US2061734A

US2061734A - Signaling system

Info

Publication number: US2061734A
Application number: US746086A
Authority: US
Inventors: Ray D Kell
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1934-09-29
Filing date: 1934-09-29
Publication date: 1936-11-24
Anticipated expiration: 1953-11-24

Description

Filed Sept. 29, 1934 4 Sheets-Sheet lllw ms: u

. 11;, 2 I O 1 3' IE W \iigfi 0.3 93 0 Nov.'24, 1936. R. D. KELL 2,061,734

SIGNALING SYSTEM SIGNAL AMPLIFIER RECEIVER INVENTO"? Ra Dlfell S'IGNHL GENERATOR Nov. 24, 1936. v R L 2,061,734 SIGNALING SYSTEM Filed Sept. 29, 1954 4 Sheets-Sheet 2 V'R/INSMITTEW ATTORNEY ,u INVENTOR Nov. 24, 1936.

R. D. KELL 2,061,734

S IGNALING SYSTEM Filed Sept. 29, 1934 4 Sheets-Sheet 3 z- AvmvA To GRID or 78 runs 34 (FIG. 2.)

I/VVENTOR DERIVATIVE D. N IMPULSE GENERATOR HND case-ran 7; BY W Y ATTORNEY 4 Sheets-Sheet 4 R. D. KELL SIGNALING SYSTEM Filed Sept. 29, 1954 Nov. 24, 1936.

Patent d Nov. 24, 193.6

NITED STATES SIGNAILHNG SXSTEM Delaware Application September 29, 1934, SerialNo. 746,086

13 Claims. i. 250-6) 8, 1932, to Finch, disclose systemsof that sort.

My invention, however, is directed more particularly to apparatus for improving the ratio between signal strength and the amplitude of interfering waves or sources of noise effects.

Accordingly, it is among the objects of my invention to provide a signaling system in which a carrier may be so modulated for the transmission of intelligence that the signals may be received and translated with a high degree of fidelity and the minimum of noise accompaniment.

Further objects of my invention include the provision of suitable apparatus for transmission and reception of intelligence in which nonlinearities of the amplifiers may be neglected without causing distortion.

At the present time it is the general practice to amplitude modulate the carrier wave. The method of modulating a carrier of constant amplitude as proposed by Heising and by Finch, above cited, and possibly by others, has not been generally adopted and practiced, perhaps owing to certain operational difliculties that were encountered. it is, therefore, a further object of my invention to overcome such difficulties by the use of improved structural elements in thesystem.

The foregoing and other objects and advantages of my invention will be more clearly understood upon reference to the following detailed description when read in connection with the accompanying drawings, in which Figure 1 shows a circuit diagram of one embodiment of a radio transmitter suitable for voice modulation of a carrier wave of constant amplitude,

Fig. 2 shows a circuit diagram of a radio receiver suitable for the translation into intelligence of a constant amplitude modulated car- I prefer to employ in transmitting voice modulated signals on a constant amplitude carrier,

Fig. 4 shows a circuit detail useful for translating signals which are received according to a modification of my invention,

Figs. 5 and 6 illustrate diagrammatically the forms of certain waves which are utilized in the carrying out of my invention} Fig. '7 shows a circuit detail of a receiver when used.- according to another modification of my invention and,

Fig. 8 shows a schematic illustration of another modiflcation of my invention.

It may be stated at the outset that in the description to follow, my invention will be illustrated as having particular applicability to the voice-modulation of a carrier wave and the demodulation thereof. It will be apparent, however, that the apparatus is not limited to this use, because the same principles apply to the modulation of a carrier either for the transmission and reception of signals of any type, Whether representing sound, pictures, telegraph code, facsimiles or other intelligence.

For faithful reproduction at the receiver of the modulated signals at the transmitter, it has heretofore been considered essential that all the circuit elements should be linear; that is, that they should respond in proportion to the intensity of the applied signal. Under this limitation it has been known for a number of years that any discrimination between desired signal and noise entering the signaling system must be the result of selectivity of the circuits involved. If thesignal occupies the minimum frequency range necessary for transmission of the desired intelligence and the selectivity of the circuit used is such that it passes this signal frequency range ireelybut suppresses disturbances of all other frequencies, no further improvement in the ratio of signal to random noise can be hoped for, except of course,

by increasing the intensity of the transmitted signal.

The possibilities of improvement of the ratio of signal intensity to noise intensity I have found to lie in a departure from ordinary methods of amplitude modulation. I have found also that these possibilities may be extended by the use of non-linear circuits having an output by no means proportional to the input.

Briefly, my system contemplates the transmission of certain timing marks. The transmission of intelligences may be had without in the least depending upon amplitude modulation of the carrier. The improvement in the ratio of signal intensity to noise intensity, therefore, is obtained largelyby virtue of the fact that these timing marks can be shifted over a wide range by the desired signal, but only over a definite narrow range by any noise of less amplitude than the signal. It is admitted, however, that any interference causing noise of greater amplitude than the signal is not suppressed in the operation of my system, but, because I am able to greatly intensify the amplitude of the transmitted signals without increasing the amount'of power radiated. it will be apparent that the signal-to-noise ratio is in large part improved over what is possible when using the generally accepted present-day methods.

Referring to Fig. 1, I show a transmitter network including electric discharge tubes i and 2 which are employed as a multi-vibrator for the purpose of interrupting the carrier frequency at a constant superaudible rate; for example, 30,000 interruptions per second. These interruptions will hereinafter be referred to as a dot-frequency.

The network for causing the electric discharge tubes I and 2 to operate as a multi-vibrator is conventional. It comprises an input circuit for tube I having a grid leak I connected with the grid 8, and an input circuit for tube 2 having a grid leak 9 connected with the grid Ill. The anode circuits for these tubes are supplied with direct-current from any suitable source such as I I which may be fed through the resistors I2-I2 to the anodes I3 and I4, respectively. A capacitor I5 is interposed between the anode circuit of tube I and the grid circuit of tube 2. Another capacitor I6 is likewise interposed between the anode circuit'of tube 2 and the grid circuit of tube I. The interruption frequency produced by this multi-vibrator may be utilized in an output circuit, which is conveyed through the resistor I1 and the capacitor I8 to the grid I9 of the electric discharge tube 3. In order to maintain a proper balance between the two tubes I and 2, in view of the unsymmetrical drain of energy into the utilization circuit, a variable capacitor 20 may be inserted between the grid I0 and ground.

The output from the multl-vibrator is in the form of a rectangular wave the pattern of which is represented at a in Fig. 6. The repetition frequency of the impulses in this wave is represented as fa, the time value of which is preferably made up of two equal elements, one being a positive impulse and the other a negative impulse. It is the function of the resistor II and capacitor 20a to convert these impulses into impulses of triangular or saw-tooth pattern, as shown at b in' Fig. 6. When, in addition to the impulses id, there is superimposed a voice frequency fv, the pattern will become as shown at d in Fig. 6. A signaling wave may be impressed directly upon the dot-frequency fa from any desired source, such as the signal generator 2 I. This may be a microphone or a modulator of any desired type whether used for voice or picture transmission, or for the transmission of any other intelligence. The voice frequency fl) is amplified by the amplifier 22 and impressed through the transformer 23 between the cathode 24 of tube 3 and ground. The secondary 25 of the transformer 23 offers a high impedance to alternating currents of the dotfrequency, which are therefore shunted to ground through the low impedance resistor 30. The impress of a voice frequency upon the secondary 25, in circuit between the cathode 24 and ground is such as to swing the bias of the grid I9 to a considerable degree. The grid biasing circuit may be traced through a resistor 28 to a suitable point on the potentiometer 21, where a negative potential is supplied by the battery 28 with respect to ground. A capacitor 29 is also provided for filtering purposes, as is usual.

The fixed bias of the tube 3 is adjusted to a certain value which, in the absence of output from the multi-vibrator, would suppress the transmission of negative waves from the modulator 2I. Referring to the curve at in Fig. 6, it will be seen that only that portioncf the composite wave 3| which lies above the base line will be transmitted through the tube 3. The cut-off point of the tube characteristic suppresses the under portions of this zig-zag curve.

The output of tube 3 may be impressed across the capacitor 32 and upon a tube 4, the bias of which is so fixed as to limit the plate current for producing square-topped waves. These waves may then be successively impressed upon the tubes 5 and B for further leveling off their peaks and for amplification. Thence they are impressed upon a radio frequency oscillator and amplifier 33 for transmission in the usual manner.

In the absence of input potentials to the system from the signal generator 2 I, the carrier is interrupted at a constant superaudible frequency, fa, determined by the adjustment of the multi-vibrator. When this dot-frequency is unmodulated the carrier assumes a pattern the envelope of which may be represented as shown at c in Fig. 6.

The time intervals when the carrier is "on are equal to the time intervals when the carrier is off". The action of the tubes 3 to 6 inclusive is such, however, as to vary the on and off" intervals in relation to one another according to the bases of the triangles of the curve III. The voice modulated dot-frequencies thus produce a carrier wave envelope such as shown at e in Fig. 6. During a single cycle of voice frequency ,fo the on time of the carrier will vary from a maximum to a minimum. The degree of variation represents the percentage of modulation. In Fig. 6 the effects of a high percentage and a low percentage of modulation are shown by the differences between the heights of the curves at and g and the corresponding breadths of the carrier envelopes e and h. The patterns (1 and e are intended to represent a modulation, whereas the patterns g and 11 represent substantially a 45% modulation. When the voice frequency is higher-pitched than as shown in the examples at d and g, a wave of frequency jw may be produced as shown by the pattern 1' in Fig. 6. The carrier envelope produced by this wave is shown at in Fig. 6.

From the foregoing it will be seen that the power radiated is a function of the modulation just as in normal amplitude modulation. In the receiver the carrier is detected either directly or heterodyned and amplified before detection, as is well known in the art. The output of the detector contains the impulses of current together with interfering impulses which would produce undesirable noise and hiss. The transmitted carrier plus the interfering waves may be represented as shown at k in Fig. 6. The signal at the output of the detector still has the interference superimposed upon the impulses, for which reason it is desirable to employ a limiting tube such that the center of the impulses will not be passed. Such a tube is'shown at 34 in Fig. 2, and it will be understood that it is so biased as to remove the undesired interfering waves at 76 pulses.

least in part and thereby to produce a wave of the form 1 in Fig. 6. A further limiting tube 85 may then be employed for removing the remaining interference constituting the ragged wave crests, so as to produce a wave of the form t in Fig. 6. When the plate current of tube assumes this pattern it is enabled to control any signal responsive device of ordinary type, such as that indicated at 36 in Fig. 2, the energy being transmitted thereto, if desired, through a capacitor 37 and audio frequency transformer 38.

Still further advantages of vmy invention by way of suppressing the effects of interference may be-obtained upon employing a modification the embodiment of which will now be described. In this embodiment the intelligence is transmitted by marking only the edges of the original im- The network shown in Fig. 3 may be used for this purpose. In place of the tube 6 of Fig. 1 this network is interposed and comprises tubes, 40, M, 42 and 43. The output from tube 5 may be impressed across the capacitor 44 upon the grid of the tube 40, thereby producing variations in plate current which develop a voltage drop across the inductance 45. This voltage is the derivative of the impulses. When the direct current flowing through the resistor M is steady, no eifefct is produced upon the grids of tubes MI and M. The signalling impulses, however, produce a voltage wave such as shown at n in Fig. 5. Here it will be seen that at the commencement of I each dot, as modulated in the manner hereinbefore described, there will be a sharp peak of positive value, and at the termination of each dot there will be a similar sharp peak of negative value. The time elapsing between positive and negative peaks is represented as the equivalent of a marking impulse m. The time interval between negative and positive impulses is likewise represented as a spacing interval 8. These positive impulses may be transmitted through the tube 4i directly to the transmitter it. The negative impulses are inverted through the cooperation of tubes M and 43 and likewise impressed upon the transmitter M. The modulation therefore assumes a pattern such as shown at p in Fig. 5. These modulations result in controlling the on" and off time of the radio frequency oscillator and amplifier output such that the carrier is on" only during the very short intervals of the sharp peaked impulses of the pattern 1). The envelope of the carrier will be represented as shown at q in Fig. 5, where alternate intervals m betweenthe impulses correspond with marking intervals cf the original signal, while intervening intervals represent the spaces s of the original signal.

- The operation of tubes ii, tt'and it is as follows: Both tubes M and it receive the same signals. The cut-ofi point of tube M is so adjusted that it will transmit the positive impulses. Tube 42 will transmit both the positive and negative impulses, and tube M will transmit only positive impulses. Tube 42 reverses the phase of the signals, impressing the originally negative impulses positively upon the grid of tube M, which then operates alternately with respect to tube M. Tube- H3 is so negatively biased that it derives no effect from tube-42 when the positive impulses are being transmitted by tube M. The combined action of

tubes

4 I, 2 and 43 is to transmit impulses at the commencement and at the termination of each marking intervalm. Hence, the carrier wave may be controlled by uniform impulses such as those shown in the diagram p and the carrier envelope is constructed as at q.

If the output from the transmitter is controlled as described in the foregoing paragraph, the "on" time of the impulses may be as little as 10% and the "off" time as much as 90%. A transmitter having a given rated continuous power output will, therefore, be enabled to transmit these discrete impulses at substantially ten times the amplitude of corresponding, continuous impulses. I have shown how, at the receiver, interference of less amplitude than that of the carrier may be suppressed. Assuming that the amplitude of this interference is not increased, it will be seen that the greatly increased amplitude of the carrier impulses provides an improved ratio thereof to the noise level represented by a factor of approximately 10.

The apparatus and the method employed at the receiver make possible a still further gain in the ratio of signal response to noise response. This will be better understood upon observing that the raggedness of the wave crests due to interference may be removed by the operation of the limiting tube 35; and interference received when the carrier is keyed off may be removed by the operation of the limiting tube M.

Transmission of sound by the means described in the foregoing paragraphs is, of course, unintelligible to the conventional radio receiver, since the power radiated is a constant. It is necessary, therefore, to translate the signals'by some such means as shown in Fig. 4. The network therein represented comprises tubes ht, 5i and b2. These tubes may be placed between the receiver it and the network of tubes it and lit, shown in 3 Fig. 2. The input energy of the signals as derived from the detector of the radio receiver Mi may be impressed across the capacitor it to an intermediate tap M between resistors 5t and ti.

Tubes iii and M are interconnected so as to form substantially a multi-vibrator operating at a frequency below that at which it may be driven when signals are impressed thereon. The bias on tube till is such that it is inoperative at the start of a cycle. The first impulse causes the grid of tube hi to go positive, which in turn causes the plate of tube 52 also to go positive. The two tubes ti and ti? therefore go into the limiting condition. The decrease in plate current in tube 52 causes the voltage on the grid of tube fit to become less negative. This result is attributed to the rise of potential at the point hit on the resistor tiin the anode circuit of tube 52. Tap it connects with a point W on the grid leak circuit for tube 5b. This point it is intermediate the resistors W and the grid biasing battery M where the cathode-to-grid circuit includes resistor 5%, battery M and a further resistor ti. Since tube b? is now blocked and the potential at the point 58 has been raised to bring the bias of the tube til almost but not quite to the cut-off point, a succeeding impulse impressed on the input circuit through the resistor hill and the capacitor M will control the tube bliso as to pass plate current through the output circuit resistor b t. This condition impresses a negative impulse upon the grid of tube M, which blocks this tube and causes the grid oftube 52 to go positive. The network of the multi-vibrator is thus tripped to the other limiting condition. This action may, therefore, be repeated to reconstruct the original impulses. then be seen to constitute signals of varying length, as shown by the marking-intervals m The energy so derived may and spacing intervals 3 in the pattern 1' of Fig. 5. Such a wave may be impressed upon a signal responsive device of any suitable type, such as aloud speaker oi" a radio receiver.

A further modification of my invention 'may be made by employing suitable means to produce impulses at the transmitter which are of unsymmetrical saw-tooth pattern. These may easily be constructed in any well known manner. When modulated they will take the form of pattern a as shown in Fig. 5. The "on" and "of!" intervals oi the carrier wave will be controlled in accordance with the bases of the triangles of the u-pattem. Here the modulated impulses come on" at variable times but terminate equidistantly in point 01' time. The results produced at the receiver, however, may be utilized in the same manner as hereinbefore. .described with respect to the first modification.

They may also be used in accordance with the second modification it the means shown in Figs. 3 and 4 are brought into play. In this case, however, it is not necessary to transmit the stationary edge of each impulse. They may be removed by omitting

tubes

42 and 43. These stationary edges may be supplied at the receiver by an oscillator H (see Fig. '7) running at approxi-' mately the same frequency as the original impulses. These two sets of impulses may then be used as hereinbefore described to reconstruct the original impulses.

Tl'reoperation of the network shown in Fig. 7 is as follows: Impulses from the detector are impressed upon the grid of tube 12 causing it to go positive. Anode current will fiow in the resistor 13, dropping the potential at the point 14 and impressing a negative surge across the capacitor 16 and -upon the grid of tube I8, causing this tube to be blocked. The anode potential across resistor ll then rises and transmits an impulse across the capacitor 18 to the grid of tube 34. (Fig. 2) which operates as hereinbefore described. A positive impulse is also impressed across the capacitor 19 causing the tube 12 to go to the limiting condition. Next, a positive impulse produced and selected by the device under control of the local oscillator H is impressed upon the grid of tube 16, causing it to go positive and tripping the operation of

tubes

16 and 12 as is well understood in respect to multivibrators. These tubes will, therefore, trip one way at variable times in response to the received signals and the other way at regular times in response to the device 86. thus rebuilding a wave of the pattern t (Fig. 6). r

Due to the fact that the signaling intelligence is represented only by the time intervals intervening between impulses, suitable amplifiers may be constructed in which non-linearities are neglected. Thus distortion due to curvature of vacuum tube characteristics becomes non-existent, thus increasing the possibility of using gas-filled tubes of any well known type so as to produce high audio power, if desired.

In Fig. 8, a modification of my invention has been illustrated in schematic form. The asymmetric saw tooth wave generator iili may be of the type illustrated in U. S. Patent 1,887,237

which issued on November 8, 1932 to J. L. Finch. The modulator I03 has been previously described in connection with thermionic tube 3 of Fig. 1. The wave shaper I 05 has been illustrated by this figure, tube 40 and its circuits is an impulse generator. The inverter I" has been described in connection with tuba l2, 4! oi. Fig. 3. The carrier wave generator ill may be any type of continuous wave generator; such as, a thermionic tube oscillator.

The receiver H3 may be a superheterodyne, or a tuned radio frequency amplifier. Such amplifier is disclosed in U. 8. Patent 1,173,079 to Ernst Alexanderson. A wave shaper network M6 for the receiver has been described in detail in connection with Fig. 4. The current limiter Ii'i may be of the type shown in U. 8. Patent 1,468,116 which issued on September 18, 1923 to Irving Langmuir. The signal indicator Mil may be a loudspeaker, cathode ray tube, facsimile reproducer or the like.

Instead of inverting alternate ones of said impulses, the uniformly timed or uniform interval impulses may be suppressed by omitting tubes at and 43 of Fig. 3. In this case, the irregularly spaced impulses III are used to key the transmitter. The regularly spaced impulses 123 are supplied by an impulse generator I25, located at the point of reception, which is operated at the same frequency as the original impulses. This impulse generator may correspond to tube st of Fig. 3.

Although I have shown several specific embodiments of my invention, it is to be understood that these are merely illustrative and that other modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

I claim as my invention:

1. A signaling system comprising a carrier wave source, a source of signals, a source of dotimpulses of supersonic frequency, means for keying the carrier wave on and off at an average frequency equal to that of said supersonic impulses and means under control of said source 01 signals for varying the time intervals between successive moments of keying the carrier wave on while maintaining constant the duration of each on" impulse.

2. A signaling system in accordance with claim 1 and comprising receptive means including a network for translating signals represented by impulses of uniform amplitude and uniform duration into signals of uniform amplitude and variable duration.

3. A signaling system comprising a carrier frequency generator, means for producing a succession of discrete impulses for modulating the output of said generator, said impulses being of substantially uniform amplitude and duration, 9. source of signals 01' lower frequency than the recurrence period of said impulses, and means for causing said signals to vary the time intervals elapsing between successive ones of said discrete impulses.

4. A system in accordance with claim 3 comprising additional apparatus for reception of the carrier wave, said apparatus having means for constructing a wave of substantially rectangular pattern having fiat tops the time components of which correspond substantially to alternate time intervals elapsing between successive modulations of the carrier wave, and said pattern having fiat bottoms the time components of which accuse ,correspond substantially to intervening time intervals, whereby said signals are reconstructed.

5. In a system for the transmission and reception of signal-modulated carrier-wave impulses of constant frequency, constant amplitude and uniform duration, a multi-vibrator for producing a rectangular wave, means for translating said rectangular wave into a wave of triangular pattern, modulating means for varying the amplitude of said triangular wave, means for truncating the peaks and valleys of said triangular wave, means for deriving impulses of constant amplitude, uniform duration and spaced apart in accordance with the moments of voltage shift of the wave originally triangular but now truncated in pattern, means for modulating said carrier wave under control of the last said impulses, means for receiving and detecting said impulse-modulated carrier wave, and means including a multi-vibrator adapted to be driven by the detected impulses for reconstructing a wave the pattern of which corresponds substantially to the wave of truncated triangular pattern.

6. A system embodying the features recited in claim and characterized in that the means for modulating said carrier wave has in combination therewith a device'for accepting alternate impulses and rejecting intervening impulses corresponding to the moments of voltage shift of the truncated triangular wave, and the means for receiving and detecting said impulse-modulated carrier wave has in combination therewith a device for supplying substitute impulses in place of the rejected intervening impulses.

7. A system embodying the features recited in claim 5 and characterized in that the means for translating the rectangular wave into a wave of triangular pattern is adapted to produce an asymmetrically triangular wave and the means for modulating the carrier wave is adapted to vaccept only the impulses derived from voltage rier wave with said discrete impulses now all of one polarity, transmitting and receiving said modulated carrier wave, and rebuilding at the point of reception a wave correspondingto the flat-topped and flat-bottomed wave aforementioned.

9. The method as set forth in claim 8 including the further steps of leveling off the peaks and valleys of the wave rebuilt at the point ofreception so as to derive a wave substantially free from undesirable noise-producing components.

10. The method of signaling which comprises producing an asymmetric saw-tooth wave, modulating said saw-tooth wave, deriving therefrom a fiat-topped and flat-bottomed wave, deriving, further, a series of alternately positive and negative discrete impulses separated by time intervals substantially proportional to the flat tops 'and flat bottoms, respectively, ofthe wave so formed,

inverting alternate ones of said discrete impulses. generating a carrier wave, keying said .carried wave on for predetermined moments and "off" for irregular time intervals elapsing between the discrete impulses, transmitting and receiving said carrier wave, and rebuilding at the point of reception a wave corresponding to the flat-topped and flat-bottomed wave aforementioned.

11. The method as set forth inclaim 10 including the further steps of leveling off the peaks and valleys of the wave rebuilt at the point of reception so as to derive a wave substantially free from undesirable noise producing components.

12. The method of signaling which comprises producing an asymmetric saw-tooth wave, modulating said saw-tooth wave, deriving therefrom a flat-topped and fiat-bottomed wave, deriving, further, a series of alternately positive and negative discrete impulses separated by time intervals substantially proportional to the flat tops and flat bottoms, respectively, of the wave so formed, suppressing those of said discrete impulses which occur at uniform intervals. generating a carrier wave. keying said carrier wave by the irregularly timed of said discrete impulses, transmitting and receiving said carrier wave, supplying at the point of reception discrete impulses at uniform intervals substantially equal to the intervals of said suppressed impulses. and rebuilding at the point of reception a wave corresponding to the flattopped and fiat-bottomed wave aforementioned. .13. The method as set forth in claim 12 including the further steps '"bf leveling off the peaks and valleys of the wave rebuilt at the point of reception so as to derive a wave substantially free from undesirable noise producing components. RAY- D. vKELL.