GB2143391A - A radio frequency modulator - Google Patents

Abstract

A radio-frequency modulator for use in a videotape recorder set includes an electronic switch and a mechanical switch 8. The electronic switch consists of a diode 11, a choke coil 12, a bias resistor 14, and a diode 15 for blocking reverse current. When the electronic switch is closed, so that diode 11 is biased on, one of two tuning coils (5,6), both of which are associated with the PAL system, is automatically inserted into the closed circuitry. Thus, a sound carrier frequency of 5.0 MHz is obtained, depending on the position of the contact of the mechanical switch 8. When the electronic switch is open, so that diode 15 is off, these two tuning coils 5,6 are separated from the closed circuitry, and a sound carrier frequency of 4.5 MHz for the NTSC system is obtained. <IMAGE>

Description

SPECIFICATION RF modulator Field of the invention The present invention relates to an RF modulator and, more particularly, to an RF modulatorwhich is used in a VTR set and equipped with a plurality of tuning coils, an electronic switch, and a mechanical switch to select any one among various sound carrier frequencies for accommodating the modulatorto various broadcast systems.

Background of the invention When the signal recorded in a VTR is reproduced on a television receiver, the signal is made to assume the same form as television waves. As well known in the art, there exist the NTSC system, the PAL system, and the SECAM system as television broadcast systems. In an RF modulator, some components are used for all the broadcast systems by switching other components between various states to make it available for every broadcast system. For example, the sound carrier frequency used in the NTSC system is 4.5 MHz, while that of the PAL system is 5.5 MHz and 6.0 MHz. A different tuning coil is selected depending on the television broadcast system by the action of a selector switch to obtain the corresponding sound carrier frequency.

The sound carrier frequency selector switch in a conventional RF modulator is shown in Figure 1, where the selector switch is switched between different conditions to obtain the frequency suited to the NTSC system or PAL system. This circuit includes a signal-generating device 1 such as a transistor, a tuning capacitor 2, a bypass capacitor 3, tuning coils 4, 5, 6, and a selector switch 7. The tuning coil 4 generates a sound carrier frequency of 4.5 MHzforthe NTSC system. The tuning coils 5 and 6 generate sound carrier frequencies of 5.5. MHz and 6.0 MHz, respectively, for the PAL system.

It is now assumed that the VTR set using the RF modulator is switched to a mode adapted to the NTSC system. When the tuning coil 5 or 6 is inserted into the closed circuitry by the action of the selector switch 7, the other circuits in the VTR set need not be subjected to switching operation, because these two coils are associated with the same PAL system.

However, when the switch 7 is operated to introduce the coil 4 into the closed circuitry for accommodating the RF modulator to the NTSC system rather than the PAL system, the reception and reproduction of a broadcast is not achieved simply by subjecting some parts of the RF modulator to switching operation.

Rather, other circuits within the VTR set are required to be subjected to switching operation so as to accommodate the NTSC system, instead of the PAL system. Accordingly, the VTR also necessitates a selector switch. In an arrangement where the RF modulator and other circuits of the VTR set are separately changed to other conditions by means of at least two mechanical selector switches for accommodation to various broadcast systems, if any one of the switches is not operated for inadvertence, the picture image will be obtained but the sound will not be reproduced, or vice versa. Accordingly, an arrangement may be suggested in which the selector switch 7 is fabricated as two circuits to permit the RF modulator and the inside of the VTR set to be simultaneously subjected to switching operation.In this scheme, however, the RF modulatorthat is only one part of the VTR set controls the VTR set in an inverted manner. This is undesirable for the design of the VTR set and limits the degree of freedom in designing the VTR set.

Summary ofthe invention It is an object of the present invention to provide an RF modulator which is free of the foregoing difficulties with the prior art device.

It is a more specific object of the invention to provide an RF modulator which accommodates itself to a plurality of broadcast systems by the operation of an electronic switch and adjusts itself to a plurality of sound carrier frequencies for an identical broadcast system by the operation of a mechanical switch in such a way that the RF module and the inside of the associated VTR set always conform with the same broadcast system and that no limitations are imposed on the design of the VTR set.

Brief description of the drawing Figure 1 is a diagram of the sound carrier frequency selection circuit in a conventional RF modulator; and Figure 2 is a circuit diagram of the circuit configuration of an RF modulator according to the present invention.

Detailed description of the invention Referring to Figure 2, there is shown the circuit configuration of an RF modulator according to the invention, and in which reference numerals 1-6 are used to indicate the same components as those in Figure 1. The circuit further includes a mechanical switch 8 for changing the conditions of the tuning coils 5 and 6, capacitors 9 and 10 for blocking flow of alternating current, a diode 11 of an electronic switch (described later), a choke coil 12 for blocking flow of RF signals, a bias power supply terminal 13, a bias resistor 14, a diode 15 for blocking flow of reverse current, and a power supply terminal 16 of the electronic switch. The electronic switch comprises the diode 11, the choke coil 12, the bias resistor 14, and the diode 15.When the electronic switch is open, the sound carrierfrequencyforthe NTSC system is obtained. When it is closed, the sound carrier frequency for the PAL system is provided.

When the voltage for accommodating the modulator to other broadcast system is not applied to the power supply terminal 16, the voltage EB applied to the bias supply terminal 13 biases the diode 11 of the electronic switch in its forward direction and thus it conducts. Hence, high-frequency signals are transmitted through this conducting device. When the switch 8 is electrically connected with the tuning coil 5, the coils 4 and 5 cooperate to generate a frequency of 5.5. MHz. When the switch 8 is electrically connected with the coil 6, the coils 4 and 6 cooperate to generate a frequency of 6.0 MHz. This frequency of oscillation depends on whether the mechanical switch 8 is electrically connected with the coil 5 or 6.When the switch 8 is electrically connected with the coil 5, the inductance of the tuning circuit is the composite inductance of the tuning coils 4 and 5 connected in parallel, and the value of this composite inductance results in the aforementioned frequency of 5.5 MHz. When the switch 8 is electrically connected with the coil 6, the inductance of the tuning circuit is the composite inductance of the coils 4 and 6 connected in parallel such that the frequency of 6.0 MHz is determined by the value of this composite inductance. In this way, whenever the switching voltage is not applied to the power terminal 16 of the electronic switch i.e., the electronic switch is closed, either the coil 5 or 6 associated with the PAL system is inserted into the closed circuitry.Under this condition, the sound carrier frequency of 5.5 MHz or 6.0 MHz can be obtained by changing the condition of the mechanical switch 8 as needed.

When another switching voltage which is higher than the voltage E5 applied to the bias power supply terminal 13 is applied to the electronic switch power supply terminal 16, the diode 11 is reverse biased and comes out of conduction, so that the tuning coils 5 and 6 are electrically separated from each other.

That is, when the electronic switch is open, the tuning circuit includes only the coil 4, as far as inductance is concerned. Thus, the sound carrier frequency of 4.5 MHz forthe NTSC system is obtained.

In this manner, by applying the voltage for selecting a specific broadcast system to the power supply terminal 16 or halting such application depending on whether the NTSC system or the PAL system is selected by the interior of the VTR set, the condition of the RF modulator is automatically switched by the switching operation which is performed in the VTR to select one television broadcast system. Consequently, the broadcast system which is selected by the RF modulator is certainly changed from the PAL system to the NTSC system or vice versa, irrespective of the position of the contact of the switch 8. In addition, no limitations are imposed on the degree of freedom in designing the VTR set, because the RF modulator is dependent for the switching operation upon the VTR set.

The description has been made by giving the NTSC and PAL systems as examples, but it is to be understood that the invention may also be applied to an arrangement which can select the SECAM system. Further, the number of tuning coils is not limited to three, but the invention may be similarly applied to an apparatus where the sound carrier frequency is selected out of several values using a larger number of tuning coils.

As thus far described, according to the present invention, selection of the sound carrier frequency according to different television systems is made by an electronic switch. For an identical broadcast system, selection of the sound carrier frequency is made by a mechanical switch. The switching operation for the VTR set is automatically followed by the switching operation for the RF modulator. Hence, the switching operation to select a broadcast system is performed with certainty. Another advantage is that no restrictions are placed on the degree of freedom in designing the VTR set.

Claims (3)

1. In an RF modulator having parts that accommodatethe modulatortoanyoneofa plurality of television broadcast systems to select a carrier frequency adapted to the television broadcast system, the improvement comprising: an electronic switch for selecting one from the broadcast systems, and a mechanical switch for selecting one adapted to the selected broadcast system from a plurality of carrier frequencies, whereby the carrier frequency is selected by these switches to accommodate the RF modulator to the plurality of television broadcast systems.

2. In an RF modulator as setforth in claim 1, the further improvement wherein the electronic switch is placed between the tuning coil 4 and the tuning coils 5, 6, and wherein the mechanical switch is so provided that the tuning coils 5 and 6 are alternately used.

3. A radio frequency modulator substantially as hereinbefore described, with reference to Figure 2 of the accompanying drawings.