US2198248A

US2198248A - Method of and means for communication

Info

Publication number: US2198248A
Application number: US98874A
Authority: US
Inventors: Clarence W Hansell
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1936-09-01
Filing date: 1936-09-01
Publication date: 1940-04-23
Anticipated expiration: 1957-04-23

Description

Apil 23, 1940.

C. W. HANSELL METHOD OF AND MEANS FOR COMMUNICATION Filed sept. 1, 1956 3 Sheets-Sheet l INVENTOR C. w. HANsz-:LL BY .l M

ATTORNEY n. Qt Q NN dmv April 23,- 1940- -c. w. HANsELl. 2,198,248

METHOD 0F AND MEANS FOR COMMUNICATION Filed Sept. 1, 1936 3 Sheets-Sheet 2 HIGH lll

ATTORN EY April 23, 1940.

c. w. HANsELL 2,198,248

METHOD OF AND MEANS FOR COMMUNICATION Filed Sept. l, 1936 3 Sheets-Sheet 5 Ely. 4,

ATTORNEY Patented Apr. 23, 1940 UNITED STATES METHOD OF AND MEANS FOR COMMUNICATION Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application September 1, 1936, Serial No. 98,874

21 Claims.

This invention concerns a novel method of and means for signalling by means of wave energy, the amplitude, or phase, or frequency of which has been modulated in accordance with signals and the resultant sidebands transmitted with or without the carrier.

The novel method of and means for signalling, of the present invention, involves utilizing each sideband separately for signal reproduction purposes to thereby obtain frequency diversity in the receiver .and all the advantages to be gained thereby, such as reduction of fading and of noise. 'I'he absence of the carrier from the received energy reduces fading usually caused by carrier amplitude fluctuation and harmonic distortion which is caused by beating of the carrier with the sidebands in the circuits or during transmission. When the carrier is suppressed at the transmitter a new carrier may be introduced at the receiver and may be regulated in amplitude so that best results are obtained when it is beat with the sidebands to render the signal.

Modulatcd wave energy from an ordinary amplitude or phase or frequency modulated transmitter may be received, but the receiver of the present invention is also adaptable to the reception of modulated wave energy from a multiplex transmitter of any type, and in particular from the multiplex facsimile transmitter of the present application' and of my United States application #756,926 led December 11, 1934 of which this application is a continuation in part. In the reception of any type of transmission` in accordance with my novel method and means, the benefits of frequency diversity afforded by frequency spacing of the sidebands, each of which alone is caused to reproduce the signal and both of which aiect the indicating means jointly is obtained.

Likewise, the amplitude diversity effect of signals on two sidebands is obtained in the reception of any type of signals and this is particularly of benefit when signals are multiplexed.

In my novel receiver, the sidebands are separated and separately beat with the received carrier or with a new carrier introduced at the receiver and separately demodulated to produce signal indications which may be combined. Since the two sidebands are demodulated separately distortions in the resultant signals cannot be caused by phase fluctuation in the sideband or carrier energy.

As stated above, my novel method of and means for demodulating wave energy may be utilized with any type of transmitter but is particularly adapted to a multiplex transmitter. 'Ihis transmitter comprises a plurality of brushes, preferably or twelve, which are arranged ina single line to scan the message which is printed on a tape having a non-conducting backing. This tape has printed on its surface the message characters in (Cl. Z50-6) control ten audio frequency currents, are thenL impressed on a common line circuit and passed through a transmitter for transmission over the ether, if a radio circuit is used, or over a line to a distant receiver if a land line circuit is used.

In order to reduce undesired fading due to the effect of carrier amplitude fluctuations, and to reduce harmonic distortions which cause interference between channels, it is proposed to eliminate the carrier at the transmitter and to send out both sidebands, and at the receiver to re-introduce the carrier before detecting the signal waves. However, I also contemplate transmission of the carrier which is beat with the sidebands at the. receiver after said carrier is brought to a desired constant amplitude. f

In'receiving, the individual audio frequencies corresponding to each channel are separated by means of filters, and two rectified currents corresponding to each channel combine to reproduce the message at a scanning head, which may, if desired, be identical with that used at the transmitter. Thus both sidebands may be used without introducing distortion due to phase fluctuations, and benefit is obtained from the amplitude diversity effect of using two sidebands on each channel.

'Ihe advantages of using such a multiplex facsimile printer arrangement as outlined above are as follows:

l. 'I'here is no need for synchronizing equipment to keep the terminal oflice transmitting and receiving equipment in synchronism, as must be done in the full page facsimile system. It is only necessary to run the receiving tape at approximately the same speed as the transmitting tape, and this may readily be judged by eye and set by hand.

2. The messages sent out by the multiplex facsimile system are relatively secret, even without the provision of any special secrecy equipment. However, the messages may be made still more secret by providing like cams at transmitter and receiver to continuously vary the relative positions of the ten transmitting and receiving points and by frequently changing the assignments of the radio channels among the ten scanning points.

3. By using the multiplex principle, the keying speed on each channel is reduced to a small .iraction of that required to scan a given area in a given time by the single channel method. If ten channels are employed, then the keying speed, or modulating frequency, ci each channel is reduced to of that required when a single channel is used. For this reason, by he use of suitable equipment, the echo phenomena caused hy multipr. transmission in the radio circuit has less distor .ig effect upon the reproduction. In addition, the reduced speed of each channel peru mits the use of relatively simple transmitting and receiving equipment. At the transmitter, for eX- ample, there may be used the new photocells manufactured by the Weston Electrical Instrument Company under the trade name Photronic, to operate relays and control audio channels directly without requi mg any vacuum tubes or battery supply. At the receiver, ordinary electromagnetically operated vibrators will record as rapidly es will be needed for most circuits.

4.. By using a single line of type sent and received, with, let us paper tape, very little, if any, scanning of blank. space is required. Therefore the total intelligence transmitted per unit frequency per unit of time is better than that obtained with the full. page facsimile system. Even on irly closely written messages the gain may easily be about two to one, and, on the average, considerably more.

5. The facsimile method, as distinguished from code op rated printers, is relatively free from errors, and even when letters are mutilated such fact will be recognized. Incorrect, unrecognizable errors and substitutions are practically absont. With the terminal equipment which may be used, the probability of whole letters being missed is greatly reduced hy staggering the pickup recording points lengthwise of the tape. For a letter to be missed entirely, a fade or interference must last for the time required to pass several letters through the equipment.

G. The operation of the multiplex facsimile communication system does not require operators specially trained to be efficient in the Morse or Continental telegraph codes. It is only nec essary to employ skilled typists to type messages on tape at the transmitter and to copy them on message blanks at the receiver. This should make possible a considerable reduction in central oice personnel cost.

'7. If it is assumed that the power of the transmitter, or its full modulation Swing, used on one channel is then if ten channels are multiplexed on it the amplitude of the modulation allowable on each channel for ideal cenditions is 10%, and the power used per channel is only 1%. In other werds. multiplexing theoretically reduces the power per channel in pro portion to the square of the number of channels. Actually, with the multiplex facsimile scheme, considerable allowance may be made for the diversity in phase of the audio modulating currents and the timing of the marking intervals on the channels. That is, the system may be so set up that all the channels never mark, and all audio frequency currents are never at their peak value with like polarity at the same time, so that one might have a considerable increase in the amount of modulation per chan nel due to the iact that the modulations do not all add together in phase at the same time, and very seldom, if ever, would all channels be modulated at once. By using the equipment without any particular regard to the phase relations oi the ten tones, the amplitude per channel can be set at onequarter the amplitude which could be allowed for a single tone. This corresponds to a pow per of instead of 1% of that allowable when all the modulation is concentrated on a single tone. Since in addition to time diversity in the channels there may be tolerated occasional slight over-modulation, the power per channel may be increased to 10% without causing undue diiliculties. Since there are ten channels, the sum total of modulation therefore may amount to 100% oi that obtainable on single channel. in other words, the power eiiiciency of the mul tipler, system may be set up to be approximately the same as the emciency ol a single modulated system.

8. The use of phase or frequency modulation rather than amplitude iodulation enables obng five or six times the power obtainable with amplitude modulation with an equipment of a given construction cost. Consequently, the overall effectiveness oi the phase or frequency modulated multiplex facsimile printer transmit er in accordance with the present invention is pproximately six times that of a single tone amplitude modulated transmitter. However, my novel transmission system may use amplitude modulation at the transmitter and demodulation at the receiver as well as phase and frequency modulation.

9. At the receiver, use of automatic tuning control with the phase or frequency modulated circuit permits the increase in selectivity ci the receiving system to a point where there is admitted a frequency band only great enough to include the useful modulation.

The foregoing advantages will be more readily apparent from a reading of the following detailed description which is accompanied by drawings wherein:

Figures l. and 2 illustrate two diierent embodiments of transmitting circuits for multiplex facsimile printer systems embodying the principles ci the present invention, the diierence in the two circuits being mainly in the kind of tape used and the manner oi sending eut phase mod ulated Waves instead of amplitude modulated waves.

Figure 3 illustrates one form of receiving apparatus which can be used to receive the phase or amplitude modulated waves.

Figure 4 illustrates simpliiied form of my receiver which is adapted to the reception oi wave energy modulated in any manner.

In Figure lv there is shown a multiplex facu simile transmitter system for ten channels. A multi-tone generator I of any well known form is arranged to generate ten different audio ircquencies f1, fz, f3, etc., one for each channel, which frequencies are individually passed by sending nlters to scanning contacts 3, which may be brushes or needles arranged to bear down on a tape L'. Tape 4 is comprised on a nonconductive ne tissue having written or printed thereon the message in conductive letters, this tape being passed underneath the scanning contacts. Each channel is shown as having a separate contact 3 associated with its filter and another Contact such as 3" which is in a com mon circuit with the other channels, whereby when the two contacts of each channel are connected together upon engaging a letter on the tape, an obvious circuit will be closed to send out the audio frequency characteristic of the Li i) channel over line 5, which line is common to all the channels.

Tape 4 may be madek of paper upon which is placed any suitable conductive solution, for example, potassium, 'ferricyanide and sodium chloride. For that matter the surface may be rendered conductive by marking with an ordinary lead pencil. The tape may consist of a thin foil mounted upon a stout paper backing and', if so, the surface of the foil should be covered with a non-conductive fine tissue and the message placed upon this tissue.. In such case a special solution of graphite may be used for rendering the tape electrically conductive where marked by the stylus or pen. When the metal foil is used the foil may serve as a common contact for all the scanning points.

Instead of using paper or a metal foil, an endless metal tape may be employed. This tape may be passed through a liquid solution, which, when dried, leaves an insulating film on the tape. Then, when writing or typewritten matter is put on the film by marking with an electrically conductive element such as graphite, in the same liquid as the insulator, the insulating material will be pushed and washed aside and its place taken by conducting material. The tape may then be run under the electrical contacts, its message sent, and subsequently it may be washed, returned to the insulating solution, and the whole process repeated.

It is preferred that the metal tape be of a color which is as different as possible from the marking material so as to make the record readily visible to the operator. For economical reasons, however, a polished steel tape is desired. The solution for the insulating and marking material should preferably be in water and slightly alkaline to prevent corrosion of the tape. The insulating material might be made of such elements as water glass, sugar, alum, borax, cadmium sulphide, camphor, dextrin, starch, Egyptian blue, glue, mucilage, etc.

The marking fluid might be a colloidal solution of any conducting material which is chemically stable in Water. For example, carbon, tungsten, silver and platinum might be used. If desired, the marking may be done with a solution which reacts chemically with the material in or on the paper to deposit or precipitate out a conducting material. If desired, solutions of deliquescent material, for example a solution of deliquescent salts which retain moisture and are conducting, might be used for passage under the transmitter contacts. In the latter case coloring matter would be added to the solution in order to make it legible on the transmitting tape. As an alternative to coloring the salt solution, substances might be put in which would decompose due to the passage of electrical current to form a visible record. The record would then be developed by ,the current flowing through it as it passed under the transmitting contact points. The record so formed would thus represent a true copy of the message as sent through the transmitter and so would be of value as a check on the functioning of the transmitter. This may be accomplished, for example, by using iron scanning electrodes and a marking fluid containing potassium iodide or a combination of potassium ferricyanide and sodium chloride.

The tape may be impressed with the message characters either by writing or by a typewriter whose keys hit upon a ribbon placed above the tape and the thin tape then passed under the sending contacts. Ify desired, the tape may be wound around a drum 6, as shown in Figure 2, which drum is rotated to move the type past the scanning contacts. In using an arrangement such as shown in Figure 2, it is preferred to use metal tape of the type mentioned above, whereupon itis only required that there be one scanning contact 3 associated with each channel, the other contact 3" in common with all of the chan nels being in contact with the tape through the drum, whereby closure of both contacts will occur when 3 engages a conductive symbol or letter on the tape.

As the tape passes beneath the sending contacts', there is produced a ycomplex wave representing the sum of the waves of frequencies fi, fa, fz, etc., at any instant, dcpendinon upon th. number of channels closed through by the configuration of the lettery at that instant, which complex wave is transferred over line 5 through a suitable audio amplification means such as l, and then through a land line 8 leading from the transmitter to the receiver in the case of wire transmission, or to a radio transmitter in the case of radio transmission.

At the radio transmitting station there is a source of carrier frequency 9 which preferably is controlled by a crystal I0. The carrier energy is supplied symmetrically to a pushpull modulator il, as shown in Figure 1 of the drawings, in consequence of which the carrier is eliminated from the output I2, and only the two sidebands resulting from amplitude modulation of the carrier and the original modulating frequencies are produced, which are then amplified by power amplifier I4 and radiated by means of an antenna I5. When desired, a filter I3 may be applied as a high pass lter which cuts off at the carrier frequency so that the modulator frequencies and one of the sidebands are eliminated, the other sideband being amplified by the power amplifier I4.

The system of Figure 2 is very similar to that of Figure 1, except that herein a steel tape is used instead of a paper or metal foil tape, and phase or frequency modulation is employed by means of suitable well known apparatus, herein conventionally indicated by box I 6. It is preferred to employ the transmitter system of this figure instead of that of Figure 1, because of the greater amount of power which may be obtained over each channel.

For receiving signals sent out over the system of Figure 1, any suitable arrangement may be employed wherein a carrier Wave is reintroduced at the receiver and the audio frequencies characteristic of the various channels filtered out to record the signals on a suitable scanning head which preferably is identical with that used on the transmitter. One suitable way of accomplishing this is disclosed in my United States Patent #1,751,584 to which attention is invited.

.Figure 3 illustrates the preferred form of receiver for receiving phase modulated waves transmitted over the circuit of Figure 2. This apparatus comprises a special phase modulation telephone receiver having two sets of audio frequency filters and two sets of rectiiiers or Thyratrons operated from the output of the receiving filters. A description of this receiver and the operation thereof will now be given:

The incoming phase modulated radio frequency energy picked up by antenna I is increased in power in a high frequency tube amplifier 2, and the amplified energy is beat down by means of oscillations from 0 to an intermediate frequcncy in a detector of the electron discharge type The intermediate frequency energy is further amplified in tube amplifier 4. The output of amplifier i is then divided into two portions, one of these portions being the carrier wave and the other being the side frequency waves produced by the multiplex modulation of carrier at the distant transmitter. The new carrier wave is separated out from the complex signal by means of a neutralized piezo-electric crystal filter 5 whose selectivity is so great that in the output there appears only the unmodu- 1'ated carrier the mean intermediate frequency. This carrier is then amplified and limited in iter t so that its strength will remain constant :ardless of variations in the strength of the carrier picked up by the antenna. The limiting to eliminate the eects of carrier fading. limiting, the carrier, whose strength is relatively large, is split into two compohaving equal strength but differing in phase by 0 or 180, one of these components being applied cophasall.y to the control electrodes of the tubes of detector and the other component being applied co-phasally to the control electrodes of the tubes of detector 8. The strength of the carrier introduced as described into the two detectors is made relatively large.

carrier may also be supplied from a separate local oscillator O' connected by switch S to the lines L leading to detectors 'l and 8 in which case the lines are disconnected from G by switch S'.

Another portieri of the intermediate frequency energy is taken from the output of amplifier 4 and applied to high and low pass filters 9 and i3. These filters separate those frequencies lying above and below the carrier frequency into two independent circuits. The carrier of interi'ate frequency may or may not be permitted to pass through the filters but this s not important since if carrier energy does pass through these filters it will reach the two detectors 'l and with a strength very much less than the strength of the carrier that is applied from the lil iter i to the detectors. Assuming that there .3, er and lower side frequencies produced by the multiplex modulation and these side freci s separately flowing in the two circuits, 1 e energy of one group of side frequencies is i p 'ed to the control grids of the tubes of both .I `s of detectors 'i and 8 in like phase relation. other set of side frequencies is applied to ie control grids of the tubes of the two detectors and fl reversed phase relation. Under these onditions, taking into account the fact that detectors i and 8 are supplied with carrier energy :Tn l or 180 phase relation, the outputs from the and 8 will differ in audio phase two detectors i 'cy O or i800. The outputs from the two detectors are passed through transformers Il and I2, each of which has two secondary windings. One secondary winding of each transformer is connected in series with a similar winding on the other sformer with like polarity, and the other ondary winding of each transformer is conin series with the corresponding winding Aie other transformer with reversed polarity.

Under thc-se conditions, if the conductive confecions from the low pass filter l0 are interrupted, it can be seen that the energy from high pass filter 9 will produce outputs from both detectors and 8 which will add together in one cf the serially connected output transformer circuits but oppose each other in the other serially connected output transformer. If, on the other hand, the energy from high pass filter 9 is in stead interrupted but that from filter l0 allowed to pass then there will be obtained output energy in the secondary circuits of the output transformer in the other of the two serially connected output circuits. In other words, in each of the two output circuits energy is obtained corresponding to only one of the side bands produced by the modulation. When the mcdu lation consists of intermittent audio tones of different frequency produced by the scanning of the tape at the transmitter one should normally find a corresponding audio modulation in both of the serially connected output circuits. However, if radio conditions are such that one side frequency or sido band has faded out or been reduced in amplitude the corresponding side frequency or frequencies on the other side of the carrier frequency may still be present. Only one side frequency corresponding to any one modulated frequency needs to be present in the signal picked up by the receiver in order that there may be a corresponding output in one or the other of the serially connected output circuits. If desired, the series of keyed tone frequencies in each output circuit may be taken, separated from one another by means of band pass filters and then the energy of each tone converted into a direct current. A similar treatment may be given to the several tones in the other output circuit. Under fading conditions the tone outputs in either of the output circuits may fail fairly frequently but only occasionally will the outputs from both output circuits fail simultaneously. In other words, there is a diversity effeet against fading in using the two sidebands independently. If it is attempted to detect the modulation with the ordinary detector scheme wherein both the side frequencies are used to produce each audio output, very frequent failures will occur due to the phase relation of the carrier wave varying with respect to the sideband waves during transmission over the radio circuit.

In the description of the operation of Figure 3, so far given, it has been emphasized that each side band produces an output in one of the serially connected output circuits and the other side band produces an output in the other of the serially connected output circuits. It may also be interesting to note the manner in which the two detectors I and 8 of Figure 3 cooperate to bring about this result.

If at any particular time we have equal upper and lower side band inputs which produce outputs from both of the two detectors then these outputs from the two detectors are made up of modulation frequency currents which are 90o different in phase. The 90 phase relation comes about automatically due to the reversed polarity of input to the two detectors from the low pass nlter. It is caused by the fact that reversal of polarity of one side band causes a change of one quarter cycle, at each modulation frequency, in the time required for a pair of conjugate side frequencies to reach any specified phase relation. In other words, if we are receiving a carrier wave modulated by a single audio frequency then, if we could reverse the polarity of side frequency current on one side of the carrier, the effect would be to advance or reta-rd the time at which the two side frequency currents add (or oppose) by a time interval equal to a quarter cycle of the modulating frequency.

Since the outputs of

detectors

1 and 8, when both have an output, are always 90 different in phase, due to reversed polarity of one of the side band inputs, it follows that the output in each of the serially connected output circuits is made up of two output components of current, for each modulating frequency, and that these components must be added at 90, vectorially, in order to obtain the resultant output in each circuit. Since the serial output connection is reversed, in the output of one detector in one of the serially connected output circuits, it follows that we must reverse one of our 90 vectors before we make the vector addition to find the resultant current or potential in this circuit. It is this reversal which gives us addition of the components due to only one side band in each of the serially connected output circuits. 'I'herefore, if we could interrupt first one and then the other of the inputs from the high and low pass filters to the detectors we would interrupt first one and then the other of the outputs to the two serially connected output circuits. That is, in each of the output circuits the components of output due to only one side frequency are added and the components due to the other side frequency are opposed.

If we consider now the effect of the phase relation of the carrier supplied to the detectors, with respect to the modulation vector, or vector sum of side frequency components, we will nd that there is a difference of 90 in the relations of carrier and modulation vector in the inputs to the two detectors. In other words, when the carrier and side frequencies cooperate to produce maximum amplitude modulation of the carrier in the input to one detector they produce maximum phase modulation in the input to the other detector. Also, if the phase relation of the carrier, with respect to the side frequencies, could be varied continuously first one and then the other detector would be provided with maximum amplitude modulation. The -amounts of amplitude modulation in the inputs to the two detectors would vary up and down sinusoidally, and differentially, if the phase of the carrier were moved at a constant rate.

In long distance short wave radio communication the phase relation of the received carrier, with respect to the received modulation component, does vary in a more or less random fashion so that a wave transmitted with amplitude modulation may arrive at the receiver with either amplitude or phase modulation, or both, in varying degree. The receiving system of Figure 3 gives a useful output when the received wave is either amplitude or phase modulated because one or both detectors always provide an output regardless of the carrier phase.

We may think of detector 1 asan amplitude modulation detector and of detector 8 as a phase modulation detector, or vice versa, depending on specific circuits and adjustments. One detector may then detect amplitude modulation and the other phase modulation. Then, by adding the two detector outputs, keeping in mind their constant 90 difference in phase brought about by reversing one side band input to one detector, we may obtain an output from either of the two serially connected output circuits which is constant in value regardless of variation in the carrier phase which turns amplitude modulation into phase modulation, and vice versa, in varying amounts as observed in practice on long distance short wave circuits.

It would be fortunate if we could combine the outputs from the two serially connected output circuits to obtain a single modulation frequency output such as We would like to have for long distance short wave telephone communication. Unfortunately this can not be done because the relative phases of the outputs in the two serially connected output circuits vary in accordance with variations of phase between the conjugate side frequency components which carry the modulation. i At times, due to random iiuctuations in relative phase or timing of the two side frequencies, corresponding to each modulating frequency, the two outputs would add and at times they would oppose. However, by keeping the two outputs separate and using only their energy components, independent of phase, we may cause them to cooperate in providing a conmlon useful signal for purposes such as telegraph or facsimile communication. We may rectify the two alternating current outputs in the two serially connected output circuits to obtain direct currents which can be added or we can use each current to control a relay device such as the Thyratrons in Figure 3, which cooperate in producing a useful record or other indications of a transmitted message. t

We may note that the vector sum of the two detector outputs in either of the two serially connected output circuits remains unchanged even if the phase relation of the carrier, with respect to the side frequencies, is continually changing. For this reason there is no need for the usual fixed relation in phase between carrier and side frequencies. Large carrier phase variations, during radio transmission, do not produce the usual modulation fading. Also, if desired, the carrier may be suppressed at the transmitter and its place taken by a. non-synchronous carrier generated and introduced at the receiver.

The non-synchronous carrier must, of course, have a frequency close to the exact frequency which a synchronous carrier would have. Otherwise the beat frequency outputs between the carrier and each of the two side frequencies will not have the same frequency. If the carrier frequency changes relative to the side frequencies there will be a differential variation in beats between the vcarrier and the side frequencies. If the variations in the beat frequencies are very great then filters I3 and I4 of Figure 3, for eX- ample, may not be wide enough to pass the signal output energies on their changed frequencies. In practice, it is not very difficult to provide constancy venough to hold the output beat frequencies within the filter bands. On the other hand, it is virtually impossible to introduce a locally generated carrier into a double sideband signal and maintain correct phase relations for producing constant modulation of the carrier by the side frequencies. The frequency tolerance permissible in this case 1s zero and has never been attained. For these reasons the possibility of using a non-synchronous carrier at the receiver in the present rsystem makes double sideband carrier eliminated communication possible and so provides a greatly improved system. It is no longer necessary to provide a transmitter capable of the power output of both carrier and side frequencies in order to obtain useful double sideband transmission.

Instead of converting each tone frequency from each output circuit to direct current in the ordinary manner, it is preferred to use these energies to control a pulsating direct current more suitable for actuating the vibrator of a multiplex facsimile recorder. Assuming, for example, that one of the multiplexed channels employs a keyed tone frequency of 1000 cycles, then under normal conditions 1000 cycle energy will appear in both of the receiver output circuits. If band pass filters I3, i4 will pass 1000 cycles, then keyed 1000 cycle energy will reach the grids of two 'Ihyratrons cr grid controlled glow discharge tubes l5, I6. The presence of audio excitation on the grids of the Thyratrons will cause the gas within the tubes to be ionized when anode voltage is applied. This will permit anode current to pass. If the anodes of the two Thyratrons are excited differentially by means of 60 cycle alternating current from the transformer il, as shown, then so long as a 1000 cycle tone is present in both output circuits both tubes will carry current on the positive half of the 60 cycle voltage applied to their anodes. As a result, a pulsating direct current which is modulated at the rate of "120 cycles will be applied to the coil of the recorder vibrator I8 and will cause the vibrator to record marks on a suitable recording tape corresponding to the markings on the transmitter tape at the sending station. If either side frequency in the incoming signal fades out, the recorder vibrator will still be operated if the opposite corresponding side frequency is still present though the recording will be perhaps slightly less efficient. A pair of audio frequency band pass filters corresponding to lters I3, I4 will be used for each tone frequency used in the multiplex facsimile transmission. Of course, if desired, the tubes l5, i6 may be of the high vacuum type or they may be replaced by oxide rectifiers or any other suitable means for controlling a direct or a pulsating current, which can be used for recording purposes. Preferably ten or twelve transmitting channels and ten or twelve recorder vibrators would be used. The recorder vibrators may print the message on a paper tape by tapping a typewriter ribbon or a strip of carbon paper, or by any other means by which electrical energy may be used to make markings on the paper.

It will be noted that the output of the special receiver noted above gives two sidebands which are composed of the ten transmitted tones in a single complex wave. The audio filters I3, i4 are really wave analyzers, there being one pair for each of the two tones, so that each of the compleX sidebands are resolved into ten component tones, therefore, since there is an upper and a lower complex sideband, there will be an upper and a lower tone sideband for each transmitted tone. Figure 3 shows only one of ten units coinposed of audio filters I3, I4, Thyratrons I5, I6, and recorder vibrator I6. As explained above, each audio lter passes one tone sideband. One tone sideband is impressed on the grid of one Thyratron and the other tone sideband is impressed on the grid of the other Thyratron. Thus, either sideband will cause a half wave rectied current to flow through the recorder vibrator winding in pulses 60 per second. Each pulse causes the vibrator to move against a light spring, the spring lifting the vibrator between impulses. The ten vibrators are arranged so as to account for that element of the signal mark corresponding to the tone which was produced by scanning a single element at the transmitter. The vibrator engages a carbon paper tape or typewriter ribbon, which rests in contact with a white paper tape, to reproduce the signal mark scanned.

In order that the receiver may be made more efficient, lt is preferred to apply an automatic tuning means for correcting the receiver adjustments and holding optimum conditions for reception. 1n doing this a phase detector i9 may be used for detecting the phase relation between radio frequency energies of the carrier before and after the crystal lter 5. Several different methods are well known in the art for varying the value of a direct current by means of the phase relation between two alternating current energies. Any one of these schemes may be used in the detector I. A direct current output is taken from detector IS which varies in strength or polarity or both in accordance with the phase relation of radio frequency energies across input and output of the crystal filter. The direct current energy is then used in any one of several well known schemes for Varying the frequency of the first beating oscillator in the receiver. When connections and adjustments are made correctly, any change in the frequency of the intermediate frequency energy will be accompanied by a phase shift in the radio frequency energy on the two sides of the crystal lter and this phase shift will automatically react upon the first eating oscillator to reduce the change in intermediate frequency. Thus, once the receiver is adjusted to the transmitter, it will keep itself correctly adjusted without manual manipulation.

When the usual modulated wave, such as used in telephony is used, the two sidebands each produce in the output of l' and 3 signal currents which may be used separately or jointly in the recording apparatus shown or other indicating means such as phones.

Figure 4 illustrates a simplified forni of receiver accomplishing substantially the same results as that shown Figure 3. It will be noticed that the receiver in Figure i differs from that shown in Figure 3 principally in the arrangement of the detecting circuits. A description of this receiver and its operation is as follows:

The incoming modulated radio frequency energy picked up by antenna I is increased in power in a high frequency tube amplifier 2, and the amplified energy is converted by modulation with oscillations from oscillator O in first detector 3 to a suitable intermediate frequency. The intermediate frequency energy is further amplified in tube amplifier 4. The output of amplifier is then divided into two portions, one of these portions being the carrier wave and the other being the side frequency waves produced by the modulation of the carrier at the distant transmitter. The new carrier wave is separated from the com piex signal by means of a neutralized piezo electric crystal filter 5 whose selectivity is so great that in the output there appears only the unmodulated carrier cf the mean intermediate ircquency. This carrier is then amplified and liniited in limiter ES so that its strength will remain constant regardless of variations in the strength of the carrier picked up by the antenna. The limiting tends to eliminate the effects of car'ier fading. After limiting the carrier energy is further amplied and applied through separate coupling means to detectors l and 8. 1t will be noted that in this receiver it is not required that the carrier have any particular phase relation l'or the proper operation of the detectors as oppose to the specific phase requirements of the receiver illustrated in Figure 3.

Another portion of the intermediate frequency energy is taken from the output of amplier 4 and applied to high and low pass filters 9 and I 0. These filters separate those frequencies lying above and below the carrier frequency into two respective independent circuits. The carrier or intermediate frequency may or may not be permitted to pass through the filters but this is not important since if carrier energy does pass through these filters it will reach the two

detectors

1 and 8 with a strength very much less than the strength of the carrier that is applied from the limitor 6 to the detectors. It is now Observed that in each of the detectors I and 8 are present only one sideband and the carrier frequency and thus for detection purposes we have the equivalent of single sideband reception wherein it is unnecessary to maintain a specific co-relative phase relation between carrier and side frequencies in order to obtain proper detection action. It will be further observed that the system will function equally satisfactorily for either amplitude or phase modulation as the segregation of the sidebands into separate detectors converts either type of modulation into equivalent single sideband modulation.

The outputs of detectors 'I and 8 are separately coupled through their respective output circuits Il and I2 to suitable recording devices, in the circuit illustrated, to a group of selective frequency multiplex recorder vibrators, the operation of which was described with reference to Figure 3.

One advantage obtained in this receiver is that it is substantially insensitive to co-relative phase relations of carrier and sidebands. Thus there is no discriminatjon in output resulting from differential phase changes of carrier and side frequencies caused by fading or multipath transmission phenomena. Further, it is equally responsive to either phase or amplitude modulation for the same reason.

Another advantage obtained in this receiver as in the receiver of Figure 3 is the advantage of frequency diversity in that the receiver will deliver a satisfactory output so long as the carrier and one of the sidebands is present and it is well known to the art that fading phenomena over long distance transmission path discriminate between frequencies differing by only a fraction of a percent in such a manner that it is seldom that both side frequencies of a particular modulation will be simultaneously low as the result of fading but rather at a particular instant due to the nature of the fading phenomena it is likely that one side frequency may be enhanced and the other reduced. Thus, the total output of this type of receiver may be eX- peeted to remain reasonably cons-tant in the presence of severe fading conditions Whereas in a common type of receiver wherein carrier and sidebands are applied together` to one detector, co-relative phase distortion resulting from fading will cause a wide variation in signal strength and objectionable harmonic distortions in the output although the respective frequencies may be present in substantially proper strength.

It will be noted that the phase detector i9, controlling the frequency of the heterodyning oscillator O is not essential to the proper operation of this receiver. However, it will be of considerable advantage in maintaining alignment of the carrier and sidebands in their respective filter circuits as it is recognized that it is somewhat diflicult to maintain alignment of the car- -rier in the piezo-electric filter if an unstabilized heterodyning oscillator is used, particularly in the reception of high frequency signals.

I claim:

1. A system of radio communication including a signal modulated carrier wave transmitter in which the carrier wave is suppressed but both sidebands of the signal modulated carrier are transmitted, a receiver including means for beating each sideband independently with a locally generated carrier wave to produce two independent outputs whose strength is substantially independent of the exact phase or frequency of the locally generated carrier and means responsive to the two outputs cooperatively to reproduce intelligible signals carried by the modulation.

2. A system of radio communication including a transmitter in which a carrier wave is modulated in phase by signal potentials and the carrier wave is suppressed but both sidebands of the signal modulated carrier are transmitted, a phase modulation receiver including means for producing local oscillations of controlled amplitude and for beating each sideband independently with the locally produced oscillations to produce two components of different phase whose strength is substantially independent of the exact phase or frequency of the locally generated carrier and circuits responsive to the two components to reproduce intelligible signalscarried by the modulation.

3. A system of radio communication including a transmitter having means for modulating the length of a carrier Wave in accordance with signals to produce at least two side frequencies corresponding to each signal frequency, means for transmitting the resultant sideband frequencies, a receiver including a source of oscillations of a frequency of the order of the frequency of the modulated carrier and meansI for separately beating each sideband with oscillations from said source of oscillations whereby each sideband is detected independently of beats between the local oscillations and the other' sideband to produce two independent outputs, each substantially reproducing the original modulation regardless of fluctuations in length of the received carrier, and means connected with said beating means for recording the sum of the two outputs irrespective of their phase relation to render intelligible signals.

4. The method of reducing the effects of fading in radio communication which includes the steps of modulating a carrier wave at the transmitter to produce two side frequencies corresponding to each modulation frequency while suppressing the carrier wave energy, transmitting the side frequencies, receiving the side frequencies, producing local osciliations of large amplitude as cornpared to the amplitude of the side frequencies, beating the local oscillations of large amplitude with each side frequency to separately dernodulate said side frequencies and combining the resultant energy to reproduce the signals.

5. A system of radio communication including a transmitter in which the carrier wave is suppressed and both sidebands of thesignal modulated carrier transmitted, a receiver including means for producing oscillations the amplitude of which may be regulated for beating with the said sidebands, means for beating each of said sidebands independently with said produced amplitude regulated oscillations and means for utilizing the two energies produced by said beating action to produce the signal.

6. A system of radio communication including a phase modulation transmitter in which the carrie wave is suppressed and both sidebands of the sig al modulated carrier transmitted, a receiver including a separate phase modulate wave fic-modulator for each sideband, a common means for producing oscillations the amplitude or" which may be regulated for beating with the respective sidebands, means in each demodulatcr ior beating each of said sidebands independently with said produced amplitude regulated oscillations and means coupled with the output ci' each demodulator for utilizing the energies produced by said beating action to reproduce the signal.

'7. The n' n"hed oi reducing the of lading in radio communication which includes the steps of modulating carrier wave at the transmitter to produce at least two side frequencies corresponding to each modulating frequency, transmitting said side frequencies, receiving said side frequencies, separately 1seating said side frecuencias with oscillations of controlled amplitude to produce two detected outputs, controlling the frequency of said oscillations in a sense to compensate ior phase or frequency va "iatlons of the received wave, and utilizing said outputs cooperatively to reproduce intelligible signals.

8. Ln a system for receiving Wave energy modulated in phase and comprising carrier and sideband components, means for separating said carrier and sidehand components each from the other and passing the separated components over separate paths, apair of detectors, each detector comprising two discharge devices each having a control electrode and an output electrode, means ior coupling one of the paths in which sideband energy news to the control electrodes in the devices of one of said pairs oi detectors, means coupling the path in which the other sideband energy 'lows to 'the control electrodes in the devices of 'une other of said pair of detectors, means coupling the path in which carrier energy flows to corresponding electrodes in each or" the devices l: `th detectors and an output circuit coupled to the cutout electrode in each of the devices of both or s d :airs of detectors.

9. lli a ase modulation receiver, a plurality ci pairs ol" electron discharge devices each having a control a cathode and an anode, a source or" phase modulated oscillatory energy comprising carrier sidebands, means for applying one of said sidebands in phase opposition to the control grids of the tubes oi each of said pairs of uces, means i'or applying the other of said sidebands in phase opposition to the control grids of the tub s of each of said pairs oi tubes, the phase e other said hands applied to the consrol grids ci the tubes ci one of said pairs of tubes being substantially reversed relative to the phase oi *he said other oi said side bands but relatively reversed phase or polarity as applied to the control electrodes ci one of the other of said of tubes, means for applying energy charac tic tubes or" said carrier wave energy in phase to the control grids of the tubes in each of said pairs of tubes, output circuits conect' ig the anodes oi each of said pairs of tubes in push-pull relation, pair of secondary Windirgs couple to each of said output circuits and r co circuits connecting a winding in each or said pairs or windings in series, the polarity of the windings in one or" said recording circuits being similar so that one component of output currents therein add, the polarity of the windings in the other ol said recording circuits being opposed so that another component of output currents therein also add.

lo. In a system for demodulating Wave energy modulated in phase and comprising a carrier and two sidebands, means for receiving said energy, iltering means connected with said last named means for separating said energy to obtain therefrom carrier energy only and upper and lower sideband energies only, a pair oi electron discharge devices each having corresponding control electrodes and corresponding output electrodes, separate means for impressing both of said sideband energies in phase opposition on the corresponding control electrodes in said devices, means for impressing carrier energy in phase on corresponding control electrodes in said devices and an output circuit connected with the output electrodes of said devices.

ll. in a system for demodulating phase modulated wave energy comprising a carrier and upper and lower side'oands and for using the difference in frequency of said sidebands to overcome iading, means for receiving and separating the carrier and sidebands each from the other, a detector ior each sideband each detector having a plurality or electrodes including an output electrode, means for impressing one sideband on electrodes in one oi said detectors and the other sideband on the corresponding electrodes in the other detector, means for impressing carrier wave energy displaced in phase relative to both sidehands on both detectors and a separate output circuit coupled to the output electrodes of each of said detectors.

l2. in a system for receiving wave energy modulated in phase and comprising carrier and sideband components, means for separating said carrier and sideband components each from the other and passing the separated components over separate paths, a pair ol' diierential detectors, each detector comprising two discharge devices having control electrodes and output electrodes, means for connecting one of the paths in which sideband energy flows to like control electrodes in th'v devices of one of said pairs of detectors, means connecting the path in which the other sideband energy flows to like electrodes in the devices oi the other or said pair of detectors, means connecting the path in which carrier energy flows to corresponding electrodes in each ol` the devices of both detectors and an indicating circuit coupled to corresponding electrodes in each of the devices in both of said detectors.

i3. A system as recited in claim l2 wherein the carrier Waves applied to the corresponding electrodes in each of the devices i both detectors are displaced in phase relative to the energy in both of said sidebands.

in a phase modulation receiver, a plurality or" pairs of electron discharge devices each having a control grid, a cathode and an anode, a source of phase modulated oscillatory energy comprising a carrier and sidebands, means for applying one of said sidebands in phase opposition to 'the control grids of the tubes of each of said pairs ci tuoes, means for applying the other or said side'cands reversed in phase relative to the phase of said first sidehand, in phase opposition to the contro electrodes of the tubes of each or' said pairs oi tubes, means for applying energy characteristic oi said carrier wave energy in phase to se control gr or the tubes in each or" said pairs or" tubes, the in-phase carrier energy on the control grids of one of said pairs of tubes being in phase or in phase opposition relative to the inphase carrier energy on the control grids of the other of said pairs of tubes, output circuits connecting the anodes of each of said pairs of tubes in push-pull relation, a pair of secondary windings coupled to each of said output circuits and recording circuits connecting a winding in each of said pairs of windings in series, the p0- larity of the windings in one of said circuits being similar so that currents therein add, the polarity of the windings in the other of said circuits being opposed so'that the currents therein oppose.

15. In a system for demodulating wave energy modulated in phase and comprising a carrier and two sidebands, means for receiving said energy, means connected with said last named means for separating said energy to obtain therefrom carrier and upper and lower sideband energy, a pair of electron discharge devices having corresponding control electrodes and output electrodes, separate means for impressing both of said sideband energies in phase oppositiony to the corresponding control electrodes in said devices, means for impressing carrier energy in phase on corresponding control electrodes in said devices and an indicating circuit connected with the output electrodes of said devices.

16. A system for receiving and demodulating modulated transmitted wave energy comprising two sidebands resulting from modulating a carrier at signal frequency and preventing phase variations in said carrier during transmission from reducing the modulation output comprising, a receiver including means responsive to said modulated Wave, two separate iiltering means coupled with said responsive means for deriving respectively lower sideband energy only and upper sideband energy only, means for producing oscillatory energy of a frequency of the order of said carrier frequency and of substantially constant amplitude, a separate balanced demodulating means coupled to each of said filtering means, means coupling each of said balanced demodulating means to said oscillation producing means so that oscillatory energy is beat with each sideband separately to produce two independent beat notes characteristic of each sideband, independent output circuits connected with each of said demodulating means in each of which outputs beat frequency energy substantially reproducing the original modulation regardless of phase or frequency iiuctuations of the received carrier ows, and means coupled with said output circuits for adding said output energies irrespective of the phases to render the signals.

17. In a system. for receiving and demodulating modulated transmitted wave energy comprising a carrier and two sidebands and preventing variations in the phase of said carrier during transmission from reducing the modulated output, modulated wave responsive means, sideband separating means coupled with said responsive means, carrier separating means coupled with said responsive means, a pair of balanced rectifiers, means for impressing one sideband and carrier energy on one of said balanced rectiers, means for impressing the other sideband and carrier energy on the other of said balanced rectiers, independent output circuits connected with each of said rectiiiers in each of which output circuits energy substantially reproducing the original modulation regardless of phase or frequency uctuations of the received carrier flows, and means associated with said output circuits for recording the arithmetic sumr of said outputs to render the signals.

18. 'Ihe method of demodulating transmitted Wave energy comprising a carrier and sidebands and for Apreventing phase or frequency fluctuations of the carrier during transmission from reducing the demcdulated output which includes the steps of beating two portions of each of said sideband energies with carrier energy to produce pairs of beat notes characteristic of upper and lower sideband energy, adjusting the phase relation of the carrier and sideband portions so that a displaced phase relation is maintained between said pairs of beat notes whereby beat notes characteristic of the same sideband energy do not oppose and cancel, and combining the beat notes characteristic of each sideband energy additively to reproduce the signal.

19. In a system for demodulating transmitted wave energy comprising carrier energy and a pair of sidebands and for preventing phase or frequency fluctuations of the carrier during transmission from opposing demodulation components and reducing the output, means for beating energy characteristic of said carrier energy with two portions of each of said sideband energies to produce beat note energies of displaced phase relation, and means for combining the beat note energies characteristic of each sideband additively to produce two independent outputs.

20. 'I'he method of reducing the effects of fading in radio communications which includes the steps of, modulating a carrier wave at the transmitter to produce at least two side frequencies corresponding to each modulating frequency, transmitting the resulting side band frequencies, receiving the transmitted side band frequencies, producing, by means of said received side band frequencies, new bands of wave energy characteristic of the original sideband frequencies, producing local oscillations of substantially constant amplitude and of a frequency intermediate the mean frequencies of said produced bands of wave energy, separately beating said local oscillations with each of said produced bands of wave energy to produce, by each of said beating processes, two components of diiferent phase each characteristic of signal modulations and combining pairs of said components characteristic of the same band of Wave energy additively to render the signal.

2l. In a system of radio communication, a transmitter in which the amplitude or strength of a wave is signal modulated to produce at least two side frequencies corresponding to each modulating frequency and the resultant carrier and side band are transmitted, a receiver energized by said transmitted side bands, means in said receiver for separating the side bands from the carrier and from each other and for deriving, from said side bands, bands of energy characteristic of each side band, a pair of rectifying means, means for impressing a band of energy characteristic of each side band on each of said rectifying means, means for impressing beating voltages on each of said rectifying means, means coupled with said rectifying means to produce an independent output for each side band each output substantially reproducing the original modulation irrespective of phase or frequency fluctuations of the received carrier, and means for recording the two outputs singly and together to render intelligible signals.

CLARENCE W. HAN SELL.