JPS6029263Y2 - Common filter circuit for receiver for TV audio multiplex broadcasting and FM stereo broadcasting - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a filter circuit in which a single common filter serves as a low-pass filter used in each circuit in a receiver for both television audio multiplex broadcasting and FM stereo broadcasting.

Conventionally, tuners for receiving TV audio multiplex broadcasts and tuners for receiving FM stereo broadcasts have been manufactured and used independently, and their respective audio output circuits receive unnecessary pilot signals, residual subcarriers, and high-frequency signals. To remove unwanted noise bones, low-pass filters with two attenuation poles each were used.

In other words, for TV audio multiplex broadcasting, the synchronization signal remains at 15.7
5 KHz and 31.5 KHz of the subcarrier must be particularly attenuated and removed, and for FM stereo broadcasting, the remaining portions of 19 KHz as a pilot signal and 38 KHz as a subcarrier must be attenuated.

However, recent cassette tape recorders with radios can receive TV audio multiplex broadcasts and FM stereo broadcasts.
Recordable devices began to be produced, but because the broadcast systems for both broadcasts were different, it was necessary to use separate low-pass filters for each audio output.

The present invention provides a plan to share a low-pass filter for blocking unnecessary signals and unnecessary noise used in a receiver for both television audio multiplex broadcasting and FM stereo broadcasting.

This will be explained with reference to the drawings.

FIG. 1 shows a circuit diagram of an embodiment of the present invention, and FIG. 2 shows its attenuation characteristics.

In the figure, reference numeral 1 denotes a changeover switch for switching input between a television audio signal and an FM stereo signal.

2 is a first filter circuit, and 3 is a second filter circuit.

The first filter circuit 2 includes an inductance element L1 having a tap terminal T1 to which a signal input is connected, and one end S of the inductance element L1.
Two capacitors C1 and C2 are connected in series and grounded.

The others y are connected to the next second filter circuit 3.

The collector of the switching transistor TR1 is connected to the intermediate connection point between the capacitors C1 and 02, while the emitter is grounded, and the switching control signal input is connected to the base via a resistor R□.

Next, the second filter circuit 3 includes an inductance element L2 having a tap terminal T2 and three capacitors C3 and C.
4, C6.

A signal from the first filter circuit 2 is input to the tap terminal T2, and two capacitors C3 and C4 are connected in series to one end S2 of the inductance element L2 and grounded.

The collector of the second switching transistor TR2 is connected to the intermediate connection point, and its emitter is grounded.

A switching control signal is supplied to this base via a resistor R2.

On the other hand, the other end F2 of the inductance element L2 is an output terminal and is grounded through a capacitor C5.

The present invention is constructed in this way. First, when a positive control signal reaches the switching transistors TR1 and TR2, the respective collectors and emitters are turned on, and the capacitors C2 and C2 are short-circuited. .5K) will result in an attenuation pole of Iz.

Next, when the above positive control signal disappears, the switching transistor TR□.

TR2 is in a cut-off state, and the series-connected capacitive elements C5 and C
2, C3, and C2 become their respective combined capacitances, and the capacitance becomes small, and the frequencies of the first and second attenuation poles become 19 KHz and 38 KHz.

Actual examples are C engineering = 1, 800PF, C2 =
3.9oopF', C3=3t900PF,
If the inductance elements L□, L, and 2 are appropriately designed using C4=8.200PF, the attenuation of the first attenuation pole will be approximately 40db, and the attenuation of the second attenuation pole will be approximately 65db to 70d.
b can be obtained.

FIG. 2 shows these frequency versus attenuation characteristics, where the solid line is the characteristic curve for audio multiplexing, and the dotted line is the characteristic curve for FM stereo.

As described above, according to the present invention, the frequencies of the first and second attenuation poles can be easily switched by controlling the switching transistor by sharing the same filter circuit, and the receiver can be used for both TV audio multiplex broadcasting and FM stereo broadcasting. It can be used for both purposes, saving space and reducing costs, which has great effects.

[Brief explanation of drawings]

FIG. 1 shows a circuit diagram of an embodiment of the present invention, and FIG. 2 shows its attenuation characteristics. 1... Selector switch, 2... First filter circuit, 3... Second filter circuit,
h...First inductance element, L2...
...Second inductance element, TR1, TR2...
...Switching transistor, C1, C2, C
3, C4, C5... Capacitor.

Claims (1)

[Scope of utility model registration request] In a low-pass filter having first and second attenuation poles, two capacitors are connected in series and grounded as respective capacitance elements for obtaining the first and second attenuation poles, and their respective intermediate connections are connected in series. A switching transistor is connected between the point and the ground, and by controlling the base voltage of the switching transistor, the grounding ON/OFF control of the intermediate connection point is performed, thereby setting the first attenuation pole frequency to 15.75 KHz and 19 KHz.
z and select the second attenuation pole frequency to 31.5KH.
38K Hz and 38K Hz, and is commonly used for receiving television audio multiplex broadcasts and FM stereo broadcasts.