US2469837A - Wave translating system - Google Patents

Description

May 10, 1949. M. E. MOHR 2,469,837

WAVE TRANSLATING SYSTEM Filed Sept. 26,1946

I INVENTOR I 2'- By M E MOI-IR A TTORNEY Patented May 10, 1949 UNITED STATES PATENT OFFICE WAVE TRANSLATING SYSTEM Milton E. Mohr, New Providence, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 26, 1946, Serial No. 699,555

- Claims. (Cl. 332- 29) This invention relates to circuit switching and to wave modulation. 1 r

Objects of the invention include impedance control, modulation, frequency control and frequency modulation.

In a specific aspect the invention is a frequency modulator comprising an oscillator circuit with signal responsive electric space discharge tubes, for example gas tubes, controlling rectifiersfor switching discrete resistances into and out of the oscillator circuit to vary the oscillator frequency abruptly in steps.

Also, in a specific aspect the invention is a frequency modulator responsive to a modulating signal which may be a signal of gradually varying instantaneous magnitude, to produce a frequencfi modplated wave whose frequency variations occur abruptly, in steps. When a signal, in its transmission eitheFdirectly or as a modulation on a carrier wave, is subject tog alse, the signal quality, for example, articulation in the case of speech, is degraded by the noise. In the case of a continuously or gradually varying signal (for example a speech wave or a speech modulated carrier wave) this degradation increases with the ratio of noise energy to signal energy at the receiver output (after demodulation or vention will be apparent from the following description and claims.

The single figure of the drawing shows a frequency modulator embodying a form of the invention. q v

Oscillator O is shown as a variable frequency RC oscillator of the bridge type disclosed for example in Fig. 5 of R. -O. 'Wi'sePatent 2,319,965, May 25, 1943, theamplitude control resistance L in oscillator O-being shown,however, as a gasfllled tube, as, inmy Patent-2,345,712, April 4, 1944. The oscillator comprises a vacuum tube V with feedback from the plate to the grid through a direct current stopping condenser C and a frequency determining parallel-T feedback network N of the general type shown in Fig. 23 (d) on page 918 of "Radio Engineers Handbook," by F. E. Terman, published by McGraw-HillBook 00., Inc., N. Y., 1943. T1 is an output transformer for the oscillator. Network N comprises, as one shunt arm, two resistances R and R in parallel. The resistance R comprises transformer T and its load. T comprises a primary winding P and a secondary winding S. Its load comprises equal resistances R1 and R2 in series, and comprises a variable portion of resistances II to 20 and respectively equal resistances 3! to 40. Resistances detection oi" the signal in thecase of carrier-- l1-to20'and3l"to' l'llbanbe efiectively'connected transmission), or in other words the graph plotted with articulation or quality as ordinates and noise-to-signal ratios as abscissae, has a negative slope. However, when the signal is transmitted in stepped wave form, (i. e., with the signal variations occurring in abrupt steps of sub-' stantial magnitude), then for noise-to-signal ratios up to a considerable magnitude the quality degradation (due to the noise), though greater than in the case of the gradually varying signal, is substantially uniform, and for higher noise-to-signal ratios it is not materially difierent from the degradation inthe case of gradually varying signal. It is found that, due apparently to this closer approach to uniform quality over a considerable range of noise-to-signal ratio, the signal is received, for example, by a listener when the receiving apparatus is an electroacoustic reproducer, with greater ease, accuracy and intelligibility when transmitted in stepped wave form than when transmitted as a continuous or gradual variation. 7

It is also an object of the invention to .improve ease, accuracy and intelligibility of reception of signals subject to disturbance by noise where the noise-to-signal ratio varies widely.

Other objects, aspects and features of the inacross S by diodes 2| to 40 under control of gasfilled discharge tubes l to ill shown by way of example as triodes, which are controlled by signals from signal source M and by switching means, for example of electronic or mechanical type, represented for simplicity by mechanical rotary switches 1| and 12.

Plate current limiting resistances 5| to 60, which may have values of the order of 20,000 ohms, for example, connect the plates of tubes I to lo with cathodes of the diodes as shown.

Discharge currents of the triodes pass from their cathodes through switch II, plate current supply'source B, resistances R1 and R2 in parallel, the resistances ii to 20 and 3| to 40 and the associated diodes, and from the diode cathodes through the limiting resistances to the triode plates. If desired, switch 1| and battery B may be shunted by a high resistance 13, of the order of a megohm, for example. This resistance serves to prevent ionization of the tubes I through 9 by possible small leakage across the insulating section 83-.

The grids of tubes l to lo are connected to ad-' justable contacts of potential dividing resistances or potentiometers 6| to 10, preferably through resistors as shown, which serve to prevent inter- 3 action between ionized and unionized tubes through the mutual output impedance of the vogad 80. This occurs because the grid of a tube which has ionized is held at a positive potential which must be kept from ionizing the following tubes. These latter resistors may have values of the order of one megohm, for example. If desired, the potentiometer resistances may be equal. They may have values of the order of 50,000 ohms, for example. The grid circuits extend through the potentiometers, a high resistance 16, of the order of a megohm, for example, and negative grid biasing sources 15 and 14, to the triode cathodes. The path through resistance 16 and source 15 is shunted by switch 12. The voltages of sources I4 and 15 may respectively have values of volts and 146 volts, for example. Across the potentiometers are connected a positive grid biasing source H in series with secondary winding 18 of signal input transformer T2 for the triodes. The primary winding 19 of the transformer is fed from signal source M. The signal source may be, for example, a microphone or telephone transmitter delivering speech currents. A vogad 80 may be connected between the microphone and the winding 19, so constant volume signals are supplied to the winding 18. The vogad may be a system such, for instance, as that disclosed in H. L. Barney Patent 2,285,794, June 9, 1942, or the paper entitled A Vogad for radio telephone circuits," by S. B. Wright, S. Doba and A. C. Dickieson, Proceedings of the Institute of Radio Engineers, April 1939, page 254.

The voltage of source 11 may be substantially half the value of the maximum range (negative to positive) of the voltage induced in winding 18, so that, when signals are being delivered by transmitter M, the voltage input to potentiometers 6| to 10, is a direct current or unidirectional voltage varying between the limits zero and a positive value equal to the maximum range of the voltage induced in winding 78. ,For instance, with this maximum range or swing (nega tive maximum to positive maximum) adjusted to r 60 volts (by means of the vogad) the voltage of battery 11 may be volts.

Te switches H and 12 are shown by way of cated by the arrow headed arcs at a speed of the order of 4,000 revolutions per second, for example. The shaft is driven by any suitable means (not shown). Arm 8| cooperates with stationary contact 82 which, with a short are of insulation 83, forms a ring or circle. Arm 9| cooperates with stationary contact 93 which, with a long are of insulation 92 forms a ring. If desired, the arcs 83 and. 93 may be approximately equal (in degrees or in-radians). Each may be of the order of ten or more per cent of 360 degrees. However, the switch 12 is arranged to close a short time after the switch II has closed, as indicated in the drawing, wherein the arm 8| is shown as having just left the insulation 83 and the arm 9| is shown as just arriving at the contact 93. The time interval between the departure of 8| from 83 and the arrival of 9| at 93 may be of the order of one or two percent of the time required for one revolution of shaft D, for example.

Resistances R1 and R2 may each be of the order of 5,000 ohms, for example. The resistances II to 20 and 3! to 40 may each be of the order of 200,000 ohms, for example; but they will begiven values appropriate to yield linear or any 4 other desired modulation law, and this will not in general result in equal resistances.

The potentiometer contacts are adjusted to respectively supply successively lower potentiometer voltages tothe grids of the associated tubes. as indicated in the drawing. If desired, these decrements may be equal to each other and to the potentiometer voltage applied to the grid of tube III.

In operation of the system, when switch 'll opens, it removes the plate voltage supplied by battery B from the plates of tubes to ID, whereupon all ten of the tubes are non-conducting. After switch 1| again closes, they are maintained non-conducting by grid bias batteries 15 and 14 for an instant until switch 12 closes and thereby short-circuits battery 15 and resistance 16, thus eifectively removing battery 15 from the grid circuits. This removal of battery 15 conditions the tubes to ID to be controlled. throughout the short period in which switch 12 remains closed, by the signal voltage across winding 18, the voltage of battery 11 and the voltage of battery 14.

If that signal voltage is zero, battery 11 will fire tubes l to 5. If, on the other hand, that signal voltage is equal and opposite to the voltage of battery 11, all of the tubes to ill remain unfired, with a negative bias of 10 volts from battery 14. If that signal voltage is positive and equal to the voltage of battery 11, all of the tubes i to ID fire. Whatever that signal voltage, the tubes to I0 are controlled in accordance with the magnitude of such voltage. Thus, in accordance with such magnitude, any of the tubes may fire, the number of tubes that fire being dependent on such magnitude, and the magnitude required to fire any given tube exceeding that required to fire the lower numbered tubes.

After switch 12 again opens, the tubes that fired during the time switch "was closed remain fired until switch 1| again opens to complete a cycle of operation; but during this interval between "the opening of switch 12 and the opening of switch 1|, battery I5 prevents firing of any other tube.

During a cycle of operation such as that just considered, whenever tube is in the conducting condition diodes 2| and 4| effectively connect resistances it.l -and iiin seriesacrosswinding s because of the flow of plate current of tube I from battery B through the branched circuit comprising R1, H and 2| in parallel with R2, 3| and 4| and thence through resistance 5|, tube and switch II to battery B. Whenever tube is not in the conducting condition, the circuit across winding S through II and 3| in series is effectively open at 2| during those half-cycles of the oscillator wave induced in S 'that have a given sign and is eifectively open at 4| during the alternate half cycles. The oscillator then returns to minimum frequency, a value determined by network N with only R1 and R2 across S.

In the same way that tube controls the switching of an associated pair of oscillation frequency control resistors through an associated pair of diodes, so does each of the tubes 2 to H1. Thus, the resistance R, and consequently the oscillation frequency of the oscillator O, is varied in discrete steps in accordance with the continuous changes in intensity of the signal wave induced in winding I8. That is, the oscillations generated by oscillator 0 are frequency modulated at the signal frequency in abrupt steps, by the gradually or continuously varying signal:

and the stepped envelope of the frequency modulated wave conforms .to the modulating signal wave (except during the interval, which from a practical standpoint may be neglected, during which 8| is on segment 83 so the oscillator returns to the reference condition).

In the case, for example, in which the modulation law for which the resistances II to and 3| to 40 are proportioned is that of linear modulation, a smooth curve drawn as nearly as possible through the centers of the tops of the steps of the envelope of the curve of oscillator frequency versus time would have substantially the same form as the modulating signal wave or the curve of signal intensity versus time. Any desired width, 1. e., time duration, of the steps may be provided by appropriate choice of the angular velocity of contacts 8| and 9|. Any desired number of different values of oscillator frequency adjustment may be provided by appropriate-choice of the number of gas tubes and their associated diodes and resistances.

When any of thetriodes I to II! are in the conducting condition, the conducting triodes are conjugate to winding S. Therefore, the currents through these tubes do not flow through winding S. Consequently the starting or stopping of the direct current through tubes 1 to it does not produce transient currents in S which, acting through winding P, could apply to the oscillator grid a transient voltage that would be transferred by the oscillator tube to the output through transformer T1. In the system shown, the oscillator frequency is controlled by resistance R, which in turn is controlled by the resistance shunts placed across winding-S. The diodes act as switches activated by the gas tubes passing current through them. As one gas tub after another fires, more and more shunt resistances are-placed across the coil secondary and the oscillation frequency increases, yet no disturbing pulses are transmitted to the oscillator.

controlling impedances for connection in varying 1 number in said circuit to vary th oscillator frequency abruptly in steps in accordance with the number of saidimpedances simultaneously in said circuit, a source of modulating signals, means responsive to, said signals for varying the number of said impedances simultaneously in said circuit in accordance with th magnitudes assumed by said signals at prescribed instants, and

control apparatus rendering said means responsive to said signals at only said instants regardtieusls of magnitude changes in said signalsat other 2. In a frequency modulation system, an oscillator circuit comprisingdiscrete resistances for connection in varying number in said circuit to correspondingly vary in steps the oscillator frequency, a source of modulating voltage of. continuously varying instantaneous magnitude,

means responsive to said voltage at moments separated by prescribed relatively long time inof said resistances'connected in said circuit in accordance with the instantaneous magnitude of said voltage, and timing circuits for determining the lengths of said intervals.

3. A wave translating system comprising a wave generator having a frequency controlling circuit, discret impedances for connection in said circuit to vary the frequency of the generated waves, means comprising grid controlled gas tubes respectively associated with said impedances for switching each impedance into said circuit when the gas tube associated with the impedance becomes conductive, means for supplying periodically reduced potential to the plates of said gas tubes, a source of modulating voltage, means operative momentarily after each period of reduced plate potential for rendering said gas tubes subject to triggering by said voltage, and

means for rendering the magnitude of said voltage required to trigger said tubes different for each of said tubes.

4. A frequency modulator comprising an oscillator having two terminals, discrete impedances for connection in varying number between said terminals to modulate the frequency of said oscillator, a pair of rectifiers individual to each of said impedances for switching the impedance into and out of circuit, a grid controlled gas tube individual to each pair of rectifiers for controlling the switching operation of, the pair, each ofsaid gas tubes having its plate circuit conjugate to said terminals, means for supplying plate potential to said gas tubes, means for supplying a modulating signal tothe grids of saidgas tubes, with a different magnitude for each grid, and means for periodically interrupting the supply of plate potentials momentarily and after each restoration of the plate potentials conditioning momentarily the grid circuits of said gas tubes for controlling the ionization of said gas tubes in accordance with the modulating signal. u r

ality a of each bridge comprising a resistance and a tervals of length independent of magnitude changes insaid voltage forchanging the number 5. A modulating system comprising a pl of Wheatstone bridges having one diagonal in common and having a pair of adjacent ratio arms in common, each of the .two remaining arms rectifier, in series, said rectifiers in each bridge being oppositely poled with respect to said one diagonal, gas tubes having control grids, means connecting the internal plate-cathode paths'of" said tubes in the other diagonals of the respective bridges, means for supplying periodically inter rupted plate potential to said tubes, a source of modulating voltage, means for connecting said source in the grid-cathode circuits of said tubes,

means for causing the magnitude of said voltage ance-capacityfrequency determining circuit in-.

cluding said one diagonal.

- 1 M'IL' 1ON E. MOHR.

I REFERENCES CITED The following references are of record in the file of this-patent:

UNITED STATES PATENT Number Name f Date 2,262,468 Percival Nov. 11,1941 2,278,658 Kroger Apr. 7, 1942 2,321,269

Amt June 8, 1943

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