JPS62115934A - Band switching device for an stereo receiver - Google Patents

Abstract

PURPOSE:To improve an option characteristic by making the function of the 2nd filter inoperable only when an Am stereo demodulator circuit selects a stereo state and receives a stereo signal. CONSTITUTION:An iF filter 6 is constituted of a serial connection between the 1st filter 15 composed of a wide band and the 2nd filter 16 composed of a narrow band. If a mode changeover switch 9 is turned ON, an AM stereo demodulator circuit 8 takes a monaural state, envelope-detects an inputted iF signal, demodulates it in a monaural state, and supplies a signal L+R to subsequent stages, whereby the iF signal passes through the 2nd filter 16 with a narrow band. On the other hand, if the mode changeover switch 9 is turned OFF, the AM stereo demodulator circuit 8 attains a stereo state, and the envelope and phase of the inputted iF signal are detected. Thus in a stereo reception state, the high frequency of an audio output is expanded to enjoy music with affluent stereo feelings, while in a monaural reception state an audio output can be received at an S/N similar to a conventional monaural reception.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a band switching device for an AM stereo receiver capable of receiving signals in both a monaural state and a stereo state.

(Prior art) Recently, AM stereo broadcasting has been started in some countries including the United States, and based on this AM stereo broadcasting has been started, it is possible to receive AM stereo signals.
A stereo receiver has been developed.

This type of conventional AM stereo receiver transmits an AM stereo signal received by an antenna to an intermediate frequency (iF
) is converted into a signal and then supplied to an AM stereo demodulation circuit via an iF filter and an iF amplifier, where it is controlled to be demodulated in a mode specified by a mode changeover switch, that is, in a monaural state or in a stereo state.

(Problem to be Solved by the Invention) However, in this type of AM stereo receiver, if a narrow band pass filter is used for the iF filter considering only selectivity, the frequency characteristics will be equal to the selectivity characteristics of this iF filter. As a result, the level of the high frequency range is significantly lowered, and as a result, even though the L channel and R channel can be sufficiently separated by demodulating the stereo condition, there is a problem that a stereo feeling cannot be obtained. On the other hand, if a wide band pass filter is used instead of the narrow band pass filter with emphasis on the stereo effect, the selectivity characteristics will deteriorate and the problem of interference will occur.

(Means for Solving the Problems) The present invention provides an iF
A double signal is converted and this iF double signal is passed through an iF filter.
In an AM stereo receiver that is supplied to an M stereo demodulation circuit and demodulated to either a monaural or stereo state, the iF filter is connected in series with a first filter having a wide band and a second filter having a narrow band. The function of the second filter can be controlled by a switching element to be enabled or disabled, and the function of the second filter is disabled only when the AM stereo demodulation circuit selects a stereo state and receives a stereo signal. This is what is done.

(Function) In the stereo reception state, the filter function of the second filter is disabled and the pass band of the iF frequency is widened, thereby extending the high range of the audio output and producing a sound full of stereo feeling. To enable the filter function of a filter, cut the high frequency range of audio output, and make the S/N similar to that during normal monaural reception.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 2 is a block diagram showing the overall configuration of an AM stereo receiver according to the present invention.

In FIG. 2, an AM stereo signal received by an antenna 1 is transmitted through a radio frequency amplification (RF) stage 3, a mixing circuit 4,
The front end section 2 consisting of a local oscillator circuit 5 converts the signal by iF, and the A
The signal is input to the M stereo demodulation circuit 8. The L channel and R channel signals output from the AM stereo demodulation circuit 8 are adjusted by the volume and balance adjustment circuit 12 via the corresponding preamplifier IOL, IOR, )-on control circuits 11L and IIR, and then adjusted by the amplifier 13. The power of the signal is amplified and outputted from the respective speakers 14L and 14R. The mode changeover switch 9 is a switch for switching between a stereo reception state and a monaural reception state, and when turned on, the monaural reception state is instructed to the AM stereo demodulation circuit 8.

FIG. 1 shows the iF filter 6, the iF amplifier 7 and the AM
A specific circuit configuration of the stereo demodulation circuit 8 is shown.

The iF filter 6 is configured by a first filter 15 having a wide band and a second filter 16 having a narrow band connected in series, and further includes a band switching switching diode D1 that shorts both ends of the second filter 16 in an alternating current manner. is connected. The iF amplifier 7 is a transistor Q2
and a resistor that determines the bias of this transistor Q2.

The AM stereo demodulation circuit 8 is composed of one integrated circuit (for example, M
C1320P type), and the terminal (2) is an oven collector and outputs a different level depending on whether the receiving state is monaural or stereo, and is switched to the "L" level when in the stereo receiving state. Furthermore, when the terminal (2) is at the rLJ level, the monaural state is forced. Terminals (2) and (2) are terminals connected to the preamplifiers IOL and IOH, and terminal (2) is a power supply terminal. An LED constituting an AM stereo indicator and a current limiting resistor R2 that limits the current flowing through the LED are connected between the terminals ■ and ■. Further, a band switching transistor Q1 is connected to the terminal (2), which makes the band switching switching diode D1 conductive when the output of the terminal [phase] is at the rLJ level, that is, in a stereo reception state. Resistor R3 is for limiting the base current of this transistor Q+. The mode changeover switch 9 is provided at terminal (3), and when this mode changeover switch 9 is turned on, the AM
The stereo demodulation circuit 8 takes a monaural state, performs envelope detection of the input iF times signal, demodulates it in a monaural state, and supplies L+R signals to the next stage and subsequent stages. Then, at this time, the terminal ■ becomes the "H" level, and the LED constituting the stereo indicator goes out. In addition, the transistor Q,
is also turned off, and no current flows through the switching diode D1, resulting in a cant-off state. Therefore, since the signal passes through the second filter 16 with the iF times narrow band, a signal with a good S/N ratio and less interference is supplied to the iF amplifier 7 and the subsequent ones.

On the other hand, when the mode selector switch 9 is turned off,
The AM stereo demodulation circuit 8 is in a stereo state,
Performs envelope detection and phase detection of the input iF multiplied signal. However, when the signal is not a stereo signal, that is, when there is no pilot signal indicating that it is a stereo signal, the terminal [phase] becomes "H" level, and only envelope detection (L+R) is output. Further, in this case, since the terminal (2) is at the "H" level, the LED is off and the transistor Q is also off, resulting in an iF times signal narrow band pass type signal. However, in the case of stereo signals, envelope detection (L+R) and phase detection (L
-R), the terminal [phase] goes to the "L" level, and in the AM stereo demodulation circuit 8, L and R are separated by making use of a matrix circuit (not shown). Furthermore, since the terminal @ is at the "L" level at this time, the LED is lit to indicate that the stereo signal is being received. At the same time, the transistor Q is also turned on, so the switching diode D is connected through the inductance L for cutting the iF signal.
I, current flows through resistor R+, and switching diode D
I becomes conductive. Therefore, the narrow band second filter 16
is short-circuited in terms of alternating current, so the iF characteristic has a band characteristic defined by the first filter 15, which is a wide band filter, and frequency identification extends to a high frequency range.

(Effects of the Invention) As described above, according to the present invention, it is possible to extend the high range of the audio output in a stereo reception state and enjoy sound with a full stereo feel, and in a monaural reception state, It can be received with the same S/N ratio as monaural reception.

[Brief explanation of drawings]

FIG. 1 is an electrical circuit diagram showing an embodiment of a band switching device for an AM stereo receiver according to the present invention, and FIG. 2 is a block diagram illustrating the overall configuration of the AM stereo receiver. 2...Front end section 6...iF filter 8...AM stereo demodulation circuit

Claims (1)

[Claims] 1) Convert the AM stereo signal received by the antenna into an iF signal, supply this iF signal to an AM stereo demodulation circuit via an iF filter, and convert it into either monaural or stereo state. In the AM stereo receiver that demodulates the iF filter, the iF filter is configured by a series connection of a first filter having a wide band and a second filter having a narrow band, and the function of the second filter is enabled or disabled by a switching element. controllable, and said A
A band switching device for an AM stereo receiver, characterized in that an M stereo demodulation circuit selects a stereo state, and the function of the second filter is disabled only when receiving a stereo signal.