US2480913A - Frequency modulation television receiver - Google Patents

Description

Sept- 5, 1949- lv. J. DUKE ETAL 2,480,913

FREQUENCY MODULATION TELEVISION RECEIVER Filed Dec. 27V, 1943 4 Sheets-Sheet` 2 ucl/Vm osc/zMra/a 90m PM INVENToRs. 'e s si vee/vow' J. puffs,

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FREQUENCY MODULATION TELEVISION RECEIVER Filed Dec. 27, 1943 4 Sheets-Sheet 3 was@ LF. uw o 60 Q l l,.5

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.late the audio frequency carrier.

Patented Sept. 6, -1949 UNITED sTATEs PATENT orti-,cs

FREQUENCY MODULATION TELEVISION RECEIVER Vernon J. Duke, Rockville Centre, Y., and Robert W. Clark, Teaneck, N. J., assignors to Radio Corporation of Americana corporation vof 'Delaware Application December 27, 1943, Seria1'No..515,818

4 Claims. 1

This invention relates to an improvement 'in television systems, and more .particularly to an improvement in a television receiver for receiving a radio frequency carrier which has `been frequency modulated .by the television signals.

In conventional xtelevision systems the video or .image `and .synchronizing signals are transmitted as modulations of one carrier frequency while the accompanying audio or ,sound signals are transmitted as a modulationrof a .separate carrier, gen- .erallypositioned-slightly Vabove the video or image carrier. It is normal practice .to amplitude .mod-

ulate .the video carrier and to frequency .modu- In the amplitude modulation .of the radio `frequency .carrier by the video signal, it is standard practice to control the .carrier amplitude insuch a way .that .full carrier amplitude -(or 100%.) .is .transmitted during the presence of synchronizing signals only. Full blackrin the subject is generally transmitted at approximately 7.5% of maximum `carrier arnplitude, and full white in the ,picture is .normally transmitted at .approximately zero .amplitude of the carrier (when direct :current insertion is employed). vnecessary in television in :order that the 'background or average light :intensity maybe transmitted to the receiver .and in .order that vthe .Synchronizing signals may .be .separated from vthe image signals by .amplitude .selection or .other .appropriate means.

The .audio signals are .generally transmitted as ,a .frequency modulation .of the .carrier lprovided for that purpose,.and are .detected 'by `appropriate discrimination vcircuits lin the television receiver. Since .the .audio .and video signals are .transmitted as modulations ,ofseparate carriers, itis com- :mon :practice ,in .a television .receiver to .employ .a .single local oscillator, which when beat .against the two received radio frequency carriers `produces -two separate intermediate frequencies. One of these is a frequency Ymodulated intermefdiate .frequency carrier and includes Ythe .audio information, whereas .the other ,is an `amplitude modulated intermediate ifrequency .carrier and yincludes the video information.

In .the present .invention provision .is made whereby .both the videoand the .audio signals :are transmitted -by .frequency modulation .of lseparate v`radio frequency carriers,.and when-.this is done in accordance with the present invention, the Yconventional video vsignal present :at dthe .transmitter -is -used A`for frequency `.modulating an .appropriete carrier.

In the present invention `two .different .typesof Signal transmission in -this .manner iisreceiving circuits may .be employed. In one nstance the ,-full frequency .deviation produced by .the modulation video signal is represented by Aa sing-le intermediate frequency channel and a ,single ,discriminator is usedso that `at the .output of `the :discriminator a Icomplete composite television signal will be present including .the image signals, the blanking andthe synchronizing signals.

-Inanotlier form of the present ,inventionel .ara-te intermediate frequency Achannels are employed in the receiver, one lfor passing the `frequency `-deviations representing the image portion .of the received signals and the .other .for passing .only the frequency deviations representing the synchronizing portion of the received signals. :In this particular form of .the present invention, the complete composite video signal is `received asa frequency modulation of a single carrier, .but by providing sepa-rate intermediate .frequency channels having different band pass characterristics, the image signals and the synchronizing signals may be separated `.according to the frequency deviations of the Vintermediate frequency gcarrier. When such la system is employed, two separate detectors or discriminators are used, -oneor responding .to-,the `frequencydeviations-ocfcurring within lthe `image intermediate frequency channel and the other for responding to frequency deviations .occurring .within the ,synchronizing intermediate ,channel frequency. When separate intermediate frequency channelsare provided Vfor ithe image ,and synchronizing signals, `complete and ,convenientseparation of thesesignals is lpossible. It is, therefore, .possible to cause ,the receiver .to respond .to .synchronizing impulses of lproper amplitude -while at the same time there- .ceiver is unresponsive to transient `conditions or .undesired disturbances which produce deviations less rthan .maximum ,deviations produced by thesynchronizing impulses.

Inthe present .invention the audio signals .may .still vbe transmitted .asa frequency ,modulation of .a separate radio frequency carrier, and for the .transmission of these signals through vthe .reoeiver .a separate .intermediate frequency ampliier `channel may lbe provided.

.It is, therefore. .one purpose of the present in- .vention .to lprovide means `for Vtransmitting video .signals .as airequency modulation of a radio frequency carrier.

Another :purpose fof the present invention ,re- .sides in .the .provisionof .a television receiver responsiveto .frequency modulationsof .a radio frequency ,carrier in which the ,frequency modulations are produced in accordance with the television video signals.

Still another purpose of the present invention resides in the provision of a television receiver for receiving a frequency modulated carrier, the frequency deviations of which are produced in accordance with composite video signals, together with means for detecting and separating the image signals from the synchronizing signals.

A still further purpose of the present invention resides in the provisions of means in a television receiver for receiving a frequency modulated radio frequency carrier in which the frequency deviation-s are produced by a composite video signal and in which the receiver includes two intermediate frequency channels, the band pass of one being suincient to accommodate the frequency deviations representing the image signals, and the band pass of the other being chosen to accommodate the frequency deviations representing the synchronizing signals.

Still another purpose of the present invention resides in the provision of means in a television receiver for receiving a video signal frequency modulated radio frequency carrier in which the various intermediate frequency band pass circuits are so designed that over-modulation or excessive frequency deviation do not produce any undesired or erroneous results.

Various other advantages and purposes of the present invention will become more apparent to those skilled in the art from the following detailed description, particularly when considered in connection with the drawings, wherein like reference numerals represent like parts and wherein:

Figure 1 diagrammatically represents one form of the present invention;

Figure 2 shows a graph and curves used in explaining the operation of the system shown in Figure 1;

Figure 3 shows a diagrammatic modification of the present invention;

Figure 4 shows a graph and curves u-sedin explaining the operation of the system shown in Figure 3;

Figure 5 shows curves representing one undesired result that will take place under one set of circumstances;

Figure 6 shows curves representing another undesired result that will take place under another set of circumstances; and

Figure 7 shows curves representing desired response under conditions of over-modulation.

Referring now to Figure 1, there is shown schematically a television receiver including a radio frequency tuner l0 to which signals received upon an appropriate antenna I2 (or by way of other communication channel) are supplied. A local oscillator I4 is also provided, the frequency of operation of which may be altered in accordance with the radio frequency tuning of the receiver. The received signals and the oscillations from the oscillator I4 are supplied to a mixer in order to produce an intermediate frequency carrier which is applied to an intermediate frequency amplifier i6. If the audio signals which accompany the video signals are transmitted as modulations of a separate radio frequency carrier, the audio intermediate frequency carrier may be supplied to appropriate amplifiers and demodulators over conductor I8. Y Y

Since the present invention is not concerned with the particular audio system employed, details of that system are not shown herein.

` video signal series.

The output from the video intermediate frequency amplifier is then applied to a discriminator network 20 (preceded by a limiter, if desired), and the resulting video signals are, in turn, applied to an appropriate video amplifier 22. The discriminator also applies signals to a synchronizing signal separator 24 in order that the synchronizing signals may be separated from the image signals thereby to control the operation of horizontal and vertical deflection generators 26. The amplified video signals as supplied by the amplifier 22 are applied to the control electrode of an image producing tube or Kinescope 28 in order to modulate the current intensity of a cathode ray beam developed therein. In order that the cathode ray beam may be caused to scan a target surface or luminescent or fluorescent screen, energy from the horizontal and vertical deflection generators 26 is applied to a deflection yoke 30 associated with the image tube.

As stated above, the television receiver is designed to receive a radio frequency carrier that has been frequency modulated by the composite In order to clearly explain the operation of the system represented by Figure 1, reference is now made to Figure 2 of the drawings.

It will be assumed, for example, that the center frequency of the radio frequency carrier is 104 megacycles, with deviation limits extending 4: megacycles on each side of the carrier center as shown in Figure 2. Therefore, the lower deviation limit of the carrier is approximately 100 megacycles, whereas the upper deviation limit is 108 megacycles.

It will also be assumed that the peaksV of the synchronizing impulses are transmitted at approximately the upper deviation limit and that full white in the image is transmitted at approximately the lower deviation limit. If a standard composite video signal series is used to frequency modulate the radio frequency carrier, then full black in the image will be represented by 106 megacycles. Under these conditions a band of 6 megacycles is provided for the transmission of the image signals and, similarly, a band of 2 megacycles is provided for the transmission of the synchronizing signals.

If under these conditions the local oscillator at the receiver operates, for example, at 90 megacycles, a complete intermediate frequency band of 8 megacycles is required for the transmission of the intermediate frequency carrier, and this band extends from 10 megacycles as a lower deviation limit to 18 megacycles as an upper deviation limit. Full black in the image (or the blanking impulses) occurs at 16 megacycles in the intermediate frequency channel in the assumed exemplary system.

For responding to these frequency deviations a discriminator must be provided having a suflicient frequency band width to respond to the full 8 megacycle deviation band in a substantially linear fashion. A desired discriminator response curve is shown in Figure 2, and when frequency deviations from the intermediate frequency amplifier channel are applied to the discriminator, a series of composite video signals such as represented in Figure 2 may be produced. It is desirable that the discriminator respond substantially linearly to frequency deviations occurring between 10 and 18 megacycles, and it may also be desirable under some circumstances that the discriminator be non-responsive to signals occurother, provided no over-modulation occurs'.

ring below megacycles or above' 18 megacycles.

A discriminator having such a response curve is shown and described in U. S. Letters Patent No. 2,413,913, issued'to Vernon J. Duke on Jan.- '7,

It may be seen, therefore, that when a television receiver such as that represented in Figure 1 is providedfor the'reception of a frequency 'modulated carrier, a composite video signal may loe-produced, as shown in Figure 2, which includes the image signals, the blanking signals and the sy'n ':hronizing'V signals in theirproper intensity, and having proper values with respect to each To produce such a result it is'merely necessary to frequency modulate a radio frequency carrier by the presently use d video signals, and when such signals are received and are applied to an appropriate discriminator, a video signal series will be produced which may be employed for producing a-'visual image in a manner similar to the present amplitude modulation receivers.

In a system as describedabove, some form of 'synchronizing separation circuit must be provided for separating the synchronizing impulses from the image signals and, in addition, a relatively wide bandintermediate frequency amplifier channel and discrimnator must be provided.

In a modification of the present invention it is proposed' to provide two separate intermediate frequency channels in the receiver (in addition to the sound channel), the channels being related to each other in such aY manner that a single radio vfrequency carrier that has been frequency modulated by the video signal may be received. In this respect, reference is now made to Figure 3 wherein a receiver diagram is shown which includes a radio frequency tuner and mixer Ill as in Figure 1 to which is applied energy from an appropriate antenna I2, for example. An oscillator I4 is also provided for heterodyning with the received carrier signals to produce intermediate frequency carriers. The frequency deviations which are produced as a result of the tele- A'vision image signals are `then applied to the image vturn supplies image signals to an appropriate limagef'signal amplifier 46. The synchronizing in'- termediate frequency Vamplifier supplies signals to a'syn'chronizing signal discriminator 48 (also after' limiting, if desired) and the output of this discriminator isapplied as control signal impulses uponv the horizontal and vertical deection generators, conventionally shown at 5U. The v horizontaland vertical deflection generators sup- Aply energy to a deflection yoke 36 for properly de- "iiecting a cathode ray beam developed within the Kinescope or image producing tube 28, and the rier is assumed, for example, to be 104 meacycles. v l

Since the radio frequency carrier has a maximum deviation of 8 megacycles, the lower deviation limit is approximately 100 megacycles, whereas the upper deviation limit is approximately 108 megacycles. Since full vblack in the image (or the blanking impulses) normally has an intensity corresponding to'75%V of the intensity of the'peaks of the synchronizing impulses, 'the frequency corresponding to full blackv willb'e 106 megacycles.

With such a received radio frequency carrier (which 'corresponds to the received carrier inthe conditions assumed in Figure 2), if a radio frequency oscillator operating at megacycles -is provided in the receiver, a frequency deviation of from 10 to 18 megacycles will be produced at the output of the mixer l0. At this point in thecircuit two separate band pass amplifiers are provided, one of which is designed to accommodate the frequency deviations representing the image signals which extend from 10 to 16 megacycles, the other band pass amplifier being designed to accommodate at least a portion of the frequency deviations representing the synchronizing signals. These deviations extend froml 16 to `18 Vmegacycles. The rst intermediate frequency amplifier 46, therefore, passes aV band of frequencies extending from 10 to 16 megacycles, and after these signals have been properly amplified, they are then applied to an image discriminator which has va characteristic such as that represented in Figure 4 andV which responds to the image signal deviations. At the output of the image discriminator image signals may then be derived which may have a wave form such Vas that represented by curve 52 in Figure 4.

It will be observed that the output from theimage Ydiscriminator 44 does not include any syn- 'chronizing impulses, but includes onlyV those signals which are representative of the particular television image frequency deviations. The frequency deviationsrthat extend above, 16 megacycles and whichare a result of the transmission Vof the synchronizing signals are notimpressed on Acycle deviation band from '16 to y18 megacycles,

but may have a narrower band of. approximately one megacycle preferably chosen to extend from A1f7 to 18 megacycles as shown in Figure 4. Under these conditions f the synchronizing signal discriminator 48 need only be responsive to signals occurring within these limits, and such a discriminator may have a characteristic as that representedin Figure 4. Atthe output of the synlchronizing signal discriminator are then available 'the synchronizing signals onlyasA represented by curve 54 in Figure 4; l

There is a distinct advantage in limiting the band' Width of Athesynchronizing signal intermediate frequency amplifier 42 to frequencies" occurring within the band of from 17 to 18 megacycles, since noise disturbances caused by static,

Yautomobile ignition, etc., are therefore at least in part rendered ineffectual since the synchronizing signal band pass amplier 42 is unresponsive to .deviations occurring below 1 7 megacycles or above .18 megacycles. Since all synchronizing impulses :By using a system such as that described above .in connection with Figures 3 and 4, it may therefore beseen that a radio frequency carrier that has been frequency modulated by a standard vcomposite video signal series may be convenient- .lyreceived and applied to two separate intermediate fequency amplifier channels to result in lcomplete and convenient separation of the synchronizing signals from the image signals.

Furthermore, under these conditions the band `Width, of the intermediate frequency amplifier Vchannels need not be excessively large, and, furthermore, the discriminators are not required to respond to an excessively wide frequency deviation band. The output from the image dis- `crirninator as represented by curve 52 in Figure 4 Ymay. be applied directly to an appropriate image amplifier 46, and after sufficient amplification the signal variation may then be applied to the control electrode of the image producing tube. No synchronizing separation circuits are necessary, Ysince the separation is produced as a result of proper choice of intermediate frequency bands with the result that the output from the synchronizing signal discriminator 48 may be applied directly to the horizontal and vertical deflection generators used in the receiver.

The aboveassumed frequencies are for the purpose of explanation only, and it is obvious that .any other range of frequencies or frequency deviationy limits may be employed.

In explaining the systems shown in Figures 1 Aand 3, it is assumed that no frequency deviation foccurs outside the prescribed deviation limits.

Under actual operation it has been found that frequency deviations beyond the desired limits frequently occur, and in a system having band pass limits such as described above in connection with Figures 2 and 4, certain undesired effects will result. In order to describe these undesired effects, reference is now made to Figure 5 which shows a frequency discriminator response curve having substantially linear response characteristics between frequencies extending from fn to f1. 'The response of the discriminator falls off rapidly for frequencies below fn or above f1. With such a discriminator it is assumed that the video intermediate frequency band pass amplifier has a response characteristic such as that represented at Ell, which, as may be seen from Figure 5, extends beyond the response band of the discrirninator. If frequency deviations such as represented by curve 62 are applied to the intermediate frequency amplifier and discriminator, a Voltage variation such as represented at 64 is available at the output of the discriminator. It will be noticed that the height of the synchronizingsignal for curve 62 does not extend beyond frequency, f1, nor do any signals representingwhite extend below fo. In other words, the applied frequency deviation as represented by curve 62 is within the'response limits of the discriminator, with the result that lno distortion occurs and a proper voltage varia- Cil tion represented by the Vcurve 64 will be produced at the output of the discriminator.

. If, however, deviations beyond the limits of the discriminator arereceived at the receiver as represented by curve 66 of Figure 5, then certain undesired effects will result. If, for example, the synchronizing signal should for some reason deviate beyond frequency f1 as represented by point A of curve 66, the discriminator output will not correspond to the applied frequency deviation. This occurs by reason of the fact that the deviations extending beyond frequency fi, although wholly within the band pass limits of the inter- /mediate frequency amplifier, are beyond the response characteristics of the discriminator, with the result that the output of the discriminator drops to zero rather than being a value such as .represented by A of Figure 5. The dotted portion .output from a discriminator having a characteristic represented by ,curve 58 of Figure 5. Since .the output of the discriminator falls to zero, such a signal potential would not be suitable for syn- ,chronizing purposes, but, instead, would correspond to a picture signal representative of gray.

Similarly, if the deviation inthe direction of white extends below the deviation limit fo, then the dis- Vcriniinator response is such that the output signal is driven in the direction of black, as represented by the solid line in curve 68, rather than remaining .at point B', or at a value corresponding to full white in the image.

It will be seen, therefore, that if a television receiver employs an intermediate frequency band pass having suicient width to accommodate appreciably more than the normal frequency deviation, llout if the discriminator response will .accommodate only the theoretical or prescribed .frequencydeviatiom serious distortion of the wave form derived from the discriminator will result if the frequency deviation of the signal remains within the intermediate frequency amplifier band width but extends beyond the discriminator frequency limits.

A similar undesired distortion results, as shown in Figure 6, when the discriminator response is of sufficient width to accommodate excessive deviations, but where the video intermediate frequency band amplifier has a frequency limit only suiiicient` to accommodate the normal or prescribed frequency deviation. In this particular instance it will be assumedrthatthe video intermediate frequency band accommodates only frequencies extending from fo to fr, as represented by curve 'I in Figure 6. Itwill also be assumed that the discriminator has a wider deviation response ranging from below frequencies corresponding to fo and to above frequencies corresponding to f1. The curve of such a response is indicated at 12 in Figure 6.

If, under these conditions, the applied frequency deviations remain within the band from fo to f1, as represented by curve 74, then a corresponding and proper output voltage variation, Vas represented by curve 16, may be derived from the discriminator, since both the intermediate frequency amplifier and the discriminator will accommodate such a frequency deviation. Should the frequency deviation extend beyond these limits, however, as represented by curve 18 of Figure 6, distortion will occur, as shown in curve 80, since the voltage variation from the discriminator will be that represented by the solid rather than the dotted portion of curve 88. Should the synchronizing impulse extend beyond f1, as for example point C in Figure 6, then the output from the discriminator, instead of extending beyond black in the image as should be the case, will actually extend from biack in the direction of white to a value corresponding to approximately zero Voltage from the discriminator. The dotted portion C" shown at Figure 6, representing the synchronizing signal, will therefore be eliminated, and in the absence of such synchronizing signal the receiver may drop out of synchronization with the transmitter.

A similar situation will result if the frequency deviation in the direction of white in the picture extends below frequency fu, and as soon as the deviation extends beyond fo the voltage variation in the output from the discriminator will be abruptly driven toward zero rather than remaining of a value corresponding to white, or extending above white as shown at D', inasmuch as the band pass limits of the intermediate frequency amplier channel have been exceeded.

Accordingly, if the video intermediate frequency band pass amplifier has a band width only sufficient to accommodate the normal frequency deviation limits, serious distortion will result should the deviation, for some reason, extend beyond these limits.

Since excessive frequency deviations cannot be entirely prevented, some provision must be made in the receiver for preventing serious distortion when such excessive deviation occurs. Obviously,

if either the discriminator (as shown in Figure 5) or the intermediate frequency amplifier (as shown in Figure 6) will accommodate only frequencies occurring within the normal deviation band, distortion such as represented in Figures 5 and 6 will result. Such distortion may cause loss of synchronism, and where frequencies representative of white 'erroneously produce signals representative of black or gray in the image, considerable visible distortion will result in the produced television image.

In order to guard against such distortion, a receiver with components having response characteristics such as represented in Figure '7 may be used. In this figure it will be assumed that the normal deviation band extends from fo to f1 for the complete video signal series. In this system two separate intermediate frequency amplifier channels are used, as shown and described in connection with Figures 3 and 4, with the normal image deviation extending from fu to f2 and the synchronizing signal normal deviation extending frcm ,f2 to f1. Since frequency f2 is representative of black, the image intermediate frequency amplifier need not respond to any frequency above f2. Accordingly, the upper frequency limit of the image intermediate frequency ampliier band may coincide with frequency f2 as indicated by curve 84 in Figure 7. Frequency deviations below frequency fo which correspond to full white in the image should be passed by both the intermediate frequency amplifier and the image discriminator in order to avoid the distortion represented at B' and D in Figures 5 and 6, respectively. Accordingly, the lower frequency limit of the image intermediate frequency amplier should extend an appreciable distance below fo.

The image discriminator may have a response such as indicated at 86' in Figure 7, 'and it is l@ preferable that the response be substantially linear between the limits of fo and f2. The response of the discriminator for frequency deviations above f2 should remain at a value corresponding to the response at frequency f2 to assure th'e production of a signal corresponding to full black in the image even though frequencies above f2 cannot be passed through the image intermediate frequency amplifier, The discriminator should be responsive to signals below fo, and in order to prevent blooming on the image screen of the receiver, it is preferable that the response be maintained uniform for a predetermined amount below frequency fu, as indicated by curve 86. Accordingly, when frequency deviations representative of image` signals are applied to an intermediate frequency amplifier having characteristics indicated by curve 34 and to a discriminator having characteristics indicated by curve 86, the distortion shown in Figures 5 and 6 will not occur. A. frequency deviation such as represented by curve 88 of Figure '7 may bie applied to these elements, and even though the deviation may sporadically extend below fo as inicated at E, the voltage output from the discriminator will remain at full White as representedv at E in curve 93, rather than drop to some value in the dinection of black as in the case of Figures 5 and 6. Similarly, the presence of frequency deviations beyond fz (as occur during the synchronizing intervals) does not cause any change in the output signal strength from a value corresponding tofull black.

In the synchronizing signal intermediate frequency amplifier it is not necessary that any frequencies below f2 be passed since the frequency f2 is representative of full black and all synchronizing signals extend in a direction above frequency f2. Accordingly, the synchronizing signal intermediate frequency amplifier may have a nesponse such as indicated by curve 92 in Figure 7, with frequency f2 representing the lower frequency limit and with the upper frequency limit extending appreciably above f1, in order to accommodate any excessive frequency swing in that direction.

Similarly, the synchronizing discriminator response may be such as represented by curve 94 in Figure 7, and should preferably be substantially linear between the limits of f2 and f1, and should also be responsive to frequency deviations above f1 in order to prevent elimination of the synchronizing signals as is the case in conditions represented by Figure 5 or Figure 6. The output fromthe discriminator should remain unchanged (at zero, for example) for deviations below fz, and since frequencies below f2 cannot pass through the synchronizing signal intermediate frequency amplier, as indicated at 92, the output potential level of the discriminator, in the absence of an applied signal, should correspond to its output when frequency f2 is applied thereto.

If the frequency variations remain within the ,l limits of f2 and f1, then a normal synchronizing signal will be developed as represented by curve S6, and also if the deviation should exceed frequency f1 as indicated at F in Figure 7, a proper synchronizing signal will still be derived from the discriminator since such excessive deviation is still within the pass band of the intermediate frequency amplifier and since the discriminator will respond, with substantially uniform output, to such excessive deviation. Accordingly, an excessive deviation in the direction of the synchronizing signal Will still produce a voltage variation at the output of the discriminator corresponding to the signal produced by a normal deviation.

Intermediate frequency amplifiers having the desired band width and characteristics may readily be designed in a manner well known to those skilled in the art. Furthermore, frequency discriminators of the desired band width and having characteristics such as represented by curves 86 and 94 in Figure 7 may also be provided by appropriate design and may, in fact, form a part of the intermediate frequency amplifier channels. For example, the image intermediate frequency amplier channel may be designed as a low-pass iilter having uniform response to frequencies below fo, and having a linear attenuating effect for frequencies between fe and fz. The response should be zero for frequencies above f2. When such an amplifier channel is provided, a frequency discrimination effect results between ,fu and f2. For the synchronizing intermediate frequency amplifier channel a high-pass filter circuit may be employed having zero transmission eiiiicency (or cut-oli) below f2, a linear increase in response from f2 to f1, and a luniform output response for frequencies above f1. Amplitude modulation limiters may or may not be used in such a system, as desired.

When a television receiver is provided with image and synchronizing signal intermediate frequency band pass amplifiers such as represented by curves 84 and 92 in Figure 7, and when appropriate discriminators or frequency discriminating amplifiers are provided having characteristics such as represented by curves 86 and 94, it is possible to derive from the discriminator or frequency discriminating amplifier channels an image signal such as represented by curve 90 and a synchronizing signal such as represented by curve 96 without the distortion associated with circuit elements such as represented in Figures 5 and 6. The clipping" action produced at E and F under conditions of excessive frequency deviation is desirable, since in the image signal such clipping action prevents blooming or the production of excessive white on the receiver screen as stated above.

Furthermore, the clipping action exercised on excessive deviation in the direction of the synchronizing signals is desirable, since thevoltage variations derived from the synchronizing signal discriminator remains substantially uniform in intensity regardless of such excessive frequency deviation.

With the present invention it is, therefore, apparent that a television receiver may be provided which will respond to a frequency modulation of a radio frequency carrier where the frequency modulation is produced by a conventional composite video signal. It is also possible to utilize such a frequency modulated signal Without resulting in undesired distortion and possible elimination of the transmitted synchronizing signals.

By exercising the present invention in conjunction with frequency modulated transmission of the accompanying audio signals, the television transmission may be made completely frequency modulated with desirable and satisfactory results at the receiver.

Although the present invention is described somewhat in detail, various alterations and modiiications may be made therein without departing from the spirit and scope thereof, and it is desired that any and all such modifications be considered within the purview of the present invention except as limited by the hereinafter ap-N pended claims.

Having now described our invention, what we claim as new and desire to have protected by Letters Patent is:

1. A television receiver for receiving a frequency modulated radio frequency carrier modulated by television video signals such that the frequency deviations representative of the image signals and the frequency deviations representative of the synchronizing signals occupy adjacent bands, comprising an image signal band pass amplifying channel having characteristics such that frequency deviations representative of the image signals may be amplifier thereby, a frequency discriminator associated with said image signal amplifying channel, said discriminator having substantally linear frequency discriminating characteristics throughout substantiallythe entire image signal frequency deviation band and having substantially uniform response to frequencies outside the image signal frequency deviation band whereby television image signals may be produced by said discriminator, means for r applying the produced image signals to a translating device, a synchronizing signal band pass amplifying channel having characteristics such that frequency deviations representative of the synchronizing signals may be amplified thereby, a second frequency discriminator associated with said synchronizing signal amplifying channel, said discriminator having substantially linear frequency discriminating characteristics over at least a portion of the synchronizing signal frequency deviation band and having substantially uniform response to frequencies outside the synchronizing signal frequency deviation band whereby television synchronizing signals may be produced by said second discriminator, and a synchronizing signal responsive circuit associated with said second discriminator.

2. A television receiver for receiving a frequency modulated radio frequency carrier modulated by television video signals such thatthe frequency deviations representative of the image signals and the frequency deviations representative of the synchronizing signals occupy adjacent bands, comprising an image signal amplifying channel having low-pass characteristics with uniform response below a predetermined carrier frequency corresponding to one frequency deviation limit of the image signals and with substantially linear diminution of response in the image signal frequency deviation band whereby at the output of said amplier a series of image signals may be produced, a synchronizing signal amplifying channel having high-pass characteristics with uniform response above a predetermined carrier frequency corresponding to one frequency deviation limit of the synchronizing signals and with substantially linear diminution response in the synchronizing signal frequency deviation band whereby at the output of said syn- 1 chronizing signal amplifier a series of synchronizing signals may be produced, and signal utilizing elements coupled to each amplifier circuit.

3. A television receiver for receiving a frequency modulated radio frequency carrier modulated by television video signals such that the frequency deviation band representative of image signals is adjacent the frequency deviation band representative of synchronizing signals, comprising a band pass image signal amplifying channel having a sloping characteristic such that substantially linear frequency response discrimination is present within the image signal frequency deviation band whereby ai; the output of said amplier a series of image signals is produced, a band pass synchronizing signal amplifying channel having a sloping characteristic such that substantially linear frequency response discrimination is present within the synchronizing signal frequency deviation band whereby at the output of said synchronizing signal amplifier a series of synchronizing signals is produced, and translating devices individually responsive to the produced image and synchronizing signals.

4. A television receiver for receiving a frequency modulated radio frequency carrier modulated by television video signals such that the frequency deviation band representative of image signals is substantially adjacent the frequency deviation band representative of the synchronizing signals, comprising a first amplifying channel having low-pass characteristics with uniform response below a predetermined carrier frequency corresponding to the low frequency deviation limit of one frequency deviation band and with substantially linear decrease in frequency response within said one frequency deviation band whereby at the output of said ampliiier a first series of signals is produced, a second amplifying channel having high pass characteristics with uniform response above a predetermined carrier frequency corresponding to the high frequency deviation limit of the other frequency deviation band and with substantially linear increase in frequency response within said other frequency deviation band whereby at the output of said second amplifier a second series of signals is produced, and signal utilizing elements coupled to each amplifier circuit.

VERNON J. DUKE.

ROBERT W. CLARK.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 2,254,435 Loughren Sept. 2, 1941 2,290,517 Wilson July 21, 1942 2,413,913 Duke Jan. 7, 1947 2,435,736 Carnahan Feb. 10, 1948 OTHER REFERENCES Carnahan F-M Applied to a Television System, Electronics, Feb., 1940, pages 26, 30, 31 and 32.

Duplex Transmission of Frequency-Modulated Sound and Facsimile by Artzt and Foster. Reprint from RCA Review, No. 298, pages 89, 90, and 96.

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