KR100478413B1 - Audio signal recording and reproducing apparatus - Google Patents

Abstract

A low pass filter for removing a predetermined frequency or higher frequency component of an audio signal having a wide bandwidth, an A / D conversion circuit for converting an audio signal from the low pass filter into a digital signal, and a digitalization from the A / D conversion circuit. Storage means for recording and storing a predetermined time interval of the audio signal, a D / A conversion circuit for converting the audio signal read out from the storage means into an analog signal, a tone control circuit for performing frequency correction on the audio signal; For the audio signal recorded in the storage means or the audio signal read out from the storage means, the low frequency component of the audio signal to be reproduced is attenuated correspondingly to the high frequency component of the audio signal removed by the low pass filter, thereby remarkably unnatural. A control circuit for controlling the tone control circuit so that an audio signal with no discomfort can be reproduced; The recording and reproducing apparatus of the audio signal including is provided.

Description

Audio signal recording and reproducing apparatus

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio signal recording and reproducing apparatus for recording an audio signal having a wide bandwidth such as FM (frequency modulated) radio broadcasting in a semiconductor memory by restricting its bandwidth.

Recently, although it is not necessary to record an audio signal of a long time from several tens of minutes to a cassette tape, for example, an apparatus using a semiconductor memory as a recording medium is provided when recording an audio signal of about a few minutes. have.

For example, in an answering machine telephone set, the time interval of the voice message to be recorded from the calling party is almost all shorter than a few tens of seconds. For this purpose, there has been provided an automatic answering machine using a semiconductor memory capable of recording a plurality of voice messages, each of several tens of seconds each, as a recording medium for voice messages.

When an audio signal of a relatively long time, for example, a few minutes is recorded in such a semiconductor memory, the audio signal is recorded after generally limiting the band to a narrow band, for example, a band up to 1.5 kHz. In this way, an audio signal of a relatively long time can be recorded in a semiconductor memory having a limited memory capacity.

According to the above-mentioned answering machine, the frequency characteristic of the telephone set itself is provided with a characteristic of a narrow band of several 100 to 3 kHz as shown in Fig. 6A.

Therefore, in the case of an answering machine telephone set using the above-described semiconductor memory as a recording medium, for example, an audio signal such as a band-limited voice message in which the cut-off frequency is set to 1.5 kHz is A / D. Even when the audio signal is converted and recorded in the semiconductor memory (see Fig. 6B) and reproduced by itself, its low frequency side is removed corresponding to its high frequency side, so that the audio signal can be reproduced without noticeable unnaturalness ( 6C).

As described above, the semiconductor memory can record an audio signal of a relatively long time of about several minutes, and it can be repeatedly used for recording and reproducing, and is very small in size, so as to be mounted on various devices as a recording medium of the audio signal. come.

For example, in the FM radio receiver and the like, it has been assumed that the above-described semiconductor memory is used as a recording medium for recording and reproducing relatively short time information such as news programs, weather forecasts or traffic information. In this case, even when a wideband audio signal in the range of 15 kHz is provided as in the audio signal in FM radio broadcasting as shown in Fig. 7A, when it is recorded in the semiconductor memory as described above, For example, the high frequency region of the 1.5 kHz or higher audio signal should be removed as shown in FIG. 7B.

That is, when a wideband signal is recorded by itself in a semiconductor memory having a predetermined memory capacity, a sampling clock having a high frequency should be used to accurately record the high frequency region of the audio signal by A / D conversion. The amount of data to be made increases and the time for recording an audio signal in the semiconductor memory becomes short. Thus, by removing a high frequency region of the audio signal, for example, 1.5 kHz or more, recording of an audio signal of a relatively long time of several minutes is possible.

However, when the audio signal is recorded in the semiconductor memory after the high frequency region is removed as shown in Fig. 7B and the signal is reproduced, the high frequency portion is removed from the original audio signal as indicated by the dotted line in Fig. 7C. An audio signal with only low frequency components remaining is reproduced. Audio signals reproduced with only high-frequency components and thus low-frequency components produce unnatural sounds in which the low-frequency region is markedly and excessively exposed, and the clarity worsens, making them difficult to hear.

In view of the above disadvantages, an object of the present invention is to provide an audio signal recording and reproducing apparatus capable of reproducing an audio signal recorded in a semiconductor memory as a clear and natural audio signal.

In order to solve the above problems, according to the first aspect of the present invention,

A low pass filter for removing a high frequency component of a predetermined frequency or higher of an audio signal having a wide bandwidth;

An A / D conversion circuit for converting the audio signal from the low pass filter into a digital signal,

Storage means for recording and storing a period of a predetermined time of the digitized audio signal from the A / D conversion circuit;

A D / A conversion circuit for converting the audio signal read out from the storage means into an analog signal,

A tone control circuit for providing an A / D conversion circuit in a previous step, a D / A conversion circuit in a later step, and performing a frequency correction on the audio signal;

A control circuit for controlling the tone control circuit so that the low frequency component of the audio signal to be reproduced with respect to the audio signal read out from the storage means or the audio signal recorded in the storage means corresponds to the high frequency component of the audio signal removed by the low pass filter. Wow,

An audio signal recording and reproducing apparatus is provided.

According to this first characteristic of the audio signal recording and reproducing apparatus, when reproducing the audio signal recorded in the memory, the tone control circuit is converted into an analog signal in accordance with the high frequency component read out from the memory and removed by the low pass filter. The low frequency component is corrected for the audio signal. In this way, a well balanced audio signal is reproduced for both the high frequency region and the low frequency region.

In addition, according to the second aspect of the present invention,

A receiving circuit for demodulating an audio signal by receiving and broadcasting radio broadcast waves;

A field strength detection circuit for detecting the field strength of radio broadcast waves received and tuned by a receiving circuit;

A first tone control circuit for performing frequency correction on at least the low frequency region of the audio signal demodulated by the receiving circuit according to the detection result of the electric field strength,

A low pass filter for removing a predetermined frequency or higher frequency component of the demodulated audio signal;

An A / D conversion circuit for converting the audio signal from the low pass filter into a digital signal,

Storage means for recording and storing a predetermined time period of the digitized audio signal from the A / D conversion circuit;

A D / A conversion circuit for converting the audio signal read out from the storage means into an analog signal,

A second tone control circuit which is provided in the previous step for the low pass filter and performs frequency correction of the audio signal so that low frequency components of the audio signal are attenuated;

A second audio signal recorded in the storage means, taking into account the frequency correction for the audio signal performed in the first tone control circuit based on the detection result of the electric field intensity and the high frequency components of the audio signal removed by the low pass filter; A control circuit for controlling the tone control circuit,

An audio signal recording and reproducing apparatus is provided.

According to this second characteristic of the recording and reproducing apparatus of the audio signal, the control circuit performs frequency correction performed in the first tone control circuit according to the electric field strength of the received and tuned radio broadcast wave detected by the electric field strength detecting circuit. The second tone control circuit is controlled to perform frequency correction in consideration of the high frequency components of the audio signal removed by the low pass filter. As a result of the frequency correction, a well balanced audio signal is recorded in the memory for both the high frequency region and the low frequency region.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A description of a given embodiment of an apparatus for recording and reproducing an audio signal according to the present invention applied to a radio receiver capable of receiving FM radio broadcasting is made below with reference to the drawings. Incidentally, for the sake of simplicity, the FM radio receiver in this embodiment is given as a mono type radio receiver. However, the present invention can also be applied essentially to stereotype radio receivers.

1 is a block diagram illustrating an FM radio receiver to which the present invention is applied. As shown in FIG. 1, in this case, the radio receiver 100 includes an antenna 11, a tuner 12, a switch circuit 13, a tone control circuit 14, an amplification circuit 15, and a speaker 16. And a low pass filter (hereinafter simply abbreviated as LPF) 21, an A / D conversion circuit 22, a semiconductor memory (hereinafter simply referred to as a memory) 23, and a D / A. A recording and reproducing apparatus 20 including a conversion circuit 24, a memory controller 25, and a microcomputer 30 for controlling various parts of the radio receiver 100.

The antenna 11 receives a broadcast wave of FM radio broadcasting and supplies a received signal to the tuner 12. The tuner 12, which has received the tuning information from the microcomputer 30 according to the user's instruction input, selects a desired object from the FM radio broadcast wave, converts the wave into an intermediate frequency signal, and converts the intermediate frequency signal. After amplification and demodulation, it is output as an audio signal Au1.

According to this embodiment, the radio broadcast to be heard as described above is an FM radio broadcast, and the audio signal Au1 is demodulated as a voice signal having a wide frequency characteristic in the range of about 15 kHz.

The audio signal Au1 output from the tuner 12 is supplied to one input terminal "a" of the switch circuit 13. Also, the reproduced audio signal Au2 from the recording and reproducing apparatus 20 is supplied to the other input terminal "b" of the switch circuit 13. The switch circuit 13 is switched by a switch signal from the microcomputer 30 according to the user's mode selection. That is, in this embodiment, the radio receiver 100 is provided with a reception mode, a recording mode and a reproduction mode. In the reception mode and the recording mode, the switch circuit 13 is switched to the input terminal "a", and in the regeneration mode, the switch circuit 13 is switched to the input terminal "b". The output from the switch circuit 13 is supplied to the tone control circuit 14.

Although the tone control circuit 14 performs tone control (frequency correction) by the control signal from the microcomputer 30, the mode of correction differs depending on the reception mode, the recording mode and the reproduction mode.

The tone control circuit 14 is provided with a tone characteristic as shown in Fig. 2, and in the case of the reception mode and the recording mode for the audio signal input thereto, for example, the microcomputer 30 according to the instruction input from the user. Tone control (frequency correction) is carried out by supplying and receiving tone control information from, for example, to form an audio signal having sound quality in accordance with the user's preference.

In addition, in the reproduction mode, the tone control circuit 14 sets the low frequency side so that the reproduction signal has a frequency characteristic balance with respect to the reproduction signal of the audio signal recorded with the band limit and the high frequency side removed as described later. By this, frequency correction is performed in accordance with the control signal from the microcomputer 30. In this case, the tone control circuit 14 exhibits attenuation characteristics of about -6 dB to -8 dB at 100 Hz, for example, as indicated by a characteristic curve of -6 dB to -8 dB in the characteristic diagram of the tone control circuit 14 of FIG. Remove low frequency components to produce.

The audio signal subjected to the tone control is supplied to the speaker 16 via the amplification circuit 15, so that the audio signal or the reproduced audio signal of the broadcast program of the desired FM radio broadcast is converted into voice by the speaker 16, thereby Sound is produced.

On the other hand, the audio signal Au1 demodulated by the tuner 12 is also supplied to the LPF 21 of the recording and reproducing apparatus 20.

The LPF 21 is, for example, from the supplied audio signal Au1 in order to be able to record, for example, a predetermined time of audio signal in a memory 23 having a predetermined storage capacity. The cut-off frequency removes frequency components in the frequency region higher than 1.5 kHz and passes only low frequency components.

In this way, the band-limited audio signal is supplied to the A / D conversion circuit 22. As soon as the A / D conversion circuit 22 receives the supply of the clock signal from the memory controller 25, the A / D conversion circuit 22 converts the applied audio signal to A / D, and supplies the audio signal which became a digital signal to the memory 23.

The memory controller 25 supplies the sampling clock signal to the A / D conversion circuit 22 and the D / A conversion circuit 24 under the control of the microcomputer 30, and writes, reads, and reads the memory 23. Uh, and addressing control is performed.

The memory 23 is provided with, for example, a storage capacity of several megabits, for example, four megabits or eight megabits, so that the band can be changed as described above under the write and read control and the addressing control by the memory controller 25. A digitized audio signal having limited narrow band characteristics is stored for several minutes, and the stored voice data is read therefrom.

In this embodiment, a signal having a sampling frequency of, for example, 4 kHz is used for A / D conversion or recording / reading to or from the memory 23, and since recording is performed by a DPCM (Differential Pulse Code Modulation) system, The change in the audio signal is recorded in the memory 23 in four bits. Therefore, the recording rate is 4 bits x 4 kHz = 16 kbps, and information of 16 kilobits per second or about 1 megabit per minute is recorded in the memory 23.

In addition, the memory 23 is provided with a memory capacity of, for example, 4 Mbits to 8 Mbits, so that an audio signal having a narrow bandwidth of 4 to 8 minutes can be recorded in the memory 23.

As soon as the D / A conversion circuit 24 receives the supply of the clock signal from the memory controller 25, the D / A conversion circuit 24 converts the digital audio signal read out from the memory 23 into an analog signal Au2. This audio signal Au2 is an audio signal whose band of high frequency components is limited by the LPF 21 as shown in FIG. 3, for example. This audio signal Au2 output from the D / A conversion circuit 24 is supplied to the input terminal "b" of the switch circuit 13 as an output audio signal of the recording and reproducing apparatus 20.

The microcomputer 30 forms a control signal for controlling each part of the radio receiver 100 according to the operation input information input when the user operates the operation panel (not shown) of the radio receiver 100.

Also in this embodiment, the radio receiver 100 is provided with three modes as described above: reception mode, recording mode and playback mode, and each mode can be selected when the user operates the mode switching key installed on the operation panel. have.

For example, when the reception mode is selected, the switch circuit 13 is switched to output the audio signal au1 supplied to the side of the input terminal "a" under the control of the microcomputer 30. The tone control circuit 14 performs normal tone control in accordance with tone control instructions given by the operation of the above-described operation panel of the user. In addition, as described above, the audio signal Au1 from the tuner 12 is supplied to the speaker 16 via the switch circuit 13, the tone control circuit 14, the amplification circuit 15, and is converted into a voice therein. This produces a sound.

When the recording mode is selected, the memory controller 25 supplies a clock signal to the A / D conversion circuit 22 under the control of the microcomputer 30, and performs recording control and addressing control on the memory 23. . As a result, the band is limited by the LPF 21 and the predetermined time period of the audio signal digitized by the A / D conversion circuit 22 is recorded in the memory 23.

In this case, the switch circuit 13 and the tone control circuit 14 are controlled in the same manner as in the case where the reception mode is selected, so that the audio signal Au1 from the tuner 12 is converted into voice and thereby the sound from the speaker 16. Is made. That is, a predetermined time period of the audio signal for the sounded voice is recorded in the memory 23.

Further, when the reproduction mode is selected, the switch circuit 13 is switched to output the audio signal Au2 supplied to the input terminal "b" under the control of the microcomputer 30.

The memory controller 25 performs read control and addressing control on the memory 23 under the control of the microcomputer 30, reads out the audio signal recorded in the memory 23, and converts the audio signal to the D / A conversion circuit 24. To feed. Upon receiving the supply of the clock signal from the memory controller 25, the D / A conversion circuit 24 converts the supplied audio signal into an analog audio signal Au2 and inputs it to the input terminal of the switch circuit 13 ("b). ") To supply. This audio signal Au2 is an audio signal from which high frequency components have been removed by the LPF 21 as shown in FIG.

The audio signal Au2 is supplied to the tone control circuit 14 via the switch circuit 13. In the tone control circuit 14, attenuation characteristics of -6 dB to -8 dB are provided, for example, at 100 Hz as described above, whereby a well balanced audio signal is formed in which both the high frequency region and the low frequency region are automatically attenuated. As described above, tone control for attenuating low frequency components with respect to the reproduced audio signal Au2 is performed.

The audio signal Au2 subjected to the tone control in the control circuit 14 is supplied to the speaker 16 via the amplification circuit 15, whereby a voice that is easy to hear is reproduced.

4 is a block diagram showing another embodiment of the present invention. This embodiment also describes the case of an FM radio receiver, in which case the radio receiver 200 has the function of detecting the received field strength, and limits the band of the output signal from the tuner according to the received field strength, S / N or listening feeling characteristics can be improved by this.

In this embodiment, the FM radio receiver 200 includes an antenna 51, a tuner 52, a tone control circuit 53, a switch circuit 54, an amplifying circuit 55, and a speaker 56 ( 50, a recording and reproducing apparatus 60 including an LPF 61, an A / D conversion circuit 62, a memory 63, a D / A conversion circuit 64, and a plurality of radio receivers 200. And a microcomputer 70 and an A / D conversion circuit 71 for controlling the parts.

The tuner 52, in addition to the front end circuit 521, the intermediate frequency signal amplifying circuit 522 and the detection circuit 523, the electric field strength of the radio broadcast signal tuned from the tone control circuit 524, the intermediate frequency signals It includes a field strength detection circuit 525 to detect, and is provided with a function of suppressing the noise that is increased when the field strength of the received and tuned radio broadcasting is weak as follows. The recording and reproducing apparatus 60 has a configuration similar to that of the recording and reproducing apparatus 20 of the radio receiver 100 described above.

In addition, unlike the case of the radio receiver 100 in FIG. 1, the radio receiver 200 of this embodiment is not provided with a tone control circuit at the rear end of the switch circuit 54, and instead, the tuner 41 is used. Next, the tone control circuit 53 is provided.

The tone control circuit 53 has a circuit similar to the tone control circuit 14 in the radio receiver 100 described above, and is provided with a tone characteristic as shown in FIG.

The switch circuit 54 corresponds to the switch circuit 13 in the radio receiver 100 described above.

In the tuner 52, the FM radio broadcast wave received at the antenna 11 is supplied to the front end circuit 521. The front end circuit 521 tunes the desired FM radio broadcast wave according to the tuning instruction information from the microcomputer 70 according to the user's selection instruction, converts it into an intermediate frequency signal, and converts the intermediate frequency signal into an intermediate frequency signal. The frequency signal amplifier circuit 522 is supplied.

The intermediate frequency amplification circuit 522 amplifies the intermediate frequency signal and supplies it to the detection circuit 523 and the electric field strength detection circuit 525. The detection circuit 523 demodulates the audio signal and supplies the demodulated audio signal to the tone control circuit 524.

The electric field strength detection circuit 525 detects the electric field strength of the FM radio broadcasting wave according to the degree of the intermediate frequency signal, supplies the detected output to the tone control circuit 524 as a control signal, and converts the detected output into A / D conversion. The microcomputer 70 is supplied to the microcomputer 70 through the circuit 71.

The tone control circuit 524 is provided with a frequency characteristic as shown in Fig. 5, and according to the detection result of the supplied electric field strength, so as to suppress the noise that is increased when the electric field strength of the received and tuned FM radio broadcast is weak. From the detection circuit 523, the high frequency components and the low frequency components are corrected.

That is, in the case where the received electric field strength is a strong electric field of about 40 dBμ or more, as shown in FIG. 5 according to the frequency characteristic of the tone control circuit 524, attenuation of the high frequency components is not performed and the low frequency components are slightly reduced. Only attenuation is reduced.

In the case of a weak electric field having a received electric field strength of 20 dBμ or 10 dBμ, as shown in FIG. 5, the high frequency region is cut and the low frequency region is attenuated in consideration of the balance with the high frequency region to improve noise characteristics. .

The tone control circuit 53 corrects the frequency characteristic of the audio signal Au3 from the tuner 52 by the control signal from the microcomputer 70, and in this embodiment, the low pass filter 61 in the recording mode. Attenuate the low frequency region to create a balance with the high frequency region according to the band limit in. Further, in the reception mode, the frequency characteristic of the tone control circuit 53 depends on the operation instruction by the user as in the above-described embodiment.

On the other hand, in this embodiment of the radio receiver 200, when it is determined that the electric field received by the tuner 52 is weak, the low frequency region is already significantly attenuated in consideration of the balance with the blocking of the high frequency region for noise reduction as described above. It is becoming.

Therefore, when recording, if a constant amount of attenuation is always performed in the tone control circuit 53 for the low frequency region, the amount of attenuation on the low frequency region side may be excessively large, the balance is deteriorated, and the sound is more in view of the listening feeling. May deteriorate.

Thus, according to this embodiment, in the recording mode, the microcomputer 70 refers to the information of the electric field strength from the A / D conversion circuit 71, and when the received input electric field is strong, the tone control circuit 53 When the low frequency region is sufficiently attenuated and the electric field is weak, the attenuation of the low frequency region in the tone control circuit 524 is predicted in advance, so that the amount of attenuation with respect to the low frequency region in the tone control circuit 53 is reduced.

Therefore, in the case of the recording mode, the audio signal Au4 from the tone control circuit 53 is formed in consideration of not only the attenuation of the high frequency region in the low pass filter 61 but also the band limitation according to the electric field strength, and thus the low pass. The band of the audio signal output from the filter 61 is limited to a well balanced wave irrespective of the electric field strength. This audio signal is then recorded in the memory 63.

In the reproduction mode, a well-balanced and band-limited audio signal is read out from the memory 63, recovered from the D / A conversion circuit 64 to the analog signal Au5, and the switch circuit 54 and the amplification circuit 55. Is supplied to the speaker 56, and reproduced therein as sound, and thus, a reproduction sound having superior listening feeling characteristics can be provided.

Furthermore, in Europe, there is a broadcasting station that provides a RDS (Radio Data System) service among FM broadcasting stations, whereby a set of digital data related to broadcasting stations, programs, and the like is broadcast in addition to the unique audio signal. An RDS decoder is installed in the radio receiver 100 or 200 described above, for example, and RDS data added to the audio signal of the FM radio broadcast currently being listened to is sampled and decoded, and an identification code (PTY code) indicating the content of the program is provided. Is detected as broadcasting a specific program, for example a traffic information program, the audio signal of the traffic information program is recorded in the memory 23 or 63 mounted in the radio receiver 100 or 200, and only the latest traffic. Only information can be recorded at all times and kept in memory.

In addition, there may be a case where an FM radio broadcast is not listened to or a traffic information program is broadcast by another FM broadcasting station. Therefore, even if you are not listening to FM radio broadcasts or if the traffic information is broadcasted by other broadcasting stations, for example, RDS data added to the audio signals of each FM radio broadcasting station is searched and the audio signals of the most recent traffic information program are stored in memory. To record.

In addition, when a radio broadcast program is listened to using the radio receiver 100 or 200 described above, the audio signal of the radio broadcast program to be listened can always be recorded in the memory 23 or 63, and the audio signal is stored in the memory 23. Or 63), the newest audio signal is recorded by overlapping the predetermined time period of the most recent audio signal of the radio broadcast program being listened to in the memory 23 or 63.

In this case, when the information required by the listener was broadcast but the listener did not listen correctly, such as in the case where the notification of the application for the present was broadcast to the listener but the listener did not listen to the radio program that was listening to the user, By selecting the reproduction mode, the audio signal recorded in the memory 23 or 63 can be reproduced, whereby the information necessary for the listener can be obtained.

In the above embodiment, for the sake of simplicity, the FM radio broadcast for reception is described as being a mono broadcast, but the present invention can also be applied to the case of stereo broadcasting.

In the above embodiment, the present invention has been described in the case where the present invention is applied to a radio receiver, but the present invention is not only for radio reception, but also for electronic music apparatuses and language learning apparatuses, for example, in which audio signals having a broadband are recorded and reproduced in a memory. May also be applied.

As described above, according to the audio signal recording and reproducing apparatus of the present invention, the audio signal recorded in the semiconductor memory is subjected to frequency correction by using a tone control circuit without installing an independent circuit for frequency correction, The low frequency region and the high frequency region can be reproduced in a well balanced state.

Further, according to the audio signal recording and reproducing apparatus of the present invention, frequency correction can also be performed in consideration of the electric field strength, and does not perform an extra low frequency region correction operation according to the change of the electric field intensity.

Accordingly, an apparatus for recording and reproducing an audio signal suitable for a radio receiver for receiving FM radio broadcasting can be provided.

1 is a block diagram illustrating an embodiment of an apparatus for recording and reproducing an audio signal according to the present invention;

2 is a view showing characteristics of a tone control circuit;

3 is a view showing frequency characteristics of an audio signal recorded and reproduced in a semiconductor memory;

4 is a block diagram for explaining another embodiment of an audio signal recording and reproducing apparatus according to the present invention;

5 is a view showing frequency characteristics of a demodulated audio signal of FM radio broadcasting;

6A, 6B, and 6C are diagrams for explaining frequency characteristics of audio signals recorded in a semiconductor memory in an answering machine.

7A, 7B, and 7C are diagrams for explaining frequency characteristics of audio signals in FM radio broadcasting which are changed by being recorded in the semiconductor memory.

* Code descriptions for the main parts of the drawings

10. Receiver 11. Antenna

12. Tuner 13. Switch Circuit

14. Tone Control Circuit 15. Amplification Circuit

16. Speaker 20. Recording and playback device

21. Low pass filter 22. A / D conversion circuit

23. Semiconductor memory 24. D / A conversion circuit

25. Memory Controller 30. Microcomputer

Claims (1)

A receiving circuit for demodulating audio signals by receiving and tuning radio broadcast waves; A field strength detection circuit for detecting the field strength of radio broadcast waves received and tuned by a receiving circuit; A first tone control circuit for performing frequency correction on at least a low frequency region of the audio signal demodulated by the receiving circuit according to the result of the electric field strength detection; A low pass filter for removing high frequency components of a predetermined frequency or higher of the demodulated audio signal; An A / D conversion circuit for converting the audio signal from the low pass filter into a digital signal, Storage means for recording and storing a predetermined time period of the digitized audio signal from the A / D conversion circuit; A D / A conversion circuit for converting the audio signal read out from the storage means into an analog signal, A second tone control circuit provided in a previous step for the low pass filter and performing frequency correction on the audio signal so that low frequency components of the audio signal are attenuated; The second tone in consideration of the high frequency component of the audio signal removed by the low pass filter and the frequency correction for the audio signal performed in the first tone control means circuit for the audio signal recorded in the storage means based on the electric field strength detection result. And a control circuit for controlling the control circuit.