘ N/ P ~ (3 ~ mam. C’ AJ-rAnP. 7 non. Fleas ‘ L_ wig-Lg . ' r o_sc.. FILTER ' R I . H‘ TO U i 2 ‘5 7 T |_| NE‘ 0R ' ‘ REE ' Q ' A HEAHP. ‘ AEC. ' > 7 ' _ 7‘ , o; l 3i ' ?/XER , ‘ ‘ I . H}? F ' mm’ ‘NE-‘r MIXER v - _>MODLJI_ATOR AME - gm“ ' INVENTOR ' ‘ 0a. 29, 1946. s; 0. EILENBERGER 2,410,275 _ SIGNALING BY SUB-MODULATION Filed Dec. 13, 1941 3 Sheets-Sheet 2 4 is f: ll 51- '5“. F] Ga IO INVENTQR .BY ATTORNEY 2,410,275 f’atented Oct. 29, 19:46 UNITED STATES PArENrorFici: SIGNALING BY SUBMODULATION Stanley D. Eilenberger, Kenosha, Wis., assignor, by mesne assignments, of sixty-?ve percent to S. E. Steen, Kenosha, Wis., seventeen and one-_ half percent to L. G. Voorhees, Macedonia, Ohio, and seventeen and one-half percent tov Howard W. Taft, Balboa Heights, 0. Z. Application December 13,1941, Serial No. 422,915 22 Claims. (01. 250—6) 1 2 This invention relates to a methodof signaling ~ where the main energy is con?ned within the carrier. It is particularly applicable to the trans mission of intelligence by wire and radio and wave in which the modulation factor is increased in ‘successive steps prior to demodulation. Sixth, to provide a method of reception of a radio-frequency carrier‘ wave in which the modulation factor is increased frequency-modulated when so applied, results in an economy of the frequency space required for such transmission. . ,?. at the receiver by reducing the mean carrier fre Brie?y the invention consists of a method of frequency modulation in which the carrier fre " quency to a value less than the frequency of the signal wave recovered by demodulation. Seventh; to provide a method of communication 10 by frequency modulation by which the modula nal frequency. The carrier so modulated may be- . tion factor may be controlled at the receiver. quency is varied through a frequency range nar rower than the bandwidth of the modulating sig the radio-frequency carrier bylwhich radio trans-_ 4. Eighth, ‘to provide a method of communication mission is effected, a low-frequency carrier of a which‘ sha'll’provide a high degree of freedom mean frequency less than the frequency of the from all undesired signals, such as noise, static, modulating signal, or an intermediate carrier 15 etc. from which the low-frequency carrier is produced. Ninth, to provide a ‘method of secret commu The low-frequency carrier may be transmitted H“ nication. - directly over wire lines or used to modulate a Tenth, to provide a method of controlling the higher carrier frequency for radio transmission depth ‘of modulation in a frequency-modulation as hereinafter described. In such a frequency 20 system so that the e?iciency of recovering the modulated wave the highest density of energy original signal’ at the receiver may be uniform will always be in the carrier wave. ly high. ' ' ' , Among the numerous objects of this invention Eleventh, to provide a method of demodulation are: in a frequency modulated system so that a small First, to provide a method of producing a modu lated signal wave where the main energy is con ?ned to the carrier and the amplitude of the side bands is inversely proportional to the signal fre quency, thus reducing the interference poten tialities of the sidebands so that the frequency channel required by this modulated signal wave duringtransmission by means of wire line or radio shall be much smaller than the original signal bandwidth, thus providing a means of communication in a very narrow channel. 'Second, to provide a method of frequency modulation of a radio-frequency carrier where the bandwidth occupied by the main energy of themodulated radio-frequency carrier is less than the bandwidth of the original signal wave. . radio-frequency Twelfth, to provide a method of frequency modulation in which a high order of frequency stability is obtained. . Thirteenth, to provide a method of combin ing two or more of the above advantages into a single system. - It is understood that a basic purpose of this invention is to provide a means of controlling the 35 energy concentration ratio in an electric wave asset forth in object one above, and that such control is always obtained in combination with one or more other features of this invention, For the purposes of this application, the term sub-modulation is understood to mean a condi Third, to provide a method of reception ofa frequency-modulated 25 deviation ratio at the transmitter is made to ap pear as a large deviation ratio at the receiver. carrier tion where the lowest frequency of the modulat ing signal wave is higher than the carrier wave; this low frequency carrier is referred to as a subcarrier, and the‘ process of modulating such a and. frequency range. ‘ 45 carrier is referred to as sub-modulation, Such 7 .Fourth, to provide a method of reception of a a sub-modulated subcarrieris always obtained frequency modulated radio-frequency , carrier in combination with one or more other features wave in which the modulation factor is increased of this invention. ' wave as described, that will permit full recovery of the original signal wave both as to amplitude prior to demodulation. . The following mathematical analysis of the Fifth, to provide a method of reception of a 50 frequency modulation ‘process and the accom frequency-modulated radio-frequency carrier panying explanation of the manner in which 2,410,275 4 various factors are treated in this invention will serve to show how this invention differs from Equation 3 represents a familiar expression of a modulated wave containing both the carrier and prior frequency modulation methods, where l\T:21.-n denotes the signal frequency. F=21rf, the frequency modulated carrier. Ar the amplitude of the unmodulated carrier F. the two side band frequencies at a distance cor C11 m the modulation index de?ned by the equation. responding to the signal frequency. In contrast to amplitude modulation, the side band compo nents have opposite signs and are 90° out of phase with the carrier. The amplitude of the side bands is in the ?rst approximation propor tional to the modulation index as de?ned above. 10 Hence, for any given width of the frequency swing between the two edge frequencies fa and ft, the amplitude of the side bands will become where fa and it are the edge frequencies between which the carrier F is shifted at the rate of the smaller as the signal frequency becomes higher. This is obvious because the modulation index 112 for a given frequency swing is inversely propor signal frequency. A frequency modulated wave is represented by the following equation: ' A=Af cos [FM-12L sin Ni] tional to the signal frequency. It can therefore be stated: in a frequency mod (1) ulating system where the modulation index is small the main energy will be con?ned to the It is well known that Equation 1 can be ex panded in a series to be represented by the 20 carrier and furthermore, the amplitude of the side bands will become lower with increasing Bessel function signal frequency. In contrast .to ordinary am 71:00 plitude modulation where the amplitude of the A=l:lo(m) sin Ft+22[;,,(m) cos 2nFt+ 12:1 side bands is only determined by the depth of 2211 (QM-1) (m) sin (2n,—l)Ft:|A; (1a) 25 modulation and is independent of the value of In: In this series the term with the coeflicient Io rep resents the carrier corresponding to the Bessel the signal frequency and where, therefore, the interference of adjacent channels is solely de pendent upon the highest frequency present in the sideband,v a frequency modulated system in function of the zero order. The side bands are 30 which the modulation index m is small presents a case wherein the amplitude of the interfering of the Bessel function of the ?rst and nth order. signal from the adjacent channels will continu It can be shown that if the argument m becomes ously decrease as the frequency limits of the side small then the series converges very rapidly and band are approached. In the commonly used all the terms having the coefficients of the higher 35 ‘systems of frequency modulation, the modula order function will disappear. In such a case tion index is always larger than unity, and therethe coefficient In of the ?rst term representing fore, the energy distribution as given by the represented by the terms having the coefficients the carrier will contain the greater part of the expansion of the frequency modulated wave into energy contained in the entire series. the Bessel function series shown in Equation For purposes of illustration Equation 1 can be 40 1a, shows that where m is larger than unity, a greater part of the energy will be contained in transformed into the ?rst and nth order terms representing the sidebands. In a system according to this inven tion, m is always smaller than unity, and there It may be assumed that m is small, for instance, fore, the greater part of the energy will remain smaller than 0.3. In this case in the carrier. A=Af[cos (Ft) cos (“gen Nt)——sin (Ft) sin (N01 (2) In the discussion above no statement was made 7-3 Sin N't will be smaller than m 2 itself because the sine function is under all cir cumstances smaller than unity. Hence, the value of the cosine function (3 m __ . concerning the relationship between the frequen cies F and N. Under ordinary circumstances the frequency F is largev in comparison to fre quency N. However, it can be easily shown that if the frequency F modulated with the frequency N should form part of a beat frequency, an ex pression similar to Equation 3, for the amplitude of the resulting beat frequency B can be obtained in which 0S ( 2 sin N t) . can be approximated very well with unity with out making any great error. For similar reasons, 60 if m is small, the function sin The resulting beat frequency signal thus has the same characteristics as the original frequency modulated signal and contains the same signal frequencies in the form of sidebands. By proper selections of the values F and F1, a condition can be arrived at in which E is small in comparison sin N t) can be written as in substituting the sine function by the angle to N. s itself. With this approximation Equation 2 will The invention will be best understood from a be transformed into 70 consideration of the following detailed descrip A=Af[cos Ft—% sin (Ft) sin (N01: . Afcos (Fz)+@2A, [cos (pimps-3251, [cos (F—N)t] (3) tion in view of the accompanying drawings form ing a part of the speci?cation; nevertheless, it is to be understood that the invention is not con ?ned to the disclosure, being susceptible of such changes and modi?cations as de?ne no material 2,410,275 - 5 Plate 6 is coupled to control grid ,8 through a departure from the salient features of the inven tion as expressed in the appended claims. variable compression condenser l8 having a‘ maximum capacity of approximately ‘100 paf. ' ‘ In the drawings: and resistor l9'which~ is approximately 150,000 ohms. 20 is a resistance of .5 megohm and 4 is‘ also a resistance of .5-megohm. I2 is a'cathode radio frequency wave in accordance with this resistorof 300 ohms and I3 is a cathode ‘by-pass‘ invention. ' condenser of .01 ,uf. I4 is a series screen r'e Figure 2 shows in block form a receiver for use sistor of 30,000 ohms which is by-passed by mica‘ with the transmitter of Fig. 1. Figure 3 shows in block form a radio transmit 150K condenser l6 of .01 pf. and also electrolytic con denser !5 having a capacity of 8 cf. 2| is a ter for the amplitude modulation of the radio variable compression mica condenser having a wave by the new modulating wave produced in accordance with this invention. maximum capacity of 30 lief” while 3 is the com ‘Figure 1 shows in block form a radio transmit ter arranged to produce a frequency-modulated ' Figure 4 shows in block form a receiver for use mon ground connection and ‘AZ-23 are connec with the transmitter of Fig. 3. Figure.5 shows in block form a modi?cation of apart of the transmitter of Fig. 3. tions to the oscillator tuned circuit. The audio frequency signal applied to the modulator grid 7 ‘ Figure 6 illustrates in detail an arrangement of the reactance ‘modulator tube 9 will cause a‘ phase shift in the resistance capacity network which may be used for modulator unit C in the made 7 up by condensers l8—2l and resistors transmitters of ‘Figs. 1, 3, 5 and unit C1, in the 203 l9—-20, which will be proportional to the ampli tude of the signal applied, and the number of receiver of Fig. 2. ' shifts per second will be proportional to the fre Figure 7 illustrates the frequency versus volt quency of the signal applied. This exerts a tun-v age characteristic of discriminator unit R in the ing effect on controlled oscillator D. ' , ’ receivers of Figs. 2 and 4. Figure 8 illustrates a detailed arrangement of‘ 25 Modulator unit C is arranged to shift the fre-~ quency of oscillator D from its mean frequency discriminator unit R in the receivers in Figs. 2 and 4. of, for example, 1,000,000 C. P. S. through a range Figure 9 illustrates in detail an arrangement of 20 C. P. S. plus and 20 C. P. S. minus, a total carrier shift of 40 C. P. S. , equal to a maximum for use in ?lter units G of Figs. 2, 4 and 5. Figure 10 illustrates the frequency response 30 carrier frequency of 1,000,020 ‘and a minimum carrier frequency of 999,980 C. P. S. The maxi curve of ?lter unit G and discriminator unit R. mum shift will then correspond to the maximum Figure 11 is a curve showing the distribution amplitude of the original signal, and the total of the energy in the sidebands for two cases, with number of shifts per second will be equal to the a total frequency deviation in one direction of 100 cycles and 1000 cycles as a function of the signal 35 frequency of the original signal, the entire opera; frequency. tion being identical with that commonly em ployed in conventional frequency modulation, ex cept that the total carrier shift is extremely small, Figure 12 is a similar energy distribution curve showing the energy content in the ‘sidebands if as compared to the usual shift of '75 kc. plus and the deviation from the carrier is 20 cycles in one direction. 40 '75 kc. minus, or a total shift of 150 kc. The nar _ Figure 13 illustrates the percentage change of row deviation frequency-modulated radio-fre quency wave is then ampli?ed by a radio-fre quency ampli?er I and connected to the antenna . the instantaneous values of the unmodulated carrier due to modulation. _ Figure 14 shows an exaggerated curve repre senting one quarter cycle of a frequency modu lated wave modulated with a, signal frequency ten times the carrier frequency. ' Figure 15 shows the attenuation of the inter fering signals originating in channels adjacent to the carrier as a function of carrier separation. . Referring more more particularly to Fig. 1, A is a source of original signal, which may be a micro phone having an original audio frequency input covering the range of 250 to 3000 C. P. S., while B is an audio frequency ampli?er of conventional type, which may be any ampli?er such as is in _ K. L is a conventional power supply for the transmitter. - ’ Referring to the receiver block schematic in Fig. 2, K represents the antenna‘, vN the R. F. ampli?er, O the convertor, all of which are con , ventional; P represents the I. F. ampli?er which, for example, is tuned to a resonant frequency of 465,000 C. P. S. N and P are tuned to accept the carrier and both sidebands as in conventional practice. E represents a ?xed R. F. oscillator, tuned to a ?xed frequency of 464,900 C. P. S., 55 which is designed to have a relatively high am plitude, in comparison to the output of I. F. am pli?er P, which it is beating with. With static common use; C is a modulator unit which might conditions'at the transmitter of Fig. 1, the mean be of the magnetically controlled condenser type, beat frequency developed in the mixer-ampli?er or of the so-called reactance modulator type illustrated by Fig. 6. The object of modulator 60 unit F is 100 C. P. S., which is the beat between unit C is to provide a capacitive or inductive var iation as a padder tuning element of oscillator D. The reactance vmodulator circuit illustrated by Fig. 6 accomplishes modulation electronically and is the preferred form. The circuit shown by 65 Fig. 6 is approximately the same as used in cer tain frequency modulation’ systems of COIlVBll-r. the ?xed oscillator unit E and the I. F. output of unit P. As the transmitter of Fig. 1 is modulated; causing a frequency shift of 40 cycles maximum (with a signal at unit A of maximum amplitude), the I. F. at the output of unit P shifts by a like amount,‘ thus causing a similar shift in the beat note frequency developed in unit F, from a mini tional type, where the'ampli?ed audio frequency mum value of 80 to a maximum value of 120 signal from the output of ampli?er unit B is sup plied to the input connections I and 2 and is applied to modulator grid 1 of vacuum tube 9 which‘ might be‘ commercial type 6L7. B is the C. P. s., the total shift being dependent on the amplitude of the original signal, and the number of shifts per second being dependent on'the fre-v quency of the original signal, as has been previ ously shown for the carrier and I. F. waves. This shifting frequency, 80 to 120 C. P. S., is now acting as an audio-frequency carrier. Thisfrequency~ plate terminal of vacuum tube 9 which is fed through radio frequency choke 5 from a source of voltage II, which is approximately 250 volts. 7 $2,410,275 8 modulated audio-frequency carrier is then passed from the unmodulated wave in percent of the through ?lter G to attenuate impressed frequen cies having a frequency greater than 120 C. P. S., such as harmonics and the other modulation prod ucts of the mixer-ampli?er F. This audio fre quency carrier wave is hereinafter referred to as instantaneous value of the unmodulated-wave can of the unmodulatedv wave as a basis of compari son. Figure 13 representing the percentage the subcarrier wave. change of a 100 cycle carrier modulated with a be calculated simply by forming the difference be tween the two and taking the instantaneous value signal frequency of 1000 cycles and with a modu Fig. 9 illustrates in detail a parallel resonant lation index of 0.01 was calculated in such a way. ?lter which I have used in practice as unit G of Fig. 2. Referring more particularly to Fig. 9, 10 It might be seen that in the ?rst period of 0.00025 second the maximum deviation is only 59 and 50, represent the input terminals, from minus 0.08 percent against the unmodulated ampli?er-mixer unit F; B! is a transformer, wave. This deviation reaches its maximum value whichmight well have 2. turns ratio 1:1 and a of plus and minus 3.3 percent at; a distance equal secondary, inductance 63, which might have a value of approximately 4 henrys; condenser 65 15 ing a quarter cycle of the signal frequency be fore and after the quarter cycle of the carrier would then have an approximate value of .25 ,lf., frequency. so that the resonant circuit represented by in— ductance B3 and capacity 55 would be approxi If the resonant circuit G shall follow the mod ulation according to Equation 5, the Q factor mately in resonance at 100 C. P. S. Primary in ductance 62 could have a like value of inductance, 20 of the circuit as de?ned by the Equation 6 must be chosen accordingly. and might also be tuned to resonance with capac ity 64, if so desired, although it is not essential that the primary inductance be in resonance; vacuum tube 12 is used as a coupling element. 68 represents the common ground connection, 25 In (6), Rp denotes the equivalent parallel loss re sistance of the circuit in ohms, C the capacity in while 66 and 6'.’ represent the output terminal farads and L the inductivity in henrys. If at any connection to frequency discriminator R, or to moment the source of energy supply in circuit G succeeding stages of filter G. As previously men should be disconnected, the voltage E across the tioned such a, ?lter as illustrated by Fig. 9 will attenuate all impressed frequencies having a fre 30 circuit would not fall to zero immediately, but due to the stored energy in the circuit would drop quency greater than the pass band character according to the expression istic of the tuned resonant circuit to a far greater degree than it will attenuate the high signal fre ;_—B£ quency modulation component of the 100 cycle E: Eoe 2Q Sin (Bt+¢) (7) carrier Wave, which it is, desired to pass on to fre 35 in which e denotes the base of the natural loga quency discriminator unit R. rithms. The following will clarify the action of a par allel resonant circuit as used in ?lter G and illus trated by Fig. 9, in response to an impressed fre quency modulated wave in which the modulating signal frequency is of a, higher order than the carrier frequency. Filter unit G is resonant at If the circuit is connected to a source of energy with the terminal voltage E0 sin Bt, maximum amplitude will not be reached imme diately, but again according to the function Obviously, if any changes due to modulation the carrier frequency, which, in the example should take place in the instantaneous value of given, is 100 cycles. In a frequency modulated 45 the carrier wave, then the Q factor and the con wave represented by the equation stants of the circuit must be such that if, for in stance, the circuit was instantaneously discon nected from its source of voltage, the voltage would change by a higher percentage than that the value of the unmodulated function Ab cos ‘.Bt will coincide with the value of the modulated 50 indicated by the modulation index m. For in stance, for the values shown in Fig. 13, this would wave for any time t at which Nt=21rnt is an mean that at the point of largest difference be integer multiple of 1r. Between those points of tween modulated and unmodulated wave form coincidence the modulated wave will deviate from the resonant voltage of the circuit must decay by the values of the unmodulated wave determined more than 3.3% of its actual value within a time by the magnitude of the modulation index m. A=Ab<cos Bt-l-WZL sin Ni) (5) If B is small in comparison to N, and m is small, then the frequency modulated wave correspond ing to Equation 5 will attain a form as shown in Fig. 14. In this ?gure a quarter cycle of the car rier frequency B is shown modulated by a fre quency N equalling ten times B, where 69 is the unmodulated carrier wave 13 and ‘I0 is the modu lated wave form, ‘H representing ?xed points where the modulated and unmodulated waves coincide; the modulation index m is then 0.01. At any point where modulation occurs the slope of the wave will change to a slope corresponding to a higher or lower frequency by an amount de termined ‘by the depth of modulation. The maxi mum deviation can be expected at the point where the slope of the unmodulated wave is a maxi mum. For a cosine function this will be at a, quarter cycle of the, wave B. If the function in Equation 5 is computed for a given set of values 13, N and m, the deviation of the modulated Wave of 0.00025 second. In a generalized form the con dition —1rB/ 60 cos Bt cos Bt-cos (3H1; sin Ni) [ Q N] —_P1r P~T< 1*” Q (9) 65 must be ful?lled if the signal frequency n shall be passed. In the above expression p denotes the ratio between signal and carrier frequency. For instance, with a carrier frequency of 100 cycles resonant with the circuit G and a modulation in 70 deX of 0.01, the maximum signal frequency which could be passed is 1000 cycles provided the Q of the circuit is not higher than 8.7. If the Q is re duced to, for instance, 2.9, the maximum fre quency which the circuit will transmit as a fre 75 quency modulated signal willbe increased to 3000 2,410,275 10 cycles. This system, is, for inherent reasons and for its practical purposes, limited to low values of modulation index. For instance, if the modu carrier frequency deviates from its mean fre quency, and that the audio frequencies recovered depend upon the rate at which the carrier fre lation index should be increased to 0.1, the max quency deviates from the mean frequency. imum signal frequency which could be passed In agreement with the mathematical discus sion above, Fig. 10 shows average physical char with a carrier and a resonant circuit G of 100 acteristics of units G and R. In this figure the cycles and a Q of 8.7 would be limited to 110 cycles. line A represents the attenuation of the resonant The output of ?lter G is connected to the input filter G as a function of frequency, taking the 24 and 25 of the discriminator unit R, which- is a conventional demodulator circuit commonly 10 carrier resonant frequency as zero level and ap plies to frequencies not associated with the de called a frequency discriminator, tuned to a mean sired frequency-modulated subcarrier. 'Curve A frequency of 100 C. P. S. The values of the circuit was arrived at by impressing a constant ampli elements required for the proper functioning of tude variable frequency across input terminals 59 the demodulator in the desired frequency range and 60, and measuring ?lter response across out are disclosed in connection with the detailed de put terminals 66 and 67. Referring to the same scription of Fig. 8, where two iron core trans zero level, curve vB indicates the frequency re formers 26 and 21, with the primaries 28 and 29 sponse of the ?lter G to sidebands associated with connected in series by connecting link 30 have the frequency-modulated subcarrier and of the their secondaries 3! and 32 tuned to the frequen cies 80 and 120 cycles by means of condensers 33 20 recovered audio signal across the output termi nals 5i and 58 of the discriminator unit R. having an approximate value of l ,uf., and 34, Figure 10 shows the practical results obtained having an approximate value of .15 ,uf., provided with this invention in recovering the original secondaries 3! and 32 have an approximate like modulating signal, the frequency of which is value of inductance of 4.5 h. The common junc tion 35 of the two tuned secondaries is linked to a 25 higher than that of the carrier frequency. Figs. 11 and 12 indicate to what extent the ground connection 36. The potential ends of the energy can be concentrated in the carrier, if the tuned secondaries are connected to grids 39 and modulation index is chosen properly. Curve A 40 of two triode ampli?er tubes 44 and 45 with shows the total energy content represented in all a common cathode 4! connected to ground sidebands as a function of the signal frequency through the common cathode bias resistor 31. An for a case in which the maximum frequency outlet 38 is provided to furnish control voltage deviation from the main carrier frequency is for the automatic frequency control unit C1. selected as 100 cycles. The total energy content The plates 42 and 43 of the tubes 44 and 45 are of all sidebands is less than 10 percent, in com connected through condensers 5| and 52 to the parison to the large percentage of the energy primary 53 of an audio output transformer 54, present in the sidebands if the frequency devia the secondary 55 of which is feeding potenti tion is increased to 1000 cycles, as indicated in ometer 56. The balance of the discriminator curve B of Fig. 11. A similar curve showing the circuit is obtained by adjusting the slider 49 of energy content of all sidebands for the practical the potentiometer 4B, which might have a resist ance of 25,000 ohms, in series with the two equal 40 case where the frequency deviation is 20 cycles is shown in Fig. 12, which indicates that the resistances 46 and 4'! of approximately 100,000 maximum energy content of all sidebands is al ohms. Plate voltage is supplied to the tubes 44 ways less than 2'% for modulating signals above and 45 through the connection 50. Under condi 250 cycles. tions of balance, the audio frequency carrier volt In the example given above, the signal is moved age becomes 0, between plates 42 and 43, only the 45 through a value 20% higher than resonance and recovered signal voltage appearing at these points. 20% lower than resonance. This is many times The variable output taken off from the poten greater than is¢possib1e in a radio frequency cir ' tiometer 56 across the terminals 5'! and 58 is fed cuit. For example, if the I. F. was 4.3 megacycles either to a correcting network NET or across the high pass ?lter S to the audio frequency ampli- - ?er T. Typical working characteristics of such a circuit are shown in Fig. 7, which represents an idealized input-frequency output voltage charac teristic such as possessed by a discriminator. of the type shown in Fig. 8. Input frequencies are represented by the abscissae, and output voltages are represented by ordinates on an arbitrary scale of +1 to -1. With an input carrier frequency of 100 C. P. S., which corresponds to a point of zero amplitude of the modulating signal wave, the characteristic shows an output voltage of and, the total carrier shift in a, frequency modu lated system was 100 kc., the discriminator cir cuit would be forced to operate on a relatively small departure from resonance. If K1‘ denotes the total frequency shift for maximum amplitude of the signal, then K is de?ned as the modulation factor, indicating the percentage change of the mean carrier frequency. In the present case, the extremes above and below resonance are a high percentage of the mean frequency, and, there fore, it is-possible to develop good efficiency in recovering the audio frequency energy repre sented by the frequency swing between 80 and zero. As the signal Wave approaches maximum 120 cycles. In order to maintain the frequency . amplitude in the negative direction, the carrier of the subcarrier frequency always exactly at the frequency increases toward 120 C. P. S., and the output voltage approaches a value of -1. As the 65 same value, a conventional automatic frequency modulating signal wave varies from negative control circuit C1 might be provided which‘ is maximum to zero, completing the half cycle, the controlled by the discriminator unit R. Unit C1 is essentially the same as unit C in Fig. 1. frequency decreases from 120 C. P. S. to 100 Any deviation from the correct Value of the sub C. P. S. and the output voltage varies from —1 to 0. On the next half cycle of the modulating sig 70 carrier frequency will disturb the balance of the discriminator unit R, thus furnishing a control nal wave the frequency decreases to 80 C‘. P. S. voltage to unit C1, of such polarity and magni and returns to 100 C. P. S., yielding a like half ' tude that the correct value of the subcarrier fre cycle (0, +1, 0) of recovered voltage at the re quency is restored. This might be important for ceiver. It is thus obvious that the recovered audio the reason that the automatic frequency control voltage is dependent upon the amount that the 11 12 unit is compensating forall fluctuations in fre quency due to instability of the oscillators either on thetransmitting or on the receiving end, thus maintaining a constant subcarrier frequency. 1900 cycles, which would also be rejected "by ?lter G. 'It should be noted that ?lter G is shown as a single stage in Fig. 9, but it is obvious that any number of such stages could be used in Automatic volume control, ‘in its conventional - cascade, preferably separated by coupling tubes. form, would also be desirable, in order to main tain'a relatively constant amplitude at the out put of I. F. ampli?er P. No circuits are shown forautomatic volume control, as this would be In the example given, the carriers are separated by 1000 C. P. S. ; the receiver of- Fig. 2 must vhave suflicient selectivity in ?lter circuits G and dis criminator R to accept the desired carrier and arranged in accordance with such circuits as com 10 reject the undesired adjacent carriers; it has monly used. The output of R is passed through previously been shown that the sidebands of these :unit S, which is a conventional high-pass ?lter undesired carriers are of such low amplitude that with a cut-off frequency of 120 cycles. This re they cannot interfere with the desired carrier, moves any remaining frequencies below 120 cycles, even though the carriers were much closer than which, up to this, point have acted as an audio 1000 C. P. S. given in this example; the necessary frequency carrier for the ‘original signal, also carrier separation is strictly a function of the any additional undesired low frequencies mak selectivity of circuits G and R; this selectivity ing the overall system substantially noise free, could be increased by inserting a standard band by a combination'of ?lters. The output of unit rejection ?lter between the mixer unit F and ?lter S is coupled into a conventional audio frequency unit G, at the points marked ER in Fig. 2; such ampli?er T, and reproduced on a conventional a band rejection v?lter would be conventional in reproducer U. These units may be any type de design, and would be tuned to reject the un sired and are'not illustrated in detail. The sys desired carrier; such a ?lter would need have a tem, as described above, with the example given, rejection band only 40 cycles wide, as the un would not reproduce original audio frequencies 25 desired carrier must always have its main energy below 120 cycles, and might discriminate against within the narrow band through which it is modu frequencies just above this pass band, but would lated; if it is desired to reject more than one reproduce all audio frequencies lying between 250 interfering carrier, several band rejection ?lters might be connected in cascade, each tuned to a and ‘3,000 without discrimination, in well de 30 different frequency, as is conventional practice. signed circuits. Referring now to Fig. '3, units A. B. C and D It is obvious that the receiver of Fig. 2 may be are identical with the corresponding units of Fig. turned to resonance with the desired carrier in 1 and perform the same functions, producing at the ordinary manner, .by'tuning the radio fre the input to mixer-ampli?er F a signal frequency quency circuits of R. F. ampli?er-N and the oscil lator circuits of converter 0; this is true even 35 frequency-modulated wave of 40 C. P. S. frequency swing. This is mixed with a constant frequency though the next adjacent carriers were relatively wave from the oscillator E to produce a fre close to the desired carrier. For example, if quency modulated low-frequency wave having a the transmitter of Fig. 1 is transmitting at the mean frequency of 100 cycles, which is passed frequency of 1000 kc. and this signal is frequency modulated plus and minus 20 cycles, the R. F. 40 through low pass ?lter J, the purpose of which is to pass all frequencies below the highest signal and convertor circuits in the receiver of Fig. 2 must also be tuned to 1000 kc. in order to pro frequency, for instance-3000 C. P‘.'S., and to ‘re .move all superfluous combination frequencies duce the 100 cycle subcarrier beat note at the in and harmonics generated in the mixer circuit F. put to ?lter G; if the next adjacent carrier was separated by 1 k0,, this carrier would also be 45 The frequency-modulated low-frequency wave is then ampli?ed by audio-frequency ampli?er H ampli?ed by the R. F. ampli?er N and L. F. and caused to modulate the amplitude of a radio ampli?er P and ‘would therefore appear in the frequency wave supplied to the modulator M by mixer unit F and also beat with ?xed oscillator the constant frequency oscillator E2. The am E, which has a frequency of 464,900 C. P. 8.; this plitude-modulated wave is ampli?ed ‘by ‘a'high ‘is so because'the pass band of N and P must be frequency ampli?er I and fed to‘the antenna K. wide enough to pass all side-bands oi‘ the desired carrier; the undesired carriers, being separated from the desired carrier by 1000 C. P. S. would produce a subcarrier beat frequency, at the in put to‘?lter G, of 900 or 1100 C‘. P. 5., depending upon whether-the undesired carrier was above or As before, L is a power'supply for the radio transmitter. The ?nal signal may be On any carrier frequency desired and unit E2 may be a crystal controlled oscillator or any other type desired for a particular ‘class of service. The wave radiated from antenna K normally will have "below the desired carrier. The two carriers, sepa a band-width equal'tortwice the highest frequency rated by 1000 cycles will not beat with themselves of the frequency-modulated low-frequency car to produce a spurious beat frequency ,of 1000 cycles, provided the input characteristics of mixer 60 rier passed by the ?lter J. However. this can be reduced by approximately one-half by the use unit F are linear; the design of mixer circuits of ?lters between modulator M and the ampli?er having such characteristics is well known in the I to effect single-sideband transmission in the art. Thedesired carrier, having a subcarrier beat conventional manner. note of 1000. P. S., is accepted and passed by Referring to the receiver block diagram, as il '?lter'G and the signal recovered by discrimina lustrated in Fig. 4, as the receiving unit for the ‘tor R, as ‘both of these circuits are resonant at radio transmiter of'Fig. 3, K represents the an ‘100 C. P. ‘3.; the undesired carriers, having sub tenna; N a radio frequency ampli?er; ‘P an inter carrier beat notes of 900 or 1100 cycles would be mediate frequency ampli?er; O the convertor; Q rejected by both ?lter G and discriminator unit 70 the demodulator or detector circuit. All of these R, and therefore, no interference would be pro circuits are conventional and maybe of any type duced with the desired carrier. It is obvious desired, such as those in common use. No ‘de .that .the nextadjacent carriers, separated from :the desiredcarrierby 2..kc. above or below, would ‘alsoxproduce subcarrienbeat .notes of 2100 and tailed schematics are shown for these units. .As before, however, the pass band for the radio ,frequency and inter-mediate frequency ampli 2,410,275 v13 ?ers should accept the carrier and ?rst order sidebands. The output of I. F. ampli?er P is connected to the input of ampli?er-mixer unit F, where it is mixed with the constant frequency wave from oscillator E which is adjusted to the same fre quency as the output of I. F. ampli?er. P, which, 14 both Figs. 2 and 4. In well designed circuits this equalizing network should not be necessary but should such a network be desired for any reason. the design would follow conventional practice. The ?delity of either system would be the maxi-1 mum possible with the circuits used. If the system is to be operated as a wire line system, as in telephone service, the audio input in the example given, is 465 kc., so that the re from the originating telephone is fed into the sultant beat note is zero; inasmuch as the output of I. F. ampli?er P was amplitude modulated by 10 audio-frequency ampli?er B, of Fig. 3. The out put of audio-frequency ampli?er H, which is the low-frequency frequency-modulated carrier wave of 80 to 120 C. P. S., is fed into the wire line. At signal, the effect of beating the output of I. F. am the receiving point the line can be connected di pli?er P with its own frequency is essentially the same as demodulation, and only the 100 cycle 15 rectly to the input of a demodulator R, or if it is desired to eliminate noise or signals lying in other subcarrier frequency with its associated frequency frequency ranges on the line, the line willbe con modulated sideband will remain, the carrier hav nected to the input of ?lter G. Following the ing been eliminated by heating with its own fre the original 100 cycle subcarrier, which in turn had been frequency modulated by the original quency. Unit C1 is a reactance tuning unit ar ranged for automatic frequency control in a man ner similar to the arrangement shown for Fig. 2, so that oscillator E may be held at a constant frequency, thus resulting in a continuous zero beat, even though the frequeency at the output of unit P might vary. In the example of Fig. 2 reactance tuning unit C1 received its control volt age from discriminator unit R; in the present instance the control voltage is derived directly from mixer unit F; it is wel1 known that in any mixer stage where two frequencies are present, the DC. plate current will rise sharply as zero beat is approached, reaching a maximum at zero beat; by utilizing the voltage drop across a series plate resistor in the mixer tube of unit F. a con trol voltage is developed which will cause reac tance' tuning unit C1 to maintain the frequency of oscillator E so that Zero beat always results. This method of obtaining automatic frequency con trol is well known in the art. The output of am ?lter will be a demodulator R and such conven tional audio-frequency networks as are required to convey the recovered audio-frequency wave to the receiving telephone. As applied to carrier current telephony, the fre quency-modulated low-frequency carrier of 80 to 120 C. P. S. can be used in lieu of the original audio-frequencies to modulate a plurality of car rier frequencies each separated from the next by an amount su?icient to permit selection of the individual 40 C. P. S. carrier channels by proper ?lters at the receiving point. This process and the equipment necessary for carrying it outform the subject of a companion application and will not be described in detail herein. The methods described of modulating an audio frequency subcarrier wave, in themselves provide a new means of secret communication, as it is self evident that the frequency-modulated 100 cycle audio-frequency carrier wave or the radio wave amplitude-modulated thereby would not provide pli?er-mixer unit F is connected to the input 40 an intelligent signal on any existing equipment, and that the subcarrier wave in itself would pro of ?lter G, and from this point on the operation Vide an ef?cient masking signal, effectively con of the circuit is the same as shown for Fig. 2; as cealing the intelligence contained in the frequency previously shown, additional frequencies equal to modulation of this subcarrier wave; only a spe the carrier separation will be present: these will be attenuated in the manner previously described 45 cial receiver of the type illustrated in block sche matic form by Fig. 4 would recover the. original in detail; as in the case of Fig. 2 a band rejection audio-frequency signal. It is also obvious that ?lter tuned to the interfering carrier frequencies existing equipment would not recover the intelli might be inserted at the points marked BR, if gence transmitted by the system of Fig. 1, due this was desirable or necessary. to the small modulation index; only a receiver The tuning method just described is for use of the type shown in block schematic form by Fig. when two or more adjacent channels are oper v2 would recover the original signal. It should be ated close to each other, all channels using a sys further noted that any existing system of privacy tem similar to that illustrated by the transmitter in communication, such as the many forms of of Fig. 3 and the receiver of Fig. 4. However, such frequency inversion, might be combined with the a system might be operated as a single channel methods herein described to provide additional between two other carriers of a different type, new methods of secret communication. such as conventional amplitude modulated car A simpli?cation of the method is illustrated riers, so that this type of tuning, that is. the zero by the block diagram of Fig. 5. As in Figs. 1 beat method achieved by the use of oscillator E, was unnecessary. In this case the operation can and 3, A is a microphone, B is an audio-frequency be simpli?ed by entirely eliminating units E. F 60 ampli?er, and C is a modulator. Controlled oscillator D has a mean frequency of 100 cycles and C1, and connecting the output of I. F. ampli which is varied between the limits of 80 and ‘120 ?er P directly to a conventional demodulator unit C. P. S. to produce directly the low-frequency Q, as illustrated by the dotted lines, so that the frequency-modulated wave that. is produced by original subcarrier frequency of 100 cycles is re— covered by recti?cation; the output of demodu 65 the beat method in Fig. 3. Since a greater tuning effect is required to vary the frequency of a 100 lator Q would then be connected directly to the C. P. S. oscillator between the limits of 80 and 120 input of ?lter G, and from this point on the op C. P. S. than to vary the frequency of a high eration of the receiver is as shown above. As frequency oscillator through a range of 40 C. P. S., shown for the receiver of Fig. 2, automatic volume control might be added in the conventional man 70 a magnetic condenser might be more effective than the modulator‘ of Fig. 6. Alternatively, her, which would be desirable for reasons pre viously shown. If desired, or necessary, an equalizing network, modulator unit C‘ might consist of a- standard “varistor” as commonly used in telephone com munication, so designed as to provide a resistive designed to correct the signal frequency response might be inserted at the points market NET, in 75 variation to the frequency determining resistors 2,410,275 15 of a resistance tuned oscillator. The so called “varistor” generally consists of a plurality of copper-oxide recti?er discs arranged in the form of a Wheatstone bridge and so poled that the passage of a pulsating current through two of the opposite junctures will result in a variation in the resistance presented to a circuit connected to the two conjugate junctures. The end re sults secured by this or other alternate methods 16 low carrier level, taken at the input to low-fre quency discriminator unit R and shows the maxi mum possible amplitude which the sidebands may attain, in terms of interference with an act jacent channel, Where the two channels are sep arated by a mean frequency of 250 cycles and both carriers have the same amplitude. ‘ Figure 15 was prepared from data contained in the energy distribution curve of Fig. 12 and the ?lter of tuning local oscillator D would be the same 10 unit G attenuation curve of Fig. 10. In actual as those already described for modulator unit physical practice, the carrier separation would C and illustrated herein by Fig. 6. The output of oscillator D passes through a ?lter G and ampli ?er H to the wire line or radio modulator as above explained in connection with Fig. 3. It should be noted that no accepted de?nition exists for the depth of modulation in a frequency modulated system. It has been variously de?ned as (a) the modulation index is 1 when the total shift in carrier frequency is equal to the fre quency of the modulating signal, and (b) an ar— bitrary factor where the total shift in carrier fre quency is considered 100% modulation, regard less of the actual value of this carrier shift. It will appear logical that a more reasonable meas ure of modulation factor in a frequency modu lated wave would be the relation of the total fre quency deviation to the mean carrier frequency; this is so because in any resonant circuit of given decrement a very de?nite frequency deviation 30 from the resonant frequency would be necessary in order to create the maximum amplitude change possible, which in a frequency discriminator cir probably exceed 250 cycles and would be lim ited only by the ability of the receiver to select the desired carrier frequency. For any carrier separation of 250 cycles or greater, interference from the sideband frequencies would be neg ligible. Two alternate methods of producing a radio Wave with the energy concentrated in the ‘carrier wave have been described in detail, the method of Figs. 1 and 2 being preferred for radio applica tion. The method of Figs. 3 and 4, however, While being more complex for radio purposes, illus trates better the basic method which results in a frequency-modulated wave of lower mean fre quency than the lowest modulating signal fre quency, with concentration of energy in the car rier, which permits an economy of frequency space for both wire and radio transmission, This is so because the sideband frequencies do not have the power to interfere with closely adjacent carrier frequencies, even though all sideband fre quencies are present. Therefore, an economy of cuit such as unit R of Figs. 2 and 4, would cor‘ respond to- maximum efficiency. For the pur 35 frequency space requiredv is realized by placing adjacent carrier frequencies closer together than poses of this description the latter de?nition is the bandwidth occupied by their sideband fre used as a measure of modulation factor of a quencies, and allowing such sideband frequencies frequency modulated wave, which I have previ to overlap these closely adjacent carrier fre ously denoted as K, wherein K represents the quencies. Both methods are inherently noise free percentage change of the carrier frequency. 40 to a high degree, due to the action of ?lter unit From the de?nition of modulation factor given G in attenuating all amplitude modulation not immediately above, it is self-evident that there present in the original frequency modulation, is no relation between percentage frequency de while at the same time the original frequency viation produced by the frequency modulation in modulation is passed with only small insertion the transmitters of Figs. 1 and 3, and the carrier loss, thus rejecting, to a high degree, such am frequency, nor is there any relation between this plitude modulation as may have been produced percentage frequency deviation and the ?nal L. F. by natural or man made static, etc. Inherently, frequency in the receiver of Figs. 2 and 4. The a high order of frequency stability is obtained, carrier and L. F. frequencies might be anything due to the very small frequency deviation caused desired, without affecting the ?nal modulation by modulation; it is well known in the art that factor or the overall e?iciency of this system. ' carrier frequency stability decreases as the car The only basically important percentage relation rier frequency deviation increases. A high order ship between frequency deviation and the mean of ei?ciency in signal recovery is obtained, by resonant frequency exists at the input to fre the use of a very low subcarrier frequency, so that quency discriminator unit R of Figs. 2 and 4. the small frequency deviation caused by modula In the system of Figs. 1 and 2 this relation tion is a high percentage of the frequency of this ship is under control in the receiver of Fig. 2. low frequency subcarrier; therefore, a high This is so because the resonant frequency, that is, modulation factor is always obtained in the sub the mean beat frequency produced between the carrier wave prior to demodulation. output of I. F. ampli?er unit P and ?xed oscil lator E may be made any value desired. In the 60 example given the mean resonant beat frequency is 100 cycles ‘and the total frequency deviation produced by frequency-modulation at the trans mitter is from 80 cycles to 120 cycles. ’ The foregoing description clearly indicates that in the system of Figs. 1 and 2 the depth of modu lation or modulation factor is always under con trol in the design of the receiver of Fig. 2, andv that this modulation factor may always be made any desired value, so that in well designed cir- ' suits, the frequency ratio between maximum fre quency deviation and the mean resonant beat frequency is such that maximum amplitude is developed in the discriminator unit R. ~ ' Figure 15 represents the D. B. attenuation be— 75 The above examples are for the purpose of illus trating some of the methods and means by which the broad purposes of the invention may be car ried out and are not to be deemed as restrictive in any manner. Other modi?cations and alterna tives will occur to those skilled in the art without departing from the scope of the invention as de ?ned by the following claims. I claim: 1. The method of signaling, which comprises the steps of frequency modulating at a modula tion index smaller than unity, a high frequency carrier with a signal containing the intelligence to be transmitted, transmitting said frequency modulated high frequency carrier to a receiving point, beating the received frequency modulated 2,410,275 1:17 ,18 carrier to produce a frequency modulated audio frequency‘ subcarrier, the mean frequency of which shall be lower than the lowest signal fre 5. The method of receiving an amplitude modu lated wave modulated by a frequency modulated subcarrier wave as de?ned in claim 5, which com quency, so selecting a band offrequencies in-_ cluding the subcarrier that “the subcarrier and its associated ‘sidebands are substantially un prises the steps of receiving the amplitude modu lated wave, demodulating the desired amplitude modulated wave to recoverithe frequency modu attenuated while frequencies lying within the fre quency band occupied by the sidebands and not. lated subcarrier wave, so selecting a band of fre quencies including the subcarrier that the sub associated with the subcarrier are attenuated carrier and its associated sidebands are substan - below a desired interference level, and demodulat-v 10 tially unattenuated while frequencies lying with ing the frequency modulated audio frequency subcarrier to recover the original signal. ing the frequency band occupied by the sidebands and not associated with the subcarrierare at ' 2. The method of signaling, which comprises tenuated "below a desired interference level,‘ and the steps of frequency modulating 'at a modula demodulating the recovered frequency modulated tion index smaller than unity, a high frequency 15 subcarrier‘wave to recover the original signal. carrier with a signal containing the intelligence 6. The method of signaling, which comprises to be transmitted, mixing said frequency modu the steps of frequency modulating at a modula lated carrier with a constant frequency to pro tion index smaller than unity, a high frequency duce a new frequency modulated carrier wave of carrier with a signal containing the intelligence audio frequency, the mean, frequency of which to be transmitted, transmitting said frequency shall be lower than the lowest signal frequency, modulated high frequency carrier to a receiving transmitting said audio frequency frequency point, beating the received frequency modulated modulated-subcarrier‘wave to a receiving point, carrier to produce a frequency modulated beat so selecting a band of frequencies including the frequency, the mean frequency of which shall be subcarrier that the subcarrier and its associated lower than the lowest signal frequency, to act sidebands are substantially unattenuated while as an audio frequency frequency-modulated sub frequencies lying within the frequency band oc cupied by the sidebands and not associated with carrier, rejecting all undesired subcarrier fre quencies produced from vadjacent channels and selecting only the desired subcarrier frequency and demodulating said audio frequency fre the subcarrier are attenuated below a desired in terference level, and demodulating the subcarrier to recover the original signal. quency-modulated subcarrier to recover the orig 3. The method of signaling, which comprises inal signal. ' the steps of frequency modulating at a modula '7, The method of signaling, which comprises tion index smaller than unity, a high frequency the steps of frequency modulating a high fre-v carrier with a signal containing the intelligence 35 quency carrier with a signal containing the intel to be transmitted, mixing said frequency modu ligence to be transmitted, at a modulation index lated carrier with a constant frequency to pro smaller than unity, mixing said frequency modu duce a new frequency modulated subcarrier wave of audio frequency, the mean frequency of which lated carrier with a constant frequency wave-to produce a new frequency modulated carrier wave shall be lower than the lowest signal frequency, impressing said frequency modulated audio fre of audio frequency, the mean frequency of which shall be lower than the lowest signal frequency, quency subcarrier as a modulating signal on a transmitting the frequency modulated audio fre second high frequency carrier by amplitude quency subcarrier to a receiving point, rejecting 'modulation, transmitting said second high fre all undesired carriers and selecting only the de quency carrier as an amplitude modulated high 45 sired carrier and demodulating the frequency frequency signal modulated with the audio fre modulated audio frequency subcarrier, to recover quency frequency-modulated subcarrier to a re the original signal. ceiving point, selecting the desired carrier fre quency and rejecting all undesired carrier fre quencies, demodulating the received amplitude modulated high frequency signal to recover the desired frequency modulated audio frequency sub carrier and demodulating said frequency modu lated audio frequency subcarrier to recover the 8. The method of signaling, which comprises the steps ‘of frequency modulating a low fre ouency carrier with a signal containing “the intel ligence to be transmitted, at a modulation index smaller than unity, where the low frequency carrier has a mean frequency lower than the low est signal frequency, transmitting said low fre-‘ original signal. 55 quency frequency-modulated carrier to'a receiv 4. The method of producing an amplitude ing point, selecting the desired low frequency' modulated carrier wave,'having the main energy carrier and rejecting all others, and demodulating con?ned to a narrower band of frequencies than the frequency modulated low frequency carrierto that-occupied by the signal containing the intel recover the original signal. ' ' ligence to be transmitted, which comprises the 9. The method of signaling, which comprises steps of producing a signal of given bandwidth, the steps of frequency modulating a low fre frequency modulating a ?rsthigh frequency car quency carrier with a signal containing the intelé rier with said signal, at ‘a modulation index‘ ligence to be transmitted, at a modulation index smaller than unity, beating the so modulated smaller than unity, where the low frequency car ?rst carrier wave with a constant frequency wave, 65 rierhas a mean frequency lower than the lowest thus producing a frequency modulated audio fre signal frequency, impressing said frequency _ quency subcarrier wave accompanied with side modulated low frequency carrier as a modulating band frequencies corresponding to the signal fre signal on a high frequency carrier by ‘amplitude quencies, whereby the amplitudes of the sideband modulation, transmitting said'amplitude modu frequencies of said subcarrier wave are small as lated high frequency carrier to a receiving’ point, compared to the amplitudes of the audio fre quency subcarrier itself, and amplitude modulat reiecting all undesired carriers and selecting only the desired carrier, demodulating the'received amplitude modulated high frequency carrier by ing a second high frequency carrier wave with said frequency modulated audio-frequency sub carrier wave. ' recti?cation to recover the frequency modulated 75 low frequency carrier, and demodulating the fre 2,410,275 19 '20 quency modulated low frequency carrier to re separated .by a, narrower frequency band than the cover the original signal. bandwidth occupied by ‘the sideband frequencies 10. The method of receiving a frequency modu lated high frequency carrier, modulated with a of any adjacent modulated carriers, which com prises the steps of producing a frequency modu signal containing the intelligence to be trans lated ‘carrier modulated with a signal frequency containing ‘the intelligence to be transmitted at a mitted, at a modulation index smaller than unity, modulation index smaller than unity, and asso which comprises the steps of beating the received ciated with sidebands the energy content of which frequency modulated carrier to produce a fre is small in comparison to the energy content of quency-modulated beat frequency, the mean fre quency of which shall be lower than the lowest 10 the carrier; and in which the amplitude of any frequency component in the sideband becomes signal frequency, to act as a frequency modulated audio frequency subcarrier, ?ltering the frequency modulated subcarrier to attenuate all amplitude modulation products not originally present in the frequency modulated carrier wave, which have a frequency greater than the highest frequency reached by the frequency modulated subcarrier wave clue to modulation by the original signal, also to attenuate all other subcarrier frequencies having a frequency differing from the resonant frequency of the ?lter, demodulating the ?ltered frequency modulated subcarrier wave to recover smaller the farther said component is separated from its associated carrier, transmitting said. fre quency modulated carrier to a receiving point, beating the received frequency modulated carrier to produce a frequency modulated beat frequency, the mean frequency of which shall be lower than the lowest signal frequency, to act as a frequency modulated audio frequency subcarrier, impressing the same on an audio frequency frequency-dis criminator circuit to recover the original signal. 14. The method of separating a desired fre quency modulated carrier from a number of simi the original signal, ?ltering the recovered signal lar modulated carriers received in a common to attenuate all residual components with a fre quency lower than the highest frequency reached 25 channel, all modulated at‘ a modulation index smaller than unit, in which the frequency be by the frequency modulated subcarrier due to tween any tWo adjacent carriers is smaller than frequency modulation by the original signal and the maximum bandwidth of the side bands asso reproducing the ?ltered signal. ciated with any one of the frequency modulated 11. A method of privacy in communication, which comprises the steps of frequency modulat 30 carriers, which comprises beating the received fre quency modulated carriers to produce a plurality ing a ?rst high frequency carrier with the intelli of frequency modulated audio frequency subcar gence to be transmitted, at a modulation index rier waves, one of which having a mean frequency smaller than unity, beating the frequency modu lower than the lowest signal frequency, passing lated ?rst high frequency carrier with a constant frequency wave, to produce a frequency modu 35 this last-mentioned subcarrier wave through a lated beat frequency which shall act as an audio ?lter circuit resonant at the mean frequency of frequency subcarrier, in which the amplitude of the subcarrier and frequency discriminating said ?ltered frequency modulated audio frequency the‘ subcarrier at its'mean frequency is large as compared to the amplitude of the sidebands, amplitude modulating a second high frequency carrier with this frequency modulated audio fre quency subcarrier, transmitting the amplitude subcarrier wave to recover the original signal. l5pIn a receiver for the reception of a fre quency modulated carrier modulated with a signal frequency containing the intelligence transmitted, modulated second high frequency carrier to a re at a modulation, index smaller than unity, in ceiving point, during which transmission the fre quency modulated audio frequency subcarrier cluding a constant frequency oscillator producing a ?nal frequency modulated beat frequency sub acts as a masking signal to prevent recovery of carrier having a mean frequency lower than the the original signal by unauthorized persons, de modulating the amplitude modulated high fre lowest signal frequency, a parallel resonant cir cuit, the resonant frequency of which is equal to quency carrier to recover the frequency modu said frequency modulated subcarrier mean fre lated audio frequency subcarrier and demodulat quency, the Q factor of said circuit satisfying the ing the recovered frequency modulated subcarrier condition ' wave to recover the original signal. 12. The method of privacy in communication, which comprises the steps of frequency modu lating a high frequency carrier with the intelli gence to be transmitted at a modulation index smaller than unity so that the frequency modu in which Fb denotes the resonant frequency and PS the highest signal frequency and a frequency discriminator circuit including two resonant cir lated high frequency carrier will appear as a con stant frequency, transmitting this frequency cuits with the frequencies F1 and F2, above and modulated carrier to a receiving point, beating 60 below the frequency Fb, satisfying the same con dition when F1 or F2 are substituted for Fb. the received carrier to produce a frequency modu 16. In a receiver for the reception of a carrier lated audio frequency subcarrier, the mean fre I wave amplitude modulated by a frequency modu quency of which shall be lower than the lowest signal frequency, so selecting a band of fre lated subcarrier having a modulation index less quencies including the subcarrier that the sub 65 than unity, a resonant ?lter circuit tuned to the mean frequency of the subcarrier and having a Q carrier- and its associated sidebands are substan factor which satis?es the condition tially unattenuated while frequencies lying with in the frequency band occupied by the sidebands and not associated with the subcarrier are at tenuated below a desired interference level, and 70 demodulating the frequency modulated audio fre quency subcarrier so that authorized persons may "recover the original signal. ' 13. The method of signaling by modulated car cos Fct for any given signal frequency F5 and subcarrier mean frequency Fe, a frequency discriminator cir cuit following the resonant ?lter circuit and in rier currents in which the carrier frequencies are 75 cluding resonant circuits tuned to slightly differ 2,410,275 21 22 ent frequencies F1 and F2, above and below the subcarrier mean frequency F0 and satisfying the condition for their respective Q factors, when F1 subcarrier on. a high frequency carrier by am or F2 are substituted for Fe. point, beating the received amplitude modulated 17. In a transmitting circuit for the transmis sion of modulated carrier currents, a high fre quency oscillator, a reactance modulator modu lating said oscillator, a constant frequency os plitude modulation, transmitting said amplitude modulated high frequency carrier to a receiving carrier with a constant frequency wave the fre quency of which is equal to the received carrier frequency, thus producing a direct current com ponent corresponding to zero beat frequency and sideband components corresponding to the fre— cillator separated from the mean frequency of the ?rst oscillator by a frequency which is less 10 quency modulated low frequency subcarrier, so selecting a band of frequencies including the sub than the lowest modulating frequency, a mixer carrier that the subcarrier and its associated circuit associated with both the aforesaid oscil sidebands are substantially unattenuated while lators, a low pass ?lter circuit following said frequencies lying within the frequency band oc mixer circuit to pass all the difference frequencies cupied by the sidebands and not associated with produced in the mixer circuit. the subcarrier are attenuated below a desired in 18. In a transmitting circuit for the transmis terference level, and demodulating the frequency sion of modulated carrier currents, a reactance modulated audio frequency subcarrier to recover the original signal. 21. The method of receiving an amplitude mod stant frequency oscillator separated from the 20 ulated high frequency wave, modulated by a fre mean frequency of the ?rst oscillator by a fre quency modulated subcarrier wave having a mean quency which is less than the.lowest modulating frequency lower than the lowest signal frequency frequency, a mixer circuit associated with both modulator, a high frequency oscillator frequency modulated by said reactance modulator, a con aforesaid oscillators, a low pass ?lter to pass all and frequency modulated at a modulation index di?erence frequencies produced in said mixer cir 25 lower than unity, which comprises beating the received amplitude modulated carrier with a con cuit and a high frequency oscillator amplitude stant frequency wave the frequency of which is modulated with the output of said mixer circuit equal to the received carrier frequency, thus pro through said low pass ?lter. ducing a direct current component corresponding 19. The method of receiving an amplitude modulated wave modulated by a frequency modu 30 to zero beat frequency and sideband components corresponding to the frequency modulated low lated audio frequency subcarrier wave as de?ned frequency subcarrier, so selecting a band of fre in claim 5, which comprises the steps of receiv ing the amplitude modulated wave, beating the quencies including the subcarrier that the sub carrier and its associated sidebands are sub stant frequency wave the frequency of which is 35 stantially unattenuated While frequencies lying received amplitude modulated carrier with a con equal to the received carrier frequency, thus pro ducing a direct current component correspond ing to zero beat frequency and sideband compo within the frequency band occupied by the side bands and not associated with the subcarrier are attenuated below a desired interference level, and demodulating the frequency modulated audio fre nents corresponding to the frequency modulated audio frequency subcarrier, so selecting a band 40 quency subcarrier to recover the original signal. 22. The method of signaling, which comprises of frequencies including the subcarrier that the the steps of frequency modulating at a modula subcarrier and its associated sidebands are sub stantially unattenuated while frequencies lying tion index smaller than unity, a high frequency carrier with a signal containing the intelligence within the frequency band occupied‘ by the side bands and not associated with the subcarrier are 45 to be transmitted, transmitting said frequency modulated high frequency carrier to a receiving attenuated below a desired interference level, and demodulating the frequency modulated audio fre point, beating the received frequency modulated quency subcarrier to recover the original signal. carrier to produce a frequency modulated beat 20. The method of signaling, which comprises frequency, the mean frequency of which shall be the steps of frequency modulating a low fre 50 lower than the lowest signal frequency, to act as an audio frequency frequency-modulated sub quency carrier with a signal containing the in carrier, and demodulating said audio frequency telligence to be transmitted, at a modulation in dex smaller than unity, the low fraquency car frequency-modulated subcarrier to recover the rier having a mean frequency lower than the original signal. lowest signal frequency, impressing said frequency 55 STANLEY D. EILENBERGER. modulated low frequency carrier as a modulating

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