GB2189368A - Noise reduction in video signals - Google Patents

Description

GB 2 189 368 A SPECIFICATION picture occasionally isaccompanied bya

reduction of the frequency transmission range, i.e., a FM signal dernodulator for video signal reproducing deterioration of the signal waveform response apparatus characteristics of the picture reproducing circuits.

70 Forexample, when an attempt is madeto improve Background of the invention the S/N ratio, especially in VT13s, the signal

Field of Invention waveform response characteristics deteriorate, so

This invention reiatesto a demoduiatorfor a that a significatnt noise arises on the leading and/or frequency modulation signal (referred as FM signal trailing end portion of respective pulse-like signals, hereafter) and more particularlyto an FM signal 75 such as luminance signals in the FM signal band of demodu 1 atorfor video signals, such as luminance the composite video signal. Therefore it is important signals, of video signal reproducing apparatus. to increase the S/N ratio while maintaining the signal waveform response characteristics at a prescribed Description of thepriorart level.

Recently, in thefields of video signal reproducing 80 Conventionally, there are known following several apparatus such astelevision receivers (referred as methodsfor raising the S/N ratio of pictures in VT13s.

TV receivers hereafter) and/orvideo tape recorders (1) Increasing the amount of emphasis in an (referred asVTRs hereafter), many improvement emphasis circuit in a picture recording circuit, in have been made for raising the quality of pictures or advance of reproducing pictures.

images of the image display screen of TV receivers or 85 (2) Increasing the amount of cancelling in a noise VTRs. As is well known, both the sharpness and the canceller circuit in a picture reproducing circuit.

signal to noise ratio (referred as S/N ratio hereafter) (3) Increasing the signal componentwith a of the picture are especially important factors for relatively high C/N ratio (carrierto noise ratio) in the raising picture quality. FM signal, in other words a lowfrequency signal The sharpness of a picture is influenced bythe 90 componentwhich is lowerthan the carrier signal, for frequency response characteristics, i.e., the signal raising the S/N ratio of a baseband signal after FM waveform response characteristics of picture demodulation.

reproducing circuits in TV receivers and/or VTRs. For The conventional methods, however, have example, when the signal waveform response drawbacks as described below. When attempting to characteristics of the picture reproducing circuits are 95 increase the amount of emphasis, as in method (1), insufficient at a leading end portion and a trailing some frequency signal components fail to carry out end portion of respective pulse-shaped signals, such white level clipping and dark level clipping, so that as luminance signals in video signals, the resulting the signal waveform response characteristics pictures on the image display screen become inferior deteriorate.

in sharpness. As eil known, the luminance signal is 100 In method (2), the noise canceller circuit extracts included in an FM signal band of a composite video the high frequency component from the video signal togetherwith other signals. such as color signal, reverses the phases of the extracted signals signals. The signal waveform response after limiting the amplitudes of the high frequency characteristics of a picture reproducing circuit is component by a limiter and then addsthe extracted determined by the frequency transmission rangeof 105 signals to the original video signal. Thus, a noise the circuit. Therefore, in orderto obtain good signal with a low level and a high frequency in the original waveform response characteristics, it is necessaryto video signal is cancelled bythe noise in the extracted make the frequency transmission range of the signals. When an attempt is madeto increasethe picture reproducing circuitswider. In particular, it is amountof cancelling,the S/N ratio of signals is desired to expand the frequency transmission range 110 improved atflatwaveform portions of thewaveform.

to a frequency as high as possible. However,the noise is not be removed atwaveform Many attempts have been madefor raising the change portions in which the signal steeply changes signal waveform response characteristics of picture over a large amplitude, and has a high frequency reproducing circuits. However, it has become component. For instance. this may occur in the difficult to further increase the signal waveform 115 section where the signal changes from the dark level response characteristics, because the frequency to the white level. Moreover, the dulation of such transmission range of the picture reproducing noise increases. Therefore, the signal waveform circuits has expanded to a relatively wide range as a response characteristics deteriorate and the noise in result of recent progress in circuit design. the waveform change section becomes more Particularly. the improvement of the picture quality 120 pronounced.

in VTRs by raising the signal waveform response In method (3), when the lowfrequency signal characteristics has become difficult. This is because component,which is lowerthan the carriersignal. is the frequency transmission range of the picture increased, an inversion of the picture signal between reproducing circuits in VTRs is restricted to a the dark level and the white level occurs more easily narrower rang than the range of such circuits in TV 125 and, atthe same time. the image quality declines at receivers. the portion where drark level changes to white level.

Accordingly, the improvement of the picture More particularly, the portion of the waveform quality has been attempted from the standpoint of changing from the dark level to the white level is the the S/N ratio of the picture on the image display portion where the carrier signal of the FM signal screen. However, an increase of the S/N ratio of the 130 moves at the highest frequency. Consequently, the 2 GB 2 189 368 A 2 CIN ratio of the signal becomes worst at the amplitude without deterioration of the signal waveform change portion. Therefore, in method (3), waveform response characteristics.

which does not use a signal componentwith a bad Afurtherobject of the present invention isto CIN ratio, asthat above, although the S/N ratio in the remove the noise existing in a waveform change flatwaveform portion is improved, the waveform 70 portion where a luminance signal changes from a change portion wherethe signal changesfromthe dark level to a white level without& deterioration of the dark level to the white level deteriorates. signal waveform response characteristics.

Incidentally, one of the causes of signal deterioration Additional objects and advantages will be obvious is the fact of thatthe carrier frequency, which is from the description which follows, or may be equivalentto the frequency in the waveform change 75 learned by practice of the invention.

portion, is located in the upper end of the FM signal In orderto achieve the above objects, the FM transmission band. This is to avoid using demodulatorfor a video signal reproducing components with a low CIN ratio. That is to say, in apparatus, which is responsive to an input FM signal, method (3), the amplitude and phase of the FM includes an FM demodulation circuitfor changing signals tend to be distorted in the transmission path. 80 the input FM signal to a variable amplitude video As a result,the waveform change portion f rom the signal and a noise removing circuit for substantially dark level to the white level deteriorates, so thatthe removing noise from the variable ampUtudevideo noise in thiswaveform change portion becomes signal. The noise removing circuit is comprised of a more pronounced. clipping circuitfor removing portions of the variable As explained above, when an attempt is made to 85 amplitude video signal carrying the noise and an improve the SIN ratio of the luminance signal in prior amplitude expansion circuitfor increasing the art VT13s, the signal waveform response amplitude of the video signal at the location of the characteristics deteriorate and moreover, the noise removed portions to a predetermined level.

in the waveform change portion substantially In the FM demodulator of the present invention, increases. Therefore, the SIN ratio can only be set at 90 noise existing in the section where dark level a compromise between these two. As a result, the changes to white level can be removed by the prior art Ms have a problem in thatthe SIN ratio of clipping process. The missing signal in the changing the signal worsens in the portion of the waveform section dueto the clipping process can be which changes f rom the dark level to the white level. compensated bythe expansion process.

Figure 1 shows method (3) used in the priorart 95 Consequently, when using this invention, both the VTFIs, Graph (a) in Figure 1 shows a waveform of a SIN ratio in the changing section and thewaveform luminance signal justafter a video signal has been characteristics can be satisfied.

restored to the base-band by demodulation. Graph (b) in Figure 1 is an enlargement of section A of the Brief description of the drawings waveform shown by graph (a) in Figure 1. As seen 100 Figure 1 is a waveform diagram showing signals in from graph (b) in Figure 1, it is clearthatthere is a a prior art FM demodu latorfor video signals in Ms.

great deal of noise atthetip of the leading end Figure2 is a circuit blockcliagram showing a first portion (the portion where signal changesfrom the embodimentof the FM demodulator for video dark level to white level). When thevideo signal has signals according to the present invention; passed through a de-emphasis circuit and a noise 105 Figure 3 is a waveform diagram showing signal canceller circuit, the luminance signal with a waveforms in the circuit of Figure 2; waveform as shown by graph (c) the Figure 1 can be Figure4is a circuit diagram showing an example obtained. Graph (d) in Figure 1 is an enlargement of of a practical circuit arrangementof the FM section A of thewaveform shown by graph (c) in demodulator of Figure 2; Figure 1. As is clearfrom graph (d) in Figure 1,the 110 Figure5is a waveform diagram of signals in the noise on the tip of the leading end portion remains, circuit of Figure 4; and is nottotally removed bythe de-emphasis circuit Figure 6is a circuit blockcliagram showing a orthe noise canceller circuit. As a result,the contrast second embodiment of the FM demodulatorfor atthe boundary of pictures presented on the image video signals according to the present invention; display screen is adversely effect bythe noise, and 115 Figure 7is a waveform diagram showing signals in this leads to deterioration of picture quality. the circuit of Figure 6; Figureffis a circuit diagram showing an example Summary of the invention of a practical circuit arrangement of the FM

Accordingly, an objectof the present invention is demodulator of Figure 6; and to provide an FM signal demodu lato r for video 120 Figure9is a waveform diagram of signals in the signals which is ableto remove the noise existing at circuit of Figure 8.

a waveform change portion wherethevideo signal steeplychanges over a large amplitude without Description of thepreferred embodiment deterioration of thesignai waveform response The present invention will now be described in characteristics. 125 detail with reference to the accompanying drawings, Another object of the present invention is to namely, Figures 2 to 9. Throughoutthe drawings, provide an FM signal demodulatorfor video signals like reference numerals and letters are used to of video tape recorders which is able to remove the designate like or equivalent elements, forthe sake of noise existing in a waveform change portion where simplicity of explanation.

the video signal steeply changes over a large 130 Referring now to Figures 2 to 5, a first embodiment 3 GB 2 189 368 A 3 of an FM demodulator for video signals accordingto capacitor Cl and a resistor R4.

the present inveniton will be described in detail. The second and third transistors Q2 and Q3 figure 2 is a circuit block diagram showing thefirst constitute an inverted type operational amplifier embodiment of the FM demodulator. Figures 3 is a OPAtogether with a feedback resistor F15, a common diagram showing signal waveforms in the circuit of 70 emitter resistor R6, a collector load resistor R7, base Figure2. bias resistors R8 and R9 and a capacitor C3. The In Figure 2, a composite video signal S1,which has second and third transistors Q2 and Q3 are a prescribed FM signal band, is applied to an FM connected attheir emitter terminals to each other modulation circuit 11. The FM demodulation circuit and the emitter terminals are connected to the power 11 demodulates the FM signal band. The 75 supply source terminal PS through the common demodulated signal is applied to a low pass filter emitter resistor R6. The collector of the second (Referred as LPF hereinafter) 12 so that a baseband transistor Q2 is connected to the ground terminal G luminance signal S2, as shown by graph (a) in Figure through the collector load resistor R7. While the 2, is obtained as an output of the LPF 12. Atip of the collector of the third transistor Q3 is connected leading end portion of the waveform of this 80 directly to the ground terminal G. The base terminal baseband luminance signal S2 usually carries a of the third transistor Q3 is connected to the ground noise signal N. The luminance signal S2 is applied to terminal G through a parallel circuit of the base bias a white level clipping circuit 13. Wherein the resistor R9 and the capacitor C2. Furtherthe base luminance signal S2 is clipped at a white level terminal of the third transistor Q3 is connected to the specified as the threshold level, so thatthe tip with 85 power supply source terminal PS. The collector noise signal N is cut out or removed, as shown by terminal of the second transistor Q2 is connected to graph (b) in Figure 3. This clipped luminance signal the base terminal of the fourth transistor Q4.

S3 then is applied to a level expansion circuit 14. This The fourth transistor Q4 constitutes an output level expansion circuit 14 only responds to the buffer amplifier BA2 togetherwith an emitter load leading end portion of the clipped luminance signal 90 resistor Rl 0. The collector of the fourth transistor Q4 S3 and expands the amplitude level of the clipped is connected directlyto the ground terminal G. The luminance signal S3 is restored or compensated to emitterterminal of the fourth transistor Q4 is the original level, but without the noise signal N, as connected to the power supply source terminal PS shown by graph (c) in Figure 3. The expanded through the emitter load resistor Rl 0. Furtherthe luminance signal S4 is applied to a de-emphasis 95 emitterterminal of the fourth transistor Q4 is circuit and/or a noise cancelling circuit (not shown), connected to the base terminal of the second as usual. Then, prescribed de-emphasis and/or noise transistor Q3 in the operational amplifier OPA suppression processing is carried outforthe through the feedback resistor R5. The baseterminal expanded luminance signal S4. As a result, a signal of thefourth transistor Q4 is connected to the anode S5with excellent waveform characteristics, as 100 terminal of the diode Dl in the level expansion circuit shown by graph (d) in Figure 3, is obtained as the 14.

resulting luminance signal. in the level expansion circuit 14, the diode D1 is Figure 4 shows practical circuits of the white level connected at its anode terminal to the ground clipping circuit 13 and the level expansion circuit 14 terminal G through a series circuit of a capacitor C3 in Figure 2. In Figure 4, references (Qi (i = 1, 2,the 105 and a resistor Rl 3. Furtherthe anode terminal of the same hereinafter) represent transistors, references diodeDl is connected to the power supply source Di represent diodes, references Ri represent terminal PS through a series circuit of resistors Rl 1 resistors, references Ci represent capacitors and and Rl 4. The cathode terminal of the diode Dl is references Li represent inductors. The same connected to the emitterterminal of thefifth reference system will be used in Figure 8, mentioned 110 transistor Q5 through a series circuitof the inductor later. Ll and the capacitor C4. The series circuit of the In Figure 4, the white level clipping circuit 13 is inductor Ll and the capacitor C4 constitutes a comprised of first to fourth PNP transistors Q1, Q2, peaking circuit PE, as described later. Furtherthe Q3 and Q4 as main active elements of the circuit, cathode terminal of the diode D1 is connected to the while the level expansion circuit 14 is comprised of a 115 anode terminal through a resistor Rl 2. The fifth diode D1, a fifth PNP transistor Q5, an inductor Ll, a transistor Q5 is connected at its emitterterminal to capacitor C4, etc. In the white level clipping circuit 13, the power supply source terminal PS through the the firsttransistor Q1 constitutes an input buffer resistor R14. The base terminal of the fifth transistor amplifier BA1 togetherwith a base bias resistor R2 Q5 directly is connected to the base terminal of the and an emitter load resistor R3. The base terminal of 120 third transistor Q3 in the operational amplifier OPA the first transistor Q1 is connected to aground of the white level clipping circuit 13. The collector terminal G through the base bias resistor R2. The terminal of the fifth transistor Q5 is connected to the collector terminal of the f i rst transistor Q1 is ground terminal G through a resistor Rl 5. Further connected directly to the ground terminal G. The the collector term ina 1 of the fifth transistor Q5 is emitter term ina 1 of the first transistor Q1 is 125 connected to an outputterminal OUT.

connected to a power supplysource terminal PSwith The baseband luminance signal S2 outputted from a voltage Vccthrough the emitter load resistor R3. the LPF 12 is applied to the inverted-type operational Furtherthe emitterterminal of thetransistorQ1 is amplifier OPAthrough the input buffer amplifier connected to the base terminal of the second BA1, the coupling capacitor Cl and the resistor R4.

transisotr Q2 through a series circuit of a coupling 130 Here, the polarity of luminance signal S2 at a 4 GB 2 189 368 A 4 terminal P1 between the coupling capacitor Cl and the recording mode.

the resistor R4 is negative, as shown by graph (a) in As explained in detail above, the first embodiment Figure 5. The luminance signal S2 is clipped at itstip is designed so thatthe noise N appearing on the end at a prescribed level in the operational amplifier leading end portion of the luminance signal S2 is OPA, as described later. Thus, a clipped luminance 70 removed bythe clipping operation in the white level signal S3, as shown by graph (b) in Figure 5, is clipping circuit 13, and the removed portion due to obtained at a terminal P2 between the collector the clipping operation is compensated by the terminal of the fourth transistor Q4 and the anode expansion operation. Therefore, in this terminal of the diode D1 in the level expansion circuit embodiment, the S/N ratio can be improved without 14. The output of the output buffer amplifier BA2 is 75 causing deterioration of the waveform grounded through the capacitor C3 and the resistor characteristics. Thus a high-quality picture can be R13. Therefore, the polarity of the clipped luminance obtained without affect by the noises on the picture signal S3 atthe terminal P2 is positive, as shown by portion where the luminance signal changes from graph (B) in Figure 5. the dark level to thewhite level.

When the leading end portion of the luminance 80 In thefirst embodiment, the expansion operation signal S2 is applied tothe operational amplifierOPA, is carried outafterthe clipping operation for the potential of the emitterterminal of thefourth compensating the removed portion of the luminance transistor Q4which operates as the output buffer signal S2 due to the clipping operation. However,the amplifier BA2 is closestto the power source voltage expansion operation may be carried out priorto the Vcc. Thus, an output current of the output buffer 85 clipping operation. An embodiment for the latter amplifier BA2 which drives load elements such as case, i.e., a second embodiment of the present the capacitor C3, the resistor R13 and the like invention, will now be described in references to becomes minimum. Therefore the output buffer Figures 6 to 9. In Figure 6. the order of the white level amplifier BA2 fails to drive them. As a result,the clipping circuit 13 and the expansion circuit 14 is leading end portion of the luminance signal S2 is 90 reversed, as compared to that in Figure 2. Waveform clipped atthe prescribed level nearthe power source diagrams of signals in the circuit of Figure 6 are voltage Vcc. As a result, the clipped luminance signal shown in Figure 7. According to the second S3, as shown by graph (b) in Figure 5, is obtained at embodiment, the luminance signal S2 (see graph (a) the terminal P2. Atthis time, since the clipped in Figure 7) is first expanded in its amplitude level in luminance signal S3 is fed backto the baseterminal 95 the expansion circuit 14 so that a signal S4, as shown of the second transistor Q2 of the operational by graph (b) in Figure 7, is obtained. In the expansion amplifier OPAvia the feedback resistor R5, the operation the noise signal N on the leading end feedback signal operates to compensate the portion of the luminance signal S2 also is expanded, removed portion of the luminance signal S2. Bythis as shown by graph (b) in Figure7.

compensating operation, the signal waveform of the 100 Figure 8 shows practical circuit of the level clipped leading end portion gradually rises toward expansion circuit 14 and the white level clipping its trailing end. circuit 13 in Figure 6. In Figure 8, references Qi, Di, Ri, Furthermore, the output of the output buffer Ci and U represent transistors, diodes, resistors, amplifier BA2, Le.,the fourth transistor G4 is applied capacitors and inductors, respectively. In Figure 8, to thefifth transistor Q5which constitutes a 105 the level expansion circuit 14 is comprised of sixth to grunded-base type amplifier through the diode D1. ninth PNP transistors Q6, Q7, Q8 and G9, a second The diode D1 changes to the On state when the high diode D2, an inductor L21, a capacitor C24 etc. The amplitude level of the leading end portion of the ninth PNPtransistor Q9 itself constitutes the white luminance signal S3 is applied. Thus, a peaking level clipping circuit 13 togetherwith a third diode operation forthe leading end portion of the 110 D3. In the expansion circuit 14, the sixth transistor Q6 luminance signal S3 is carried out bythe series constitutes an input buffer amplifier BA3 together circuit of the inductor Ll and the capacitor C4, i.e., with a base bias resistor R22 and an emitter load the peaking circuit PE. If the resonant f requency of resistor R23. The base terminal of the sixth transistor the peaking circuit PE is set about 1 MHz, which is the G6 is connected to a ground terminal G through the most central component of the leading end portion, 115 base bias resistor R22. The collector terminal of the the clipped leading end portion of the luminance sixth transistorQ6 is connected directlytothe signal S3 is expanded in its amplitude level. As a ground terminal G. The emitterterminal of the sixth result, a signal S4 is obtained, as shown by graph (c) transistor Q6 is connected to a power supply source in Figure 5, which has been compensated forthe terminal PS with a voltage Vccthrough the emitter clipped portion of the luminance signal S2 due to the 120 lod resistor R23. Furtherthe emitterterminal of the clipping operation in the white level clipping circuit transistor Q6 is connected to the base terminal of the 13. Incidentally, the second to fourth transistors Q2- seventh transistor Q7 through a series circuit of a Q4 are setto the high gain states, respectively, in coupling capacitor C21 and a resistor R24.

orderto putthe diode Dl in the On state when the The seventh and eighth transistors Q7 and Q8 leading end portion of the luminance signal S2 is 125 constitute a high gain non-inverted type amplifier inputted. Also, the coupling capacitor Cl has a HGA together with a feedback resistor R24, emitter function of preventing a variation of the clipping resistors R25 and R26, a collector load resistor R28, operation due to a frequency drift of the carrier base bias resistors R29 and R30 and a capacitor C22.

signal of the FM signal band and a level drift of the The seventh and eighth transistors Q7 and Q8 are output signal of the FM demodulation circuit 11 in 130connected at their emitter terminals to each other GB 2 189 368 A 5 through the resistorR27 andthe emitterterminals operationforthe leading end portion ofthe are connectedtothe power supply source terminal luminancesignal S2'is carried out bytheseries PSthrough the emitter resistors R25 and R26, circuit of the inductor L21 and the capacitor C24, i.e., respectively. The collector terminal of the seventh the peaking circuit PE. If the resonant f req uency of transistor G7 is connected directlyto the ground 70 the peaking circuit PE is set about 1 MHz, which is the terminal G. The collector of the eighth transistor Q8 most central component of the leading end portion, is connected to the ground terminal G through the the leading end portion of the luminance signal S2'is collector load resistor R28. The base terminal of the expanded in its amplitude level. As a result, a signal eighth transistor Q8 is connected to the ground S4, as shown by graph (c) in Figure 9, is obtained.

terminal G through a parallel circuit of the base bias 75 The expanded luminance signal S4 is reversed in its resistor R30 and the capacitor C22. Further,the base polarity on the col lector terminal of the ninth terminal of the eigth transistor Q8 is connected to the transistor Q9, as shown by graph (c) in Figure 9.

power supply source terminal PS through the base When the leading end portion of the luminance resistor R29. Also, the base terminal of the eighth signal S2'is applied, the potential of the collector transistor Q8 is connected to the base terminal of the 80 terminal of the ninth transistor Q9 is closestto the seventh transistor Q7 through the feedback resistor power source voltage Vcc. Thus, the leading end R24. The col lector terminal of the eighth transistor portion of the luminance signal S4 is clipped atthe Q8 is connected to the base terminal of the ninth prescribed level nearthe power source voltage Vcc.

transistor Q9 througha coupling capacitor C23. The As a result, an output signal S3, as shown by graph the ninth transistor Q9 is connected at its base 85 (d) in Figure 9, is obtained atthe outputterminal terminal to the ground terminal G through a base OUT. As clearly seen from thewaveform of the biastransistor R 32. Furtherthe baseterminal of the clipped luminance signal S3, as shown by graph (d) ninth transistor Q9 is connected to the powersupply in Figure 9,the noise signal N appearing on thetip sourceterminai PS through another base bias portion of the leading end portion of the expanded resistor R31. The emitter terminal of the ninth 90 luminance signal S4 is removed, butthe leading end transistor Q9 is connected to the ground terminal G portion of the clipped luminance signal S4 has a through an emitter resistor R34. Furtherthe emitter sufficient amplitude without the noise N.

terminal of the ninth transistor Q9 is connected tothe As described above, the present invention cathodeterminal of the second diode D2. The second provides an FM demodulator for video signals such diode D2 is connected at its anodeterminal tothe 95 as luminance signals of VTRs, which is capable of ground terminal G through a series circuit of the removing noise in the portion where the luminance inductor L21 and the capacitor C24. The series circuit signal changes from the dark level to the white level oftheinductorL21 and the capacitor C24 constitutes without causingdeterioration of the waveform a peaking circuit for the leading end portion of the characteristics.

luminance signal, as described later. Further.the 100 Whilethere has been illustrated and described anodeterminal of the second diode D2 isconnected whatare at present considered to be preferred tothe cathodeterminal thereof through a resistor embodiments of the present invention, itwill be R35. The collector terminal of the ninth transistorQ9 understood bythose skilled in the artthatvarious is connected to the power supply sourceterminal PS changes and modifications may be made, and through a collector load resistor R33. Furtherthe 105 equivalents may be substituted for elementsthereof collector terminal of the ninth transistor Q9 is without departing from thetrue scope of the connected to the base terminal of the eighth invention. In addition, many modifications maybe transistor Q8 in the level expansion circuit 14 madeto adapt a particular situation or material tothe through thethird diode D3forsignal feedback. The teaching of the present invention without departing third diode D3 operatesto clipthe luminance signal 110 from the central scope thereof. Therefore, it is at a prescribed level, as described later. Also the intended thatthis invention not be limited to the collector terminal of the ninth transistor Q9 is particular embodiment disclosed as the best mode connected to an output terminal OUT of the circuit. contemplated for carrying out this invention, butthat The baseband luminance signal S2 outputted from the invention include all embodiments failing within the LPF 12 is applied to the high gain non-inverted 115 the scope of the appended claims.

type amplifier HGAthrough the input buffer amplifier BA3 and the coupling capacitor C21. Here,

Claims (8)

1. the polarity of luminance signal S2 at a terminal P21 CLAIMS between the

   coupling capacitor C21 and the seventh 1. An FM demodulator fora video signal transistor Q7 is negative, as shown by graph (a) in 120 reproducing apparatus, responsive to an input FM Figure 9. The luminance signal S2 is applied to the high gain non-inverted type amplifier HGA so that an signal. comprising:

   amplified luminancesignal S2', as shown bygraph demodulation means (11) for changing the input (b) in Figure 9, is obtained on a terminal P22 between FM signal to a variable amplitude video signal, the coupling capacitor C23 and the base terminal of 125 characterised in thatthe FM demodulatorfurther the ninth transistor Q9. The amplified luminance includes noise removing means (13,14) for signal S2'is applied to the ninth transistor Q9. The substantially removing noisefrom thevariable second diode D2 becomes ON statewhen the high amplitudevide signal,the noise removing means amplitude level of the leading end portion of the including clipping means (13) for removing portions luminance signals S2'is applied. Thus, a peaking 130 of the variable amplitude video signal carrying the noise, and amplitude expansion means (14) for 6 GB 2 189 368 A 6 increasing the amplitude of the video signal atthe location of the removed portions to a predetermined level.
2. 2. The FM demodulator of claim 1 wherein the video signal includes a plurality of components signals and the FM demodulator also includingfilter means (12) for separating a prescribed component signal from the video signal.
3. 3. The FM demodu lator of claim 2 wherein the clipping means (13) includes an amplitude level limiting circuit (BA1, OPA, BA2) having a predetermined threshold amplitude level.
4. 4. The FM demodulator of claim 3 wherein the limiting circuit (BA1, OPA, BA2) includes a plarality of transistors (Q1 to Q4).
5. 5. The FM demodulator of claim 2 wherein the prescribed component signal is a luminance signal, and the amplitude expansion means (14) includes a frequency resonance circuit (L1, C4) for increasing the amplitude of selected segments of the luminance signal to substantially the same amplitude levels which existed priorto the removal fo the portions carrying the noise.
6. 6. The FM demodulator of claim 5 wherein the frequency resonance circuit includes an inductor (L1) and a capacitor (C4).
7. 7. The FM demodulator of claim 6 wherein the filter means includes a low pass filter (12).
8. 8. An Mdemodulator substantially as herein before described with reference to Fig u re 2, Figure 4, Figure 6 or Figure 8 of the accompanying drawings.

   Printed for Her Majesty's Stationary Office by Croydon Printing Company (UK) Ltd,8187, D8991685. Published by The Patent Office, 25Southampton Buildings, London, WC2A 'I AY, from which copies maybe obtained.

   p IL,