US2104012A - Multiplex radio signaling system - Google Patents

Description

Jan. 4, 1938. E. H. ARMSTRONG 2,104,012

7 MULTIPLEX RADIO SIGNALING SYSTEM Filed Sept. 14, 1935 5 Sheets-Sheet 1 x m m Q 3 & R S N g S g k S 1 Q k t q k 2 a a w m a -a a Q a .g Q & 8Q Q w a "W:

: 2 Q 7 m o I INVENTOR. Edwin HAr/nsrmnq.

ATTORNEYS.

Jan .4, 193s. E. H. ARMS RONG 2,104,012v

MULTIPLE)! RADIO SIGNALING SYSTEM Filed Sept. 14, 1955 5 Sheets-Sheet 2 fi ER INVENTOR.

EdW/I? H. Arm srrang BY MM M ATTORNEYS.

Jan. 4, 1938. E. H. ARMSTRONG MULTIPLEX RADIO SIGNALING SYSTEM 5 Sheets-Sheet 3 Filed Sept. 14, 1935 ATTORNEYS.

Patented Jan. '4. 19

UNITED STATES PATENT OFFICE 8 Claims.

This invention relates to a new system of multiplex operation in radio signaling by which it is possible not only to multiplex successfully but to operate over greater distances and more efiectively with a given amount of power than if the power were divided among separate transmitters operating on separate simplex channels.

Many methods of multiplexing with frequency modulation have been proposed but none have been successfully operated, on account of dlfflculties of cross modulation between the channels, distortion and the like.

This system employs the wide band method of frequency modulation described in my U. S. Patent #1,941,069 and preferably the method of transmission described in my U. S. Patent #1,941,068. With the system of multiplexing ,of the present invention interference between the channels is eliminated to an extent that multiplex transmission and reception of broadcast programs can now be successfully practiced. Simultaneous transmission of two services such as the present sound program together with a facsimile service or the transmission of the same sound program on separate channels for producing perspective broadcasting and reproduction are also practicable.

Referring now to the figures which form a part of this specification Fig. 1 represents the basic transmitting arrangement and Fig. 2 one form of the multiplexing system for use in conjunction with it. Fig. 3 illustrates the general arrangement of the receiving system which is used when the system of Fig. 2 is employed. at the transmitter. Fig'. 4 shows an improved form of multiplex system for use in conjunction with the transmitter of Fig. 1 instead of the arrangement of Fig. 2. Fig. 5 illustrates the receiving arrangements for use when the system of Fig. 4 is used at the transmitter. Fig. 6 shows a characteristic curve of some of the operating features of the system which will be referred to hereinafter at length.

Referring now to Fig. 1, I represents a constant frequency oscillator, and 2 an amplifier of the output of this oscillator with a resistance 3 in its plate circuit which is small in comparison with the impedance of the tube. 4 and 5 are likewise amplifiers of the current produced by the master oscillator I. 42 is a transformer for differentially modulating the screen grid voltages of the amplifiers l and 5 by the signaling current which is applied to the' primary of the transformer thru the amplifying system 3l-4l. 43 and 44 are by-pass condensers for shunting be hereinafter explainedthe two halves of the secondary of the transformer 42, with respect to the high frequency currents of the master oscillator frequency. 6 and 1 are inductances whose impedance for the frequency of the master oscillator is small compared to the impedance of the tubes 4 and 5. 8 and 9 are condensers whose reactances for the master oscillator frequency are equal to the reactances of the inductances 6 and 1. III is a small inductance whose natural frequency is high compared to the frequency of the master oscillator. II is a condenser for balancing out the reactance of the coil I0. I2 is an adjustable resistance and I3 an amplifier connected across this resistance. The plate circuit of this amplifier is connected to the resistance 3 in the output circuit of the amplifier 2. I4 is an amplifier for the combined outputs of thetubes i3, and I 5, I 6 and I1 represent means for correcting the asymmetry of the transformer 3, 1, ill with respect to the 'two side bands as describedin my U. S. patent application Serial No. 40,542, filed September 14, 1935, Patent No. 2,063,074, issued December 8, 1936. I8 and I9 are amplifiers of the output of this system and a frequency doubler. 2| is a filter for the purpose of eliminating the fundamental frequency applied to the doubler as described in my U. S.

2 and patent application Serial No. 40,543, filed September 14, 1935, 22 is an amplifier of the doubled frequency, 23a second doubler and 24 a second filter for the purpose described above.

25 represents an amplifier, 26 a frequency doubler which may consist of a series of doublers or tripplers, 21 the power amplifier and 28, 29 the radiating system. 30 represents the input to the modulating system of the transmitter, 3| an amplifier forthe modulating current, 33, 34

-a balanced push pull amplifier and 35 an output transformer feeding the correction system 36,

31. 38 is an amplifier feeding a second balanced push pull amplifier 40, ll thru the transformer 39. The output of the tubes 40, 4| is supplied to the modulator tubes 4 and 5 thru the transformer 42. The general arrangement so far is substantial- 1y along the lines laid down in my U. S. Patent #1,941,068 and the improvements described in my U. S. applications for patents Serial Nos.

40,542 and 40,543. Certain changes in design, however, in this part of the system which are essential to successful multiplex operation will Referring now to e'""ifiultiplex channel. ar-

rangementsshm g. 2, represents the input of channel #1.

is an amplifier for the currents of this channel and 41 a low pass filter to confine the frequencies to be transmitted over this channel to its designed width. 40 is a mix-.

Referring now to channel # 2, 49 represents the input thereto and 50 an amplifier for the currents thereof. 52 represents an oscillator, usuallyof a superaudible frequency, but at least of a frequency which is the sum of the highest frequency to be transmitted on channel #1 and the highest modulating frequency to be applied to channel #2. Thru the coupling transformer 53, this frequency is applied to the input of a push pull amplifier 54, 55 the output of which is cumulatively fed into the band pass filter 58. The output of the amplifier 50 is supplied thru the transformer 5| to modulate the plate voltage of the amplifier 54, 55 thereby producing amplitude modulations upon the current of the frequency of the oscillator 52. The band pass filter in this arrangement is made wide enough to pass both side bands. The modulated current which passes thru the filter 58 is supplied to the mixer tube 59 and combined thereby with the currents of the output of the mixer tube 48 of channel #1.

The aforegiven description with respect to channel #2 applies exactly with respect to channel #3 with the exception that the frequency of the oscillator 64 of channel #3 is chosen to be at least the sum of the oscillator frequency of channel #2 plus the sum of the modulating frequencies of channels #2 and #3. The input to the channel is supplied at El and the output, thru the ,mixer tube II is combined with the outputs of the other two channels for application to the main modulating system of the frequency modulated transmitter at 30.

It is, of course; obvious that more channels operating on the same principle may be added.

Referring now to Fig. 3, 18-, I9 represents the receiving antenna, an amplifier for the received current, 8I--86 the rectifiers, oscillators and amplifiers of a two intermediate frequency superheterodyne. 81 is a current limiter, 88 a filter for removing the harmonics produced by the limiter. 89 is an amplifier for the second-intermediate frequency. 90, 92, 94 and SI, 93, 95 are selective networks of the type de scribed in my U. S. Patent #1,941,069 for converting the variations in frequency into amplitude variations. Connected in parallel with the reactance combinations 92, 94 and 93, 95 are two high resistances 9B, 91 for the purpose of straightening the characteristics of the combinations to insure linear conversion of the frequency variations into amplitude changes.

98, 99 are amplifiers whose input circuits are connected across the reactance combinations 92, 94 and 93,95 and whose output circuits are aperiodically coupled to the two diode rectifiers I04 and I05. In series with the output of the two amplifiers 9B, 99 and the two rectifiers I04, I05 are two high resistances I00 and IM for the purpose or securing straight line rectification.

.shift within the working range.

Two blocking condensers I02 and I03 isolate the rectifiers from the direct current plate supply of the rectifiers. I05 and I0! are choke coils sufficiently large to block the intermediate frequency applied to the rectifiers but not large enough to interfere with the fiow of the superaudible modulating currents. v I08 and I09 are a pair of equal transformers which are substantially fiat over the range of the frequencies occupied by the three channels combined at point 30 in Fig. 1. The primaries of these transformers are shunted by resistances H0 and III respectively. The secondaries of I08 and I09 are cumulatively connected for frequency modulation and connected to the grids of three amplifier tubes, H2, H6 and I21. The output of H2 passes thru a low pass filter H3 designed to pass the frequencies of the first channel, thru an amplifier H4 and thence to the output circuitof channel #1. The output of the amplifier H6 passes to a band pass filter I I! designed to pass the frequencies utilized in channel #2, and thence to an amplifier H8. The output of this amplifier is supplied to a rectifier I 2| thru the resistance I 20 and capacity H9. The output transformer I24 is connected to the rectifier thru a choke I22 with a parallel resistance I23. An amplifier I25 for the rectified current supplies the output of channel #2.

The description as given above with respect to channel #2 applies equally well with respect to channel #3 except, of course, thatthe band pass filter I28 is designed to handle the frequency employed at the transmitter in signaling over channel #3.

Having now described the transmitting and receiving apparatus of Figs. 1 and 2 their manner of operation will now be explained.

Suppose that the transmitter is designed to give a total swing of frequency of 150,000 cycles of the radiated wave which may be some ultra high frequency value. The design is carried out along the same lines as those laid down in my U. S. Patents #1,941,068 and #1,941,069 and my applications for U. S. patents Serial Nos. 40,542 and 40,543. There are, however, certain modifications. One of them is that, as the frequencies of modulation, are much higher than merely the audible range, it is important that the width of the transformers and filters in the frequency modulation system be expanded to take into-account these higher frequencies without producing a limitation of amplitude or phase A second modification lies in the use of balanced push pull amplifier stages in all parts of the correction system when high levels exist. The reason for this is as followswhe re high levels exist, as for example at 33, 34 just preceding the correction net work 35, 31, a form of cross modulation between channels takes place. the amplifying tubes acting as rectifiers of the superaudible frequency currents. As a result of this there is produced directly in the low frequency channel a current which corresponds in frequency to the modulations impressed upon the superaudible frequency channels and this current therefore modulates the main channel directly along With the proper modulation for that channel. This effect occurs even when the amplifying tubes in the correction system are operated at relatively low levels. It is eliminated by the balanced push pull amplifier because the-currents resulting from the rectification of the superaudible currents have such phase relation This is caused by v the modulating current set at such value by adjustment of the amplification of tube 46 that the frequency swing of the radiated wave is 50,000 cycles per second. This transmitted wave is of the same type as that described in my U. S. Patent #1,941,069.

Referring now to the operation of channel #2 assume for the moment that the modulation is cut off from channel #1. By adjustment of the input level to the amplifier 50 the modula tion applied to the plates of the amplifiers 54, of the superaudible frequency is raised to a point sufficient to give "substantially complete modulation of thatfrequency. By adjustment of the amplification of tube 59 the voltage applied to the input of the transmitter modulation systemis raised to a point sufficient to give 25,000 cycles complete swing of the modulated wave when no modulation is applied to thesecond channel. Under these conditions there occurs in the radiated wave a swing of 25,000 cycles at the rate of the frequency of the oscillator 52 of the second'channel. The extent of this swing of course depends on the amplitude of the voltage output of the tubes 54 and 55. Hence when the plates of these tubes are modulated, for full modulation the output voltage varies between double the unmodulated voltage and zero. Hence the swing of the transmitted wave varies between.50,000 cycles and zero, the swing still continuing at the rate of the frequency of the oscillator 52 but varying in extent at a rate corresponding to the frequency of the modulation ap-' plied to the input of channel #2 at 49.

An identical action occurs with respect to channel #3, when it is operated alone. By adjustment of the input level at El the modulation of the amplifier 66, 61 is adjusted to be substantially complete. Similarly by adjustment of the amplification of 'II the swing of the transmitted .wave is adjusted to be 25,000 cycles when no modulation is applied to the input of channel #3 and to vary between the limits of 50,000 cycles and zero when modulation is applied. The swing is, of course, at the rate of' the oscillator 64 of the third channel.

Referring now to the receiver the operation is as follows. The number I or main channel functions in the same manner described in my U.

S. Patent #1,941,069. The received signal is amplified by the superheterodyne receiver 'I886,

supplied to the current limiter 01, passed thru the filter 88, and then amplified and supplied to the selective system 00-91 which convertsthe frequency variations of the signal into amplitude rent of channel. #1 is of an audible frequency. The number of variations of the frequency oc-- curring in the transmitted wave in response to the modulations on this channel corresponds to the frequency of the modulating current and the.

extent of the deviations in frequency is proportional to the amplitude of the modulating current. Hence the variations in amplitude created by the action of the selective system -91 are of an audible frequency which correspond to the frequency of the modulating current of channel #1. After rectification by the rectifiers I04, I05 they may be supplied to the speaker in the ordinary way.

Referring now to channel #2 the operation is somewhat more involved. From the antenna to the input of the selective system 909I the receiver performs the same function as in the case of the main channel. However, because of the double modulation involved in the wave transmitted on channel #2 the action of the selective system is to convert the deviations of frequency in the received wave into superaudible' variations of amplitude in which the amplitude of these variations are not constant but vary in frequency and amplitude in accordance with the modulating current of channel,#2. When the above current is rectified by the rectifiers I04 and I05 there appears in the output circuit a current corresponding in frequency to the oscillator frequency of channel #2 which varies in amplitude in accordance with the modulations impressed taneously at the levels which have been considered here. Since the action of the transmitter and receiver both is linear in every respect the currents co-exist in all parts of the system without cross modulation and the swings of frequency of the modulated wave are a combination of the individual swings of the various channels, the total swing of 150,000 cycles occurring when all three channels are simultaneously frilly modu- -upon the input to that channel. This band of frequencies is then passed thru the filter III,

lated and the phases of the modulations such as to be additive. As a practical matter this so rarely occurs that it is possible to run each of the channels at a higher level than its one third value with a proportionate improvement in the noise level. The exact settings are readily determined experimentally. It islikewise true that where various types of transmissions are employed the permissible noise levels will be different so that a greater percentage .of the total swing may be assigned to 'the channel most subject to disturbance with a corresponding lesser percentage assigned to those services least affected by extraneous disturbances.

It is, of course obvious that While channel #1 has been referred to as an audible frequency channel and the other two as superaudible frequency channels that the choice of the different auxiliary frequency oscillators depends on the Referring now to Fig. 4 there is shown a second method of multiplexing which, while more complicated than the system of Figs. 1 and 2 has sub- 5 I range of frequencies which it is desired to trans-- stantial advantages over that system. In this system the modulations of the signals to be transmitted are impressed on the superaudible frequency channels not as modulations of the amplitude of the currents of these channels but as variations in the frequency. In effect it represents a frequency modulation within a frequency modulation and the method has great ad- -vantages in freedom from certain forms of cross modulation between the channels.

Referring now to Fig. 4, there is shown a multiplex system of two channels whose combined output it is intended to supply to the input 3110f the transmitter of Fig. 1. Channel #1 is supplied at I40 thru an amplifier MI, and a low pass filter I42 to confine the frequencies admitted to this channel to its designed range and a mixing tube. This channel is the same as channel #1 of the system already described.

Channel #2 in the present system differs from channel #2 of the first system in that the frequency of this channel is modulated instead of its amplitude. On account of .the fact that the frequency of the channel is necessarily low and that a frequency swing representing a considerable percentage of unmodulated value of frequency is required an expedient is resorted to in order to obtain an undistorted wave. I44 to I63 illustrates the master oscillator and frequency modulating arrangements of a transmitter which are identical with the arrangement of the main transmitter illustrated in Fig. I by I- I8. I64 represents a frequency doubler, I65a filter, I66 a second doubler, I61 a second filter and I68 a frequency multiplier of as many stages as necessary to secure the desired frequency change.

The action of all this part of the system is iden-v tical with the corresponding part of the main transmitter. The output of the frequency multiplier is supplied to a rectifier I69, where it is heterodyned down, by means of the oscillator I10 to the frequency of channel #2. HI is a band pass filter for eliminating disturbing currents outside the band, I12 an amplifier and I13 a mixing tube.

The modulation of the second channel is applied at I14 to the amplifier I15 which feeds the correction system I11, I18 thru the transformer I16. The voltage developed across the condenser I18 is then amplified by the tubes I19, I and applied to the screen grids of the modulating tubes I46 and I41.

I The operation of the transmitting system is as follows. The main or number one channel functions in the same way as the main channel of the arrangement of Figs. 1 and 2. The maximum swing for this channel, assuming as in the preceding case a maximum swing of the transmitter of 150,000 cycles, should be set at 75,000 cycles.

The operation of channel #2 is as follows. The oscillating and modulating circuits I44I63 together with the correction system and its amplifiers I15-I8I produces a high frequency current whose phase is shifted-by an amount directly proportional to the amplitude of the modulating current and inversely proportional to its frequency in the manner already described. By means of the frequency multiplying system I64-I68 this frequency is multiplied up to some high value in order to produce the required degree of modulation. This frequency may be of the order of 10,000,000 cycles or more. Assuming a value of 10,000,000 the oscillator I10 may be set to a frequency of 10,025,000 cycles. By means of the'rectifier I69 the output of the frequency multiplier system is heterodyned down to 25,000 cycles. Any variation in frequency in the 10,000,- 000 cycle current is therefore reproduced numerically in the 25,000 cycle current. Since a 10,000 cycle deviation in the 10,000,000 cycle current is readily obtainable it follows that the 25,000 cycle current may be swung from 15,000 cycles to 35,000 cycles. The output of the rectifier is passed thru a band pass filter to exclude extraneous frequencies and supplied thru the mixer tube I13 to the input 30 of the modulation system of the transmitter. By adjustment of the level of this current the modulation of the transmitter is adjusted to be 75,000 cycles total swing. The transmitted wave, therefore, varies in frequency continuously over a range of 75,000 cycles at a rate which is 25,000 times per second when the second channel is unmodulated and which varies between the limits of 15,000 times and 35,000 times-per second when the second channel is fully modulated. The number of times the rate of modulation of the transmitter varies between 15,000 times and 35,000 times per second correspond-s to the frequency of the modulation applied to the input of channel #2. It should be noted that the extent of the swing of the transmitted wave does not vary during the modulation of the second channel but is constant at 75,000 cycles per sec ond. Since the swing remains constant and only the frequency at which it is accomplished, is changed, the tendency to produce cross modulation between channels is greatly reduced. In practice this advantage is fully realized.

Referring now to Fig. 5 there is illustrated the general arrangement of the receiving system for this method of multiplexing. In this figure I90, I9I. represents the receiving antenna, I92 an amplifier for the received current, I93-I98, the rectifiers, oscillators and amplifiers of a two intermediate frequency superheterodyne. I99 is a current limiter, 200 a filter for removing the harmonies of the limiter and 20I an amplifier for the second intermediate frequency. 202, 204, 206 and 203, 205 and 201 are selective networks of the type described in the patent heretofore referred to for converting the variations of frequency into variations of amplitude. Connected in parallel with the reactance combinations 204, 20,6 and 205, 201 are two resistances 208, 209 for the purpose of straightening out the reactance characteristics to insure linear conversion of the frequency variations into amplitude changes. 2I0 and 2 are amplifiers whose input circuits are connected across the two reactance paths and whose outputs supply the two linear rectifiers'2 I6, 2I1. 222 and 223 represent two balanced transformers whose secondaries are connected to act cumulatively for frequency variations. 224 is an amplifier for supplying the frequencies of channel #1 to a low pass filter 225 which eliminates undesired frequencies from the output 221 of channel one. 228 is an amplifier for the current-s of the second channel, 229 a filter, 230 an amplifier, 23I a current limiter, 232 a filter for removing harmonics produced in the limiter and 233 an amplifier for the limited current. The remainder of the system from 234 to 255 represents the same kind of selective networkfor converting frequency changes into amplitude changes and the same kind of rectifying arrangement as is shown by 202223 with'the exception of course that it is designed to handle a current of variable frequency of a much lower order. The output of the two transformers 254 and 255 are similarly connected cumulatively for frequency variations and connected thru a low pass filter- 256 to the output 251 of the second channel.

The operation of the receiving system is as follows ghannel #1 operates in the normal manner and" the selective system 202- 209 converts the frequency variations into amplitude variations, which after rectification to produce the original modulating current of channel #1 are separated out by the filter 225 and pass to the output 221 of that channel.

The second modulation of frequency of the received wave is likewise converted into amplitude variations by the selective systems 202-209 and results in an amplitude variation of constant amplitude of the voltage applied to the inputs of the amplifiers 2) and 2, the frequency of which varies between 15,000 and 35,000 cycles per second. When rectified by the rectifiers M6 and 2H there results a frequency modulated current of constant amplitude varying in frequency from 15,000,

255 is then passed thru the low pass filter 256 to the output of the channel ,251.

The same advantages which were referred to in connection with the transmitter regarding the v freedom from cross modulation between channels are present in the receiver for the same reasons, that is, the uniform amplitude of the superimposed channel.

The explanation of the fact that the system will operate more efiectively with a given amount 'of power than if the power were divided among receiver increases, the signal levelremaining unchanged. When the incoming radio frequency voltage of the signal is above a certain ratio with respect to the noise level, the relation between the signal and the noise levels in the output of the receiver is a strictly quantitative one-that is, the noise level in the output of the receiver isinversely proportional to the strength of the signaling current in the antenna. When, however, the noise and signal voltages in the antenna approach equality, any further slight r decrease in the strength of the received signal results, in a great increase in the strength of the disturbances in the output of the receiver. This is clearly shown in the curve. For example. if a .single simplex channel is producing a voltage in that of either one alone will be received. This is illustrated by 0N with a corresponding noise level of OP. The extent of the improvement and the reason for it appears at once from the curve. The advantage obviously becomes the greater the more simplex transmitters that are combined into a single multiplex system.

While in this specification there has been shown two types of modulation only applied to the auxiliary channels, it is, of course clear that other types, such as phase, or single side band, or any other form may be used where desired without departing from the spirit of the invention.

I claim:

1. A frequency-modulated multiplex signaling system comprising a plurality of channels, each channel consisting of a separate band of frequencies, said system comprising means for causing the said channels to frequency-modulate the transmitted wave, and means for causing the frequency deviation of the transmitted wave produced-by each channel to be substantially greater than the frequency range of audibility, so as to reduce cross-modulation between the channels.

2. A receiver for a multiplex frequency modulated signaling system as specified in claim 1, said receiver comprising means for amplifying the received current, means for removing from said current the efiects of amplitude variations,

means for translating the frequency variations of the received current into changes in amplitude,

said means comprising a selective system adjusted to produce full amplitude changes in the amplified current only in response to the wide frequency swing of the transmitted wave, a detecting device for said amplitude modulated currents and means for separating out the frequencies of the respective channels from the output of the said detecting device.

3. A transmitter for a multiplex signaling system comprisinga plurality of sources of signaling current, means for generating a wave of the frequency to be transmitted, a source of superaudible frequency, means for modulating the super-audible frequency by one of saidsignaling currents, means for causing another of said signaling currents and the said modulated superaudible frequency current to modulate the frequency of the radiated wave, and means for causing the frequency deviations of the transmitted wave produced. by each of said. currents to be substantially greater than the frequency range of audibility so as to reduce crossmodulation between said currents.

4. A transmitter for a multiplex signaling system comprising ,a plurality of sources of signaling current, means for generating a wave of the frequency to be transmitted, a source of superaudible frequency, means for modulating theampiitude of the super-audible frequency by one of said signaling currents. means for causing another of said signaling currents and the said modulated super-audible frequency currentv to modulate the frequency of the radiatedfwave, and means'for causing the frequency deviations of the transmitted wave produced by each of said currents to be substantially greater than the frequency range of audibility, so as to reduce cross modulation between said currents,

5. A transmitter for a multiplex signaling system comprising atplurality of sources of signalingcurrent, means for generating a wave of the frequency to be transmitted,- a source of superaudible frequency, means for modulating the'frequency of this super-audible.frequency by one of 7 said signaling currents means for causing another of said signaling currents and the said modulated super-audible frequency current to modulate the frequency of the radiated wave, and means for causing the frequency deviations of the transmitted wave produced by each of said currents to be substantially greater than the frequency range of audibility, so as to reduce cross modulation between said currents.

6. A receiver fora multiplex signaling system the received wave of which is generated by the I transmitter of the character described in claim 4, said receiver comprising means for eliminating amplitude variations from the received wave,

means for translating the deviations in frequencyv the received wave of which is generated by the transmitter of the character described in claim 5, said receiver comprising means for eliminating amplitude variations from the received wave, means for translating the deviations in frequency corresponding to the modulations impressed upon the transmitted wave by the signaling current of the first channel into currents corresponding in amplitude and frequency to the amplitude and frequency of the first signaling current source, means for translating the deviations of frequency produced by the modulating current of the second channel into super-audible currents of varying frequency, and means for converting the variations in frequency into current corresponding in amplitude and frequency to the amplitude and frequency of the second signaling source.-

8. A transmitter for a multiplex signaling system comprising a plurality of sources of signaling current, means for generating a wave of the frequency to be transmitted, a source of superaudible frequency, means for modulating the frequency of this super-audible frequency by one of said signaling currents and means for causing another of said signaling currents and the said modulated super-audible frequency current to modulate the frequency of the radiated wave.

EDWIN H. ARMSTRONG.

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