US2070418A - Multiplex cable code telegraphy with diversity reception - Google Patents

Description

Feb. 9, 1937. BEVERAGE 2,070,418

MULTIPLEX CABLE CODE TELEGRAPHY WITH DIVERSITY RECEPTION Filed May 19, 1933 4 Sheets-Sheet l 7 NHL-79.5 15 wig.

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I [011 g ///a// llff E, P455 \1 Q g Q l v 1% INVENTOR H. H. BEVERAGE /f WW ATTORNEY Feb. 9, 1937.

H. H. BEVERAGE MULTIPLEX CABLE CODE TELEGRAPHY WITH DIVERSITY RECEPTION Filed May 19, 1953 l l l f 4 Sheets-Sheet 2 INVENTOR H.H. BEVERAGE ATTOR NEY Feb. 9, 1937. H H, EVERAGE MULTIPLEX CABLE CODE TELEGRAPI-Ix WITH DIVERSITY RECEPTION Filed May 19, 1933 4 Sheets-Sheet 3 KAI/0 R455 AM/La 0 F/ZTERS AMPl/F/ERS 2] 2 L'- E IZ INVENTOR H. H. BEVERAGE ATTOR N EY Feb. 9, 1937. H, BEVERAGE 2,070,418

MULTIPLEX CABLE CODE TELEGRAPHY WITH DIVERSITY RECEPTION REC.

INVENTOR H.H. BEVERAGE ATTORNEY M I I I TI I Patented Feb. 9, I937 NiE STATES PATENT OFFICE MULTIPLEX CABLE CODE TELEGRAPHY WITH DIVERSITY RECEPTION of Delaware Application May 19, 1933, Serial No. 671,808

10 Claims.

This invention relates to signalling and in particular to multiplex cable code telegraphy with diversity reception. The cable code, commonly used on transocean cables, requires three conditions for transmitting intelligence. For example, on the cable, a positive direct current impulse of unit length may correspond to a dot in the Morse code, no current represents a space, and a negative direct current impulse of unit length corresponds to a dash in the Morse code.

In the Morse code, the letter A is a dot and dash, which would be transmitted in cable code as a positive current impulse, space or no current, and a negative current impulse, all units being of equal length.

In general in radio transmission of telegraphy messages over long distances, there are only two conditions, namely, on and off for single frequency transmission. That is, markings are represented by a pulse of high frequency for a dot, a pulse of different duration, perhaps longer, for a dash, and no signal for intervals and spaces.

I propose to obtain these three conditions by radiating three frequencies or two frequencies for marking and off or no energy for spaces. One

feature of the present invention concerns means for receiving these two or more frequencies at high efliciency by what is known as diversity reception methods.

Most multiplex radio telegraph systems proposed in the past have been based on modulation methods. These methods result in low transmitter efficiency and very low power per channel, making the signals relatively unreliable due to fading, static, and other noises. Other methods use time division on a single frequency or single channel. These methods are efiicient but are readily mutilated by extraneous noises such as static. They also require synchronous commutators at both ends of the circuit.

The multiplexing methods I am proposing allow the transmitter to be used at high efficiency, and.

provide a method of signalling which has greater freedom from extraneous noises than prior methods. In some embodiments of my invention, I also eliminate the necessity of using a synchronous commutator at the receiving end.

In my United States application Serial No. 661,947, filed March 21, 1933, now United States Patent #2,025,190 issued December 24, 1935, I described a method for multiplex cable code signalling using five frequencies, or four frequency and off, for two channels, and seven frequencies, or six frequencies and oil, for three channels. In this system a commutator is used at the transmitter for separating the channels while selective filters are used at the receiver for separating the channels, thus doing away with the necessity of using a synchronous commutator at the receiver.

My present invention also contemplates a method using cable code on two channels with two or three frequencies in which synchronized commutators for time separation are used at each end with diversity reception at the receiver.

My present invention involves a method of and means for applying diversity reception to (A) single channel cable code using two or three frequencies; (B) two channel cable code with commutator at the transmitter and frequency selection at the receiver, using four or five frequencies; two channel cable code with synchronous commutators at both transmitter and receiver, using two or three frequencies.

In describing my invention reference will be made to the attached drawings, in which:

Figure 1 shows a diversity receiver of code signals transmitted on a single channel by a system similar to the system disclosed in United States application Serial No. 661,947, filed March 21, 1933, now United States Patent #2,025,190 issued December 24, 1935;

Figure 2 is a curve explaining the operation of the receiver of Figure 1;

Figure 3 is a circuit diagram of a transmitting system by means of which cable code on two channels utilizing 4 or 5 frequencies may be sent out;

Figure 4 is a receiver of the diversity type for receiving signals as sent out from the arrangement of Figure 3;

Figure 5 is a curve explaining the operation of the filters of Figure 4;

Figure 6 illustrates a modified transmitter and receiver; while,

Figure 6a illustrates a modification of a feature of the circuit of Figure 6.

The manner in which I apply diversity reception to a single channel cable code signalling system using two or three frequencies will now be described.

In Figure 1, I have shown a method for applying diversity reception to cable code signals on a single channel. When no dots or dashes are being sent out on the channel the transmitter may radiate continuously at a frequency is, or it may not radiate at all, as preferred. If a frequency 1; is radiated by the transmitter, it is suppressed at the receiver and produces no effect on the recorder circuits. Hence the frequency is or total absence of signal, i. e., off, corresponds to a space in the cable code. To transmit a dot in cable code the transmitter frequency may be changed to another frequency, assumed for purposes of description to be f1, which frequency may be higher than the frequency is. This frequency 11 may be passed through high pass filters at the receiver and may be utilized to raise the pen arm on the recorder, causing it to make a dot. To produce a dash on the cable code, the transmitter frequency may be caused to shift to a frequency f2, which may be a lower frequency than frequency f3. Frequency f2 may be passed through low pass filters at the receiver and utiliZed to cause the pen arm on the recorder to be pulled down below the Zero line, making a mark corresponding to a dash in the cable code. Any transmitter which sends out signals as indicated above may be used. Preferably, a transmitter of the type disclosed in my United States application Serial No. 661,947, filed March 21, 1933, now United States Patent #2,025,l90, issued December 24, 1935, is used to signal on a single channel.

The characteristics of the high pass and low pass filters for separating frequencies f1 and f2 and suppressing f3 are indicated in Figure 2.

Returning to Figure 1, I have shown a receiving system of the diversity type consisting of three complete receivers, associated with antennae which are spaced a distance apart on the order of 1000 feet. The signals are amplified at radio frequency by amplifier 3. The amplified signals are fed to a demodulator of the heterodyne type including a detector and oscillator 5. The demodulated signals are fed to an audio amplifier l. The amplitude of the signals in the output of the audio amplifier l is controlled by potentiometer 9, which feeds into high pass filter and low pass filter i3 in parallel. Inductance l9 and capacity 2| are connected in series across the filters and are tuned to suppress the spacing frequency f3. The characteristic of the filters are indicated in Figure 2. Each filter passes only a predetermined band of frequencies. Neither filter passes is.

The filters H and I3 are followed by rectifiers l and I1 respectively. Additional amplifier stages may be used between the filters and rectifiers, if desired.

The output terminals of the rectifiers l5 associated with the high pass filters in all receivers areconnected together and completed through a resistance 23 as shown. The IR. drop through this resistance 23 cooperates to operate the keyer unit, consisting of vacuum tubes 25, 3'! and 39, and associated equipment.

The output terminals of the rectifiers l! are connected together and completed through a resistance 23 as shown. The IR drop through resistance 23' cooperates to operate as the keyer unit including the tubes 25, 31, 39' and associated circuits as shown. The keying unit connected with filters l5 operates as follows:

When no signal is being received no signals are passed through the high pass filters I5. Under these circumstances tube 25 is biased to be conductive and is passing plate current from battery 21 through resistance 29. Battery 3| is adjusted to such a value that the voltage of battery 3| added to the IR drop across resistance 29 is sulficient to bias the grids of tubes 3'! and 39 to such a negative potential as to make the tubes nonconductive and completely cut oil the plate current in push- pull tubes 31 and 39. Consequently, the audio tone from oscillator 35 tuned to the frequency of filter 45 is unable to pass through the push-pull tubes and through the high pass line filter 45 to the line 5|.

Now, if frequency 1 is emitted by the transmitter, current passes through the high pass filters H to the rectifiers l5, and the rectified currents from all receivers pass through resistance 23. If the sum of these rectified ctu'rents is large enough to produce an IR drop across resistance 23 of sufficient intensity to negatively bias the grid of vacuum tube 25 to a point at which cut off takes place, no plate current flows in 25 and no current passes through resistance 29. The grids of the pushpull tubes 31 and 39 are now biased by the amount of battery 3| alone, since there is no IR drop in resistance 29. v

The bias on the grids of 31 and 39 due to battery 3| alone is such that the pushpull tubes 3'! and 39 will amplify the oscillations impressed on the input circuit of said tubes from oscillator 35. This amplified current passes through transformer 43 and high pass line filter 55 adjusted to pass oscillations of the frequency of the oscillator 35 to the line 5|. Thus during the time a signal of frequency fl is emitted and received a tone of a frequency determined by the oscillator 35 is sent over the line 5|.

At the central oifice, this tone is received from the line 5| through transformer 53, passes through high pass filter 55, and is amplified in amplifier 51. The grid electrodes of rectifier tubes 6| and i9 are normally biased to a negative value by the battery connected thereto so as to cut off or render said tubes nonconductive. Amplified tone from amplifier 5'? applied to the grid electrode of tube El overcomes the normal negative bias thereon to make the tube conductive. This causes plate current to flow through winding 63 of the polar relay, pulling up the tongue 65 to close a circuit including source 39, lead H, ground, winding 85, contact 65 and lead 67. Positive current passes through the coil 85 of recorder 93. This positive current causes the pen 89 to rise, making a mark on the moving tape or recording medium 9|, as long as frequency f1 is emitted by the transmitter and received by any of the several receivers.

If the frequency sent out by the transmitter under keying action now shifts to ix this frequency is suppressed by the inductance-capacity circuit I 92| at the receivers and no tone is received over the line. Tube 25 is conductive, current passes through resistance 29, tubes 3! and 39 are non-conductive, and no current goes over line 5|. Consequently, the current in both tubes 9| and i9 falls to zero, equal or no currents flow in windings 63 and 8|, and the tongue 55 of the polar relay takes up a central position and no current flows through the recorder coil. The recorder pen makes the spacing or no signal line.

Now if the transmitter is keyed to send out frequency f2, current passes through low pass filter i3 and rectifier ll of all receivers, and operates the second keying unit. Tube 25, normally conductive, is biased to cut off by current flowing in resistance 23. The absence of current in 29' causes the potential on the grids of tubes 31 and 39 to become more positive so that these tubes are now conductive. The tone from oscillator 4'! is passed through low pass filter 49, adjusted to pass the frequency of the oscillator 41, and over the line 5|, through low pass filter l3, and causes tube 19 to become conductive and to pass current through relay Winding 8|. This pulls tongue 65 down and completes a circuit. through battery 69 and winding 85 so that negative current passes through 85, causing the recorder pen 89 to be pulled down below the spacing line during the time frequency ii is emitted and received. The operation just described has formed the letter A in cable code on the recorder slip, as shown. The three letters indicated on the recorder slip in Figure l are A, B and C.

From the description of the operation of. #1 keyer unit, it will be seen that the intensity of the line tone from oscillator 35 is independent of fading as long as the sum of the rectified currents from the three receivers is sufiicient to bias tube 25 to out off. If the sum of the rectified currents exceeds this value, nothing happens, since tube 25 can not produce any further effect after its plate current is reduced to zero. Consequently, if the input to resistance 23 is adjusted to cut off tube 25 with the minimum of the sum of the three rectified currents, steady signals will be obtained on the line.

It has been found by experience that short wave signals received on antennae spaced some distance apart fade independently. Hence the rectified current from one receiver may be strong when the current from another receiver fades out and vice versa. Consequently, the use of several receivers greatly reduces the chances of all of the rectified currents dropping to such a low value that they will not initiate operation of the keying units.

Three receivers are normally used in practice, and the extension of the system to use more receivers will be obvious to one skilled in the art.

Some added advantages of the system just described will now be enumerated.

Since in this system dot and dash signals occur at two different frequencies, we have frequency diversity in addition to space diversity. We might get a partial fade on one frequency, but no fading on the second frequency, in which case, for example, the recorder might show a mutilated dot but a perfect dash, which would readily identify the letter.

In this system all characters, i. e., dots, spaces, and dashes, are of equal length so that the absence of a character due to fading or other cause is immediately noticeable.

In this system there is no possibility of mistaking a dot for a dash or vice versa.

Furthermore, if static is present, it would have a tendency to come through both the high pass and the low pass filters more or less equally, thus tending to excite both windings of the relay and causing it to remain in its neutral position. Therefore, there would be fewer static crashes registered on the recorder tape, particularly during spacing intervals.

In my United States application Serial No. 661,947, filed March 21, 1933, now United States Patent #2,025,190, issued December 24, 1935, I disclosed a method of using asynchronous commutator at the transmitting station with arrangements for transmitting four or five frequencies formultiplex cable code signalling on two channels. In this system no commutator is necessary at the receiver because the signals on the several channels are separated by using suitable filters for the four or five frequencies.

In Figure 3 I have shown a new means for and a method of transmitting these frequencies. The system and method shown is a modification of the method and system disclosed and claimed in my aforementioned application.

In Figure 3, I I5 is a commutator with odd alternate segments connected together and to slip ring I09 contacting with brush I05. The remaining even numbered segments are connected with slip ring III and brush IU'I. To brush I35 is con nected the tongue I I9 of relay I III associated with automatic sending head I02 for channel #1, while brush Ili'l is likewise connected to the tongue I63 of relay IOI associated with automatic sending head I04 for channel #2. The sending heads have sprockets which pull the perforated tapes I06 and IE8 respectively over the keying heads. On channel #1, for example, contacts I I2 and 5 I4 make contact with I I 3 if there are holes in the paper tape or other perforated medium. If there are no holes in the perforated medium, no contact takes place. Therefore, this device sends positive and negative impulses from battery H6 to relay III) in accordance with the code letters punched on the tape or perforated medium. These automatic keying devices may be the same as the automatic devices used in my United States Patent #2,025,190 issued December 24, 1935, or any type such as the Wheatstone, Creed, or Morkrum-Kleinschmidt The relay contact II9 makes contact through resistance R1 or R4 according to whether the winding III) is connected in series with IM or I I2 and receives a positive or negative current impulse from battery H6. The circuit may be traced from R1 through brush I05, slip ring Its. a #1 segment of the commutator to brush I 29, battery IZI, winding I23 to ground, thus completeing the circuit. The current through winding I23 of transformer I29 changes the saturation of the iron, and this changes the inductance of winding I3I which is in series with oscillator circuit I33I35. Hence the frequency generated by tube I39 is changed to some definite value f1, which may be further amplified by power amplifier I43 and radiated on antenna I45.

In like manner when one of the contacts in the keying head M14 is closed a positive or negative current impulse passes through I ill to close the armature I03 on one contact. If at the same instant the brush I28 rests on one of the #2 segments current determined in part by either R2 or R3 passes through I23 and a different frequency f2 or f3 is developed in I 39 and radiated.

The resistances R1, R2, R3 and R4 are individually adjusted for such current values as may be required to produce the necessary degree of saturation in the transformer I29 to cause the oscillator I33 to oscillate at the desired frequencies f1, f2, f3 and f4. If both channels are spacing, there is no current in the saturation winding I23, and the transmitter radiates at its lowest frequency f5.

In Figure 3, the resistances R1, R2, R3 and R4 are numbered according to the frequencies emitted. Channel #1 would correspond to II for dot, ft for dash, and ft for space. Channel #2 uses f2 for dot, f3 for dash and 5 for space.

The commutator I I 5 and transmitter heads I32 and I04 must be driven at the same speed, preferably from the same drive shaft as shown, which keeps the commutator H5 and transmitter contacts in I02 and IM operating in perfect phase relation. This is necessary because channel #2 must mark during the spacing intervals on channel #1 and vice versa, and it is obvious that the relay tongues H9 and I83 must operate during the intervals when the brush I20 is on a segment corresponding to the channel which is marking.

The letters A, B, C in cable code are shown on Figure 3 for channel #1 and X, Y, Z

are shown for channel #2, with their corresponding transmitter frequencies. Below these curves is shown the composite resulting frequencies radiated by the transmitter. The spacing frequency is occurs only when both channels are spacing, and, obviously, f5 might be omitted entirely if arrangements were made whereby zero current through the saturation transformer allowed a relay to open, causing the transmitter to cease radiating. However, it is somewhat easier to allow the transmitter to continue to radiate at some frequency is. Obviously, if desired, is may be made adjustable by passing the proper amount of current from battery I2l through the saturation winding I23. This could be accomplished by placing a suitable resistance between the brush I29 and ground.

The arrangements for receiving the transmissions from the transmitter of Figure 3 will be readily understood, as it is an extension of the system already described for single channel operation.

One suitable arrangement is indicated in Figure 4. Here three receivers are shown, each receiver being equipped with five band pass filters, adjusted to correspond with the spacing of the transmitter frequencies f1, f2, f3, f4 and is. The filter characteristics are indicated in Figure 5.

Assume that the transmitter of Figure 3 has been put in operation and the automatic keying heads E02 and I04 are running and start to send out the letter A. A frequency f1 is radiated first.

The output of #1 filter on each receiver is associated with an amplifier-rectifier and all #1 rectifiers are connected in parallel across the input resistance of #1 keying unit, from which #1 tone passes to the line L. At the central office, #1 tone passes through #1 filter, #1 amplifier, #1 rectifier and causes the pen of #1 recorder to rise, marking a dot on the recorder slip. During the next interval, the transmitter radiates is, so #1 recorder drops back to spacing. If is is traced through the receiver of Figure 4, it will be noted that #2 recorder pen is pulled down making a dash on the slip.

In like manner, the other frequencies sent out by the transmitter of Figure 3 may be traced through Figure 4 to show how the characters are formed, as the receiver of Figure 4 is assumed to be responding to the signals sent out by the transmitter of Figure 3.

In Figure 4, telephones are shown connected across filter #5. This is for the purpose of lining up the received frequencies with the corresponding filters. For example, the frequencies might be radiated differing by a fixed amount, such as 170 cycles. At the receiver the oscillators in the detectors may be set so that the spacing frequency is is, say, 595 cycles. Then f4=765 cycles, f3=935 cycles, f2=ll05 cycles, and f1=1275 cycles.

Alternatively, it may be adjusted for zero beat and the other frequencies adjusted to fall in their respective filters. Furthermore, arrangements could be made whereby f5 may be used to hold the receiver oscillators automatically at the right frequency. This may be accomplished by utilizing frequency is to actuate mechanism to automatically tune the oscillators at the receiver to a frequency which when combined with the received frequency results in a constant frequency beat note. Such a correcting device has been described in United States application Serial No. 692,092, filed October 4, 1933, and in United States application Serial No.

675,536, filed June 13, 1933. This arrangement would automatically keep the frequencies lined up in their corresponding filters.

The system disclosed in Figures 3 and 4 also has the advantages of frequency diversity, static balancing in spaces, etc., as enumerated in connection with the single channel system of Figure 1.

Some advantages may be gained in some cases both at the receiver and at the transmitter if a commutator is used to separate the signal elements at the receiver as well as at the transmitter In this case both commutators must operate in synchronism. When using codes, like the Baudot code, where all letters have the same number of unit elements, it is feasible to send synchronizing impulses for periodically correcting the speed of the receiving commutator. In the case of the cable code, however, the letters contain variable numbers of elements, and it is more difficult to fit in the synchronizing correction impulses. Therefore, it is preferable to use independently driven commutators at the transmitter and at the receiver operated from an extremely accurate source of frequency. It is relatively simple to independently drive the two commutators at speeds differing as little as 1,000,000 to 1, in which case occasional manual correction of the phase relations between the commutators at the transmitter and receiver should produce satisfactory results.

Where synchronous commutators are used, three frequencies instead of five frequencies may be used for two channel multiplex code signalling. In this case the receiver of Figure 1 may be used instead of the more complicated receiver of Figure 4.

An arrangement for two channel cable code signalling with independent synchronous commutators at the receiver and the transmitter is shown in Figure 6. Here I49 and I86 are the transmitting and receiving commutators respectively which revolve in synchronism. The automatic senders shown schematically at I52 and I54 are driven from the same source as the transmitting commutator I 49, and said drive is coordinated with the drive of I49 so that the relay tongue I56 is always marking when relay tongue I5! is spacing, and vice versa. The relays are connected through brushes I60 and I6I to slip rings connected to alternate commutator segments, as shown. Brush I63 bearing on the commutator I49 is always in contact with one of the #1 segments when the tongue I56 of #1 relay is marking, and passes either positive or negative current to the winding of relay I66 according to whether the letter character being transmitted throws the tongue I56 to battery IE5 or I66. The tongue I61 of polar relay I64 applies a different potential to the grid of tube I68 according to which of the contacts connected with source I69 is closed by the tongue I67, thus varying the impedance or conductance of the tube I68 and thereby varying the effective capacity of condenser I10. Condenser I10 and tube IE8 in series are, in effect, connected across an oscillating circuit connected as shown between the anode and cathode of tube Varying the capacity of the condenser I19 varies the electrical characteristics of the oscillation circuit connected with HI and thereby varies the F tune of the said circuit. This circuit determines the frequency of the oscillation generated by I? I. Therefore, the frequency generated by tube I'II is dependent on the position of the tongue I61 of relay I64, and may generate any one of three fre- Ill.-

quencies according to whether relay tongue I6! is closed down, closed up, or is open, or in a neutral position. The three frequencies may be designated as ii for dot, f2 for dash and is for space, to correspond with the receiver of Figure 1.

The oscillations developed by the oscillator ill are amplified by power amplifier H5 and radiated by antenna 176. I

It is obvious that battery !59 may be adjusted to give any desired spacing between the frequencies f1, f2 and is. The positive and negative arrangement shown, however, produces f1 higher, and f2 lower, than is to correspond with Figure 1.

The arrangement shown here, including the tubes I68 and H6 and associated circuits including the capacity I10, all of which are used for shifting the frequency generated by the tube iii? and its circuits, are similar to some extent to the arrangement disclosed in United States application Serial No. 563,725, filed September 19, 1931, now Patent #2,033,231 issued March 10, 1936.

The receiver E of Figure 6 is identical with one of the complete receivers of Figure 1, and the relay tongue 83 of Figure 6 corresponds to relay tongue 65 of Figure 1. For simplicity I have shown a single receiver connected to relay 82, although it will be obvious that as in Figures 1 and 4 several receivers similar to l 80 may be connected with the relay I82 for diversity purposes. Instead of operating a single recorder directly, however, tongue 183 of Figure 6 is connected to a brush E84 bearing on the segments of synchronous commutator I86, thence through brushes l8! and 189 to recorders I and l9l respectively.

The operation of the system is obvious. For example, supposing the transmitter is producing or making a dot on channel #1, relay tongue I55 will swing over to positive battery I55, and brush: 153 will be in contact with a #1 segment on commutator I49. Positive current in the winding of relay I64 will throw tongue I 6'! down, putting negative bias on the grid of tube I68. This decreases the effective capacity of condenser I'IB, thereby causing frequency f1 to be produced and radiated. At the receiver l 80, f1, will pass through the high pass filters (not shown here but shown in detail in Figure 1) and throw relay tongue I83 up to complete a circuit through the positive battery I19. Brush I84 will be in contact witha #1 segment on commutator l 86 and will complete a circuit for positive current through brush 1 81 to recorder #1 causing the pen to rise and make a dot corresponding to that originally sent. Recorder #2 will be spacing because no current is reaching any of the #2 segments from brush H34.

The manner in which the dashes and intervals are sent out from the transmitter of Figure 6, picked up by the receiver, and made to produce desired markings on the recording medium should be obvious from the above description and a statement of said operation is thought unnecessary and superfluous at this point.

While I have shown the transmitters and receivers as including thermionic tubes of the triode type, it will be understood that by such showing I do not intend to limit the invention to the use of triodes since, obviously, any tubes of the appropriate characteristics in use today may be included in said circuits with or in place of the triodes. For example, screen grid tubes, pentodes, etc., are extremely well adapted to utilization in the circuits.

In the arrangement illustrated in the several figures in the drawings magnetic means and variable capacitive means including a capacity controlled through a thermionic tube have been utilized to produce the frequency shift of the 0scillations sent out at the transmitter. It will be quency of the crystal of which may be shifted by mechanical means which varies at the keying frequency, the air gap of the crystal. Furthermore, the frequency of the crystal controlled osciilator may be shifted or varied by connecting the variable capacitor illustrated in Figure 6 in parallel with a piezo-electric crystal. In this arrangement extremely constant frequency oscillations, the frequency of which may be shifted to produce the keying indications, may be obtained. The frequency may also be varied by connecting a vacuum tube in series with an inductance instead of a capacity, as shown in Figure 6a, or by shunting a vacuum tube across a capacity or inductance in series with another capacity or inductance. It is obvious to one skilled in the art that varying the control voltage on the grid of the vacuum tube causes it to act as a variable resistance, thereby varying the current flowing through the elements in series or in shunt with the vacuum tube, which, in turn, varies the constants of the associated vacuum tube oscillator and causes the frequency to vary by a predetermined amount depending on the value of the bias applied to the grid of the vacuum tube.

Having thus described my invention and the operation thereof, what I claim is:

1. Means for receiving multiplex signals on several channels composed of impulses of different frequencies on each channel comprising, a plurality of signal receiving means located at spaced points, each of said signal receiving means including signal demodulating means, a plurality of filters connected with each demodulating means of each receiver, there being a filter con nected with each receiver for each frequency used, signal recording means, a plurality of thermionic oscillators, thermionic blocking means interposed between each of said oscillators and said recording means, and thermionic coupling means interposed between the filters of said receivers and each of said blocking means.

2. Means for receiving multiplex signals comprising, high frequency oscillations, the frequency of which is shifted to produce signal markings on several channels including, a plurality of receivers located at spaced points, each receiver including a signal demodulating means, a plurality of filters connected with each receiver, there being a filter for each frequency on each channel, a plurality of keying units, each of said keying units being connected with a filter in each of said receivers, there being a keying unit for each frequency used, each of said keying units including a continuously operative oscillation generator, a recording device, thermionic blocking means interposed between each of said oscillators and said recording device, and a thermionic coupling tube connecting each of said blocking means to similar filters in each of said receivers.

3. Means for receiving multiplex signals comprising, high frequency oscillations, the frequency of which is shifted to produce impulses of high frequency oscillations, the frequencies of which are characteristic of signal markings on several channels including, a plurality of receivers located at spaced points, each receiver including a signal demodulating means, a plurality of filters connected with each receiver, there being a filter for each frequency used on each channel, a plurality of tone keying units, each of said keying units being connected with a filter in each of said receivers, a continuously operative oscillation generator in each of said tone keyers, a recording device, thermionic amplifiers interposed between each of said oscillators and said recording device, a circuit for biasing said amplifiers to cut off, and a thermionic coupling tube connecting each of said amplifiers to similar filters in each of said receivers, said coupling tubes each having a circuit, a portion of which is common-to the biasing circuit of the amplifier to which it is coupled.

4. In a system for receiving multiplex signals, on several channels, composed of impulses of different frequencies on each channel, a plurality of signal receiving means located at spaced points, each of said signal receiving means including signal demodulating means, filtering means connected with each demodulating means of each receiver, signal recording means, a plurality of sources of oscillations, blocking means interposed between each of said sources of oscillations and said recording means, and coupling means interposed between the filters of said receivers and each of said blocking means.

5. In a system for receiving multiplex signals comprising high frequency oscillations, the frequency of which is shifted to produce impulses of high frequency oscillations, the frequencies of which are characteristic of signal markings on several channels, a plurality of receivers located at spaced points, each receiver including a signal demodulating means, a plurality of filters connected with each receiver, there being a filter for each frequency used on each channel, a plurality of tone keying units, each of said keying units being connected with a filter in each of said receivers, a continuously operative oscillation generator in each of said tone keying units, a plurality of recording devices, blocking tubes interposed between each of said oscillation generators and said recording devices, a circuit for biasing each of said blocking tubes to cutoff, and a thermionic coupling tube connecting each of said blocking tubes to similar filters in each of said receivers, said coupling tubes each having a circuit a portion of which is common to the blocking tube to which it is coupled.

6. The method of multiplex signalling on several channels simultaneously by time separating the several signals on the severalchannels, and frequency separating the signal elements on each channel at the transmitter which includes the steps of, receiving signals on all of said channels at a plurality of spaced points, separating out like signal elements received at the several points, rectifying the same to produce rectified currents, combining the rectified currents, producing oscillations, and recording said oscillations as long as said combined rectified currents are above a predetermined value.

'7. The method of receiving multiplex signals on several channels which signals are formed by simultaneously time separating the several signal elements on the several channels and frequency separating the signal elements on each channel, which includes the steps of, simultaneously receiving energy portions characteristic of all of the signal elements at spaced points, segregating the signal elements transmitted on the several channels and separating the signal elements on each channel and producing energy characteristic of each signal element on each channel to reproduce the original signal when the energy received at one or more of said points exceeds a predetermined intensity.

8. A diversity system for receiving multiplexing signals produced by time separation of the signal elements on several channels and frequency separation of the signal elements on each channel comprising, a plurality of receivers, filter circuits connected with each receiver for separating signal elements of like frequency, there being a filter for each frequency used and several filters for each receiver, a rectifier connected with each filter, signalling recording means, normally inoperative energizing means connected with said recording means, there being an energizing means for each channel, and means connected with said rectifiers and said energizing means for rendering the same operative on the receipt of signals on any of said receivers or several of said receivers which produce currents of a predetermined intensity in the output of one or more of said rectifiers.

9. In a multiplex transmission system, a plurality of sources of direct current impulses, each source comprising impulses of different polarity which polarity is characteristic of signalling markings, an electron discharge tube having anode, cathode and control grid electrodes, oscillation generating circuits connected between said control grid and cathode and between said anode and cathode, a reactance connected with one of said circuits, a magnetic relay having a winding, a movable contact and a pair of fixed contacts, an additional electron discharge tube having an anode, a cathode, and a control grid, a connection between the anode and cathode of said additional tube and said reactance, a connection between the control grid of said additional tube and the movable contact of said magnetic relay,

sources of potential of different polarity connected to said fixed contacts, circuits connecting the winding of said magnetic relay to said first named direct current impulse, sources and means in said last named circuits for closing and opening the same at signal frequency.

10. In a system for receiving multiplex signals comprising high frequency oscillations, the frequency of which is shifted to produce impulses of high frequency oscillations, the frequencies of which are characteristic of signal markings on several channels, a plurality of receivers located at spaced points, each receiver including a signal demodulating means, a plurality of signal filters connected with each receiver, there being a filter for each frequency used on each channel, a plurality of tone keying units, each of said keying units being connected with a signal filter in each' of said receivers, an oscillation generator in each of said tone keying units, each of said generators producing oscillations of a different frequency, a plurality of recording devices, a blocking tube connecting each of said oscillation generators to at least one of said recording devices, a filter circuit for each of said generators in said last named connections, and an electron discharge tube connecting each of said blocking tubes to similar signal filters connected with said receivers, said coupling tubes each having a circuit a portion of which is common to the blocking tube to which it is coupled.

HAROLD H. BEVERAGE.

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