US2264019A

US2264019A - Signal-translating system

Info

Publication number: US2264019A
Application number: US365459A
Authority: US
Inventors: Nelson P Case
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1940-11-13
Filing date: 1940-11-13
Publication date: 1941-11-25
Anticipated expiration: 1958-11-25

Description

W5 @www mq BIEST AVAILAELE can??- uw SIGNAL-TRANSLATING SYSTEM Filed-Nov. 13, 1940- INVENTOR A NELSON P. CASE side of a desired carrier signal.

'Patented News, 1941 UNITED s'rli'rlzs PATENT ori-"ice 2,264,019 i SIGNAL-TRANSLATING SYSTEM Nelson P. Case, Great Neck, N. Y., assigner to l-Iazeltine Corp., a corporation of Delaware Application November 13, 1940, Serial N0. 365,459

(ci. 25o-20) i v11 Claims.

The present invention relates to signal-translating systems and, more particularly, to such systems having a controllable translating characteristic. While the invention is of general apl lencing the receiver during intervals when the" signal input to the detector is below the prede-- tei-mined level necessary for satisfactory reproduction, either because the received signal is near the prevailing noise level or because the receiver is appreciably mistuned from a strong desired signal, in which latter case the reproduction oi signals is also distorted by such mistuning. The silencing of a frequency-modulated carriersignal receiver when the receiver is appreciably mistuned from a desired carrier signal is additionally desirable to suppress the reproduction of the spurious responses resulting from the frequency-responsive -characteristic of the carriersignal channels provided in the receiver.

Many of theprior art noise suppression systems heretofore used in amplitude-modulated carriersignal receivers are not adaptable for use in frequency-modulated carrier-signal receivers. The former class of receivers include means for deriving from a received carrier signal an automatic amplication control bias or equivalent, which is generally used also to control the operation of the noise suppression system if included in the receiver. The automatic amplification control bias developed in a frequency-modulated carrier-signal receiver, on the other hand, is not suitable to control a noise suppression system'of the prior art since it has a substantially constant amplitude over a broad tuning range von either This constantamplitude characteristic results from the broad pass-band characteristic of the carrier-signal amplifiers necessarily used in frequency-modulated carrier-signal receivers. Any attempt to narrow' the range over which the automatic amplification control bias or equivalent is substantially constant has generally proven unsatisfactory since any such attempt must be predicated upon the integration of the energy of the modulated-carrier signal only at and in proximity to its mean frequency and it is well known that such carrier signals have a relatively small portion-of their total energy concentrated Within thisV limited frequency range.' Moreover, the sharply-tuned circuit. necessary to providethe limited .constant-amplitude range of the automatic amplification control bias can be maintained in proper alignment with the frequency detector of the .receiver only with exceedingly great difficulty.

It is an object of the present invention, therefore, to provide a new and improved signal-translating system having a controllable translation characteristic and one which avoids one or more .of the above-mentioned disadvantages and limitations of the priorvart systems.

It is a further object of the invention to provide a signal-translating system wherein a signaltranslating characteristic of the system is controlled in accordance with variations of the mean frequency of a frequency-modulated carrier signal from a predetermined frequency.

It is an additional object of the. invention to provide a signal-translating system wherein variations of a characteristic of a frequency-modulated carrier signal from a predetermined value control the operation of an'oscillation generator, the output of Which controls, in turn, a charactcristic of the signal-translating system.

In accordance with the invention, a signaltranslating system comprises an input, circuit adapted to have signals applied thereto, and a signal-translating channel coupled to the input circuit. The system includes means for deriving a rst unidirectional potential the magnitude of which varies with' a characteristic of a signal applied to the input circuit, and means for deriving a second unidirectional potential the magnitude and polarity of which vary with variations of the signal characteristic in opposite senses from a predeterminedvalue. The system additionally is provided with a source of oscillations, means responsive jointly to the derived unidirectional potentials for modifying a characteristic of the oscillations of the source,v and means responsive to the characteristic of the oscillations for controlling a characteristic of the signal-translating channel.

-For a better understanding of the present invention, together with other and further objects y thereof, reference is had to the following description .taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims. f

Referring now tothe drawing, Fig. 1 is a cirthe operation of the latter; Fig. 3 is 'a circuit diagram representing a modified form of the invention; and Fig. 4 is a graph comprising curves representing frequency-.voltage relations occurring at,certain points in the arrangement of Fig. 3. Referring now more particularly to Fig. 1,`

arnpliiier I4 of one or more stages, an amplitude limiter I5, a frequency-modulation detector and A. V. C. supply I6, an audio-frequency amplifier II of one or more stages, and a sound reproduce'r l8r Coupled to the A. V. C. supply and to the output of the frequency-modulation detector` of unit I3 is an automatic control system I9, more fully described hereinafter. the output of which is coupled to a control electrode of one or more of the tubes of the audio-frequency amplifier-I1.

An automatic amplification control or A. V. C. circuit is connected between the output of the A. V. C. supply of unit I6 and the control elec-y trodes of one or more of the tubes of the radiofrequency amplifier I0, the oscillator-modulator I3, and the intermediate-frequency amplifier I4,

in conventional manner.

It will be understood that the various units Ill-I9, inclusive, just described may, with the exception of the unit I9 presently tobe considered, be of a conventional construction and operation, the details of which are well known in the art, rendering detailed description thereof unnecessary. Considering briefly the operation of the receiver as a whole, and neglecting for the moment the operation of the automatic control system I9, presently to be described, a desired frequency-modulated carrier signal is selected and amplified by the radio-frequency amplier I0, converted to a frequency-modulated intermediate-frequency carrier signal in theoscillatormodulator I3, amplified in the intermediate-frequency ampliiier I4, limited to a predetermined substantially constant amplitude by the limiter I5, and detected by the frequency-modulation detector of unit I6, thereby to derive the audio-frequency modulation components. The audio-frequency components are, in turn, amplified in the audio-frequency amplier II and are reproduced by the sound reproducer I8 in a conventional manner.

.The automatic amplification control or A. V. C. bias derived from the A. V. C. supply of unit I8 is effective to control the amplication of one or more of the units I0, I3, and I4 to maintain the signal input to the limiter I5 within a relatively narrow range for a wide range of received signal intensities. l

Referring now more particularly to the portion of the system embodying the present invention, the audio-frequency amplier II comprises a signal-translating channel and it is desirable that a characteristic of the channel be controlled in accordance with a characteristic of the carrier signal applied from the limiter I5 to the input circuit of unit I6. More specifically, it is desired that the ampliiication of the audio-frequency amplifier I'I be controlled in accordance With either or both the intensityv of a received carrier signal and the difference between the mean frequency of the frequency-modulated intermediatefrequency carrier signal and that of band-pass selectors of thereceiver, the control being such that the receiver is silenced when the received carrier-signal intensity is too low or the aforesaid rent amplier tube 32.

frequency dii'erence has a value corresponding to a predetermined mistuning of the receiver from a desired carrier signal. 'I o this end, the A. V. C. supply of unit I6 comprises means for deriving a unidirectional potential the magnitude of which varies with a characteristicof the vfrequency- -modulated intermediate-frequency carrier signal applied to the input circuit of unit I6. The frequency-modulation detector of unit I6 comprises means for deriving a second unidirectional potential, the magnitude and polarity'of which vary with variations of the frequency of the intermediate-frequency signal carrier on either side of the mean frequency of the selector circuits. In this connection, it will be appreciated that the frequency of the intermediate-frequency signal deviates over-a predetermined range centered about its mean or nominal frequency.

The automatic amplification control system I9 includes a source of oscillations comprising an oscillation generator 20 which may be of any Well-known type. but .is shown as a single tube multivibrator of the type disclosed in United States Letters Patent No. 2,203,519, granted June 4, 1940 to Madison Cawein. Briefly, the generator 20 comprises a vacuum tube 2I of the pentode type having, its screen and suppressor directly coupled together through a condenser 22, the suppressor being provided with a grid leak 23. The screen and anode of tube 2I are energized from a source of space current +B through suitable

load f resistors

24 and 25, respectively. A condenser 26 is connected between the anode and ground, the condenser being periodically and alternately charged through the anodelload resistor 25 from the space current source- +B and discharged through tube 2|. A cathode-bias resistor 2'I is included in. the cathode lead While the control grid of tube 2I iscoupled through a resistor 28 to the A. V. C. supply of unit I6, is coupled through a filter circuit comprising a series resistor 2S and a shunt condenser 30'to the output of the frequency detector of unit I6, and, lastly, is coupled through a resistor 3I to the anode of a direct cur- A negative-bias potential is developed across the suppressor grid-leak resistor 23 when the tube 2l is generating oscillations and this potential is applied through a lter network comprising a, series resistor 33 and a shunt condenser 35 and through a resistor 34 to an amplication control circuit of the audio-frequency amplifier I1. In practice, the amplification control circuit of unit I'I generally comprises the control-grid circuit of the rst amplifier stage of unit I1.

-The input circuit of the direct current ampliiier 32 is coupled through an audio-frequency filter network comprising a series resistor 36 and shunt condenser 3'I to the output circuit of the frequency detector of unit I6. The `output circuit of amplifier 32 includes a load'resistor 38- and a source of space current +B. Positive .values of potential applied from the output of the frequency detector of unit I6 to the control grid of amplier 32 develop a large potential drop across resistor 36 by grid-rectification and consequently are not reproduced in the output of ampliiier 32. Negative values of the detector output potential are amplied, however, thereby to derive in the output of amplier 32 a unidirectional potential having positive polarity. Y Thus, amplifier 32 comprises means responsive to the detector unidirectional potential output of one polarity for deriving a second unidirectional potential of opposite polarity. That is,

amplifier 32 comprises means for deriving a unidirectional potential; the magnitude of which varies with variations in one sense of thefrequ'ency of thesignal' carrier from a` predetermined value, the polarity of this potential and the sense of variation thereof being opposite to that of the detector output potential with variation inthe one sense of the .signal-carrier frequency from the aforesaid predetermined value.

In considering the operation of the circuit just described, reference may be had to the aforementioned Patent 2,203,519 for a description of the detailed operation of the oscillation generator 20, per se. Brieiiy, however, for purposes of the present description of the system operation, it may be stated that the anode current of generator is of pulse wave form and that the generator generates a voltage o f substantially saw-tooth wave form across the condenser 26 in its output' circuit. The oscillations of the anode circuit cause a potential of pulse Wave form to be produced in the circuit of the screen grid, which potential is applied through condenser 22 to develop across the suppressor grid resistor 23 an alternating potential having small positive half-cycles and relatively large negative Ahalfcycles.- This particular type of generator possesses the important characteristic that the amplitude of the output saw-tooth oscillations developed, and the grid bias-developed across grid resistor 23 by grid rectification, are, for purposes of the present invention, substantially independent of variations of bias applied to the input electrodes of the generator.

The generator 20 possesses the additional important characteristic that the generation of oscillations is suddenly terminated when the negative biasing potential applied tothe control grid of tube `2I exceeds a predetermined magnitude and the generator remains inactive until such negative bias is reduced toa value lower than that at which the generator ceased to oscillate. Thus, this type of generator has, more or Assuming that the intensity of a received carrier signal is constant, the curves of Fig. 2 show the magnitudes ofthe unidirectional potentials developed by the frequency detector and by the A. V. C. supply of unit I6, that developedin the output circuit of ampliiier 32, and the approximate resultant control bias applied to the grid of oscillation generator 20 with variations of frequency of the intermediate-frequency carrier signal, the value of the approximate resultant control bias beingarrived at under the assump- .tion that no grid current is drawn by generator 20 when the grid is biased positively. Curve A represents the unidirectional potential derived by the A. V. C.- supply of unit I6, the magnitude of this potential usually being substantially constant, as indicated, over the range of .frequency less inherently, the property of either oscillating strongly or not at all, and changes in the magnitude of the .control-grid bias do not, insofar as the purposes of the present invention are concerned, change the amplitude of the generated oscillations except by a form of switch action from zero amplitude to full amplitude, or vice versa. Therefore, the speed of control of generator 20 is controlled only by the effective time constant of its input circuit, comprising the

circuit elements

28, 29, 30, 3l, 36, 31, 38 and the circuit elements included in the output circuits ofthe A. V. C. supply and the frequency detector of unit I6, and by the time constant of its output circuit, comprising the

circuit elements

23, 33, 34, 35 and the circuit elements included in the control circuit-of unit II. constants can be made so small that, to the ear,

the initiation or termination of the generation of oscillations is substantially instantaneous.

It follows, from the definite "on-off action of generator 20 that there is no transition period where generator 20 is partially operating. Since it is the presence of a transition region which causes distortion at a critical carrier-signal intensity, or critical value of frequency of the intermediate-frequency carrier signal, it is apparent that the control system I9 is free from such distortion. Obviously, there also can be no undesirable volume control effect when the automatic control bias applied to unit I1 Vis either normal or far beyond cutoff. This further ad- These timeV deviation of the intermediate-frequency carrier signal. Curve B represents the unidirectional potential derived from the intermediate-frequency carrier signal by the frequency detector of unit I6 which, it is seen, varies linearlybut in' opposite senses, on either side of the mean resonant frequency fo of the selector circuit of unit I6. This latter potential is applied to the input circuit of amplier 32 and there is derived at the anode thereof a unidirectional potential ,represented by curve C. From curve C it Will .amplifier 32, and that the anode potential inl-V creases linearly with frequency for increasingly negative values of detector output potential.

Since theoutputs of the A, V. C supply and the frequency detectorof unit I6 and the output of amplifier 32 are all applied tothe control grid of generator 20, the resultant potential applied to the control grid is a resultant voltage, the

, values of which are generally represented by curve D of Fig. 2. It will be seen from the resultant curve D that the control grid of generator 20 is negative as long as the frequency o f the intermediate-frequency signal carrier lies within the limited frequency range f to -f and that the control grid is positively biased for other values of frequency of the intermediatefrequency carrier signal.

`frequ'ency of the intermediate-frequency carrier signal lies substantially within the frequency range ,f to'-f. It may here be noted that, while the frequency of the intermediate-frequency signal deviates over a range wider than. the limited from time to time, this effect is avoided by filtering out the modulation components by the

filter networks

36, 31 and 29,'30. When the mean frequency of the intermediate-frequency signal carrier lies outside of the frequency range f to -f,

vantage of the present invention follows from'A the lack of a transition region.

the grid of generator 23 is biased positively, it

oscillates in a normal manner, a large negative bias is developed across the suppressor grid resistor 23, and the audio-frequency amplifier 'l1 is biased beyondcutoif, thereby silencing the receiver. e

The receiver is similarly silenced in the event that a desired received carrier signal does not have sumcient intensity to ensure satisfactory reproduction, the operation being substantially lthe same as in the foregoing described operation except that the slope of the detector output charunit I6 and the control grid of generator 20. Thus, the rectifier device 39 comprises means for deriving from the detector output potential a unidirectional potential having zero value when the frequency of the Signal carrier is above a predetermined value and having a given polarity when the signal-carrier frequency is below this predetermined value.

acteristic, represented by curve B of Fig. 2, is not as great for carrier signals of low intensity and' the magnitude of the A. V. C. bias, represented by curve A of Fig. 2, is correspondingly smaller.

The arrangement of Fig. 2 also includes a tuning indicator device, which may be of the 6E5 type of vacuum tube, the control grid of which is coupled through a resistor 4| to the conti-ol grid of tube 2|. The tuning indicator device 40 is energized from a source of space current +B.

The operation of this modified form'of the invention isessentially similar to that of the-Fig.

Thus' it will be evident that generator 2U continues to oscillate, thereby silencing the receiver, until such time as the received carriersignal is accurately tuned in and has a suiiciently large intensity that th'e negative bias derived by the A. V. C. supply of unit I6 is larger in magnitude than the positive biasv applied to the control grid ofl tube 2| from amplier 32.

From the foregoing description of theopera.- tion of the invention, it will be seen that the control grid of generator and its associated control circuit comprise means responsive to the unidirectional potentials derived by the A. V. C. supply and frequency detector of unit i6 for modifying the amplitude characteristic of the controlling' the amplifying characteristic of the audio-frequency amplifler i1. It will further be seen that the detector and A. V. C. supply of unit Iii. and the amplifier 32 together constitute means responsive to the frequency characteristic of the intermediate-frequency signal carrier for deriving a unidirectional potential having a negative polarity when the frequency characteristic of the signa1 carrier has a predetermined range of values and having a positive polarity when the value ef the carrier frequency is above or below the predetermined range of values.

-It is evident from curve D of Fig. 2 that the bias applied to the control grid of generator 20 may become negative, thereby to allow the audiofrequency amplifier I1 to operate with full output, in the event that the receiver is so badly mistuned from the desired carrier signal that the mean frequency of the intermediate-frequency signal carrier departs farther than a frequency f1 from the desired intermediate frequency. of course, is a condition of operation which in certain cases may be undesirable and which arises from the fact that the potential of the anodeof amplier 32 increases only to a predetermined value of potential; thereafter, increasingly larger negative values of detector output eventually bias the grid of generator 20 negatively, as indicated by curve. D. Such an operating condition is avoided by the-arrangement of Fig. 3 which is a circuit diagramrepresenting a modified form of the automatic control system essentially similar to the corresponding system I! of Fig. 1.

1 arrangement except that only positive values of unidirectional potential are applied from the detector of unit I6 to the control grid of gndirectional voltages and the resultant unidirectional voltage applied to the control grid of gen- 'erator 20, the value of the resultant voltage being A arrived at under the assumption, as before, that th'e grid of generator 20 does not draw grid current when positively biased. Curve A1 represents .the unidirectional potential applied to the control' grid from the A. V. C. supply of unit I6.

Curve-,B1 represents the unidirectional potential y i applied to the control grid from the detector of unit I6,- there being only positive'values of this voltage by virtue of the device 39. Curve C1 represents the unidirectional voltage applied to the control grid from the output of amplifier 32. The magnitude of the resultant of these several voltages. is represented, in general, by curve Di from which it is evident that the control grid of generator 20 can becom'e negative only when the intermediate-frequency signal carrier has a fremean frequency lying within the limited-frequency range f to -f. Consequently,A the receiver is silenced at all times except when the intermediate-freqeuncy signal carrier has a frequency very close to that of the desired intermediate frequency. Thus, amplifier 32 and the rectifier device 39 comprise means for deriving from the unidirectional voltage of the detector of unit I6 a second unidirectional voltage the magnitude of which varies positively from a predetermined value with variations of the frequency of the intermediate-frequency signal carrier in opposite senses from` the predetermined desired intermediate frequency.

The bias applied to the control grid of tube 2| is also applied to the grid of the tuning indicator .signal. Thus, the condition of tuning of the receiver to a desired carrier signal is audibly indicated'by the operation of -the audio amplifier |1 similar circuit elements being designated by similar reference characters. In this embodiment the negative values of unidirectional potential developed in the output of the detectorof unit I I are prevented from reaching the control grid of generator 20 by the insertion of a unilaterally conductive or rectifier device 39 in the coupling circuit between the outputf the detector of the same l.

channels. Thus, in the arrangement of Fig. 3,

'when the receiver is appreciably detuned from a carrier signal, the output of both the detector and A. V. C. supply of unit I6 is zero and the only bias applied to the control grid of vacuum tube 2| is a small constant positivebias derived from the output of the amplifier 32. This posi-v tive bias, of course, causes the generator 20 to generate oscillations, thereby to produce a negative bias across the resistor 23' which blocks the audio-frequency amplier .I1 to silence the output of the receiver. This described operation applies also to the Fig. 1 arrangement except for a small range of carrier-signal frequencies in the vicinity of the frequency f1, Fig. 2, where, as explained above, the generator 20 may cease to generate oscillations due to the negative values of curve D for such frequencies.

While in the arrangementsa of Figs. 1 and 3 the negative bias applied through ythe resistor 28 to the control grid of tube 2l is derived from the A. V. C. supply of unit I6, this bias may al-v While there have been described what are at present considered to be the preferred embodil in the appended claims to cover all such changes ternatively be derived from, a source of constant e potential, as, for example, a battery. However, where the bias is derived from a source of constant potential, the bias applied to the control grid of tube 2| is negative and has the same value either when the receiver is accurately tuned to a desired carrier signal or when the receiver is not tuned to any carrier signal as whenI tuned between carrier-signal transmission channels. In certain cases, this type of operation may be undesirable since the resultant negative bias applied to the control grid of tube 2| causes the generator 20 to cease generating oscillations, whereby the audio-frequency amplier Il has normal amplication, whenever the receiver is not tuned to proximity with a carrier signal. Thus interchannel noise is received when tuning between stations, a condition of operation which is avoided by the arrangements of Figs. 1 and 3.

As illustrative of a specic embodiment of the invention, the following circuit constants are given for an embodiment of the invention shown in Fig. 1:

Vacuum tube 2| Type BSJ'I Vacuum tube 32 Type 6SF5 Resistor 23 megohm-- 1 Resistor 24 ohms 68.000 Resistor 25 do 50,000 Resistor 21 do 220 Resistor 28 do 670,000 Resistor 29 megohms 1 lResistor 3| ..do 15 Resistor 33 do 3 Resistor 34 do 10 Resistor 36 do 1 Resistor 38 do 15 Condenser 22 micro-microfarads 3,000 Condenser 26 do 2,000 Condenser microfarad 0.02 Condenser do 0.01 Condenser 31 do 0.02 +B ;volts 250 Mean intermediate frequency megacycles 4.3

The circuit constants for an embodiment of the modified form of -the invention shown vin Resistor 28 megohms 3.3

and modications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A signal-translating system comprising, an input circuit adapted to have signals applied thereto, a signal-translating channel coupled to said input circuit, means for deriving a first unidirectional potential the magnitude and polarity oi which vary with variations of a characteristic of an applied signal in opposite senses from a predetermined value, means for deriving a second unidirectional potential the magnitude of which varies with variations in one sense of said char-f acteristlc from said predetermined value, the polarity of said last-named potential and the sense of variation thereof being opposite to that of said rst-named potential with variations in said one sense of said characteristic fromsaid predetermined value, and means responsive jointly to said derived unidirectional potentials for controlling a characteristic of said signalvsaid characteristic is above a predetermined Condenser 30 microfarad 0.01" Device 39 Type GHG vacuum tube value and having opposite polarity when said characteristic is below said predetermined value, means for deriving from said unidirectional potential of said one polarity a second unidirectional potential having said opposite polarity, and means responsive jointly to said unidirectional potentials of said opposite polarity for controlling a characteristic of said signal-translating channel.

3. A signal-translating channel comprising, an`

input circuit adapted to have signals applied thereto, a signal-translating channel coupled to said input circuit, means responsive to a characteristic of'an applied signalfor deriving a unldirectional control potential having one polarity when said characteristic has a predetermined value and having opposite polarity when the value of said characteristic differs above or below said value by a predetermined amount, a source of oscillations, means responsive to said derived unidirectional potential for. modifying a characteristic of the oscillations of said source, and means responsive to said characteristic of said oscillations for controlling a characteristic of said signal-translating channel.

4. A signal-translating system comprising, an input circuit adapted to have signals applied thereto, a signal-translating channel coupled to said input circuit, means for deriving a rst unidirectional potential the magnitude and polarity of which vary with variations of a characteristic' of an applied signal in opposite senses irom a predetermined value, means for deriving a second unidirectional potential the magnitude of which varies withvariations `in one sense of said characteristic from a predetermined value, the polarity of said last-named potential and the sense of the variations of magnitude thereof being4 opposite to that of said rst unidirectional potential with variations in said one sense of said characteristic from said predetermined value, a source of. oscillations, means responsive jointly to said derived unidirectional potentials for modifying a characteristic of the oscillations of said source, and means' responsive to said characteristic of said oscillations for controlling a characteristic of said signal-translating channel.

5. .A signal-translating system comprising, an input circuit adapted to have signals applied thereto, a signal-translating channel coupled to -sad input circuit, means responsive to a characteristic of an applied signal for deriving a unidirectional potential having one polarity when said characteristic is above a .predetermined value and having opposite polarity when said characteristic is below seid predetermined value,

vary with variations of a characteristic of an applied signal in opposite senses from a predeterminedvalue, means for deriving a second unidirectional potential the magnitude of which varies with variations in one sense of said characteristic from said predetermined value, the polarity of said second unidirectional potential and the sense of variation oi the magnitude thereof being opposite to that of said rst unidirectional potential with variations in said one sense of said characteristic i'rom said predetermined value. a source oi oscillations, means responsive jointly to said demeans for deriving from said unidirectional posaid input circuit, means responsive to a characteristic of an applied signal for deriving a unidirectional potential having one polarity when said characteristic is above a predetermined value and having opposite polaritywhen said characteristic is below said predetermined value, a direct current ampliiler having an input circuit coupled to said last-named means and having an output circuit in which is derived from said unidirectional potential of said one polarity a second unidirectional potential having said opposite polarity, a source of oscillations, means responsive jointly to said unidirectional potentials of said opposite polarity for modifying a characteristic of the oscillations of said source, and means responsive to said characteristic of said oscillations for controlling a characteristic oi said'signal-translating channel.

7. A signal-translating system comprising, an input circuit adapted to have carrier signals applied thereto, a signal-translating channel coupled to said input circuit, a frequency detector coupled to said input circuit for deriving from an applied signal a unidirectional potential having one polarity when the frequency of said carrier signal is above a predetermined frequency and having another polarity when the value of said carrier-signal frequency is below a predetermined frequency, an audio-frequency iilter network,A a

direct current ampliiier coupled through saidnetwork to said frequency detector for deriving from said unidirectional potential of said one polarity a second unidirectional potential having said opposite polarity, a source of oscillations,

means responsive jointly to said derived unidio sponsive jointly to said second and third unidlrectional potentials oi said opposite polarities for modifying a characteristic of the oscillations of said source, and means responsive to said characteristic of said oscillations for controlling a characteristic of said signal-translating channel.

8. A signal-translating system comprising, an input circuit adapted to have signals applied thereto, a signal ampliler coupled to saidinput circuit, means for deriving a rst unidirectionalv potential the magniimde and polarity of which lived unidirectional potentials for modifving a characteristic oi.' the oscillations of said source,

and means responsive ,to said characteristic of said oscillations for controlling the ampliilcation of said signal ampliiler.

9. A signal-translating system comprising, an input circuit. adapted to have signals applied thereto, a signal-translating channel coupled to said input circuit, means responsive to a characteristic of an applied signal for deriving a first .unidirectional potential having one polarity when said characteristic is above a predetermined value and having opposite polarity when said characteristic is below said predetermined value, means for deriving from said first unidirectional potential of said one polarity a second unidirectional potential having said opposite polarity, means for deriving from said rst unidirectional potential a third unidirectional potentialhaving zero value when said characteristic is above said predetermined value and having said opposite polarity when said characteristic is below said predetermined value, a source oi oscillations, means responsive jointly to said second and third unidirectional potentials for modifying a characteristic A of the oscillations of said source, and means responsive'to said characteristic of said oscillations for controlling a characteristic of said signal-translating channel.

10. A signal-translating system comprising, an input circuit adapted to have signals applied thereto, a signal-translating channel coupled to said input circuit, means responsive to a characteristic oi' an applied signal for deriving a rst unidirectional potential having one polarity when said characteristic is above a predetermined value and having opposite polarity when said characteristic is below said predetermined value. means for deriving from said iirst unidirectional potential of said one polarity a second unidirectional potential having said opposite polarity, means including a unilaterally conductive device for deriving from said rst imidirectional potential a third unidirectional potential having said zero value when said characteristic is above said Dredetermined value and having opposite polarity when said characteristic is below said predetermined value, a source of oscillations, means rerectional potentials for modifying a characteristic lof the oscillations of said source, and means responsivevto said characteristic of said oscillations for controlling a characteristic of said signaltranslating channel.

11. A signal-translating system comprising. an input circuit adapted to have signals applied thereto, a signal-translating channel coupled to said input circuit, means for deriving a ilrst unidirectional potential the magnitude of which varies with a characteristic of an applied signal,

means responsive to a characteristic of said applied signal for deriving a second unidirectional Potential having one polarity'when said lastresponsive jointly to said ilrst,A second, and third unidirectional potentials for modifying a characteristic .of the oscillations of said source, vand means responsive to said characteristic of said oscillations for controlling a. characteristic of said signal-translating channel.

. NELSON P. CASE.