US2020327A - Single side band transmission - Google Patents

Description

Nov. 12', 1935. E. s. PURINGTON SINGLE SIDE BAND TRANSMISSION Filed sept. 6, 195o 5 Sheets-Sher'et 1 259,15

INVENTOR ELLlSON S. PURlNGTON BY 7% m ATTORN EY E. S. PURINGTON ,SINGLE SIDE BAND TRANSISSION Nov. i2, 1935. l

3 Sheets-Sheet 2 Filed Sept. 6, 1930 Nov. 12, 1935. E. s. PU'RlNGToN 2,020,327

` SINGLE SIDE BAND TRANSMISSIO-N v Filed sept. e, 1930 :s sheets-sheet 3- Ff'g, 6'

INVENToR :Lusen s PumNcnou v BY ATTORNEY I Patented Nov. l2, 1935 PATENT 'oFFicE f lsuman sms BAND 'rnaNsmssroN Ellison S. Purlngton, Gloucester, Mass., assigner to John Hays Hammond, Jr.

Application september' e, issu', serai Na 480,165-

4zi cisims.- (ci. 11s-111) This invention relates to methods of single side band transmission. More particularly it relates to the production of a transmission consisting of a single side band or a single side band and a car- 5 `rier in which the frequency components radiated are produced synthetically by various high frequency components modified in various manners as will be more clearly-set forth hereinafter.

The usual type of radio transmission by voice modulation methods, as is well known, utilizes a carrier wave and two side bands, one on each side of the carrier wave. It is well known that a system involving a carrier wave and one side band only, is superior to the usual system as respects: (1) tone quality, since the beating of the v side bands with each other is therebyeliminated;

(2) freedom from fading, since the phase relations of the carrierv and side bands cannot be differentially disturbed to give at certain times,

phase opposition of the currents produced by beating of the carrier on one side band with respect to that produced by beating produced by the other side band; and, (3) less spectral space is required for transmitting a signal capable of being received by the usual type of radio receiver.

Further, single side band transmission gives even greater advantages over carrier `and single side band transmission with respect to quality,

fading, and compactness of radiation, with the further advantage of greater transmission range per wattk of the' transmitter power.

Thel development of carrier and single side band circuits, although acknowledged'to be desirable, has not come about because of the com- 85 plexity of the circuits' required, involving radio frequency lters of a. great degree of selectivity and also requiring several stages of modulation. 'Ihe development of single side band circuits at present meets with the same dlfliculties. 40 It is the object of this invention to provide methods and systems for producing carrier and single side band radiation.

It is, more particularly, the object of this invention to overcome the above noted difiiculties and provide direct means for producing carrier and single side band radiation without the use of multiple stages of modulation and radio frequency flltering.

It is a further object to produce single side band transmission by utilizing the single side band and carrier methods in push-pull arrangement of two circuits whereby the single side bands become additive and carriers neutralize 55 each other. y

These and further objects will become apparent from the following'speciilcation taken yin connection with the appended drawings.

Four methods are involved in this invention as 50. follmvs:

' 1. The combination of amplitude modulation and phase modulation.

2. The combination of amplitude modulation and frequency modulation.

3. Separate amplitude modulations involving l reversal of one of the side bands produced and combining of the two spectra in various manners.

4. Direct amplitude modulation by methods utilizing two phase sources for the modulated and modulating frequencies. 1o`

The last mentioned method is being covered in my copending application Serial No. 480,166, tiled September 6, 1930, concurrently herewith, and is being included in this description merely for the purpose of coordination of the various sys- 15 tems. The present specification therefore, principally concerns itself with methods 1, 2 and '3.

For the two simultaneous methods outlined above under methods 1 and 2,'there are required the following: f

For method 1, means for the simultaneous phase and amplitude modulation of a carrier wave with the extent of phase modulation in radians from the mean value equal to the degree of amplitude modulation but with the two modu- 25 lations quadrature related.

Method 2 requires means ,for the simultaneous frequency and amplitude modulation of a `carrier wave with the two modicatlons produced in phase, and with the extent of the frequency vari- 30 ations from the mean equal to the degree of amplitude modulation multiplied by the frequency of modulation. l

Method 3 may be divided into 4three combinations as follows:

A. Means for producing an amplitude modul lated wave form with carrier and two side bands,

means for producing a wave form similar to the above wave form with all three equivalent waves of equal magnitude in the two. wave forms, but 40 with one side band of one Wave form in phase opposition with the corresponding side band in the other wave form, and means for combining the two wave forms by addition vwhereby 'one side band will be eliminated and the other side band and the carrier will remain. y

B. Means'for producing a wave composed of ak carrier and two side bands by the usual amplitude modulation methods, means for producing a wave of carrier and two side bands similar to the above, except that one side band of one wave form must be equal and of opposite phase to the corresponding side band of the other wave form, and means for the subtraction of the two wave forms where-v by the'carrier and one side band will be elimi- 55 l r'iated,` resulting in the productionof a wave cornposed of a single side band.

C. Means for producing a composite wave form by the carrier suppression system consisting of two side bands only, means for producing similar composite wave by the carrier suppression system with the two side bands only, and with one side band of this wave form equal and opposite to the corresponding side band of the other wave form, and the other side band equal to the corresponding side band in the other wave form, and in phase therewith, and means for addition whereby one side band is eliminated, and there is thus produced a wave form comprising a single side band. A

For methods 1 and 4, as given above, namely the method involving combined phase and amplitude modulation, and the method involving producing two phase sources of modulated and modulating frequencies, a necessary tool is a means for producing a two phase audio frequency source from a single phase audio frequency source. With a single frequency audio source, a two phase current may be readily produced, but with a band of from 250 to 4,000 cycles or about 4 octaves, certain diiiiculties though encountered are nevertheless overcome by the methods of the present invention and that of my copending application noted above.

One method of producing a two phase audio current which will be described in the present application, involves stepping up the frequency of the speech band to a'n'intermediate frequency band by the usual push-pull method and then filtering to produce the desired phase lag. 'I'hus a band of 250 to 4,000 cycles could be used to modulate the 6,000 cycle current and the side band of from 6,250 to 10,000 cycles retained. A suitable modulator for accomplishing this is disclosed in my copending application, Serial No. 364,222, filed May 18, 1929. A filter of high pass construction having a cut off at 6,125 cycles may be rather easily constructed.

The 6,000 cycle current is split into two phases and a single side band passed through the filter is combined separately with one of the phases of the 6,000 cycle current. As a result, the detecting currents are of the original audio frequency but quadrature related for all frequencies an'd of the same output at all frequencies.

Having thus brieiiy described my invention, attention is invited to the accompanying drawings in which;

Figs. 1a and 1b represent diagrams showing the vectorial and spacial relationship of the components of an amplitude modulated wave.

Figs. 2a and 2b represent vectorial and spacial relationships of the components of a frequency modulated wave.

Fig. 3 is a circuit illustrating one method of producing phase related audio frequency currents over a wide band of frequencies.

Fig. 4 is a diagram of a circuit representing a filter method of producing phase related audio frequencies over a wide band of frequencies.

Fig. `5 is a circuit for producing a high frequency carrier, phase'modulated in accordance with an audio frequency. N

Fig. 6 -is a circuit for producing related phase modulated and amplitude modulated high frequency currents, modulated in accordance with the same audio frequency.

Fig. 7 represents a circuit for producing related audio frequency amplitude modulated and frequency modulated high frequency current.

Fig. 8 is a general diagram of a transmitter involving the principle of related frequency and amplitude modulation of a carrier frequency current.

Since there exist practically no references as to the true nature of phase modulation and frequency modulation in comparison with amplitude modulation, a brief analysis of this subject is believed to be desirable at this point.

From a general point of view, if a continuous wave is modified periodically with suitable relationship between the radio frequency and the frequency of the modification, the resulting wave form will be labsolutely recurrent. If any wave form is constructed of a series of radio oscillations, with successive oscillations differing in general in nature as to amplitude, and time interval between crossing of the zero reference line, and if, for example, twenty three such oscillations occur each .001th of a second, and if the construction of the radio frequency oscillations of the same natlle is continued so that any radio frequency" con tion at any time T will be reproduced at any time T+.001 sec., T+2X.001 sec., T+3X.001 sec., etc., then the wave form is absolutely recurrent every one thousandth of a second.

It is apparent by Fouriers theorem that the recurrent wave will produce the same identical effect as a plurality of sine wave forms with different amplitudes and with frequencies which are of a fundamental of 1,000 cycles and with higher .frequencies which are integral multiples of a thousand cycles when these are added together.

For a sinusoidal wave form of amplitude modulation, it is well known that the amplitude of the fundamental frequency of a thousand cycles will be zero, and also that all harmonics will be zero except three in the region of the radio frequency oscillations, namely the 22nd, 23rd, and the 24th harmonics. These harmonics are designated as the lower side band, the carrier frequency, and the upper side band currents.

If the ratio of the radio frequency to the modified frequency is not an exact integral, nevertheless, the wave form will be recurrent for a sufficient number of modifying cycles as, for exwould be 1%)=34.5 cycles, l

and the harmonics of this fundamental frequency which have other than zero amplitude would be in the vicinity of the 858th. 887th, and 916th. In other words, the exactness of the ratio of a carrier to the modifying frequency is not necessary, as the Fouriers analysis would merely involve a lower fundamental than that actually used physically in producing the modification.

It is thus mathematically apparent that whether the modification is in amplitude, phase, or frequency, the modified wave form is entirely equivalent in this effect to the sum of a plurality of readily determined frequencies.

It is well understood that a pure slnusoidally amplitude modulated wave produces the exact effect of a carrier and an upper or lower side frequency. Furthermore, it is well known that the upper and lower side frequencies are equal in magnitude and different in frequency from the carrier by the modulation frequency. However, it is not so thoroughly appreciated that for a pure amplitude varied wave, an additional relation must exist with respect to the three component waves. This relationship is that if three continuous waves represent an amplitude modulated wave, not only must the upper and lower waves be different in frequency from the mid frequency wave by the same amounts, but also at 4each and every instant of time lthe vectorial sum of half the mid wave and the` lower wave must equal the vectorial sum off-half the mid wave and the high-wave.

From another point of view, the necessary'conditlons may be shown by representing the three waves as C the carrier, S+ for the upper side frequencyand S- f or the lower-side frequency.

' In a vectorial representation, as is shown in Fig.

la, the three waves may be represented as having a common origin 0, it being understood that each A waves are maximum positive value, and therefore, modulation is at the peak.

Fig. 1b is a spectral representation, of the modulated wave form, and the corresponding formula. .l

Any arrangement of `three continuous waves, other than in'the proper 'relation to represent an amplitude modulated wave, will representa phase cr frequency modulated. wave orcombin.-4

\ tion of phase or frequency and amplitudemodulISO .instants of time. y

The three wavesinthe displaced position add lated wave. y

In particular, the greatest change that canbe` made in the arrangement 'of the three continuouswaves of Fig. la and lb to make the composite wave which they represent least like an amplitude modulated wave, is made by changing the phase ofS+ or S- by 180 degrees or of C by degrees. The methodlof doing this and thus altering the arrangement of the group of three `complished by using apush-pull modulation to produce the side' bands and by using the phase shift to produce a ldisplaced crrier for@ combining with the side bands to give the final wave form. 4'

Akchange of 180 degrees of one side frequency 'is indicated in Figs. 2a and 2b, the lower side band having been reversed from the condition shown in Fig. 1. For Fig. 2a the line bisecting the angle between the lines S+' and S- lies at right angles C and will continue to do so at future up to a composite current as follows: y

if:- cosl (w'-p).t+cos atei-goce (w-l-pjf =cos wt-kain pt sin at' i Amit is, the waveform or ng. 2 'could be produced from a carrier cos wt by amplitude modulationA according tol the amplitude function bined by addition to give,

' .single side f A j combined with. phase modulation according to the phase cycle, t=tan1 k sin pt. a

If we start with lc=1, for example, and decrease k toward a small value, the amplitude function becomes small very much more rapidly 5 than the phase function. If k=l corresponding to modulation on an amplitude basis, after shifting the side band the amplitude varies between I and that is, from 1.0 to 1.41 corresponding to 18% modulation. ptvaries from +45 to -45. If k= Mg, corresponding to 20% modulation, the amplitude after 'shifting becomes varied from 1 to corresponding to 1.5% modulation. The value of t then ranges from +14 -30' minus 14 -30. 'I'hat is, by cutting k from 1 to V4 of a. degree of amplitude, modulation is reduced from 18% to 1.5%, while the maximum departure of phase from the mean is reduced from 45 to `14 -30'; or roughly by cutting k by a factor of 4, the amount ofphase modulation reduces 'by a factor of 3 but the amount of amplitude modulation reduces by a factor of l2 toa nearly negligible quantity.

Accordingly, the wave forms of Fig. 2 may be produced by phase modulation methods with 30 considerable degree of accuracy providing the amount of phase modulation is not more than 10 to 25 electrical degrees either way from the mean value. Therefore, the wave form represented 'is termed a quasi-phase modulated wave form, because of its close approximation'to the arrange'- ment o f a true phase'modulated wave form. The exact representation of a true sinusoidal phase modulated wave form is =cos (M4-pm sin pt) in which w represents the carrier frequency, and p the modified frequency. There is no necessary limitation upon om the maximum departure of' the phase from the mean value O.

A sinusoidal frequency modulated wave form results whenever there is a sinusoidally phase 45.

modulated wave form. The instantaneous frequency is determined by the rate of change ,of the angle of which we take the cosine to give the instantaneous current. Thus the wave form expression, i= cos (t+msin pt) represents a cur- 50 rent with instantaneous frequency,

f=;d-t ait-hp., sin pt]=[w+p cos pt] and therefore, a phase modulation according to 55 the function msin pt produces theA exact wave form as a frequency modulation accordimr, to the frequency variation, f

4 F(t)=2 rp Cos pf That is, we may producethe same `identical result by phase modulation methods or by fref quency modulation methods with due attention shown for an amplitude modulated wavey and a quasi-frequency modulated wave mayv be com# (i1-Fla) =2 008 t+ k 00B (cH-p) f which isl a carrier one side band, or by subtraction to give (i1-iz) =k cos (-p)t which isa It is further obvious that if a single side band only is desired, the values of k in the two wave forms may be any value whatsoever, but equal in magnitude, in particular zero, in which case the wave forms reduce to those which can be produced by suppressed carrier systems. Also, if the carrier and a single side band are desired then the carriers need not be equal and one may be zero if desired.

There are many procedures by which this part of my invention may be practiced. Two modulated wave forms corresponding to amplitude modulated and quasi-phase modulated, signals Pmay be produced independently and added to or .subtracted from each other, or the two processes may be simultaneously arranged.

One method of production which involves a simultaneous process may be accomplished as follows:

A carrier and a single side frequency may be expressed mathematically by 1`=cos wt-l-k cos (w-I-p)t= (l-i-k cos pt) cos wt-Ic sin pt sin wt= k sin pt COS pt COS (wt-*"tan im For lc2 1 that is, k is a small quantity, this becomes cos wt-i-k cos (w+p)t= (l-i-k cos pt) cos (wt-i-k sin pt) That is, the amplitude variation must be made in accordance with the function (l-i-k cos pt) while the phase variation must be made according to the function k sin pt (phase function).

The percentage of modulation, and the amount of phase modulation expressed in radians must be the same amount but the two modulations must be quadrature related, since one is a sine function of time and the other is a cosine function of time.

on the other hand, 1f the phase variation is made by frequency variation means, the frequency function is:

That is, both frequency and amplitude variations must be simultaneously made, in phase, but the amount of frequency variation must be proportionally greater for high frequencies than for low frequencies.

The summation of the requirements for carrying out the phase of this invention in accordance with the above desired methods, has been included in the introduction of this specification.

Having thus discussed in general the features and principles underlying my invention, attention is now invited to the drawings which show means for carrying it out.

Particular attention is now invited to Fig. 3 which is a diagram of a circuit which is similar to that shown in my copending application, Serial No. 480,166 filed concurrently herewith.

In this figure the audio frequency current having a frequency say from 250 to 4,000 cycles per second is supplied through leads I and 2 of the input of a transformer, the outputs of which are adapted to supply the grids of push-pull modulator devices 4 and 5 in push-pull fashion. An auxiliary current of say 6,000 cycles is supplied by the generator 3, through the split secondary II of transformer I to the plate circuits of the devices 4 and 5 through impedances I2 and I3 .in Fig. 3.

as shown. The modulator, as described, operates in accordance with the principles of my modu- 'lator, as disclosed in my copending application above noted, and Is adapted to supply the side frequencies resulting from said modulation to 5 the input impedance I9 of the filter 20.

This filter is of the high band pass type and is adapted to transmit frequencies of say from 6,125 to 10,000 cycles with substantially no diminution and eliminate entirely all low frequencies. 10

After amplification by an amplifier circuit, as shown, the frequency passed through the band pass filter is supplied^to the input circuit of the push-pull detectors I4 and I5 in parallel.

The oscillator 3 also is adapted to supply the l5 grids of the push-pulldetector I4 and I5 with the modulating current of 6,000 cycles, but the inputs of the respective devices are so arranged that the impedance 6 which acts to supply the device I5 is included in a capacity circuit in- 20 cluding the capacity I and resistor 6, whereas the impedance 9 across which the grid circuit of device I4 is connected, is in circuit with the inductive reactance 8 and resistor 9. Thus the auxiliary frequency of 6,000 cycles is supplied to the two tubes I4 and I5 in quadrature relationship.

The detectors I4 and I5 operate on the beat detection principle and are arranged to supply the detected low frequency to the output windings I6 and I `I constituting the primaries of secondary transformers I6-2I and I'I-22 respectively. The detected current will be out of phase by 90 regardless of the frequency of the currents supplied through leads I and 2 and thus the terminals A and B are adapted to supply audio currents of 3 the same frequency at phase quadrature regardless of the frequency. 'I'he operation of this circuit is fully covered in my copending application as above noted.

Attention is now particularly invited to Fig. 4 40 which shows another method of producing two phase currents which method however, is not so4 perfect theoretically as the indirect method shown Here the audio current is supplied through the transformer 40 to a filter compris- 45 ing a number of sections. The output of a part of the filter, which part acts merely as an inductance, is supplied through the connection at 42 to the grid of amplifier 44 across the variable impedance 46. The part of the filter circuit thus in the circuit of device 44 is substantially inductive. The entire filter has a capacitive character and the output of it is supplied to the input of the amplifier 45. The variable impedance 46 is so arranged as to supply varying voltages to the grid of 44 in order to compensate for the attenuation of the current being supplied to the grid of 45.

While a. simple resi-Stance, condenser, and coil l arrangement will not give a 90 shift in phase 60 for any considerable band of frequencies, a more complex network will be capable of accomplishing the desired result. The simple filter networks of the low pass type produce phase lags with the greatest lag at the higher frequencies near the cut off points. Simple filters of the high pass type produce phase advances with the greatest advance for the lower frequencies near the cut off point. By a considerable combination of high and low frequency filter sections, it is possible to obtain substantially 90 advance or lag over the desired range, with but little attenuation so that two points on the filter network are produced with 90 phase relation and with practically the same attenuation ratio for all frequencies. After ammade to operate upon the circuits for 'duction of single side hand or carrier and single side band transmission, as will-bedescribed hereinafter.

Either of these produce the two phase source of audio frequency which is required in the development of my invention. and'in certain of the remaining' figures of the drawingfthe two phase source is indicated ratherthanshownindetail, it being understood that either ofthe systems Just disclosed by the amplifier Il. The input audio frequency is supplied through the leads Il. and the trans# former'to the variable impedance devices li, l2. In the output circuit of the last mentioned devices are inductances Il and M which comprise the secondarles of a transformer which includes also the primary Il driven from theoutput of a amplifier il. VInductances i8 and M are wound in such a' fashion that they neutraliae each other' 'asto tone frequencies beingtransferred to inductance l regardless of the value of their input. Thus no energy is' transferred from the-inductances il and M to the inductance Il, but the current flowing through inductances Il and M being altered at tone frequencies. serves toalter the impedance of li and cause the phase relationship between the input and output circuits of the ampliner l., to be varied.v The high frequency thus varied in accordance with the voice currents is amplified by the power amplifier l1 and radiated by means of appropriate output circuits 59 and antenna circuit il.

In other words, a constant frequency source is provided with an amplier circuit' which has a.`

variable phase characteristic and controlling meansforchangingthephaseinaccordancewith speech currents. A limiting .Y or compensating means may be provided in the output of ampli'- iier 56 for maintaining a constant'am'plitude revgardless of variations in phase. f

In operation, the phase angle ofthe impedance in the plate circuit ofthe ampliiier 5I is varied at speech lfrequencies, and therefore, the phase of the voltage delivered to the gridof the high power amplifier l1 is varied with respect to the phase of the master radio source Il. As the speech is impressed upon the grids of the shifter tubes 5i, 52, the impedance of these tubes is lowered and raised in accordance with speech currents, and the phaseangle of their external impedance is varied accordingly. The speech currents are prevented from developing an audio voltage in the radio circuit by the opposition of the plate windings of the shifter device which prevents audio frequency energy from entering the circuits of amplifier 5l, but does not prevent the reaction due to the changing impedanceof yAs the impedance of the shiftertubes varies..v

changes occur not only in phase but in magnitude of the energy being ampliiied by device l0. The

' magnitude of this effect may be by any lof the following devices;

l. Operating the radio transformer lcircuit at two methods may be used to 4 the master radio circuit of rather low radio fren modulations. v

3. Use of 'afeurrent limiting device. as sug- 5 gested above, for the coupling to the high power amplifier l1. l l

4. By the use of small'changes of phase for quency, and the use of vmultiple means as by power rectifiers whereby the phase change of, the multiplied radio frequency will be great compared with the phase change of the master radio frequency. l

With this type of transmission, a radiation correspondingfto that condition in Fig. 2 may be produced except that the upper and lower side frequencies, as above noted, are replaced by upper and lower side bands.

For producing carrier and single side band only, z 0

as before indicated, the output of .a phase varied radio circuit maybe combined with the output of an amplitude varied radio circuit with phase and amplitude variation properly related in phase and degree of modification. From the previous discussion, it will be -noted that in orderthat these two vmodulations may be properly combined, they must result from modulation of the sameaudio l current but at phase quadrature. Also, as noted above, the quadrature related audio current may ao be produced by eitherthe method of Fig. 8 or Fig. ias outlined above.

Attention is now invited to Fig. 6 which shows a simple type of circuit for accomplishing these effects. 'Ihis ligure is based upon the method of a5 .l

phase modulation shown in Fig. 5. 'I'he connecy tions from a source' of audiocurrent il, supply a phase splitter 62, which phase` splitter is aryranged to split the phase of the audio ciurent .of the amplifier 6i in accordance with phase A of the audio current. In this diagram, B8 is the radio frequency source and 81 is the high power so amplifier. Thus-phase A is adapted to operate 4 the radio' phase shifter device: Audio current of phase B is adapted to be amplified by the amplifiers 6l and 6l and operate into the grid circuit of the modulator device 1li., the output circuit 1I 55 of which isincluded in the plate circuit of the radio high power ampliner 01." The tuned out`y put circuit 12- of amplifier 01 is inductively related to the inductance 1| of the antenna circuit which latter serves to radiate the high frequency 00 energy thus produced.. As a result of this, -the output of the radio amplifier l1 is modulated by audiophaseB,and theradiophaseisshiftedf by the audio phaseA, thus producing a carrier and a single side band as outlined above.'

A similar method and apparatus may be used to combine the amplitude and frequency modulated radiant energy'with, however. the necessary addition of arrangements for producing the` proper proportions of frequency and amplitude A means for producing frequency variations in accordance with an laudio current are condenser microphone devices and other arrangements ineluding the method disclosed in Patent #1,599,586 or by any similar arrangement Referring now to Fig. '1, there is shown an arrangement for obtaining an audio frequency current for modulation purposes which is in phase with corresponding frequency variations. This circuit comprises a source of high frequency energy including an oscillator 1I and its associated circuits. Included in the frequency determining circuit of the oscillator 1l is the condenser microphone 15 which is thus adapted to vary the frequency produced by the said oscillator in accordance with the voice impulses. In series with the condenser microphone 15 is the impedance 18 through which the current will be proportional to the displacement of the condenser microphone diaphragm, or in other words, will vary in accordance with the voice impulses. The output of the oscillator is across the impedance 18 and frequency modulated high frequency energy may ybe supplied by the terminals C to the proper amplifying devices before combining or remodulating, as will be described hereinafter.

Impedance 18 serves to supply the combined amplitude and frequency modulated high frequency current to a filter 11 arrangement-which includes in its input a rectifier 18. The output terminals D of the filter 11 are thus supplied with a direct current, the intensity of which varies in accordance with the voice frequency.

As the capacity of the condenser microphone changes, not only will the frequency of the modulator frequency change, but also there will be changes in the amount of the high frequency alternating current passing through the microphone and through the impedance 18. Therefore, through this impedance is a current of varying amplitude of varying frequency with the two variations occurring simultaneously, since both the frequency and the current depend upon the value of the capacity of the condenser microphone. However, the amplitude and frequency changes are directly proportional so that the current in 19 is not the equivalent of the carrier and a single side band for all rates of frequency change. The current flowing through 19 by means of the detector 18, and filter 11, to remove the radio frequency components, causes a current to flow through the output impedance and thus produces a varying audio current voltage across the terminals D. This audio frequency voltage will correspond with the rate and extent of the radio frequency changes caused by the condenser microphone 15.

While the production of an audio voltage, which is in phase with the frequency variation, is easily accomplished, it is further necessary for my purposes to regulate the relative amount of audio current so that the amplitude modulation may be greater at the higher frequencies. This regulating must be such that the phase of the amplitude modulation and of the frequency modulation will be the same in the radiated wave. This may be done along the general lines of using two'phase shifts, one in the mechanism that produces an audio current with certain phase relationship, in other words, in the filter 11 and of strength relative to the frequency modulation, and one in the mechanism that regulates the audio current for producing the greater amplitude modulation of the higher frequencies.

Referring now more particularly to Fig. 8, there is shown a schematic diagram of means for combining the frequency modulated and the amplitude modulated high frequencies to produce a transmission comprising'a single side band. Here the audio frequency source 80 acts upon the radio g frequency oscillator 8| to produce a wave varied in frequency which wave is amplified by the amplifier 82, and also to produce an audio current having a definite relation to each frequency present as to its strength and phase with respect to the audio frequency variation in the radio frequency. That is, in general the audio currents need not be absolutely in phase with the rates of variation in radio frequency nor need the amplitude be proportional to the amount of radio frequency variation. The audio frequency current however, is amplified in the amplifier 84 and scaled as to intensity in a regulating device 85, which is used to shift the phase and regulate the amplitude of the different frequencies.

The audio current, thus scaled, is used to modulate in the modulator 83 the radio frequency which has been frequency modulated and amplified by the amplifier 82.

The degree of accurate elimination of the side band would depend upon how correctly the amplitude variations have been related to the frequency variations. Probably the most accurate method of producing single side band transmission is by the fourth method listed at the beginning of this specification which, as has been noted, is covered in my copending application Serial No. 480,166 filed concurrently herewith. This method, which utilizes two phase source of modulating and modulated currents with amplitude modulation methods, permits using well known methods of construction except as to the phase splitting of the speech band. g

The mathematical basis of the process involved in this invention is shown by the equation:

The first term is the first radio phase push-pull modulated by the first speech phase, and the second term is the second radio push-pull modulated by the second speech phase. The sum of the two terms is a single side band current.

It is obvious that by changing the method of combining the two phase radio and two phase audio currents, either the upper or lower side band can be obtained as desired. By permitting some of the carrier through or by passing it to the circuit subsequent to the push-pull modulation, it is evident that the carrier could be transmitted together with the single side band. This method, however, need not further be described here.

Having thus described my invention, attention is invited to the fact that various modifications may occur which fall within its scope, and that I am therefore not to be limited to the specific ernbodiment described and shown for the purpose of illustration, butby the actual scope of my invention as determined and set forth in the appended claims.

I claim:

l. The steps in a method of producing single side band transmission by the synthetic method which comprise generating a high frequency current, frequency modulating said high frequency current in accordance with a voice signal to be transmitted, producing a frequency modulated high frequency current, simultaneously producing an audio frequency component in phase with the frequency varying component of the frequency modulated energy, suitably adjusting the amplitude of said high frequency component, and amplitude modulating said frequency modulated transmission synthetically whichcomprises producing an audio frequency current representa.-

high frequency energy by said audio frequency component.

2. 'Ihe steps in a method of. producing single side band transmission synthetically which comprise generating a high frequency current, generating a low audio frequency, splitting the phase of said audio frequency, shifting the phase of said high frequency in accordance with one of the low frequency phases'tbus produced, and ampli'- tude modulating the phasc'varied high frequency in accordance with the other of said audio frequencyv phases. y

3. 'Ihe steps in a method of synthetically producing single side band transmission which comprise producing a high frequency carrier current, producing a low frequency current representative of the voice energy to be transmitted, splitting the phase of said last mentioned current, varying the phase of said high frequency current in accordance with one phase of the laudio frequency energy thus produced, adjusting the amplitude of the other phase of said audio frequency current, and amplitude modulating the phase shifted high frequency energy in accordance with the audiofrequency current as thus adjusted.

4. The method of producing single side band tive of the intelligence to be transmitted, generating an intermediate frequency current, pushpull modulating said intermediate frequency current bysaid audio frequency current, selecting' one of the side bands-.produced by said modulation, splitting the phase of said intermediate frequency current, demodulating the selected side band by combining with each of the phases ofsaid intermediate frequency current as thus produced, .thus producing a two phase audio frequency current, frequency modulating a high frequency carrier current byone phase of said audio frequency current, amplitude modulating the high frequency current by the other phase of said -audio frequency current, and radiating the energy thus produced. y a

5. The method of producing single side band transmission synthetically which comprises pro- `clucing an audio frequency current representaduced, thus producing a two phase audio frejquency current, generating a high frequency cur- A rent, shifting the phase of said high frequency in accordance with one ofthe audio frequency phases, and amplitude modulating the phase varied high frequency in accordance with the other of said audio frequency phases.

6. 'I'he method of producing single side band transmission synthetically which comprises producing an audio frequency current representative of the intelligence to be transmitted, generating an intermediate frequency current, pushpull modulating said 'intermediate frequency current by said audio frequency current, selecting one of the side 'bands produced by said modulation splitting the phase of said intermediate frequency current, demodulating the selected side band by combining with each of the phases of said intermediate frequency current as thus produced,

thus producing a two phase audio frequency current, producing a high frequency carrier current,

varying the phase of the high frequency current, in accordance with one pha-e of the audio frequency, adjusting the amplitude of th\e other 5 phase of said audio frequency current, and amplitude modulating the phase shifted high freinl accordance with one phaseof the audio frequency current as thus adjusted.v '1. The method of `producing single side band 10 transmission by the synthetic method which com, prises producing a-two phase intermediate frequency current, modulating one ofthe phases of said intermediate frequency current by the audio frequency to be transmitted, selecting one of the I5 side bands thus produced, detecting said side. band by each of thephases of said intermediate frequency current, and thus producingtwo phase audio frequency current, frequency modulating a' high frequency carrier current byon'e phase of zo said audio frequency current, amplitude modulating the high frequency current bythe other phase of said audio frequency current, and radiating lthe energy thus produced.

8. The method of producing. single side bandiiiir transmission by the synthetic method which comprises producing a two phase intermediate frequency current, modulating one' of the phases of said intermediate frequency current by the audio frequency tobe transmitted, selecting one of the 30 side bands thus produced, detectingsaid side band by each of the phases of said intermediate frequency current, and thus producing a two phase audio frequency current, generating a high frequencyv current, shifting the phase of said high `frequency in accordance with one ofthe audio frequency phases, and amplitude modulating the phase varied high frequency in accordance with the other of said audio frequency phases.

9. The method of producing single side band 40 transmisfz-ion -by the synthetic method 'which comprises producing .a `two phase Vintermediate frequency current, modulating one of the phases of said intermediatefrequency current by the audio frequency tobe transmitted, selecting one t5 of the side bands thus produced, detecting said side band by each of ythe phases kof said intermediate frequency current, and thus producing two phase audio frequency current, producing a high frequency carrier current, varying the phase |y of said high frequency current in accordance with one phase of .the audio frequency, adjusting the amplitude of the other phase of said audio frequency current. and amplitude vmodulating the phase shifted high frequency energy in accordance with the audio frequency current as'thus adjusted.

1.o. The method or syntheticauy producing single side band transmission which comprises iproducing a single side band representative of the audio frequency to be transmitted, demodulating said single side band by each of two phases of the high 'frequency used in producing said side band, and thus producing an audio frequency `current similarly disposed as tophase, as the 05 two phases above mentioned are disposed, frequencymodulating a high frequency carrier current by one phase of said audio frequency current, amplitude modulating the high frequency current by the other phase of said audio frequency current, and radiating the energy thus produced.

1l. The method of synthetically producing sin-- gle side band transmission which comprises producing a single side band representative of the v audio frequency to be transmitted, demodulating said single side band by each of two phases of the high frequency used in producing said side band, and thus producing an audio frequency current similarly disposed as to phase, as the two phases above mentioned are disposed, generating a high frequency current, shifting the phase of said high frequency in accordance with one of the audio frequency phases, and amplitude modulating the phase varied high frequency in accordance with the other of said audio frequency phases.

12. The method of synthetically producing single side band transmission which comprises producing a single side band representative of the audio frequency to be transmitted, demodulating said single side band by each of two phases of the high frequency used in producing said side band, and thus producing an audio frequency current similarly disposed as to phase, as the two phases above mentioned are disposed, producing a high frequency carrier current, varying the phases of the high frequency current in accordance with one phase of the audio frequency, adjusting the amplitude of the other phase of said audio frequency current, and amplitude modulating the phase shifted high frequency energy in accordance with the audio frequency current as thus adjusted.

13. The method of producing a two phase audio frequency current which comprises filtering an audio frequency current sequentially, inductively, and capacitively, and amplifying said current as thus filtered to produce one audio frequency phase, and filtering said audio frequency current inductively, simultaneously with the inductive filtering of said first produced phase, and amplifying said last mentioned audio frequency current as thus filtered and adjusting it to compensate for the attenuation 4of the first mentioned audio frequency phase to produce the second audio frequency phase.

14. Means for producing two phase audio frequency current which comprises a filter having an inductively reactive portion and a capacitively reactive portion, an input for said filter, a thermionic device adapted to be supplied by the youtput of said filter, a second thermionic device adapted to be supplied by the finductively reactive portion of said filter, and means for compensating with respect to said last mentioned device for the attenuation of current in the input of said rst mentioned thermionic device, whereby a two phase audio frequency current will exist having substantially quadrature relationship regardless of variation in frequency. 1

15. Means for the modulation of high frequency y ing the energy thus phase shifted.

16. Means for the modulation of high frequency energy by the phase shifting method which comprises a source of high frequency energy, a source of modulating energy, 'a push-pull amplifying device related to said last mentioned source, a

translating device for translating and amplifying said high frequency energy, an inductance related to said translating device, and an out-j put for said-push-pull device so related as to change the impedance of said last mentioned inductance, and thus shift the phase of the current as translated by said translating device relative to the source of high frequency energy.

n 17. Means for producing single side band transmission which comprises a source of audio frequency energy, means for splitting the phase of` audio frequency energy, a source of high frequency energy, means for varying the phase of said high frequency energy in accordance with one of the phases of said audio frequency energy, means for adjusting the volume of the other phase of said audio frequency energy, and means for amplitude modulating the high frequency energy as thus phase shifted in accordance with the adjusted audio frequency energy.

18. Means for modulating high frequency energy by the phase shifting method which comprises a source of high frequency energy, a source of modulating energy, an amplifying device rey lated t'o said last mentioned source and comprising a pair of electronic devices, a common input circuit for said electronic devices coupled to thel sourceof modulating energy and arranged so that the modulating energy is impressed upon the input circuits of both said devices simultaneously and in like sense, said devices being provided with an output circuit connected in push-pull fashion, a translating device for translating and amplifying said high frequency energy and including an input circuit through which the high frequency energy is applied thereto and an output circuit, an impedance device in the output circuit of said translating device coupled to said push-pull output circuit whereby variations in the flow of current through the push-pull Youtput circuits caused by the modulating energy causes alterations in the impedance of said inductance device.

19. In a system for modulating high frequency energy by the phase modulation method, a source of high frequency energy, an electronic tube provided with an input circuit and an output circuit, means for coupling said source of high frequency energy to said input circuit, an impedance device in said output circuit, a utilizing circuit and means for coupling said output circuit to said vutilizing circuit, a source of modulating energy,

means for coupling said source of modulating energy to said output circuit comprising a. pair of variable impedance devices each thereof being provided with input circuits and output circuits, means for coupling the input circuit of each of said devices to said source of modulating energy, means for coupling said last named output circuits to the output circuit of said electronic tube in opposing sense whereby voltages impressed upon the electronic tube output circuit from the output circuit of one of said variable impedance devices neutralizes voltagesA impressed upon said electronic tube output circuit from the output circuit of the other of said variable impedance devices, the impedance of said variable impedance device being adapted to be varied simultaneously and in like sense in accordance with the energy transferred to the input circuits thereof from said modulating source whereby varying impedance effects are transferred from said pair of variable impedance devices to the output circuit of said electronic tube without causing any substantial variations in the e n K 9,020,827

amplitude of the high frequency energy in saidelectronic tube circuits due tothe modulating frequency energy.

420. In a system for modulating high frequency energy by the phase modulation method as described in the next preceding claim characterized by the fact that the output circuit of the electronic tube includes two branches one of which includes the impedance device and the other the means for coupling the output circuit of the electronic tube to the utilizing circuit and wherein said impedance device is utilized for coupling the two variable impedance devices to the output circuit of the electronic tube.

2i. In a phase modulation system, a source ofl carrier energy. a source of modulating energy, an electronic relay device having an input circuit and an output circuit, means for applying the carrier energy to be modulated to the input of the electronic relay device and means for applying to the outputl of the electronic relay device variable impedance eiects varying in accordance with the modulating energy while preventing any actual transfer ot energy from the modulation energy source to the output of the electronic relay device.

ELLISON S. PURINGTON.

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