JPS6141321Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an AM stereo receiver, and particularly to an AM stereo receiver that takes countermeasures against interference such as noise into consideration.

Generally, in AM receivers, due to the nature of AM waves, there is a lot of interference and noise, so an intermediate frequency filter with a fairly narrow bandwidth is used, or even an audio amplifier cuts off the high frequency range to improve the S/N ratio. However, with this configuration, an intermediate frequency filter is always inserted regardless of the electric field strength, and high frequencies are cut off, resulting in poor sound quality even in strong electric field areas. There is.

Therefore, in order to eliminate such inconvenience, the receiver
A method of switching the frequency characteristics of the audible frequency and the band of the intermediate frequency filter using the AGC voltage, improving the frequency characteristics without removing high-frequency components in a strong electric field, and reducing noise by cutting off the high-frequency components in a weak electric field. has been proposed in the past, and FIG. 1 is an example of this.

That is, in FIG. 1, a broadcast radio wave received by an antenna 1 is mixed with a local oscillation signal from a local oscillation circuit 3 in a mixing circuit 2 and converted into an intermediate frequency signal, and this intermediate frequency signal is sent to an intermediate frequency amplification circuit 4. After being amplified, the signal is detected by a detection circuit 5, and an audio frequency signal is output from the detection circuit 5.

In the case of a weak electric field, this extracted audio frequency signal is filtered by a low-pass filter 6 to cut off high-frequency components, and then passed through a switch 7.
The signal passes through the contact a side of , and is output from the output terminal 8 to the audio frequency stage (not shown).

On the other hand, in the case of a strong electric field, the audio frequency signal obtained at the output side of the detection circuit 5 does not pass through the low-pass filter 6.
The signal is directly outputted from the output terminal 8 to the audio frequency stage through the contact b side of the switch 7.

The switch 7 is switched in accordance with the result of comparing the AGC voltage obtained from the detection circuit 5 with the reference voltage by the level detection circuit 9, and the S/N is
When the ratio deteriorates and the noise level (AGC voltage) exceeds the reference level, switch 7 is switched to contact a side, and low-pass filter 6 is inserted before the audio frequency stage. The AGC voltage is also supplied to the intermediate frequency amplification circuit 4, and the band of a built-in intermediate frequency filter (not shown) is switched so that it becomes wide when the electric field is strong and narrow when the electric field is weak.

By the way, when we consider the various situations in which AM receivers are used, we find that even in strong electric fields, they may be exposed to even stronger external noise, or that they may be subject to interference or interference, especially at night, even though the signal they are trying to receive is quite strong. Beat signal interference is rare. Therefore, in the case of an AM receiver configured as shown in FIG. 1, there was a drawback in that under such circumstances, it was not possible to cut off high frequency components and a good reception condition could not be maintained.

As an AM stereo receiver that solves this drawback, an AM stereo receiver as shown in FIG. 2 can be considered. Referring to FIG. 2, parts in FIG. 2 that correspond to those in FIG. 1 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

Generally speaking, when looking at the energy distribution of signals and noise, the signal has the characteristic that its level decreases as it reaches higher frequencies, as shown in Figure 4A, while noise has the characteristic of interference, as shown in Figure 4B. When there is beat disturbance, the level increases as shown by the broken line, but normally the level is approximately constant over the entire frequency range as shown by the diagonal line. Therefore, in this example, an amplitude limiter 10 is provided after the intermediate frequency amplifier circuit 4, and after the AM component is removed by the amplitude limiter 10, the phase detector 11 detects sideband waves containing noise components. This sideband wave is then supplied to a level detection circuit 13 through a bandpass filter 12 that has a predetermined bandwidth, for example, 100 Hz, and passes a predetermined frequency, for example, 8 kHz or higher.

This level detection circuit 13 connects the switch 7 to the contact b side when the output level of the bandpass filter 12 is less than the reference level, and connects the switch 7 to the contact a side when the output level of the bandpass filter 12 exceeds the reference level. It works like switching. The reference level of the level detection circuit 13 can be set arbitrarily; for example, the bandwidth of the receiver is set to 10kHz, the bandwidth of the bandpass filter 12 is set to 100kHz as described above, and the level detection circuit 1
3 to generate an output signal that switches the switch 7 from the contact b side to the a side with a signal 40 dB lower than the 0.3 radian phase modulation, the S/ of the signal appearing on the output side of the intermediate frequency amplifier circuit 4 will be substantially The N ratio
When the noise drops below 20 dB, a low pass filter 6 is inserted to remove such noise.

Therefore, the purpose of the present invention is to provide a differential signal detection circuit.
This makes it easy to detect noise components in an AM stereo receiver without increasing the circuit configuration.

FIG. 3 shows an embodiment of the AM stereo receiver of the present invention, and one embodiment of the AM stereo receiver of the present invention will be described below.

The AM stereo receiver shown in FIG. 3 is a receiver for receiving, for example, Magnabox type AM stereo broadcasting. In FIG. 3, parts corresponding to those in FIG. 2 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In this embodiment, an amplitude limiter 10 and a phase detector 1
1 is characterized in that it is not provided exclusively as shown in FIG. 2, but also serves as a device for extracting the difference signal (LR). That is, an amplitude limiter 10 and a phase detector 11 are provided after the intermediate frequency amplifier circuit 4. The phase detector 11 includes, for example, a multiplier 11a for phase comparison, a low-pass filter 11b, and a voltage-controlled oscillator 1.
A PLL detector consisting of 1c is used. Then, the signal from which the AM component has been removed by the amplitude limiter 10 is subjected to phase detection by the phase detector 11, and a difference signal (L-R) is extracted and supplied to the matrix circuit 14, where it is subjected to envelope detection by the detection circuit 5 and extracted. When the output level of the bandpass filter 12 is less than the reference level of the level detection circuit 13, the switch 7L and 7R
A main channel signal, that is, an L signal, and a sub-channel signal, that is, an R signal are outputted to output terminals 8L and 8R from each contact b side of .

The bandpass filter 12, the level detection circuit 13, the bandpass filter 16, and the level detection circuit 17 are provided to detect noise superimposed on the difference signal (LR) output from the phase detector 11, respectively. . That is, the filtering characteristics of the bandpass filter 12 are set so as to extract a frequency signal in which the ratio of the noise level to the signal level is high. For example, frequencies above 8kHz are passed, and the bandwidth is set to 100kHz. When the output level of the bandpass filter 12 is higher than the reference level of the level detection circuit 13, the switches 7L and 7R are both switched to the contact a side, and the L signal and R signal taken out from the matrix circuit 14 are passed through the low-pass filter, respectively. Noise components superimposed on the L and R signals are removed by 6L and 6R, and the signals are output to output terminals 8L and 8R via the contact a side of switch 7L and the contact a side of switch 7R, respectively.

The bandpass filter 16 is also provided to detect noise superimposed on the difference signal (L-R), and is particularly provided to detect noise based on beat components with adjacent stations. It is something. This is because noise based on so-called beat sounds generated by interference with broadcast waves of neighboring stations is noticeable even at low levels. For this reason, a bandpass filter 16 whose filtering characteristics are set is provided after the phase detector 11 in order to extract the beat component with the adjacent station.
The filtering characteristics of the bandpass filter 16 are set so as to pass frequencies of, for example, 9 kHz or higher, and the beat components are extracted. Also, the level detection circuit 17
The reference level is set to a level lower than the level set by the level detection circuit 13 described above. Switches 7L and 7R are also controlled by the output signal of this level detection circuit 17, and when there is beat interference from an adjacent station, low-pass filters 6L and 6R are inserted to adjust the beat frequency of, for example, about 9kHz. Try to block it. This allows the bandpass filter 1
In addition to the noise detected by the level detection circuit 2 and the level detection circuit 13, even if there is noise based on the beat sound, it can be effectively removed.

As described above, in the AM stereo receiver of the present invention, noise components are detected from the difference signal formed based on the intermediate frequency signal. Therefore, the noise component is superimposed on the difference signal component between the right channel signal and the left channel signal, and the ratio of the noise component to the signal component is the ratio of the noise component to the sum signal component of the right channel signal and left channel signal. This makes it easier to detect noise components.

Furthermore, in the present invention, since the difference signal detection circuit provided in the AM stereo receiver is also used to detect noise components, it is possible to minimize the increase in the number of circuits required for noise detection.

In each of the above-mentioned embodiments, the case where the output signal of the level detection circuit is used to change the switch to control the insertion and removal of the low-pass filter has been explained. It may be possible to switch.

Furthermore, the level detection circuits 13 and 17 may have hysteresis, or may be configured such that the high frequency range is continuously cut off.

Further, as the bandpass filter 12, a high-pass filter that filters frequencies higher than the corresponding frequency may be used.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of a conventional circuit, FIG. 2 is a block diagram showing an example of an AM stereo receiver,
FIG. 3 is a block diagram showing one embodiment of the present invention, and FIG. 4 is a diagram for explaining the operation of FIGS. 2 and 3. 4 is an intermediate frequency amplifier circuit, 6, 6L, 6R are low pass filters, 7, 7L, 7R are switches, 10 is an amplitude limiter, 11 is a phase detector, 12 and 16 are band filters, 13 and 17 is a level detection circuit.

Claims (1)

[Claims for Utility Model Registration] 1. A sum signal demodulation circuit for demodulating a sum signal from a signal output from an intermediate frequency amplification circuit, and a sum signal demodulation circuit for demodulating a difference signal from a signal output from the intermediate frequency amplification circuit. a difference signal demodulation circuit; a matrix circuit to which the signals output from the sum signal demodulation circuit and the difference signal demodulation circuit are supplied to form a right channel signal and a left channel signal; and a right channel output from the matrix circuit. A frequency band switching means for switching the frequency bands of the signal and the left channel signal, and a noise detection circuit for detecting a noise component included in the difference signal output from the difference signal demodulation circuit, the noise detection circuit comprising: By controlling the frequency band switching circuit in accordance with the output signal, if a noise component exceeding a predetermined amount is superimposed on the received signal, the right channel signal and the left channel signal output from the matrix circuit are An AM stereo receiver characterized by limiting the frequency band. 2. The AM stereo receiver according to claim 1, wherein the frequency band switching means switches the frequency band of an intermediate frequency filter provided in an intermediate frequency amplification circuit. 3. The AM according to claim 1 of the utility model registration claim, characterized in that the frequency band switching means switches the frequency band of a filter circuit provided for controlling each signal output from the matrix circuit. stereo receiver.