GB2301245A

GB2301245A - RF modulator

Info

Publication number: GB2301245A
Application number: GB9607757A
Authority: GB
Inventors: Kyung-Soo Kwon; Sho-Young Lim
Original assignee: Samsung Electro Mechanics Co Ltd
Current assignee: Samsung Electro Mechanics Co Ltd
Priority date: 1995-04-18
Filing date: 1996-04-15
Publication date: 1996-11-27
Anticipated expiration: 2016-04-15
Also published as: GB2301245B; DE19615390A1; GB9607757D0

Description

2301245 RADIO FREQUENCY MODULATING CIRCUIT FOR USE IN PAL METHOD

FIELD OF THE INVENTION

The present invention relates to a radio frequency (RF) modulating circuit for video tape recorders (VTR), game apparatuses and the like belonging to the PAL method. In particular, the present invention relates to an RF modulating circuit for use in the PAL method, in which different audio carrier frequencies can be automatically selected in an easy manner based on the B/G method, the I method, the D/K method and the like among the PAL method, and at the same time, precise TSG, signals are formed by utilizing the audio carrier frequency.

DESCRIPTION OF THE PRIOR ART

Generally, as the television broadcasting methods, there are known the NTSC method, and the PAL and SECAM methods, and the former is used in Korea, the United States, and Japan, while the latter are used in European countries. The PAL method of the European countries consists of various methods such as the B/G method, the I method, and the D/K method, the method being different depending on the countries. Further, the audio carrier frequencies for the different methods are also various, and includes 5.5 MHz, 6.0 MHz, and 6.5 MHz.

In the PAL method, if a broadcasting channel is. selected 1 by a VTR, and if the reproduced signals of the VTR are RFmodulated into television signals suitably to the relevant broadcasting channel to output them to the television, then the reproduced picture of the VTR appears on the screen of the television, provided that the relevant channel of the television is vacant.

The general conventional RF modulating circuit is as shown in FIG. 1.

Referring to FIG. 1, video signals Vin which are composite video signals reproduced by a VTR or a game apparatus pass through a clamp circuit section 1 which fixes the synchronization tip potential to a constant level. Then the video signals Vin pass through a switching section 3 which selects one set of signals from among the video signals and TSG signals. Then the selected signals are mixed together by a mixer 5 so as to be outputted through an RF amplifier 6 to the television. Further, audio signals Aln of the audible band pass through an audio amplifier 2 and a frequency modulating section 4 so as to be frequency-modulated. Then the modulated audio signals are mixed by a mixer 5, and pass through the RF amplifier 6 to be outputted to the television.

The TSG signals are called "test signals" or "test pattern signals", and include synchronizing signals, and video signals having black and white levels.

In the conventional RF modulating circuit, in order to 2 make different audio carrier frequencies suitable to the broadcasting method of the PAL method, a mechanical switch Swi has to be installed in a hidden place such as the rear side of a VTR or a television, and this is a troublesome task.

Further, although the general consumers rarely use, in order to test the products, the TSG signals are formed during the manufacturing process. For this purpose, the conventional RF modulator is provided with a capacitor, a SAW resonator, and an oscillator separately, with the result that the manufacturing cost is increased.

FIG. 2 illustrates another conventional RF modulating circuit.

The circuit of FIG. 2 is disclosed in U.S. Patent 5,136,387. This RP modulating circuit of FIG. 2 carries out the channel setting without using a test pattern generator.

The RF modulator of FIG. 2 includes: an A/D converter for converting a television channel setting data into digital signals; an oscillation control circuit for controlling both an audio channel oscillation and an RF channel oscillation based on the digital channel setting data of the A/D converter; a signal merging means for merging amplitude-modulated video signals (which are modulated suitably to the RF carrier oscillation frequency) with frequency/amplitude-modulated audio signals which are modulated suitably to the audio carrier oscillation; a channel capturing means for capturing the central portions of the channels 3 of the channel setting data; and a signal blocking means for blocking the video input signals over the end portions departing remote from the channel capturing means.

In the above described conventional circuit, a clip control circuit 33 is used instead of a test pattern control circuit 34, and the clip control circuit 33 performs the same function as that of the test pattern control circuit 34. Further, an AM modulator 22 includes a clamping circuit 22a, a clipping circuit 22b, and a modulating circuit 22c.

This conventional circuit includes the clip control circuit 33 instead of the test pattern control circuit, and the clip control circuit performs the same function as that of the test pattern control circuit as described above. That is, the clip control circuit performs the function of passing or blocking the incoming video signals. However, there is included no test pattern generator, and therefore, this conventional circuit can be used only on an apparatus in which signals usable in place of the test pattern signals, such as blue black or the like, are outputted. Further, the channels are selected manually by means of a variable resistor VR, and therefore, it is inconvenient to use the apparatus.

SUMMARY OF THE INVENTION

The present invention is intended to overcome the above described disadvantages of the conventional techniques.

4 Therefore it is an object of the present invention to provide an RF modulating circuit for use in the PAL method, in which proper audio carrier frequencies can be easily selected in accordance with the BIG method, the I method, and the D/K method of the PAL method.

It is another object of the present. invention to provide an RF modulating circuit for use in the PAL method, in which the audio carrier frequency can be frequency-divided even without separate resonance device, capacitor and oscillator, so as to form TSG signals.

It is still another object of the present invention to provide precise TSG signals in which errors are minimal and accuracy is high, since the errors included in the TSG signals are subjected to a frequency division together with the TSG signals, in the RF modulating circuit for use in the PAL method in which the audio carrier frequency can be frequency - divided even without separate resonating device, capacitor and oscillator, so as to form TSG signals.

In achieving the above objects, the RF modulating circuit for use in the PAL method according to the present invention includes: a TSG generating section for generating TSG signals based on a particular incoming frequency; a clamping circuit for fixing the potential of video signals Vin at a constant level; an audio amplifier for amplifying audio signals; a tank circuit section consisting of three audio carrier tank circuits based on the broadcasting method; a frequency modulating section for f requency-modulating audio carriers from an oscillator by means of output signals of the audio amplifier; a mixer f or mixing output signals of the frequency modulating section and output signals of the clamping circuit section and the TSG generating section respectively with the frequency of an oscillator; and an RF amplifier for amplifying the output signals of the mixer, and the RF modulating circuit further includes:

a control section for reading externally inputted channel selecting signals so as to output switching control signals and frequency dividing control signals; and an oscillating section for selecting and oscillating one of a plurality of oscillated resonance frequencies by means of the plurality of the tank circuits of the tank circuit section and by means of the switching control signals of the control section.

In another aspect of the present invention, the RF modulating circuit for use in the PAL method includes: a TSG generating section for generating TSG signals based on a particular incoming frequency; a clamping circuit for fixing the potential of video signals Vin at a constant level; an audio amplifier for amplifying audio signals; a tank circuit section consisting of three audio carrier tank circuits based on the broadcasting method; an oscillating section for oscillating the audio carriers of the tank circuit section; a frequency modulating section for frequencymodulating audio carriers from an oscillator by means of 6 output signals of the audio amplifier; a mixer for mixing output signals of the frequency modulating section and output signals of the clamping circuit section and the TSG generating section respetively with the frequency of an oscillator; and an RF amplifier for amplifying the output signals of the mixer, and the RF modulating circuit further includes:

a control section for reading externally inputted channel selecting signals so as to output switching control signals and frequency dividing control signals; and a frequency-dividing section for frequency-dividing the oscillation frequency of the oscillating section based on the frequency dividing control signals of the control section so as to output the frequency- divided output to the TSG generating section.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and other advantages of the present invention will become more apparent by describing in detail the preferred embodiment of the present invention with reference to the attached drawings in which:

FIG. 1 is a block diagram showing the constitution of a conventional RF modulating circuit; FIG. 2 is a block diagram showing the constitution of another RF modulating circuit; FIG. 3 is a block diagram showing the constitution of the RF modulating circuit according to the present invention; 7 FIG. 4 is a detailed circuital illustration of a control section of FIG. 3; FIG. 5 is a detailed circuital illustration of an oscillating section of FIG. 3; and FIG. 6 is a detailed circuital illustration of a frequency-dividing section of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments of the present invention will be described referring to FIGs. 3 to 6.

FIG. 3 is a block diagram showing the constitution of the RF modulating circuit according to the present invention.

Referring to FIG. 3, reference code 40 indicates a control section. The control section 40 reads externally inputted channel selecting signals CS1 and CS2 so as to supply switching control signals S1, S2 and S3 to an oscillating section 60, and so as to supply frequency dividing control signals C11, C12, C21, C22, C31 and C32 to a frequency-dividing section 70.

Reference code 50 indicates a tank circuit section, and this circuit section includes first tank Cl and Ll, second tank C2 and L2, and third tank C3 and L3, thereby forming an audio carrier resonance circuit based on the broadcasting method.

Reference code 51 indicates a clamping circuit, and this circuit fixes the potential of the synchronization tip of video signals Vin (which are composite signals) at a constant level.

8 Reference code 52 indicates a switching section, and this section switches the signals of the clamping circuit 51 and the signals of a TSG generating section by means of an external adjusting switch SWO. In the case where the output signals of the TSG generating section 80 are selected, the switching section 52 has the functions of disabling an audio amplifier, and enabling the frequency-dividing section 70.

Reference code 53 indicates the audio amplifier, and this amplifier amplifies audio signals Ain which belong to an audible frequency band.

Reference code 54 indicates a frequency modulating section, and this section modulates the audio carriers which are selected from the output signals of the audio amplifier 53 by the oscillating section 60.

Reference code 55 indicates a mixer, and this mixer 55 mixes the output signals of the frequency modulating section 54 and the output signals of the switching section 52 respectively with the oscillation frequency of the oscillator 34 so as to output a radio frequency. Reference code 56 indicates an RF amplifier, and this amplifier amplifies the output signals of the mixer 55.

Reference code 60 indicates the oscillating section, and this section 60 performs switching operations by the help of the switching control signals S1, S2 and S3 of the control section 40 so as to oscillate by selecting one frequency from among resonance frequencies Fl, F2 and F3 of the tank circuit section 50.

9 Reference code 70 indicates the frequency dividing section, and this section 70 is enabled by a signal of the switching section 52, and frequency-divides the oscillation frequency of the oscillating section 60 by the help of the frequency-dividing control signals C11, C12, C21, C22, C31 and C32 of the control section 40. Thus the frequency dividing section 70 supplies a constant frequency to the TSG generating section all the time.

Reference code 80 indicates a TSG generating section, and this section 80 which is a test signal generator forms TSG signals by utilizing the frequency-divided frequency of the frequency dividing section 70.

FIG. 4 is a detailed circuital illustration of a control section of FIG. 3.

The control section 40 includes: a first comparing circuit section 41 consisting of a plurality of transistors Q11 Q33 for comparing externally inputted channel selecting signals CS1 and CS2 with an internal reference voltage Vrefl so as to output switching control signals S1, S2 and S3; and a differential type second comparing circuit section 43 consisting of a plurality of transistors Q11 Q2, Q3, Q4, Q5 and Q6 for comparing the output signals of the first comparing circuit section 41 with an internal reference voltage Vref2 so as to output frequency dividing control signals C11, C12, C21, C22, C31 and C32.

The frequency dividing control signals C11, C12, C21, C22, 931 and C32 of the control section are formed in pairs of C11 and C12, C21 and C22, and C31 and C32, and they are needed in an emitter- coupled logic circuit.

FIG. 5 is a detailed circuital illustration of an oscillating section of FIG. 3.

Referring to FIG. 5, the oscillating section 60 includes: electronic switching sections Q11 - Q14, Q21 - Q24, and Q31 - Q34 for connecting one of the tank circuits Cl and Ll, C2 and L2, and C3 and L3 of the tank circuit section 50 to oscillating circuits Q41 and Q43 of the oscillating section 60 in accordance with the switching control signals S1, S2 and S3 of the control section 40; and the oscillating circuits Q41 and Q43 for oscillating the resonance frequency of the connected tank circuit.

FIG. 6 is a detailed circuital illustration of a frequency dividing section 70 of FIG. 3.

The frequency dividing section 70 includes: first, second, third and fourth counters 71, 72, 73 and 74 for carrying out counting operations in accordance with the sets of the frequency dividing control signals C11, C21 and C31, or C12, C22 and C32 of the control section 40; a NAND section 75 for NANDgating the signals from the counters; a latching section 76 for maintaining the output of the NAND section 75 for a certain period of time; and a duty adjusting section 77 for adjusting the duty ratio of the output signals of the latching section 76 to a certain value.

11 Reference codes CK1 and CK2 indicate clock signals for enabling the counters, Q11 - Q34 indicate transistors for comparing signals with each other, and R1 - R35 indicate bias resistors.

Now the present invention will be described as to its operation and effects referring to FIGs. 3 to 6.

Prior to describing the operation of the present invention, it is noted that the channel selecting signals CS1 and CS2 have the opening and grounding conceptions, because they are connected or disconnected between the circuit terminals and the ground in its structures such as the electric circuits and switches. However, for the sake of convenience of the description, the status signals of the opening and grounding will be described in the form of logic signals, i.e., high "i" and low ss 0 Referring to FIG. 3, first the control section 40 reads the incoming channel selecting signals CS1 and CS@. Then the differential comparing circuit 41 compares the read channel selecting signals CS1 and CS2 with an internally set reference voltage Vref 1. In accordance with the compared results, the switching control signals S1, S2 and S3 which have a high or low level are outputted to the oscillating section 60.

Referring to FIG. 4, the operation of the circuit will be described in further detail.

If a voltage of about +5 V is supplied to a power source 12 terminal +Vcc of the control section 40, then a voltage of about +4 v is supplied to the common collector terminal of the transistors Q1, Q2 and Q3 and to the common collector terminal of the transistors Q4, Q5 and Q6 of the f irst comparing circuit section 41. At the same time, a voltage of about +2 V as a reference voltage Vrefl is supplied to the base terminals of the transistors Q3 and Q4. Further a voltage of about +2.5 V as a reference voltage Vref2 is supplied to the common base terminal of the transistors Q11, Q12, Q21, Q22, Q31 and Q32 of the second compaiing circuit section 43.

In the above described case, the case where the channel selecting signals are inputted in the form of 1,01" will be described. Owing to the channel selecting signal CS1 and the reference voltage Vrefl, the transistors Q1 and Q2 of the first comparing circuit section 41 of the control section 40 are turned off, while the transistor Q3 is turned on. Further, owing to the channel selecting signal CS2 and the reference voltage Vrefl, the transistors Q5 and Q6 of the first comparing circuit section 41 of the control section 40 are turned off, while the transistor Q4 is turned on.

In accordance with the operation of the first comparing circuit section 41 of the control section 40, the switching signals S1, S2 and S3 are outputted in the form of "010".

Simultaneously with the operations of the first comparing circuit section 41, the transistors Q33 and Q32 of the second 13 comparing circuit section 43 are turned on, and the transistor Q31 is turned off, with the result that the frequency dividing control signals are outputted in the form of 1,1011. Further, the transistors Q23 and Q22 are turned off, and the transistor Q21 is turned on, with the result that the frequency dividing control signals C21 and C22 are outputted in the form of "01". Further, the transistors Q13 and Q12 are turned off, and the transistor Q11 is turned on, with the result that the frequency dividing control signals C11 and C12 are outputted in the form of '101".

The operation will be analyzed based on the above description.

If the channel selecting signals CS1 and CS@ are inputted in the form of "1011, then the frequency dividing control signals C31 and C32 are outputted in the form of "01", the frequency dividing control signals C21 and C22 are outputted in the form of "10", and the frequency dividing control signals C11 and C12 are outputted in the form of "10".

If the channel selecting signals CS1 and CS2 are inputted in the form of "ll", then the frequency dividing control signals C31 and C32 are outputted in the form of #00149, the f requency dividing control signals C21 and C22 are outputted in the form of "10", and the frequency dividing control signals C11 and C12 are outputted in the form of "01".

Meanwhile, in the tank circuit section 50 of FIG. 3, 14 the f irst tank Cl and Ll is made to resonate to 5.5 MHz, the second tank C2 and L2 is made to resonate to 6. 0 MHz, and the third tank C3 and L3 is made to resonate to 6.5 MHz. One of these resonant frequencies is selected by the oscillating section 60 in accordance with the switching control signals S1, S2 and S3 of the control section 40, so as to carry out oscillations. This will be described in detail below.

The switching control signals of the control section 40 of FIG. 3 are supplied to the oscillating section 60 of FIG. 4. In the switching control signals S1, S2 and S3 of the control section 40, only one of them has a low level, while the rest of them have a high level.

One of the resonance frequencies Fl, F2 and F3 which are outputted from the tank circuit section 10 is selected in accordance with the low leveled control signal of the control section 40 and based on the continuous operations of each group of the transistors Q11 - Q16, Q21 - Q26, and Q31 - Q36 of the electronic switch section. Then the selected frequency is supplied to the oscillating circuits Q41 and Q43. Thus the resonance frequency of the tank circuits, which is selected by the electronic switch sections Q11 - Q36 of the oscillating section 60 is oscillated by the oscillating circuits Q41 and Q43 before being outputted.

The channel selecting signals CS1 and CS2 which are inputted into the control section 40 are either control signals is produced by an external microcomputer, or status signals formed by an external mechanical means. Therefore, the RF modulating circuit according to the present invention can be controlled by an external mechanical means or an external electronic means such as a remote controller. a, In accordance with the frequency dividing control signals C11, C12, C21, C22, C31 and C32 of the control section 40, the frequency dividing section 70 divides in the ratio of 1/22 if an audio carrier of 5.5 MHz is inputted, divides in the ratio of 1/24 if 6.0 MHz is inputted, and divides in the ratio of 1/26 if 6.5 MHz is inputted. Consequently, under the control of the control section 40, the frequency dividing section 70 supplies only 250 KHZ to the TSG generating section 80 all the time. The TSG generating section 80 outputs TSG signals having the same frequency as that supplied by the frequency dividing section 70.

Among the frequency dividing control signals C11, C21, C22, C31 and C32, the pairs C11 and C12, C21 and C22, C31 and C32 have mutually inverse logic levels respectively.

C12, and The required logic levels are different depending on systems, and therefore, two mutually inverse logic levels are provided. it will be assumed that the frequency dividing control signals C12, C22 and C32 are used in the present invention.

Referring to FIG. 6, there will be described the case in which the frequency dividing control signals C31 and C32 are outputted in the form of "1V from the control section 40, the 16 frequency dividing control signals C21 and C22 are outputted in the form of I'01" from the control section 40, and the frequency dividing control signals C11 and C12 are outputted in the form of I'01" from the control section 40. That is, if the frequency dividing control signals C12, C22 and C32 have the value of H1100, then the first counter 71 of the frequency dividing section 70 does not carry out a frequency dividing operation, and the second and third counters 72 and 73 carry out a frequency operation in the ratio of 1/2, while the fourth counter 74 carry out a frequency dividing operation in the ratio of 1/2 all the time.

Therefore, a frequency division in the ratio of 1/11 occurs in the final counter 74 and the NAND section 75. These 1111-divided signals pass through the latching section 76 to be supplied to the duty adjusting section 77 in which they are divided in the ratio of 1/2, and then finally, are divided in the ratio of 1/22. Meanwhile, the frequency of 5. 5 MHz which is inputted from the oscillating section 60 is divided in the ratio of 1/22. As a result, signals of 250 KHz are supplied to the TSG generating section 80.

Meanwhile, based on the above described operating principle, if the frequency dividing control signals C12, C22 and C32 have the form of "001", a frequency division in the ratio of 1/12 occurs in the final counter 74 and the NAND section 75. These 1/12-divided signals pass through the latching section 76 to be supplied to the duty adjusting section 77 in which they are divided 17 in the ratio of 1/2, and are finally divided in the ratio of 1/24. Meanwhile, the f requency of 6. 0 MHz which is inputted f rom the oscillating section 60 is divided in the ratio of 1/24. As a result, signals of 250 KHz are supplied to the TSG generating section 80.

Meanwhile, if the frequency dividing control signals C12, C22 and C32 have the form of "101", a frequency division in the ratio of 1/13 occurs in the final counter 74 and the NAND section 75. These 1/13-divided signals pass through the latching section 76 to be supplied to the duty adjusting section 77 in which they are divided in the ratio of 1/2 to be finally divided in the ratio of 1/26. Meanwhile, the frequency of 6. 5 MHz which is inputted from the oscillating section 60 is divided in the ratio of 1/26. As a result, signals of 250 KHz are supplied to the TSG generating section 80.

In the conventional circuit, if 500 KHz is used, the deviation is about 10 KHz. In contrast to this, in the present invention, if 500 KHz is used, the deviation is about 1 KHz, but the present invention uses 250 KHz, and theref ore, the deviation reduced to a half of 1 KHz. Therefore, the TSG signals which are provided in the present invention is very low in the deviation.

Meanwhile, the potential of the synchronization tip of the video signals Vin is fixed to a certain level by the clamping circuit section 51, and the switching section 52 carries out 18 switching functions to selectively supply one set of the video signals or the TSG signals to the mixer 55.

If the switching section 52 operates in such a manner that the TSG signals are selected through an external adjusting switch SWO, the switching section 52 turns off the power source of the audio amplifier 53, and activates the frequency dividing section 70. Therefore, there is eliminated the possibility of variation of the frequency of the audio carriers in accordance with the incoming audio signals in the case where the audio carrier frequency is frequency-divided to use it as TSG signals.

Accordingly, only in the case where the video signals are selected by the switching section 52, the audio signals Ain are amplified to a certain level by the audio amplifier 53 so as to be outputted to the RF modulating section 54. The RF modulating section 54 modulates the frequency of the audio carrier waves to audio signals as described above so as to output them to the mixer 55.

The mixer 55 either mixes the inputted video signals and the modulated audio signals with the oscillated frequency of the oscillator 34, or mixes the TSG signals with the oscillated frequency of the oscillator 34. The signals thus mixed are amplified to a certain level by the RF amplifier 56 so as to be outputted to an electronic apparatus such as TV and the like.

According to the present invention as described above, a separate resonator or a separate capacitor is not required in 19 producing the TSG signals. The audio carriers are frequencydivided in accordance with the broadcasting method, and thus, a constant frequency is supplied to the TSG circuit all the time so as to produce TSG signals. Therefore, whereas the conventional tank circuit and oscillator show a deviation of about 10 KHz, the present invention drastically reduces the deviation because the deviation is also frequency-divided together with the oscillated frequency. Consequently, the TSG signals become more precise.

Further according to the present invention, different audio.carrier frequencies can be easily selected in accordance with the different broadcasting methods, i.e., the BIG method, the I method, and the D/K method of the PAL method. Thus by utilizing the audio carrier frequencies, the TSG signals are formed.

Further, according to the present invention, more precise TSG signals can be formed without using separate resonators or capacitors in forming the TSG signals. Further, the broadcasting method can be easily switched over by means of an electronic switch of the oscillating section.

The above descriptions are for showing a preferred embodiment of the present invention, and therefore, any variation and modification should come within the scope of the present invention.

Claims

WHAT IS CLAIMED IS

1. An RF modulating circuit f or use in the PAL method, comprising: a TSG generating section for generating TSG signals based on a particular incoming frequency; a clamping circuit for fixing the potential of video signals at a constant level; an audio amplifier for amplifying audio signals; a tank circuit section consisting of three audio carrier tank circuits based on the broadcasting method; a frequency modulating section for frequency-modulating audio carriers from an oscillator by means of output signals of said audio amplifier; a mixer for mixing output signals of said frequency modulating section and output signals of said clamping circuit section and said TSG generating section respectively with a frequency of an oscillator; and an RF amplifier for amplifying output signals of said mixer, and the RF modulating circuit further comprising: a control section for reading externally inputted channel selecting signals so as to output switching control signals and frequency dividing control signals; and an oscillating section for selecting and oscillating one 21 of a plurality of oscillated resonance frequencies by means of the plurality of said tank circuits of said tank circuit section and by means of the switching control signals of said control section.

2. The RF modulating circuit as claimed in claim 1, further comprising: a switching section for switching the signals of said clamping circuit and the signals of a TSG generating section by means of an external adjusting switch; and frequency dividing section for f requency- dividing an oscillation frequency of an audio carrier oscillator by the help of the frequency-dividing control signals of said control section so as to supply the divided frequency to said TSG generating section.

3. The RF modulating circuit as claimed in claim wherein said control section comprises: a first comparing circuit section consisting of a plurality of transistors for comparing externally inputted two channel selecting signals with an internal reference voltage so as to output three switching control signals; and a differential type second comparing circuit section consisting of a plurality of transistors for comparing output signals of said first comparing circuit section with another internal reference voltage so as to output frequency dividing 22 control signals.

4. The RF modulating circuit as claimed in claim 1, wherein said oscillating section comprises:

electronic switching sections for connecting one of said tank circuits of said tank circuit section to oscillating circuits of said oscillating section in accordance with switching control signals of said control section; and said oscillating circuits oscillating the resonance frequency of said connected tank circuit.

5. The RF modulating circuit as claimed in claim 1, wherein said channel selecting signals inputted into said control section are control signals from an external microcomputer.

6. The RF modulating circuit as claimed in claim 1, wherein said channel selecting signals inputted into said control section are level signals from an external mechanical switch.

7. The RF modulating circuit as claimed in claim 2, wherein, if output signals of said TSG generating section are selected, said switching section disables said audio amplifier, and enables said frequency dividing section.

8. The RF modulating circuit as claimed in claim 2, 23 wherein said frequency dividing section comprises: first, second, third and fourth counters for carrying out counting operations in accordance with the sets of the frequency dividing control signals C11, C21 and C31, or C12, C22 and C32 of said control section; a NAND section for NAND- gating the signals from said counters; a latching section for maintaining an output of said NAND section for a certain period of time; and a duty adjusting section for adjusting the duty ratio of output signals of said latching section to a certain value.

9. comprising:

An RF modulating circuit for use in the PAL method a TSG generating section for generating TSG signals based on a particular incoming frequency; a clamping circuit for fixing the potential of video signals at a constant level; an audio amplifier for amplifying audio signals; a tank circuit section consisting of three audio carrier tank circuits based on the broadcasting method; an oscillating section for oscillating the audio carriers of said tank circuit section; a frequency modulating section for frequency-modulating the audio carriers from an oscillator by means of output signals 24 of said audio amplifier; a mixer for mixing output signals of said frequency modulating section and output signals of said clamping circuit section and said TSG generating section respectively with a frequency of an oscillator; and an RF amplifier for amplifying output signals of said mixer, and the RF modulating circuit further comprising: a control section for reading externally inputted channel selecting signals so as to output switching control signals and frequency dividing control signals; and a frequency-dividing section for frequency-dividing the oscillation frequency of said oscillating section based on the frequency dividing control signals of said control section so as to output the frequency-divided output to said TSG generating section.

10. The RF modulating circuit as claimed in claim 9, further comprising: a switching section for switching the signals of said clamping circuit and the signals of a TSG generating section by means.of an external adjusting switch; and an oscillating section for carrying out oscillations by selecting one of resonance frequencies by means of a plurality of tank circuits of said tank circuit section in accordance with switching control signals of said control section.

11. The RF modulating circuit as claimed in claim 9, wherein said control section comprises:

a first comparing circuit section consisting of a plurality of transistors for comparing externally inputted two channel selecting signals with an internal reference voltage so as to output three switching control signals; and a differential type second comparing circuit section consisting of a plurality of transistors for comparing output signals of said first comparing circuit section with another internal reference voltage so as to output frequency dividing control signals.

12. The RF modulating circuit as claimed in claim 9, wherein said frequency dividing section comprises:

first, second, third and fourth counters for carrying out counting operations in accordance with the sets C11, C21 and C31, or C12, C22 and C32 of the frequency dividing control signals of said control section; NAND section for NAND-gating the signals from said counters; latching section for maintaining the output of said NAND section for a certain period of time; and a duty adjusting section for adjusting the duty ratio of 26 output signals of said latching section to a certain value.

13. The RF modulating circuit as claimed in claim 9, wherein, if output signals of said TSG generating section are selected, said switching section disables said audio amplifier, and enables said frequency dividing section.

14. The RF modulating circuit as claimed in claim 1, wherein said oscillating section comprises: electronic switching sections for connecting one of said tank circuits of said tank circuit section to oscillating circuits of said oscillating section in accordance with switching control signals of said control section; and said oscillating circuits oscillating the resonance frequency of said connected tank circuit.

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