US2290517A - Frequency-modulation television signal receiver - Google Patents

Description

July 2l, 1.942. J. c. WILSON FREQUENCY-MODULATION TELEVISION SIGNAL RECEIVER Filed Aug. 2, 1940 OKNJ ATTORNEY Patented July 21, 1942 FREQUENCY-MODULATION TELEVISION SIGNAL RECEIVER John C. Wilson, Bayside, N. Y., assignor to Hazeltine Corporation, a corporation of .Delaware Application August 2, 1940, Serial No. 349,541 8 Claims. (CL 178'l.`3)

This invention relates generally to frequencymodulation television signal receivers and, particularly, to such a receiver which is substantially unresponsive to interfering signals of frequencies l in the vicinity of the frequency which represents the black shade value of the image to be reproduced.

Received undesired signals generally have a a modulated-carrier television receiver. Thus,

, detrimental eiect on the image reproduced by received undesired signals having a reproducible characteristic similar to the reproduced characteristic of a desired signal component corresponding to the black shade value of the image being received have the eiect of introducing white spots within the darker portions of the picture. Specifically, in certain cases, ignition interference orthe like can produce an eiect in a frequency-modulation receiver which is similar to that of a spurious frequency modulation. This is very detrimental to the effective quality of reproduction for the reason that the human eye is particularly susceptible to such White spots on a dark background, Dark spots on a White background are much more tolerable. It is, therefore, very desirable in a television receiver to provide means for minimizing the reproduction of such undesired signals as appear as white spots on the darker portions of -the reproduced image. This can be done to some extent in an amplitude-modulation receiver by translating and reproducing a negatively-modulated carrier signal; that is, a signal in which increasing carrier amplitudes correspond to decreasing light values of the translated image. The reason that such a system is unresponsive to the undesired signals of the 'type mentioned above is that received undesired signals generally have the effect of increasing the amplitude of the signal which tion of a television system as a blacker-thanblack component of the modulated-carrier sign tomary to transmit the synchronizing informanal. Therefore, a receiver which is particularlyl susceptible to undesired received signals in the vvicinity' of a desired received signal level corresponding to the black shade valueoi the reproduced image is also very likely to be difficult to synchronize for the reason that the synchronizing signals themselves lie on the opposite side of a demarcation level, but in the immediate vicinity of the darker shade values of the transmitted picture. Furthermore, under such conditions it is difficult or impossible to stabilize the signal input to the signal-reproducing means at the amplitude level corresponding to the black shade value of the transmitted image.

It is an object of the invention, therefore, to provide an improved. frequency-modulation television receiver which is effective to eliminate one or more of the, disadvantages of the prior art mentioned above.

It is a further object "of the invention to provide a frequency-modulation television receiver in which the tendency -of Aundesired received signals to cause white spots in the darkerportions of the reproduced image is minimized. I

It is still another'object of the invention to provide a frequency-modulation television receiver in which the tendency of undesired signals to interfere with the synchronization of the receiver or maintenance of the black level of the picture is minimized;l `,In particular, this may imply that the'tendency of undesired signals to prevent clean separation of synchronizing and video components of the desired signal, or to prevent proper stabilization of the video signal, is minimized. v i

In accordance with the invention, a television receiver adapted to'receive a frequency-modulated carrier signal, in which a predetermined frequency deviation of the received signal represents a black shade value of the transmitted image, comprises a detector, for deriving a videofrequency signal from the received .carrier signal, which is less responsive to undesired amplitudemodulation components for one portion of its operating range than over another portion thereof; The receiver also includes means for so applying received signals to the detector that components thereof corresponding to the black shade value of the received image are detected within the above-mentioned portion of the operating range of the detector. Means are also provided for reproducing/the detected video-frequency signal.

For a better understanding of the invention, together with other and further objects thereof,

' of the image-reproducing device I8.

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.

Fig. 1 of the drawing is a circuit diagram, partially schematic, of a complete frequency-modulation television receiver of the superheterodyne type embodying the invention; Fig. 2 comprises a graph utilized to explain the operation of the portion of the receiver circuit of Fig. 1 comprising the present invention; while Fig. 3 illustrates a modification of a portion of the receiver circuit of Fig. l.

Referring now more particularly to the drawing, Fig. 1 comprises.- a frequency-modulation carrier-signal television receiver of the superheterodyne type for receiving a signal in which a predetermined carrier-frequency deviation represents the black shade value of the transmitted -image and including an antenna system III, II

connected to a radio-frequency amplier I2, to which are connected in cascade, in the order named, an oscillator-modulator I3, an intermediate-frequency amplifier I4, a. limiter I5, a detector I6, a video-frequency amplifier I1, and an image-reproducing device I8. A line-,scanning circuit I9 and a field-scanning circuit 20 are coupled to the output circuit of detector. I6 through a synchronizing-signal separator 2i and to the defiecting elements of image-reproducing device I8 in a manner well understood in the art. A sound-signal reproducing apparatus 8 is coupled to an output circuit of limiter Il for reproducing sound signals accompanying the received television program. The stages or units 8-I5, inclusive, and I'I-2I, inclusive, may all be of conventional well-known construction so that detailed illustration and description thereof are deemed unnecessary herein.

Referring briefly, however, to the operation of the system described above, a frequency-modulated television signal, in which a predetermined carrier-frequency deviation of the received signal represents the black shade value of the transmitted image, is intercepted by the antenna circuit Ili, II, is selected and amplified in radiofrequency amplifier I2, and translated to the oscillator-modulator I3 wherein itis converted into an intermediate-frequency signal which, in turn, is selectively amplined in the intermediatefrequency amplifier I4, is limited to a substantially constant amplitude in limiter I5 in order to eliminate spurious amplitude-modulation components, and is delivered to detector I6. The modulation components of the signal are derived by detector I6 and are supplied yto the videofrequency amplifier I l, wherein they are amplifled and from which they are supplied, in the usual manner, to a brilliancy-control electrode 'I'he intensity of the scanning ray of the device I8 is thus modulated -or controlled in accordance with the video-frequency voltages impressed upon the control electrode, in the usual manner.

Scanning waves are generated in the linescanning and field-scanning circuits I9 and 20, respectively, which are controlled by synchronizing-voltage pulses supplied from detector I6 through synchronizing-signal separator 2|, and are applied to the scanning elements of the -image-reproducing device I8 to produce electric scanning fields, thereby to deflect the scanning ray in two directions normal to each other, so as to trace a rectilinear scanning pattern on the screen and thereby to reconstruct the transmitted image. Sound signals are reproduced by apparatus 9 in a manner well understood by those skilled in the art.

Referring now more particularly to the portion of the system of Fig. i embodying the present invention, detector IB comprises a transformer having a primary winding 25 and a secondary winding 26 tuned by a condenser 21 to the mean frequency of the signal to be received; that is, to the mean frequency of the intermediate-frequency channel of the receiver. Sum and difierence voltages of the primary winding 25 and secondary winding 26 are applied, respectively, to the anodes of diodes 28 and 29 through a coupling condenser 30. Load resistors BI and 32 by-passed, respectively, by condensers 33 and 34 are provided for the diodes 28 and 29. The common junction of condensers 33 and 34 is returned to a point common to the coupling condenser 38 and a tap 38 on winding 26, while the opposite terminal of condenser 34 is grounded. The common terminal of resistors 3| and 32 is connected to the common terminal of condensers 33 and 34.

In considering the operation of the detecto circuit just described, it will be seen that, if tap 38 is adjusted to the mid-point of winding 26, the detector described above is, per se, of a conventional type in which sum and difference voltages of the primary winding 25 and secondary winding 26 are individually applied to diodes 28 and 29. Such a, frequency detector has a wellknown signal-input frequency-signal-output characteristic as represented by curve A of Fig. 2; that is, the detector circuit. comprises a. means for deriving from the receivedl signal a first voltage, the sum of the primary and secondary voltages, which increases with frequency inthe vicinity of the mean frequency of the received signal and a means for deriving from the received signal a second voltage, the dierence of the primary and secondary voltages, which decreases with frequency in the vicinity of the mean frequency of the received signal. 'I'hese voltages are individually applied to diodes 28 and 29 to provide a characteristic A which is balanced or symmetrical about the mean carrier frequency fo and which has a substantially linear portion over the normal operating frequency range f1, fz.

range and is substantially unresponsive to noise amplitude-modulation components at this frequency; that is, the detector is less responsive to undesired amplitude-modulation components over the portion of the range including frequency fo than over another portion thereof.

However, it has not heretofore been proposed to make the frequency of the carrier signal corresponding to the black signal'level fall on the balance point fo of the detector characteristic or, in most prior art arrangements, on any particular point of such characteristic, but to traverse the characteristic, including its unbalanced portions, in accordan-ce with the nature of the signal being translated.

Since, as pointed out above, undesired signals are most objectionable when they are primarily effective in the region of the black shade value of the reproduced image, such prior art frequency-modulation receivers have been subject to disturbing reproduction of spurious noise components in the black region of the reproduced image.

Applicant, therefore, has provided a means Such a detector circuit is inher` l ently -balanced` at the mean frequency fo of the fortranslatihg the derived voltages in the detector so that components of the received signal having a frequency corresponding to a black shade value are always detected within a portion of the operating range of the detector over which the detector is substantially less responsive to undesired signals than over the remaining portion of its operating range. That is, applicant has provided a means for so applying the received signals to the detector that the frequency of the carrier signal corresponding to the black shade value falls on'the balance point of the detector characteristic. and frequency components of the signal corresponding to the black shade values are detected within the above-mentioned less-responsive portion of the operating range. In accordance with the preferred embodiment illustrated in Fig, 1, this means comprises tap 38. By adjustment of the tap, the output characteristic of the detector may be made substantially asymmetrical with respect to the mean carrier frequency fo, as illustrated by curve B of Fig. 2. A detector having the characteristic represented by curve B is inherently balanced at a frequency f3 and is, therefore, unresponsive to undesired signals over a portion of its operating range in the vicinity of frequency f3. Therefore, the circuit of the invention is so adjusted by means of the adjustment of tap 38 that the balance point of the detector coincides with the carrier-signal frequency coresponding to the black shade value of the transmitted image. Alternatively, the receiver may be adjusted to procure this result by adjustment of oscillator-modulator section I3 and theintermediate-frequency selectors to locate the carrier-frequency component of the signal translated through the intermediate-frequency channel of the receiver corresponding to black at the proper point on the detector characteristic.

The detector I6, therefore, includes a means for deriving from the signal a first voltage which increases with frequency in the vicinity of the mean frequency of the received signal, and a meansy for deriving from the received signal a second voltage which decreases with frequency in the vicinity of the mean frequency of the signal and,

when tap 38 is displaced from its center positien, one of the derived voltages is of substantially lesser magnitude at the meanv frequency of the received signal than the other of the derived voltages. The tap 38 on secondary winding 26 thus comprises a means for adjusting the relative values of the derived voltages at the mean frequency of the signal.

In Fig. 3 there is illustrated a/modification of a portion of the circuit of4 Fig. 1 which utilizes a diierent type of a detector circuit. Elements of Fig` 3 which are similar to elements of Fig. 1 have identical reference numerals. It will be understood that the circuit of Fig. `3 can be connected into the receiver circuit of Fig. l simply by removing element I6 and connecting the terminals of element I6 to the corresponding'ly marked terminals of the circuit of Fig. 1.

The detector circuit illustrated in Fig. 3 is of the type comprising a tuned circuit resonant above the frequency band of the signals to be detected and a tuned circuit resonant below the band. Specifically, tuned circuit 40 is resonant above the band and tuned circuit 4I is resonant below the band, each of the circuits being coupled by mutual inductance to Winding 25. o'rder to vary the balance point of the detector I6', the mutual inductance between tuned circuit 40 and winding 25 is made variable.

In considering the operation of the circuit of Fig. 3, it will be seen that, if equal voltages at the mean resonant frequency of the system are coupled from tuned circuits 48 and 4I to diodes 28 and 29, respectively, a balanced characteristic in accordance with that of Curve A of Fig. 2 is provided. Furthermore, it will be seen that the frequency at which thecircuit is balanced can be changed to provide the characteristic B of Fig. 2, by an adjustment `of the mutual inductance between winding 25 and tuned circuit 40. It is believed that the operation of the circuit of Fig 3 will be readily apparent to those skilled in the art from this description, taken in connection with the description of the operation of thecircuit of Fig. 1 given above.

It will be seen that the arrangement of the present invention provides distinct advantages with regard to stabilization of the signal at the black level. That is, since undesired signals have a relatively small effect in the circuit of the invention upon signals corresponding to the black shade value of the transmitted image, a signal Which is stabilized at the black level is not materially unstabilized by-the reception of undesired signals so` that the resultant signal is more nearly stabilized at the input circuit of image-reproducing device I8. Furthermore, if an arrargement is provided in the video-frequency channel of the receiver for providing stabilization, this stabilization is more nearly exact due to the small influence undesired signals have upon the video-frequencysignals corresponding to the black shade values of the transmitted image.

A residual stabilization may be effected in the intermediate-frequency channel of the receiver, for instance, by providing a sharply-tuned circuit resonant at a frequency in the immediate vicinity of the intermediate frequency corresponding to the tips of the synchronizing signals of the transmitted signal and adapted to be excited by the intermediate-frequency signal, together with a suitable detector coupled to the tuned circuit, the output of which is utilized to control the frequency of the oscillator section of oscillator-modulator I3, to provide the desired residual stabilization .in a manner analogous to automatic frequency control of conventional ainplitude-modulation receivers.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to I those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore,

`aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A television receiver adapted to receive a frequency-modulated carrier signal in which a predetermined carrier-frequency deviation of the received signal represents the black shade value of the transmitted image comprising, a detector for deriving a video-frequency signal from said received carrier signal, said detector being less responsive to undesired amplitude-modulation components over one portion of its operating range than over another portion thereof, means for so applying received signals to said detector that frequency components thereof corresponding to said black shade value are detected within said one portion of said operating range, and means for reproducing said video-frequency signal. v

2. A television receiver adapted to receive a frequency-modulated carrier signal in which a predetermined carrier-frequency deviation of the received signal represents the black shade value of the transmitted image comprising, a detector, means included in said detector for deriving from said signal a first voltage which increases with frequency in the vicinity of the mean frequency of the received signal, means included in said detector for deriving from said received signal a second voltage which` decreases with frequency in the vicinity of the mean frequency of said signal, said detector being less responsive to undesired amplitude-modulation components over one portion of its operating range than over another portion thereof, means for so trans- -lating said derived voltages in said detector that frequency components of the received signal corresponding to said black shade value are detected within said one portion of said operating range, and means for reproducing said video-frequency signal.

3. A television receiver adapted to receive a.

frequency-modulated carrier signal in which al predetermined carrier-frequency deviation ofthe received signal represents the black shade value of the transmitted image comprising, means for deriving from said. signal a first voltage which increases with frequency in the vicinity of the mean frequency of the received signal, means for deriving vfrom said received signal a second voltage which decreases with frequency in the vicinity of the mean frequency of said signal, two diodes, means for applying one of said derived voltages to one of said diodes and the other of said derived voltages to the other of said diodes to provide a frequency-modulation detector less'responsive to undesired amplitudemodulation components over one portion of its operating range than over another portion thereof, means for .providing that frequency components of said received signal corresponding to said black shade value are detected within said one-portion of said operating range, and

frequency-modulated carrier signal in which a predetermined carrier-frequency deviation of the received signal represents a black shade value of the transmitted image comprising, a transformer having a primary winding and a secondary winding tuned to the mean frequency of said signal, two diodes, means for individuallyapplying to said diodes sum and difference voltages derived fron: said windings to provide a detector for deriving aV video-frequency signal from said received carrier signal, said detector being less 'responsive to undesired amplitudemodulation'components over one portion of its operating range than over another portion thereof, means for so applying said received signal to said detector th'at frequency components thereof corresponding to said black shade value are detected Within said one portion of said operating range, and means for reproducing said video-frequency signal.

6. A television received adapted to receive a frequency-modulated carrier signal in which a predetermined carrier-frequency deviation of the received signal represents a black shade value of the transmitted image comprising, a transformer having a primary Winding and a secondary winding tuned to the mean frequency of said sgnal, two diodes, means for individually applying to said diodes sum and difference voltages derived from said windings which are substanvtially different at the mean frequency 'of said signal to provide a detector for deriving a videofrequency sgnal from said received carrier signal, said detector being less responsive to undemeans for reproducing said video-frequency sigpredetermined carrier-frequency deviation of the received signal represents the black shade value of the transmitted image comprising, a detector, means included in said detector for deriving from said signal a first voltage which increases with frequency in the vicinity of the mean frequency of the received signal, means included in said detector for deriving from said received signal a second voltage which decreases with frequency in the vicinity of the mean frequency of said signal, said two last-named means being so proportioned that one of said derived voltages is of substantially lesser magnitude at the mean frequency of said signal than the other of said voltages, said detector being less responsive to undesired signals over one portion of its operating range than over another portion thereof, means for so translating said derived voltages in said detector that frequency componentsof the received signal corresponding to said black shade value are detected within said one portion of said operating ra'nge, and means for reproducing said video-frequency signal.

5. A television receiver adapted to receive a sired amplitude-modulation Acomponents over one portion of its operating range than over another portion thereof, means for so applying Said received signal to said detector that frequency components thereof corresponding to said black shade value are detected within said one portion of said operating range, and means for reproducing said video-frequency signal.

7. A television receiver adapted to receive a frequency-modulated carrier signal in wh'ich a predetermined carrier-frequencyV deviation of the received signal represents a black shade value of the transmitted image comprising, a transformer having a primary winding and a secondary winding tuned to the mean frequency of said signal, two diodes, means for individually applying to said diodes sum and 'difference voltages derived from said windings to provide a detector for deriving a. video-frequency signal from said received carrier signal, said detector being less responsive to undesired signals over one portion of its operating range than over another portion thereof, means including a tap on said secondary winding for adjusting the relative values of said derived voltages at the mean frequency of said signal, thereby to vary the portion of the operating range over which the detector is less responsive to undesired signals, means for so applying said received signal to said detector that frequency components thereof corresponding to said black shade value are detected within said one portion of said operating range, and means for reproducing said video-frequency signal.

8. A television receiver adapted to receive a frequency-modulated carrier signal in which a predetermined carrier-frequency deviation of the received signal represents the black shade Y value of the transmitted image comprising, a

-detector including a circuit resonant above the range of signals to be detected and a circuit ing less responsive to undesired amplitude-modui0 lation vcomponents over `one portion of its operating range than over another portion thereof, means for translating said derived voltages in said detector so that frequency components of the received signals corresponding to saidblack shade value are detected within said one portion of said operating range, and means for reproducing said video-frequency signal.

JOHN C. WILSON.

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