US3647950A

US3647950A - Variable trap device for television receivers and the like

Info

Publication number: US3647950A
Application number: US802804*A
Authority: US
Inventors: Sadayoshi Yoshikawa
Original assignee: New Nippon Electric Co Ltd
Current assignee: New Nippon Electric Co Ltd
Priority date: 1968-06-26
Filing date: 1969-02-27
Publication date: 1972-03-07
Anticipated expiration: 1989-03-07

Abstract

An intermediate-frequency video amplifier circuit in which trap circuits are provided to attenuate sound components in intermediate-frequency video signals. Frequency variations in the intermediate-frequency video signals are detected and converted into control signals which control the tuning of the trap circuits which are thereby adjusted to attenuate the sound components despite variations in the frequency thereof.

Description

United States Patent [151 mamw Yoshikawa Mar. 7, W72

[54] VARIABLE TRAP DEVICE FOR [56] References Cited 'ITJEZJEEVISION RECEIVERS AND THE UNITED STATES PATENTS 3,473,128 10/1969 Kiser ..325/422 [721 Invent mm!" Osaka Japan 3,029,339 4/1962 Pan ..325/468 73 Assignee: New Nippon c i c Ltd. Osaka, 3,281,698 /1966 R088 8! 3i. ..325/420 Japan Primary Examiner-Robert L. Richardson [221 Filed: Feb. 27, 1969 Assistant Examiner--George G. Stellar PP 802,804 Attorney-Roberts&Cohen [57] ABSTRACT Foreign Application Priority Data An intermediate-frequency video amplifier circuit in which June 26, 1968 Japan ..43/43s57 circfitsvm allenuate 9? l intermediate-frequency video signals. Frequency variations m the intermediate-frequency video signals are detected and 2% 2 1 converted into control signals which control the tuning of the 1 I o a i u: I I! l circuits are thereby adjusted to attenuate the Sound [58] Field of Search ..l78/5.8 A, 333/18, 325/420, components despite variations in the frequency thereofi 7 Claims, 17 Drawing Figures SOUND 0E7. CA7.

P344455 2 -L 4 user/41. w

2 7 5/48. J, sum

Patented March 7 1972 3,647,950

'7 Sheets-Sheet 1 Am Pcs 2%0 125 -'0.75 0 2' 3' 3. 5a 4.5 im- FREQUE/VC Y F/G.2 BRMMI INVIIN'I'OR. SADAYOSHI YOSHIKAWA dmfazw ATTORNEYS Patented March 1 1972 3,647,950

7 Sheets-Sheet 3 '5 I 2205/14/{2 l's' 7 I INVEN'I'OR.

SADAYOSHI YOSHIKAWA IIY 4 ATTORNEYS Patented March 7 1972 3,647,950

7 Sheets-Sheet 4.

F/G.8(a)

. INVI'IN'IUR.

SADAYOSHI YOSHIKAWA 4 ATTORNEYS Patented Mgrch 7 1972 '7 Sheets-Sheet 5 FIGJO IiY ATTORNEYS F/GJ/ Patented March 7 1972 '7 SheetsSheet 6 FIG/2 [50 001/0 (MHZ) IN VI'JN'I'OR. SADAYOSHI YOSHIKAWA ATTORNEYS VARIABLE TRAP DEVICE FOR TELEVISION RECEIVERS AND THE LIKE DRAWING FIG. I is a diagram showing carrier frequency distribution of a color television signal as adopted by the Japanese broadcasting system;

FIG. 2 is a partial circuit diagram showing a conventional trap for a color television receiver;

FIG. 3 illustrates a frequency characteristic curve for an intermediate-frequency amplifier circuit of conventional color television receiver;

FIGS. 4 and 5 are frequency characteristic curves showing the positions of intermediate-frequency signals appearing when the local oscillation frequency of a conventional tuner circuit has drifted;

FIG. 6 shows the picture of a television receiver illustrating the interference caused by the signals of FIGS. 4 and 5;

FIG. 7 is a schematic diagram of part of a circuit of a color television receiver incorporating an embodiment of this invention;

FIGS. 8(a) and 8(b) are respectively a schematic diagram of a phase discriminator and a chart which shows the characteristics of a phase discriminator used in the circuit of FIG. 7

FIG. 9 shows the' characteristics of a variable-capacitance diode used in the circuit of FIG. 7;

FIGS. 10 and II are frequency characteristic diagrams showing the positions of intermediate-frequency signals obtained according to the embodiment of the invention as shown in FIG. 7; and

FIGS. 12-16 correspond respectively to FIGS. 3, 4, 5, 10 and l 1 but are charted for the system employed in the United States.

DETAILED DESCRIPTION 'subcarrierf is located 3.58 MHz. away from the video-signal carrier f and the sound-signal carrier f is located 4.5 MHz. away from carrier I Also shown in FIG. I is zone Y which represents the band distribution forthe brightness signal, and zones I and O which indicate the band distribution for the color signal component.

Because the frequency interval between the color signal 'carrierf and the sound signal carrier f is relatively narrow (i.e., 920 kHz.), these two carrierstend to interfere with each other. For example, when a television signal involving these carriers is detected by a television receiver, the brightness signal willinclude a difference signal (i.e., a 920 kHz. beat) between the color signal subcarrier f and the sound signal carrier f This difference signal is mixed with the color signal to cause unpleasant color strips in'the picture. At the same time, a beat is also produced between'the video signal carrier and the sound signal carrier of the adjacent channel. This causes stripes in the picture and lowers the picture quality.

To prevent such interference, a trap circuit is provided respectively on the input and output sides of the intermediatefrequency video amplifier, thereby attenuating the intermediate-frequency sound signal. I

A conventional trap circuit'is shown in FIG. 2. It includes an antenna 1 which is connected to a tuner' 2. This tuner, as known, comprises a high-frequency amplifier tube as well as a mixer and local oscillator tube by which the television signal is converted into an intermediate-frequency signal. The output terminal of the tuner 2 is connected via a coupling capacitor 3 to one end of a parallel circuit consisting of a resistor 4 and the primary winding of a transformer 5. The other end of the parallel circuit is connected to one end of the secondary winding of the transformer 5, and the other end of the secondary winding is connected to a known intermediate-frequency video circuit 9. I

The other end of the parallel circuit consisting of the resistor 4 and the primary winding of the transformer 5 is connected to a parallel resonance circuit consisting of a capacitor 6 and a coil 7. An intemtediate tap 8 of the coil 7 is connected to a known AGC circuit for obtaining an automatic gain-control voltage.

An intermediate-frequency video amplifier circuit is constituted by the transformer 5, the circuit 9 and a transformer 10, which will be hereinafter described. The transformer 5 serves as the video circuit's first stage intermediate-frequency transformer, and the transformer 10 as the video circuits last stage intermediate-frequency transformer. The circuit 9 comprises a conventional amplifier circuit. The intermediatefrequency amplifier circuit as a whole is formed by the use of a circuit, having a favorable frequency band characteristic, such as the so-called stagger amplifier circuit.

The intermediate-frequency transformer 5 is of the bifilar winding type. This transformer 5, in combination with the resistor 4, capacitor 6 and coil 7, forms a known trap circuit which is called a bifilar-T trap. The parallel circuit comprising said capacitor 6 and coil 7 is tuned to the sound intermediate frequency (actually, this parallel circuit is tuned to a slightly higher frequency than said sound intermediate frequency.) The purpose of the resistor 4 is to adjust the degree of attenuation.

An output terminal 22 of the circuit 9 is connected to a terminal 20 of a 8+ power source by way of the primary winding of a video intermediate-frequency transformer 10 which is located at the last stage of the video intermediate-frequency amplifier circuit. Said 8+ power source provides a positive voltage for the plate of the amplifier tube of the circuit 9 to which the terminal 22 is connected. The terminal 22 is, further, connected to the terminal 24 of the sound detector circuit via a capacitor 23.

One end of the secondary winding of the transformer 10 is connected to the cathode of a video detector diode 16 via a parallel resonance circuit consisting of a capacitor I2 and a coil 11 wound on said transformer 10. The anode of the detector diode 16 is grounded by way of a parallel circuit consisting of an intermediate frequency bypass capacitor 17 and a load resistor 18. The anode is also connected to a known video amplifier circuit (not shown) located in the stage after a terminal 19.

The other end of the secondary winding of the transformer 10 is grounded by way of a parallel circuit consisting of a coil 13 and a variable resistor I4. A sliding terminal 15 of the variable resistor 14 is also grounded. The secondary winding of the transformer 10, coil 11, capacitor 12 and variable resistor 14 form a known trap circuit, which is called a bridge mutually coupled trap circuit. The parallel resonance circuit consisting of the coil II and capacitor 12 is tuned to the sound intermediate frequency (actually, this parallel resonance circuit is tuned to a slightly higher frequency than said sound intermediate frequency.) The function of the variable resistor 15 is to adjust the degree of attenuation.

The circuit described above is for a color television receiver and is the part which is improved by the present invention. Further description as to those parts not related to the invention is omitted for purposes of simplification. Also, the trap circuit for attenuating the intermediate-frequency signal of the adjacent channel is not illustrated for the same reason.

In the conventional circuit as has been described above, a television signal receiver by the antenna 1 is applied to the tuner 2 from which the signal for the desired channel is taken out. The signal is then mixed with the local oscillation frequency and is converted into a video intermediate-frequency signal which is fed to the intermediate-frequency video amplifier circuit. At the same time, in the resonance circuit consisting of coil 7 and capacitor 6 and tuned to the intermediate sound frequency, the voltage across the coil 7 and the voltage across the secondary winding of the intermediate-frequency transformer cancel each other out due to their having mutually opposite phases. As a result, the intermediate-frequency sound signal disappears from terminal 21. Actually, more than 40 db. of attenuation is effected by this operation.

After this step, the intermediate-frequency signal which has attenuated the sound intermediate frequency is amplified by the circuit 9 and transmitted to the primary winding of the intermediate-frequency transformer 10 of the last stage. At the same time, this intermediate-frequency signal is applied to the terminal 24 of the sound detector circuit via the coupling capacitor 23. The intermediate-frequency video signal appearing at the secondary winding of the transformer 10 is applied to the video amplifier circuit of the stage located after the terminal 19 via the video detector diode 16.

Before this process, however, the sound carrier is attenuated by the resonance circuit consisting of coil 11 and capacitor l2. Specifically, in the resonance circuit consisting of coil 11 and capacitor 12 tuned to thesound intermediate frequency, the current flowing in the coil 11 and the current flowing in the secondary winding of the transformer 10 cancel each other out due to their having mutually opposite phase. As a result, the intermediate-frequency sound signal is attenuated by the detector 16. In fact, the degree of this attenuation is more than 50 db.

Let it be assumed that the color signal for channel III of the Japanese Broadcasting Corporation is received by the use of the circuit described above. In this case, the video carrier f is 103.25 MHz., the sound carrier f, is 107.75 MHz. and the color subcarrier f is about 106.83 MHz., and the Japanese standard video intermediate frequency f, is 26.75 MHz. Accordingly, the local oscillation frequency of the channel III should be 130 MHz. Hence the output of the tuner is:

video intermediate frequency f,. l30l03.25=26.75 MHz.

sound intermediate frequency f l30l07.75=22.25

MHz. color subcarrier intermediate frequency f l30l06.83=

23.17 MHz. Also, since the sound carrier of channel II is ll.75 MHz., the intermediate frequency f',,, is: l30-l0l.75=28.25 MI-Iz. In this case, if the local oscillation frequency is tuned correctly to I30 MHz., the position of the intermediate-frequency signal under said condition is as shown in FIG. 3.

Referring to FIG. 3, the intermediate-frequency video signal f, and the color subcarrier intermediate-frequency signal f are located 6 db. away from the peak of the characteristic. The intermediate-frequency sound signal f, is attenuated by 40 to 50 db. more than the intermediate-frequency video signal and the color subcarrier intermediate-frequency signal, due to the bridged mutually coupled trap circuit and bifilar T-trap of FIG. 2. Under this condition, therefore, the intermediatefrequency sound signal f, and the color subcarrier intermediate-frequency signal f will not interfere with each other.

When the local oscillation frequency of the tuner drifts to a higher frequency, however, the intermediate-frequency signal will be positioned as in FIG. 4 when said signal passes through a fixed tuning trap device as in FIG. 2. In this case, the deviation in the local oscillation frequency is, by way of example, 200 kHz. As shown in FIG. 4, the attenuation of the intermeidate-frequency sound signal f, becomes smaller, and the level difference between the intermediate-frequency sound signal f, and the color subcarrier intermediate-frequency signal f becomes smaller. In this case, said two signals do interfere with each other, and a color beat of 920 kHz. is produced on the cathode-ray tube screen. This deteriorates the picture quality.

If the local oscillation frequency of the tuner drifts to a lower frequency, the position of each intermediate-frequency signal is as shown in FIG. when the signal passes through the fixed tuning trap device of FIG. 2. In this operation, the deviation in the local oscillation frequency is, by way of example, 200 kI-lz.; however, the attenuation of the intermediatefrequency sound signal f, becomes smaller, and the level difference between the intermediate-frequency sound signal f, and the color subcarrier intermediate-frequency signalf also becomes smaller. Therefore, the two signals interfere with each other to cause a color beat of 920 kHz. on the picture face. FIG. 6 shows a stripe pattern and beat stripes appearing on the edge of the letter A and on the picture face due to said 920 kHz. color beat.

Since the horizontal scanning frequency is 15,750 Hz. in the Japanese TV system, nearly 60 irregular stripe patterns are produced on the picture face when a beat signal of 920 kHz. is caused. This pattern moves at all times and gives a visibly uncomfortable picture. In addition, a color beat is brought about, for example, on the edge of the letter image on the picture face. This deteriorates the sharpness of the picture as a whole.

As has been explained, the cause of the stripe pattern lies in the variation in the local oscillation frequency of the tuner. Such variation is generally due to change in temperature at the start of operation of the tuner and also due to a secular change of the parts used therein. Besides these causes, the stripe pattern can be due to erroneous tuning of the local oscillation frequency by the audience. In some cases, the stripe pattern arises under the condition where the local oscillation frequency is shifted to the lower side in order to reduce noise in the area of a weak electric field. To solve this problem, it is considered important to maintain the local oscillation frequency constant. For this purpose, an automatic frequency control device is provided. According to this idea, however, the improvement is limited to only one point, and the parts used are still subject to secular change. In addition, the picture quality deteriorates when the input signal is weak.

The present invention has been conceived in consideration of the above-noted disadvantages. According to this invention, a variable impedance device such as a variable capacitance semiconductor is used in the intermediatefrequency sound trap and also for the adjacent channel intermediate-frequency sound trap, and the capacitance of said variable capacitance semiconductor is automatically changed according to variation in the local oscillation frequency. This permits the tuning frequency to follow the sound intermediate frequency and thus normalizes the attenuation, all of which prevents the occurrence of a sound beat and the attendant stripe pattern on the face of the picture tube.

The invention will next be explained in detail with reference to the appended drawings commencing with FIG. 7 which shows one embodiment of the invention. The parts used in FIG. 7 which are common to FIG. 2 are indicated by identical references. Those not shown in FIG. 2 but employed in accordance with this invention will hereafter be explained.

The lower connection point 4' (at which the primary winding of the transformer 5 on the input side of the intermediatefrequency amplifier circuit is connected to the resistor 4 which adjusts the degree of attenuation of the trap) has connection to one end E1 of the coil 7. Point 4 is also coupled to the other end E2 of the coil 7 via a capacitor 41, variable capacitance diode 43 and capacitor 42 which are connected in series with each other. An AGC voltage is applied from the known AGC circuit to an intermediate tap of the coil 7 via terminal 8. A resistor 45 is connected to the junction .11 at which the capacitor 42 is connected to the anode of the variable capacitance diode 43. To junction J1, a DC voltage is applied via a terminal 47 so that a constant bias voltage is supplied to the diode 43. A resistor 44 is connected to the junction .I2 at which the variable capacitance diode 43 is connected to the capacitor 41. A DC voltage obtained at output terminal 50 of a phase discriminator 49, which will be hereinafter explained, is applied to a terminal 46 which is connected to junction J2.

The

capacitors

41 and 42 are to prevent the DC voltage from being applied other than across the variable capacitance diode 43. At the same time, the

capacitors

41 and 42, in combination with the capacitance of the variable capacitance diode 43 form with coil 7 a tuning circuit.

One end of the secondary winding T of the transformer 10 located in the last stage of the intermediate-frequency video amplifier circuit is coupled to the video detector diode l6. According to the invention, a circuit comprising a coil 11 in parallel with series-connected capacitor 51, variable capacitance diode 53 and capacitor 52 is used as a trap tuning circuit which One end of the secondary winding 10' of the transformer 10 located in the last stage of the intermediate-frequency videoamplifier circuit is coupled to he video detector diode l6. According to the invention, a circuit comprising a coil 11 in parallel with series-connected capacitor variable capacitance diode 53 and capacitor 52 is used as a trap tuning circuit which is inserted between said secondary winding of the transformer 10 and the diode 16.

A constant DC voltage is applied via a terminal 56 and a resistor 54 to junction J3 by which the anode of a diode 53 is connected to the capacitor 51. This DC voltage determines the bias voltage applied to the diode. The output terminal 50 of the phase discriminator 49 is connected via a terminal 57 and a resistor 55 to a junction J4 at which the cathode of the variable capacitance diode 53 is connected to the capacitor 52. A control signal or DC voltage is supplied to junction J4 in a manner which will be hereinafter described.

The

capacitors

51 and 52 serve to prevent said DC voltage from being applied other than across the variable capacitance diode 53. The

capacitors

51 and 52, in combination with said diode 53 and the inductance coil 11, form a tuning circuit. The terminal 22 (which is coupled to the anode of the output tube of the last stage of the intermediate-frequency amplifier circuit) is connected to the terminal 24 of the sound detector circuit (not shown) via a coupling capacitor 23, and is also connected via the coupling capacitor 48 to the phase discriminator 49 which is a known circuit but which will be hereinafter described by way of example with reference to FIG. 8(a). By way of capacitor 48, therefore, an intermediatefrequency video component is applied to the phase discriminator 49.

When the intermediate-frequency video signal is normal, the phase discriminator 49 applies a reference DC output voltage to terminal 50. When this intermediate-frequency video signal is varied, the DC output voltage at terminal 50 changes proportionally to the variation in frequency in said intermediate-frequency video signal.

FIG. 8(b) shows the correlation of DC output voltage of phase discriminator 49 with variation in frequency in the input signal applied thereto. According to FIG. 8(b), the DC output voltage increases in positive direction when the input frequency deviates towards the higher side, whereas the DC output voltage increases in negative direction when the input frequency deviates towards the lower side.

If, for example, the regular local oscillation frequency of channel III of Japanese television system (which normally is 130 MHz.) deviates to the higher side by, for example, 200 kHz., the resultant'frequency is l30.2 MHz. The intermediatefrequency video component f which has been converted by the tuner circuit 2 is 26.95 MHz., the intermediate-frequency sound component f, is 22.45 MHz., and'the intermediatefrequency sound component 1, of the adjacent channel is 28.45 MHz. The intermediate-frequency video signal is amplified through the intermediate-frequency video-amplifier circuit and is applied to the phase discriminator 49 via the terminal 22 and capacitor 48.

In the phase discriminator 49, a positive DC voltage is produced at the output terminal 50 in correspondence with said frequency deviation. This DC voltage is applied to the te rminal 46 and is thus applied as a reverse bias to the cathode of the variable capacitance diode 43 via the resistor-44. In this operation, the potential across the variable capacitance diode 43 is increased and its capacitance is decreased as shown in FIG. 9. It follows that the combined capacitance of the diode 43 and

capacitors

41 and 42 is decreased and, as a result, the tuned frequency (i.e., the trap frequency) determined by the coil 7 and said combined capacitance rises to 22.45 MHz. which is equal to that of the varied intermediate-frequency sound component. Thus, the intermediate-frequency sound component is attenuated and this attenuation is effected in the illustrated' circuit by an amount of more than 40 db.

The positive DC voltage at the outputterminal of the phase discriminator 49 is moreover applied to the terminal 57, thereby effecting a reverse bias on the variable capacitance diode 53. As a result, the capacitance across the diode 53 is decreased as in the foregoing case and the combined capacitance of the diode 53 and

capacitors

51 and 52 is also decreased. The trap frequency of the tuning circuit consisting of the secondary coil of transformer 10 and said combined capacitance is increased, and the tuned frequency becomes equal to the varied intermediate-frequency sound component f, at 22.45 MHz. Thereby the intermediate-frequency sound component of the output of the intermediate frequency video amplifier circuit is attenuated in the illustrated circuit by more than 50 db. FIG. 10 shows the state of this intermediate frequency situation.

Let it next be assumed that the local oscillation frequency of the tuner deviates by 200 kHz. to the lower side. Then, in the channel III, the local oscillation frequency becomes 129.8 MHz. Therefore:

intermediate-frequency video component f, 26.55 MHz.

intermediate-frequency sound component f, 22.05 MHz.

adjacent channel intermediate-frequency sound component 28.05 MHz.

The intermediate-frequency video signal is applied to the phase discriminator 49 via capacitor 48 and the phase dis criminator 49 produces, at its output terminal 50, a DC voltage which increases in the negative direction as shown in FIG. 8(b), this corresponding to the lowered frequency. This DC voltage is applied to the cathode of the variable capacitance diode 43 from the terminal 46 via the resistor 44 and serves to bias the diode 43 in the forward direction. Therefore, the potential across the diode 43 is decreased and the capacitance of the diode 43 is increased as shown in FIG. 9. This means that the combined capacitance of the diode 43, capacitors 411 and 42 is increased. The frequency of the tuning circuit consisting of said combined capacitance and coil 7 is decreased below the reference frequency and becomes equal to the deviated intermediate-frequency which is 22.05 MHz., so that more than 40 db. of attenuation can be effected. In this operation, the intermediate-frequency component of the output of the intermediate-frequency video-amplifier circuit is also attenuated by more than 50 db. by a similar varying of the capacitance of the variable capacitance diode S3. The state of this intermediate frequency situation is shown in FIG. 1 ll.

In the circuit of the illustrated embodiment of this invention, the trap circuit for the adjacent channel is not illustrated. However, it is to be noted that the intermediate-frequency sound component can also be attenuated by more than 40 db. through the foregoing arrangement.

According to this invention as has been explained, the frequency variation of the intermediate-frequency video signal is detected by a phase discriminator, and the amount of said variation is converted to a DC voltage output. Further, the capacitance component of the trap circuit is made up with capacitors and a variable capacitance diode. The voltage across the variable capacitance diode is then controlled by said DC voltage, thereby varying the capacitance of the variable capacitance diode. As a result, the tuned frequency of the trap circuit is moved in correspondence with the variation in the intermediate-frequency video signal (which is the same as the variation in the intermediate-frequency sound component). Thus, the intermediate-frequency sound component and the adjacent channel intermediate-frequency sound component are appropriately attenuated.

In contrast to the conventional trap circuit in which the tuning frequency is fixed with respect to the variation frequency, the trap circuit of this invention has a superior control effect since the tuned frequency is automatically changed.

In the embodiment of FIG. 7, frequency variation in the intermediate-frequency video signal is detected. However, since variations would be the same in other intermediate-frequency signals or the like, the circuit may also be arranged to detect such other signals.

The detector circuit of this invention has been exemplified with a phase discriminator. However, other types of detector circuits may be used (for example, a slope detector circuit) if provided with means for deriving a DC voltage corresponding to the frequency variation. Needless to say, an automatic frequency control circuit (AFC) which is to stabilize the local oscillation frequency of the tuner may be used in combination with the circuit of this invention.

The examples given above have been based on the Japanese frequencies. However, the system is equally applicable to other systems such as in the United States the system for which is illustrated in FIGS. 12-16 which correspond respectively to FIGS. 3, 4, 5, and 11.

Finally, reference has been made above to the use of a phase discriminator. Phase discriminators are generally known. However, details of a phase discriminator which can be employed in the above-described circuit of the invention are illustrated in FIG. 8(a). This phase discriminator comprises an input terminal 70 and an output terminal 72 between which are coupled a pentode 74 and transformer 76. The cathode 78 of the pentode is connected to ground by a selfbiasing resistor 80 and a bypass condenser 82. The control grid 84 of the pentode 74 is connected to input terminal 70 via capacitor 86 and to ground via a tuned circuit consisting of coil 88 and capacitor 90. Screen grid 92 is connected to ground by a capacitor 94 and via resistor 96 to terminal 98 which is coupled to a source of positive voltage. Shield 100 is grounded. Anode 102 is connected via primary winding 104 of transformer 76 and resistor 96 to terminal 98 and also to junction 106 between

capacitors

108 and 110 which are connected across secondary winding 112 of transformer 76. Diode 114 connects one end of secondary winding 112 to output terminal 72 via resistor 116 whereas diode 118 connects the other end of secondary winding 112 to ground.

Capacitors

120 and 122 connect opposite ends of resistors 116 to ground while

resistors

124 and 126 connect

diodes

114 and 118 to opposite ends of secondary winding 112. Input signals, varying in frequency, which are applied to terminal 70 will cause control voltages of varying magnitude to appear on control grid 84 thereby varying the DC output voltage which appears at output terminal 72.

Commercially available variable capacitance semiconductors which are, by way of example, suitable for use in circuits are, for example, RD7H, RD8H, and RDIOH, which are manufactured by NBC.

What is claimed is:

1. A television circuit comprising first circuit means for utilization of a video component of a composite television signal which includes a sound component which is to be attenuated, trap circuit means operatively coupled to said first circuit means and adapted for being tuned to attenuate the sound component of said composite television signal, and second circuit means coupled to said trap circuit means to adjust the tuning of the latter automatically in accordance with the frequency of the sound component whereby to account for frequency deviations in said sound component and to attenuate said sound components; the first circuit means comprising an intermediate-frequency video amplifier circuit including input and output sections, said trap circuit means comprising trap circuits coupled to said input and output sections, said second circuit means comprising a detector circuit for detecting the frequency deviations in said sound component passing through the video amplifier circuit and for controlling tuning of the trap circuits.

2. A circuit as claimed in claim 1 wherein said second circuit means is responsive to the frequency of the sound component of said composite signal to produce a control signal and wherein said trap circuits each include a tuned circuit includmg a variable impedance coupled to said second circuit means and responsive to said control signal to control the attenuation of said sound component.

3. A circuit as claimed in claim 2 wherein said detector is a phase discriminator responsive to frequency variations in said composite signal to produce a corresponding DC voltage.

4. A circuit as claimed in claim 2 wherein said tuned circuit includes a variable capacitor controllable by an electrical signal and coupled to said second circuit means for being controlled by said control signal.

5. A circuit as claimed in claim 4 wherein said capacitor is a variable capacitance semiconductor.

6. A circuit as claimed in claim 4 comprising means to apply a DC bias and said control signal to said capacitor and wherein said tuned circuit includes further capacitors connected on opposite sides of said variable capacitor.

7. A circuit as claimed in claim 1 comprising means coupled to at least one of the trap circuits to vary the attenuation effected thereby, an automatic gain control circuit coupled to the trap circuit in the input section, and means to tap an intermediate-frequency sound signal from said output section.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORREQ'HON Patent No. 3 4 950 Dated March 7 1972 Inventor(s) Sadayoshi Yoshikawa It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 43: change "video of distribution" to --video-frequency distribution-- Column 2, line 68: change "receiver" to -received- Signed and sealed this ZL th day of October 1972.

(SEAL) Attestz' EDWARD MFLETQHERM'R. ROBERT GOTTSCHALK Attestlng Officer Commissioner of Patents ORM 1 0-1050 (10-69) uscoMM-Dc 60376-P69 fi' U.S. GOVERNMENT PRINTING OFFICE: 1959 03663JA

Claims

2. A circuit as claimed in claim 1 wherein said second circuit means is responsive to the frequency of the sound component of said composite signal to produce a control signal and wherein said trap circuits each include a tuned circuit including a variable impedance coupled to said second circuit means and responsive to said control signal to control the attenuation of said sound component.