JPS63174436A - Am stereo receiver - Google Patents

Abstract

PURPOSE:To improve fidelity and stability, by automatically switching the band area width of an intermediate frequency stage at the time of receiving a stereo broadcast and a monophonic broadcast. CONSTITUTION:A high frequency signal received at an antenna 1 is tune- amplified by a high frequency amplifier 2, and is converted to an intermediate frequency signal at a mixing circuit 3. The intermediate frequency signal is supplied to a broad-band filter 5 and a narrow-band filter 6. At the time of receiving the stereo broadcast, an output from the broad-band filter 5 is supplied to an AM stereo demodulation circuit 11, and at the time of receiving the monophonic broadcast, the output from the narrow-band filter 6 is supplied to the AM stereo demodulation circuit 11. An AND gate 12 outputs a signal of high level when the level of a received input signal exceeds a reference level, and also when a stereo pilot signal exists in the received input signal, and automatically switches a common contact (c) to a fixed contact (a) on the broad-band filter 5 side by controlling a switching circuit 7.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an AM (amplitude modulation) stereo receiver that automatically switches the passband width of an intermediate frequency between a wide band and a narrow band, and particularly relates to an AM (amplitude modulation) stereo receiver for detecting a stereo pilot signal. The present invention relates to an AM stereo receiver in which the bandwidth is switched by using the AM stereo receiver.

Conventional technology-i When receiving stereo broadcasts with an AM stereo receiver, in order to improve the sound quality, the passband width of an intermediate frequency filter (IF filter) provided in the intermediate frequency stage is determined based on the fidelity and stability (frequency shift). It is necessary to provide a wide band so as to sufficiently satisfy the margin for

However, in AM broadcasting, the channel separation is 9 KHz in Japan, so if the passband width of the IF filter is wideband (for example, 9 KHz), it is easily affected by adjacent interference caused by radio waves of adjacent channels, and noise due to this effect The problem arises that it is very difficult to listen to the sound because it appears as it is as the speaker output. Furthermore, such adjacent interference is likely to occur when the receiving input level of the receiving broadcast station becomes low.

For this reason, the conventional IF filter has a wide band (for example, 9 KH
z) and narrowband (e.g. 5KHz), and manually switch the switch when adjacent interference occurs on an adjacent channel, set the IF filter to narrowband, remove adjacent interference waves, and pass only the desired wave. There was one designed to suppress noise.

Problems to be Solved by the Invention However, in this conventional configuration, the switch was changed manually, which made the switch operation extremely troublesome.In particular, it was not safe for the driver to operate the switch in a car-mounted receiver. It was also unfavorable from a driving standpoint.

The present invention has been made in view of the above-mentioned conventional problems, and its main purpose is to provide an AM stereo receiver that automatically switches the passband width of an intermediate frequency between a wide band and a narrow band. The objective of the present invention is to provide an AM stereo receiver that automatically switches to a wide band when receiving stereo broadcasts where sound quality is a problem, and to a narrow band when receiving monaural broadcasts where sound quality is not a problem.

Another object of the present invention is to provide a stereo broadcasting system in which the level of the received input signal is high to some extent and from the viewpoint that adjacent interference is likely to occur and the sound quality is also deteriorated when the level of the received input signal decreases. The purpose is to automatically switch the passband width of the intermediate frequency to a wide band only during reception, and also automatically switch to stereo reproduction.

Means for Solving the Problems In order to solve the above problems and achieve one of the above objects, the present invention provides a pilot signal detection circuit for detecting a stereo pilot signal included in a received input signal, and the detection circuit. switching means for switching the bandwidth of the intermediate frequency stage according to the output signal of the stereo pilot signal, and when the stereo pilot signal is detected by the pilot signal detection circuit, controlling the switching means to widen the bandwidth of the intermediate frequency stage; This AM stereo receiver is characterized in that a narrow band is used when a stereo pilot signal is not detected.

Further, in order to solve the above problem and achieve the other objects mentioned above, the present invention has a level detection circuit that detects the level of a received input signal, and a stereo pilot signal included in the received human input signal. A pilot signal detection circuit, a switching means for switching the bandwidth of the intermediate frequency stage according to the output signals of the level detection circuit and the pilot signal detection circuit, and a monaural/stereo switching means for switching between stereo reproduction and monaural reproduction, When the level of the received input signal is above a predetermined value and a stereo pilot signal is detected by the pilot signal detection circuit, the switching means is controlled to widen the bandwidth of the intermediate frequency stage, and the monaural/ A characterized in that the stereo switching circuit is controlled to switch to stereo playback.
It is an M stereo receiver.

According to the present invention, a stereo pilot signal is detected, and the bandwidth of the intermediate frequency stage is changed to a wide band when a stereo pilot signal is present, that is, when receiving stereo broadcasting, and to a wide band when a stereo pilot signal is not present, that is, when receiving a stereo broadcast. Since each broadcast is automatically switched to a narrow band when receiving broadcasts,
It becomes possible to automatically switch the bandwidth of the intermediate frequency stage according to practical use.

Furthermore, in other configurations, the received input signal level is detected in addition to the stereo pilot signal, and the bandwidth of the intermediate frequency stage is widened only when the received input signal level is above a predetermined value and the stereo pilot signal is present. Since the switching is performed automatically and the reproduction is automatically switched to stereo playback, it is possible to automatically switch the bandwidth of the intermediate frequency stage and automatically switch the monaural/stereo switching circuit, which is extremely effective in practice.

Embodiment of the Invention Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

The figure is a block diagram of an AM stereo receiver according to the present invention, for example, the Motorola system (C-QUAM system).
, that is, the formula (1+L 1R) C05(ωct+φ
) is applied to an AM stereo receiver that can receive signals in the form shown in FIG.

In the figure, 1 is an antenna, 2 is a high frequency amplification circuit (RF
Amp), 3 is a mixing circuit, 4 is a local oscillator, 5 and 6 are a broadband filter and a narrowband filter, respectively, and 7 is a switching circuit, which has fixed contacts a, b and a common contact C.

8 is an intermediate frequency amplifier circuit (IF Amp), 9 is a smoothing circuit that smoothes the intermediate frequency signal, 10 is a comparator that compares the output signal level of the smoothing circuit 9 with a reference value ■, and 11 is an AM stereo demodulation circuit. This is a well-known Motorola AM stereo demodulation circuit. 12 is an AND gate, 13 is an amplifier, and 14 is a speaker.

Next, the operation will be explained.

A high frequency signal received by an antenna 1 is tuned and amplified by a high frequency amplifier 2, mixed with a local oscillation signal from a local oscillator 4 in a mixing circuit 3, and converted into an intermediate frequency signal. This intermediate frequency signal is supplied to a wideband filter 5 and a narrowband filter 6, the output signal from the wideband filter 5 is supplied to a fixed contact a of a switching circuit 7, and the output signal from the narrowband filter 6 is supplied to a fixed contact a of a switching circuit 7. Each is supplied separately. The output signal of the wideband filter 5 or the narrowband filter 6 appearing at the common contact C of the switch circuit 7 is supplied to the AM stereo demodulation circuit 11 via the intermediate frequency amplification circuit 8.

Further, a part of the output signal of the intermediate frequency amplification circuit 8 is supplied to a smoothing circuit 9, smoothed, and then supplied to a non-inverting input terminal of a comparator 10.

The comparator 19 compares the level of this intermediate frequency signal with the reference level ■
, and when the level of the intermediate frequency signal is higher, a high level signal is supplied to one input terminal of the AND gate 12.

The intermediate frequency signal supplied to the AM stereo demodulation circuit 11 is converted to left (L) and right by an envelope line detector 21 as described in, for example, Japanese Patent Publication No. 56-8528 as a Motorola type AM stereo demodulation. A sum signal (L+R) of the (R) information signals is extracted, and the output signal of the envelope detector 21 is supplied to a stereo decoder circuit 22 comprised of a matrix circuit. Further, a part of the intermediate frequency signal is supplied to the synchronous detector 23, and a signal expressed by the formula (LR+5P)CO3φ is extracted. Further, the intermediate frequency signal is also supplied to the limiter circuit 24, and the phase modulation component CO3 is
(ωct+φ) is extracted, and the output signal of the limiter circuit 24 is supplied to a phase locked loop (PLL) circuit 25 and a cosine phase detector 26. Output signal CO of PLL circuit 25
Sωct is supplied to a synchronous detector 23 and a cosine phase detector 26. The output signal COS φ of the cosine phase detector 26 is supplied to a divider 27, which divides the output signal (L-!-R+5P) CO3φ of the synchronous detector 23 by this signal COS φ. Then, the output signal of the divider 27 (LR+SP)
is supplied to the stereo decoder circuit 22 via the monaural/stereo changeover switch 28, and the output signal of the stereo decoder circuit 22 is further supplied to the speaker 1 via the amplifier 13.
It is made into sound by 4. Furthermore, the output signal of the divider 27 (
L−R+SP) is also supplied to the bandpass filter 29,
Here, the stereo pilot) (sP (frequency 25Hz in the Motorola system) is taken out. The output signal of the bandpass filter 29 is supplied to the other output terminal of the AND gate 12, and the output signal of the AND gate 12 is sent to the switching circuit. 7 and a monaural/stereo changeover switch 28 as a changeover control signal.

Therefore, the AND gate 12 determines that the level of the intermediate frequency signal, that is, the level of the received input signal, is equal to or higher than the reference level.
In addition, when the stereo pilot signal SP is present in the received input signal, it outputs a high-level signal, controls the switching circuit 7, automatically switches the common contact C to the fixed contact a on the broadband filter 5 side, and switches the monaural/stereo The changeover switch 28 is controlled to automatically switch to stereo playback, and at other times, the common contact C of the changeover circuit 7 is automatically changed to the fixed contact of the six narrowband filters, and the monaural/stereo changeover switch 28 is switched to monaural playback. Automatically switches to playback.

According to this configuration, stereo broadcasting is performed only when the level of the received input signal is equal to or higher than a predetermined level and the received input signal includes the stereo pilot signal SP, that is, when reproduced sound with good sound quality can be obtained. Since the intermediate frequency passband width is wideband and stereo is reproduced only during reception,
A very practical intermediate frequency stage bandwidth and automatic monaural/stereo switching becomes possible.

Furthermore, in this embodiment, a configuration has been described in which the bandwidth of the intermediate frequency stage is switched by detecting both the level of the stereo pilot signal SP and the reception input signal, but the switching circuit 7 is switched by detecting only the stereo pilot signal SP. control,
The bandwidth of the intermediate frequency stage may be automatically switched to a wide band only when the received input signal includes the stereo pilot signal SP. Even with this configuration, when the stereo pilot signal SP exists, that is, when the stereo pilot signal SP exists,

During REO broadcast reception, the bandwidth of the intermediate frequency stage is wideband and the sound quality of the reproduced sound is good.Furthermore, when the stereo pilot signal SP is not present, that is, during monaural reception, the sound quality is not so much of a problem. Adjacent interference can be prevented by narrowing the bandwidth of the intermediate frequency stage, and it becomes possible to automatically switch the bandwidth of the intermediate frequency stage in accordance with practical use.

Note that the present invention is not limited to this embodiment. For example, the wideband filter 5 and the narrowband filter 6 may be configured as one filter, and the filter characteristics may be varied by changing the filter constants. The switching means in this case can also be understood as means for switching the filter constant. Such a configuration is called DSP (Digital
This can be easily realized by applying the Signal Processor (Signal Processor) technology.

Effects of the Invention As explained in detail above, according to the present invention, the bandwidth of the intermediate frequency stage can be widened when receiving stereo broadcasting, and
Since each channel is automatically switched to a narrow band when receiving monaural broadcasting, it is possible to automatically switch the bandwidth of the intermediate frequency stage according to practical use.

Further, according to another configuration, stereo broadcasting is performed only when the level of the received input signal is equal to or higher than a predetermined level and the received input signal includes the stereo pilot signal SP, that is, when good sound quality can be obtained. Since stereo reproduction is performed with the intermediate frequency pass band width set to a wide band only during reception, it is possible to automatically switch the bandwidth of the intermediate frequency stage and monaural/stereo, which is extremely practical.

[Brief explanation of the drawing]

The figure is a block diagram showing an embodiment of the present invention. In the figure, 1 is an antenna, 2 is a high frequency amplifier circuit, 3 is a mixing circuit, 4 is a local oscillator, 5 is a wideband filter, 6 is a narrowband filter, 7 is a switching circuit, 8 is an intermediate frequency amplifier circuit, 9 is a smoothing circuit , 10 is a comparator, 11 is an AM stereo demodulation circuit, 12 is an AND gate, 13 is an amplifier, and 14 is a speaker.

Claims (2)

[Claims] (1) A pilot signal detection circuit that detects a stereo pilot signal included in a received input signal, and a switching means that switches the bandwidth of an intermediate frequency stage based on an output signal of the detection circuit, wherein the pilot signal detection circuit detects a stereo pilot signal. An AM stereo receiver characterized in that the switching means is controlled to set the bandwidth of the intermediate frequency stage to a wide band when a signal is detected, and to set a narrow band when a stereo pilot signal is not detected. (2) A level detection circuit that detects the level of a received input signal, a pilot signal detection circuit that detects a stereo pilot signal included in the received input signal, and an intermediate frequency a switching means for switching the bandwidth of a wave stage; and a monaural/stereo switching means for switching between stereo reproduction and monaural reproduction; AM stereo reception, characterized in that when the switching means is detected, the bandwidth of the intermediate frequency stage is widened, and the monaural/stereo switching means is controlled to switch to stereo reproduction. Machine.