JPH0426572B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an AM receiver, and in particular, to an AM receiver that allows stereophonic listening in strong electric fields and improves the S/N ratio in weak electric fields. Regarding machines.

<Prior Art> Typically, in an AM receiver, a monaural signal, which is the output of an AM tuner, is input to speakers of both left and right channels to output audio. However, with this method, it is not possible to listen with a stereophonic sound with a spacious sound. Therefore, a conventional method called AM stereo has been adopted to enable AM listening with a stereo feel.

<Problem that the invention is trying to solve> However, the S/N ratio of stereo is about 20 dB worse than that of monaural, so there is a problem that as the received field strength becomes weaker and the noise level increases, good listening cannot be achieved. be. Furthermore, even in the case of a strong electric field, there is a problem in that good listening cannot be achieved if interference signals are generated from adjacent stations.

In light of the above, an object of the present invention is to provide an AM receiver that allows listening with a stereophonic effect in a strong electric field and that can improve the S/N ratio in a weak electric field.

Another object of the present invention is to provide an AM receiver that allows good AM broadcast listening even when interference signals are generated from adjacent stations.

<Means for Solving the Problems> FIG. 1 is a block diagram of an FM stereo receiver according to the present invention.

11 is an antenna, 12 is an AM tuner, 13 is a pseudo-stereo circuit, 14 is a switching circuit, 15 is a level detection circuit for detecting the received electric field strength, and 16 is a time constant circuit.

<Operation> The pseudo-stereo circuit 13 receives the monaural signal S _M which is the output of the AM tuner 12 and outputs the L and R signals.
Pseudo-stereo signals S _L ′ and S _R ′ of both channels are output.

The switching circuit 14 switches the pseudo stereo signal S _L ′, when the received electric field strength detected by the level detection circuit 15 is equal to or higher than a predetermined threshold level E _S .
S _R ′ is output, and a monaural signal S _M is output when the received electric field strength is less than a predetermined threshold level E _S .

Note that the received electric field strength is at the threshold level E _S
Even if the above is the case, if a monaural signal S _M is output when an adjacent station interference signal is generated, the current S/M
The N ratio can be improved, making listening easier.

<Embodiment> FIG. 1 is a block diagram of an AM receiver according to the present invention.

In the figure, 11 is an antenna, 12 is an AM tuner,
13 is a pseudo stereo circuit, 14 is a switching circuit, 15
is a level detection circuit for detecting the received electric field strength, 16
is a time constant circuit.

The pseudo-stereo circuit 13, as shown in FIG.
An amplifier 13a, a comb filter 13b having the frequency characteristics shown in FIG. 3, and a differential circuit 13 that outputs the difference between the comb filter output and the amplifier output.
c, and the output of the comb-shaped filter 13b is L.
- Channel pseudo signal S _L ′, differential circuit 13
The output of signal c is used as the R-channel pseudo signal S _R ', and a stereo effect can be obtained compared to monaural.

The level detection circuit (level meter) 15 is
It is connected to an intermediate frequency amplifier (not shown) in the tuner 12, and outputs a level meter signal LMO having a value corresponding to the received electric field strength (antenna input level) E as shown in FIG. In FIG. 4, SG and NZ are the response characteristics of the signal and noise with respect to the received electric field strength E, respectively.

The time constant circuit 16 maintains the level meter signal LMO, which is the output of the level detection circuit 15, for a predetermined time period T1 _or more to a predetermined threshold level L _S (corresponding to the received electric field strength E _S , see FIG. 4). When the level is above or below, set the level of switching signal SW1 to "1" →
Invert from "0" or "0" to "1". Note that the predetermined time T ₁ is determined by the time constant of the time constant circuit 16.

Now, when the received electric field strength E is a strong electric field higher than the threshold level E _S (approximately 30 dBμ), the switching signal SW
Since 1 is at high level, the switching circuit 14
outputs pseudo-stereo signals S _L ′ and S _R ′. As a result, AM with a stereo feel (sound spread)
You can listen to broadcasts.

On the other hand, the received electric field strength E continues for a predetermined period of time or more.
When the electric field becomes lower than E _S (level L _M ≦L _S of the level meter signal LMO), that is, when the weak electric field state continues for a predetermined time T ₁ or more, the switching signal SW1, which is the output of the time constant circuit 16, is inverted to a low level. Switching signal SW
1 goes to low level, the switching circuit 14 becomes monaural.
Output _SM . As a result, even if the electric field becomes weak and the noise level increases, the S/N ratio improves, so that better listening can be achieved than in the case of pseudo stereo.

Furthermore, when the received electric field strength exceeds E _S in the monaural state, the first switching signal SW1 becomes high level and pseudo stereo signals S _L ′, S _R ′ are output.

By the way, above we have explained the case where the pseudo stereo state or monaural state is set as appropriate based only on the received electric field strength, but it is also possible to switch between the pseudo stereo state and the monaural state by taking into account the presence or absence of interference signals from adjacent stations. It can also be configured as

FIG. 5 is a block diagram of the AM receiver according to the present invention when interference signals from adjacent stations are taken into account;
The figure is a block diagram of an adjacent interfering station detection circuit.

5 differs from FIG. 1 in that (i) an adjacent interfering station detection circuit 21 and a time constant circuit 22 are provided, and (ii) the second switching signal SW2 is transmitted from the time constant circuit 22 to the switching circuit. (iii) The switching circuit 22 outputs the first and second switching signals SW.
1. A pseudo stereo signal or a monaural signal is output as appropriate based on SW2.

As shown in FIG. 6, the adjacent interfering station detection circuit 21 includes an upper adjacent station signal detection circuit 21a, a lower adjacent station signal detection circuit 21b, and detectors 21c and 21d.
and an OR circuit 21e. In FIG. 6, 12a is an AM high frequency circuit composed of a high frequency amplifier, a local oscillator, a mixing circuit, etc., and 12b is an intermediate frequency amplifier.

If the frequency of each station is at 10KHz intervals, AM
The mixed circuit output MS of the high frequency circuit 12a includes a 455KHz intermediate frequency component corresponding to the desired station, a 445KHz frequency component corresponding to the upper adjacent station, and a 465KHz frequency component corresponding to the lower adjacent station. .

Therefore, the mixing circuit output MS is input to the upper and lower adjacent station signal detection circuits 21a and 21b, respectively, and the 445 KHz component and 465 KHz component are extracted and input to the next-stage detectors 21c and 21d. Each detector 21
c and 21d convert the input signals into direct current and input them to the OR circuit 21e composed of diodes D1 and D2, and the OR circuit 21e converts the detected signal of the larger of the input signals into an adjacent interfering station signal.
Output as NOB.

Now, in FIG. 5, the first switching signal SW1 is at a high level when the received electric field strength E is a strong electric field equal to or higher than the threshold level _ES . or,
When there is no adjacent interfering station signal NOB (when the adjacent interfering station signal is below a predetermined level), the second switching signal
SW2 is at a high level. Therefore, in this case, the switching circuit 14 outputs the pseudo stereo signals S _L ′ and S _R ′ input from the pseudo stereo circuit 13 .

However, even when there is a strong electric field, the adjacent interfering station signal cannot be detected.
When NOB occurs and its level exceeds a predetermined value, a second switching signal is output from the time constant circuit 22.
SW2 immediately becomes low level. As a result, the switching circuit 14 outputs the monaural signal S _M that is the output of the AM tuner 12. Note that the reason for outputting a monaural signal is that the S/N ratio deteriorates due to the generation of an adjacent interfering station signal (noise) NOB, so outputting a monaural signal improves the S/N ratio.

After that, if the adjacent interfering station signal NOB continues for a predetermined period of time and falls below a predetermined level, the second switching signal is activated.
Since SW2 becomes high level, the switching circuit 14 outputs the pseudo stereo signals S _L ′ and S _R ′ again.

On the other hand, the received electric field strength E continues for a predetermined period of time or more.
When it becomes less than E _S , that is, the weak electric field state is
If it continues for _T1 or more, the first switching signal SW1, which is the output of the time constant circuit 16, is inverted to low level. When the first switching signal SW1 becomes low level, the switching circuit 14 outputs the monaural signal S _M regardless of the second switching signal SW2. As a result, even if the electric field becomes weak and the noise level increases, the S/N ratio is improved and the broadcast becomes easier to listen to.

<Effects of the Invention> According to the present invention, since the pseudo-stereo state is changed to a monaural state, or the monaural state is changed to a pseudo-stereo state, as appropriate depending on the received electric field strength, it is possible to listen to AM broadcasting with a stereo feeling even in a strong electric field. Moreover, when the electric field is weak, the S/N ratio can be improved and good listening can be achieved.

Furthermore, according to the present invention, when an interfering signal from an adjacent station is generated, the pseudo stereo is switched to monaural, so even if an adjacent interfering station signal is generated, AM broadcasting can be listened to in good quality.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the first AM receiver according to the present invention, FIG. 2 is a block diagram of a pseudo-stereo circuit, FIG. 3 is a frequency characteristic diagram of a comb-shaped filter in the pseudo-stereo circuit, and FIG. FIG. 5 is a block diagram of a second AM receiver according to the present invention, and FIG. 6 is a block diagram of an adjacent interfering station detection circuit. 11...Antenna, 12...AM tuner, 13...
Pseudo stereo circuit, 14... Switching circuit, 15... Level detection circuit, 16... Time constant circuit, 21... Adjacent interfering station detection circuit.

Claims (1)

[Claims] 1. An AM tuner, a level detection circuit for detecting received electric field strength,
A monaural signal S _M, which is the output of an AM tuner, is input, and a pseudo stereo signal S _L ′ of both L and R channels is generated.
A pseudo-stereo circuit that outputs S _R ′ and a received electric field strength at a predetermined threshold level.
AM comprising a switching circuit that outputs pseudo-stereo signals S _L ′ and S _R ′ when the threshold level is higher than E _S and outputs a monaural signal S _M when the threshold level is lower than the threshold level.
Receiving machine. 2 An AM tuner, a level detection circuit that detects the received field strength, an adjacent jamming station detection circuit that detects jamming signals from adjacent stations, and a monaural signal S _M , which is the output of the AM tuner, input to both the L and R channels. pseudo stereo signal
A pseudo-stereo circuit that outputs S _L ′ and S _R ′, and a received electric field strength at a predetermined threshold level.
If the received field strength is above a predetermined threshold level, the pseudo stereo signals S _L ′ and S _{R ′} are output when the signal from the adjacent interfering station is not generated. An AM receiver comprising a switching circuit that outputs a monaural signal _SM when a signal is generated and when the received electric field strength is below the threshold level.