JP5350351B2 - COMMUNICATION SYSTEM, TRANSMISSION DEVICE, RECEPTION DEVICE, AND COMMUNICATION METHOD - Google Patents

Abstract

Provided is a communication system that can have increased secrecy abilities and can reduce the audio level of leaked audio output from a reception device in a broadband system. The communication system (1) is configured from a wireless microphone (2) that is a transmission device, and a speaker device (3) that is a reception device. When a broadband audio signal is input into the wireless microphone (2), the wireless microphone (2) performs balanced modulation processing that reverses the high and low frequencies of the audio signal, and transmits an audio signal wherein, of the audio signal having reversed high and low frequencies, the signal of the band that is at or below a baseline frequency has been cut. When the speaker device (3) receives the audio signal transmitted from the wireless microphone, the speaker device (3) performs an inverse balanced modulation that re-reverses the high and low frequencies of the received audio signal, and outputs the audio signal having the re-reversed high and low frequencies.

Description

  The present invention relates to a communication system that transmits / receives a wideband audio signal, and more particularly to a technique for improving confidentiality.

  In a transceiver, a home cordless phone, and the like, a communication system that transmits and receives a narrowband audio signal including only an audio band (300 Hz to 3 kHz) is used. Conventionally, in such a narrow-band communication system, as a technique for providing confidentiality at a relatively low cost, a balanced modulator is provided in both the transmitter and the receiver, and a voice signal whose frequency is inverted is transmitted on the transmitter side. A method has been proposed in which the received voice is re-inverted and returned to the original state on the receiver side (see, for example, Patent Document 1).

  According to such a conventional method, even if another receiver (a receiver not having a balanced modulator) receives an audio signal transmitted from a transmitter, the frequency of the audio signal is inverted. Therefore, it is difficult to hear the contents of the voice. Conventionally, in a narrow-band communication system, confidentiality is maintained in this way.

JP-A-7-307721

  However, since the conventional communication system can transmit and receive only a narrowband audio signal including only the audio band (300 Hz to 3 kHz), the sound quality is lower than that of the wideband including the audio band (100 Hz to 15 kHz). There was a problem.

  Therefore, in order to improve sound quality, it is conceivable to apply the method used in the conventional system (narrowband communication system) to the broadband communication system as it is. However, simply by applying the conventional method to a broadband communication system, it is not possible to obtain the secrecy that was obtained in a narrow band communication system. In an audio signal after balanced modulation (after frequency reversal), the electrical characteristics of the electronic components that make up the balanced modulator are shifted from ideal symmetry due to variations in manufacturing, temperature changes, etc. Audio (audio signal before balanced modulation) is included as “leakage audio”. This “leakage voice” is included in the low frequency side (mainly, the voice band of 300 Hz to 3 kHz) of the voice signal after balanced modulation.

  The low frequency side (low frequency side) of the audio signal after balanced modulation corresponds to the high frequency side (high frequency side) of the audio signal before balanced modulation, but the input to the microphone is mainly audio. In some cases, the original sound (the sound signal before the balanced modulation) has few components on the high frequency side exceeding the sound band, and therefore, the low frequency component of the sound signal after the balanced modulation has few. Therefore, when other receivers (receivers that do not have a balanced modulator) receive the audio signal transmitted from the transmitter, the low frequency side of the audio signal after balanced modulation is other than “leakage audio” As a result, there is a problem that it is easy to hear "leakage voice" and the secrecy cannot be maintained.

  The present invention has been made in order to solve the above-described conventional problems. A communication system capable of reducing the voice level of leaked voice even in a wideband system (high-quality sound system) and enhancing the secrecy. The purpose is to provide.

  The communication system of the present invention is a communication system including a transmission device and a reception device for transmitting and receiving a wideband audio signal including a frequency band higher than the audio band, and the transmission device receives a wideband audio signal. An audio input unit, a balanced modulation unit that performs a balanced modulation process that inverts the frequency of the audio signal, and a filter unit that cuts a signal in a band equal to or lower than a first predetermined frequency out of the audio signal having the inverted frequency And a transmission unit that transmits an audio signal in which a band equal to or lower than the first predetermined frequency is cut, and the reception device receives the audio signal transmitted from the transmission device; It has a configuration including an inverse balanced modulation unit that performs an inverse balanced modulation process that reinverts the frequency of the audio signal, and an audio output unit that outputs the audio signal with the frequency inverted again.

  With this configuration, it is possible to transmit and receive a wideband (for example, 100 Hz to 15 kHz) audio signal including a frequency band higher than the audio band (for example, 300 Hz to 3 kHz) with enhanced secrecy. In this case, balanced modulation processing for inverting the frequency of the wideband audio signal is performed by the transmission apparatus (for example, a wireless microphone), and the low frequency band (for example, 3 kHz or less) of the audio signal whose frequency is inverted The signal is cut. Therefore, even if the audio signal after balanced modulation (after frequency inversion) includes the audio signal before balanced modulation (before frequency inversion) as “leakage sound”, the frequency band (the lower frequency band) It is possible to suppress the leakage signal from being output from the receiving device (for example, the speaker device). In this way, in a broadband system (high sound quality system), the level of leaked voice output from the receiving device can be reduced, and the secrecy can be improved.

  In the communication system of the present invention, the transmission device includes a noise superimposing unit that superimposes a noise signal in a band equal to or lower than a second predetermined frequency in the audio signal with the frequency inverted, and the reception device It has a configuration including a noise cut unit that cuts a signal in a band of the second predetermined frequency or less from the received audio signal.

  With this configuration, a noise signal (for example, a melody or another sound) is superimposed on a low frequency band (for example, 3 kHz or less) of the audio signal whose frequency is inverted by a transmission device (for example, a wireless microphone). The Therefore, even if the audio signal after balanced modulation (after frequency inversion) includes the audio signal before balanced modulation (before frequency inversion) as “leakage sound”, the frequency band (the lower frequency band) This signal is masked on the audibility by the noise signal, making it difficult to hear the leaked sound output from the receiving device (for example, the speaker device). In this way, in a broadband system (high sound quality system), the leaked voice output from the receiving apparatus can be masked, and the secrecy can be improved.

  In the communication system of the present invention, the transmission device includes a tone signal superimposing unit that superimposes a tone signal including a control signal for controlling the receiving device on the audio signal transmitted by the transmission unit, The receiving apparatus has a configuration for controlling the operation of the inverse balanced modulation unit or the noise cut unit based on the control signal transmitted by the tone signal.

  With this configuration, a tone signal is superimposed on an audio signal transmitted from a transmission device (for example, a wireless microphone), and a control signal for controlling the reception device is carried by the tone signal. A receiving device (for example, a speaker device) can control the operation of the inverse balanced modulation unit or the noise cut unit according to the on / off of the secret function on the transmitting device side according to the control signal.

  Moreover, in the communication system of the present invention, the transmission device includes a tone signal superimposing unit that superimposes a tone signal for carrying code key data on an audio signal transmitted by the transmission unit, and the receiving device includes: When the verification of the code key data carried by the tone signal is successful, the configuration includes a code key data verification unit that outputs the voice signal from the voice output unit.

  With this configuration, a tone signal is superimposed on an audio signal transmitted from a transmission device (for example, a wireless microphone), and code key data is carried by the tone signal. A receiving device (for example, a speaker device) outputs an audio signal only when the verification of the code key data is successful. As a result, the receiving device can output only the audio signal transmitted from the specific transmitting device (the transmitting device having the matched code key data).

  In the communication system of the present invention, the transmission device is based on the operation / non-operation of the balanced modulation unit, the level of the voice band of the voice signal, or the magnitude of the frequency component of the voice band included in the broadband voice signal. Thus, the filter unit is configured to switch whether or not to cut a signal in a band equal to or lower than the second predetermined frequency.

  With this configuration, the transmission apparatus switches whether to cut a signal in a low frequency band (for example, 3 kHz or less) among audio signals whose frequencies are inverted. This switching is appropriately performed based on the operation / non-operation of the balanced modulation unit, the level of the voice band of the voice signal, or the magnitude of the frequency component of the voice band included in the wideband voice signal.

  The transmission device of the present invention is a transmission device used in a communication system for transmitting and receiving a wideband audio signal including a frequency band higher than the audio band, and the transmission device is an audio input to which a wideband audio signal is input. A balanced modulation unit that performs a balanced modulation process that inverts the frequency of the audio signal, a filter unit that cuts a signal in a band that is equal to or lower than a predetermined frequency, and a frequency that is equal to or lower than the predetermined frequency. And a transmission unit that transmits an audio signal with the band of which is cut, and the reception device provided in the communication system receives the audio signal transmitted from the transmission device and re-inverts the frequency of the received audio signal In other words, the audio signal having the frequency inverted again is output.

  Also with this apparatus, as described above, it is possible to reduce the voice level of the leaked voice in a wide-band system (high sound quality system) and improve the secrecy.

  A receiving apparatus according to the present invention is a receiving apparatus used in a communication system for transmitting and receiving a wideband audio signal including a frequency band higher than a voice band, and in the transmitting apparatus provided in the communication system, a wideband audio signal is The input signal is subjected to balanced modulation processing for inverting the frequency of the audio signal, and the audio signal whose frequency is inverted is cut out of a band below a predetermined frequency, and the band below the predetermined frequency is cut off. An audio signal is transmitted, and the receiving device receives a voice signal transmitted from the transmitting device, an inverse balanced modulation unit that performs an inverse balanced modulation process to reinvert the frequency of the received audio signal, And a voice output unit that outputs a voice signal whose frequency is re-inverted.

  Also with this apparatus, as described above, it is possible to reduce the voice level of the leaked voice in a wide-band system (high sound quality system) and improve the secrecy.

The communication method of the present invention is a communication method used in a communication system including a transmission device and a reception device for transmitting and receiving a wideband audio signal including a frequency band higher than the audio band,
In the transmitter, a broadband audio signal is input, balanced modulation processing is performed to invert the frequency of the audio signal, and a signal in a band equal to or lower than a predetermined frequency is cut out of the audio signal in which the frequency is inverted, An audio signal in which a band below the predetermined frequency is cut is transmitted,
The reception device receives the audio signal transmitted from the transmission device, performs an inverse balanced modulation process that reinverts the frequency of the received audio signal, and outputs the audio signal with the frequency reinverted .

  Also by this method, as described above, it is possible to reduce the voice level of the leaked voice in the wide band system (high sound quality system) and improve the secrecy.

  The present invention provides a communication system having the effect of reducing the voice level of leaked voice output from a receiving apparatus and enhancing the secrecy in a broadband system (high sound quality system). It is something that can be done.

The block diagram which shows the structure of the communication system in the 1st Embodiment of this invention. Explanatory drawing of operation | movement of the transmitter (wireless microphone) in the 1st Embodiment of this invention Explanatory drawing of operation | movement of the receiver (speaker apparatus) in the 1st Embodiment of this invention The block diagram which shows the structure of the communication system in the 2nd Embodiment of this invention. Explanatory drawing of operation | movement of the transmission apparatus (wireless microphone) in the 2nd Embodiment of this invention. Explanatory drawing of operation | movement of the receiver (speaker apparatus) in the 2nd Embodiment of this invention. The block diagram which shows the structure of the communication system in other embodiment of this invention.

  Hereinafter, a communication system according to an embodiment of the present invention will be described with reference to the drawings. In this embodiment, a case of a communication system used for a wireless microphone system or the like is illustrated.

(First embodiment)
A configuration of a communication system according to a first exemplary embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a communication system according to the present embodiment. As shown in FIG. 1, the communication system 1 includes a wireless microphone 2 as a transmitting device and a speaker device 3 as a receiving device. In the communication system 1, a wideband (100 Hz to 15 kHz audio band) audio signal is transmitted and received. In the following description, the case of a wideband audio signal (100 Hz to 15 kHz) will be described as an example, but there may be a case where there is no lower limit, that is, (˜15 kHz).

  The wireless microphone 2 includes a microphone 4 to which an audio signal is input, an AF amplifier 5 that amplifies the input audio signal, a balanced modulation circuit 6 that performs a process of inverting the frequency of the audio signal (balanced modulation process), and a balanced A low-pass filter 7 (LPF) that cuts a frequency component higher than the modulation clock frequency in the modulation process, a high-pass filter 8 (HPF) that cuts a frequency component below a first predetermined frequency (for example, 3 kHz), and a high-pass filter 8. A filter control unit 9 that controls switching of operation (on / off), a tone signal superimposing unit 10 that superimposes a tone signal on the audio signal output of the high-pass filter 8, an FM modulation circuit 11 that FM modulates the audio signal, and audio A transmission unit 12 that transmits a signal from an antenna is provided. Here, the case where the cut-off frequency is 3 kHz is exemplified as the first predetermined frequency of the high-pass filter 8, but the cut-off frequency of the high-pass filter 8 is of course not limited to this. That is, any frequency between 0 and 3 kHz may be used. For example, when sending male voices with a heavy bass instrument in the background, set the cut-off frequency low, and when sending female voices with the treble part of the piano, etc., set the cut-off frequency high. Good.

  The code key data is carried by the tone signal superimposed by the tone signal superimposing unit 10. The tone signal carrying the code key data may be transmitted only once (superimposed on the audio signal) when the main power of the wireless microphone 2 is turned on, or the main power of the wireless microphone 2 is turned on. During transmission, it may be transmitted at a predetermined time interval (superimposed on the audio signal).

  The filter control unit 9 operates the balanced modulation circuit 6, LPF 7 and HPF 8 when on based on on / off of the secret control, and does not operate the balanced modulation circuit 6, LPF 7 and HPF 8 when off. Control to let through. This control is basically turned on when the secret function is used as the communication system 1 and is turned off when not used. However, when the voice level of the voice signal is equal to or higher than a predetermined reference level, You may make it turn on secret talk control. Further, when the audio signal includes a predetermined frequency component, the secret control may be turned on. This is because in these cases, it is considered that the leaked sound component after the balanced modulation becomes large.

  On the other hand, the speaker device 3 includes a receiving unit 13 that receives an audio signal from an antenna, an FM demodulation circuit 14 that performs FM demodulation on the received audio signal, and a tone signal extraction unit 15 that extracts a tone signal from the FM demodulated signal. , A balanced modulation circuit 16 that performs processing for inverting the frequency of the audio signal (balanced modulation processing), a low-pass filter 17 (LPF) that cuts a frequency component higher than the modulation clock frequency in the balanced modulation processing, and AF that amplifies the audio signal An amplifier 18 and a speaker 19 that outputs an audio signal are provided. The balanced modulation processing performed by the balanced modulation circuit 16 of the speaker device 3 re-inverts the frequency of the audio signal whose frequency has already been inverted (the audio signal whose frequency has been inverted by the balanced modulation circuit 6 of the wireless microphone 2). Is. Therefore, the balanced modulation process performed by the balanced modulation circuit 16 of the speaker device 3 can also be called an inverse balanced modulation process, and the balanced modulation circuit 16 of the speaker device 3 can also be called an inverse balanced modulation circuit.

  When the tone signal extraction unit 15 extracts the tone signal superimposed on the audio signal and determines that the secret microphone control is performed on the wireless microphone 2 side based on the tone signal, the tone signal extraction unit 15 also performs reverse balanced modulation and LPF processing on the speaker device side. Thus, the sound can be output correctly to the speaker. When the code key data of the speaker device 3 desired to be received is conveyed by the tone signal, the code key data is collated and the code key data is successfully collated, that is, the code key data received from the wireless microphone By outputting the audio signal only when the code key data provided in advance in the speaker device substantially matches, it is prevented from being illegally received by another speaker device having an anti-balance circuit. You may do it.

  In addition, about the method of transmitting the control signal of the speaker apparatus 3 using a tone signal, you may transmit a tone signal using a some frequency. For example, when the secret control is ON, 30 kHz is used as the frequency of the tone signal, and 30 Hz is used as the frequency of the tone signal when the control signal is OFF. In addition to the above frequencies, for example, frequencies between 60 Hz to 80 Hz and 15 kHz to 90 kHz can be used as the frequency for tone signal transmission.

  Further, the method for transmitting the control signal of the speaker device 3 using the tone signal uses the level and phase of the tone signal superimposed on the audio signal, or the presence / absence of the tone signal superimposed on the audio signal. Also good. In addition to ASK (Amplitude Shift Keying), PSK (Phase Shift Keying), FSK (Frequency Shift Keying), QAM (Quadrature Amplitude Modulation), AM, PM, FM, etc. Also good.

  The operation of the communication system 1 configured as described above will be described with reference to the drawings.

  FIG. 2 is an explanatory diagram of the operation of the wireless microphone 2 of the present embodiment. As shown in FIG. 2, in the wireless microphone 2 of the present embodiment, when a wide-band audio signal including an audio band (300 Hz to 3 kHz) is input from the microphone 4 (see FIG. 2A), A balanced modulation process for inverting the frequency of the audio signal is performed (see FIG. 2B). The low frequency side portion (sound band portion) of the audio signal after balanced modulation may contain leaked speech, but in this embodiment, this portion (the portion including leaked speech) is a high-pass filter. An audio signal that is cut at 8 and does not include leaked audio is transmitted (see FIG. 2C). In this embodiment, the voice band (300 Hz to 3 kHz) is described as an example, but a case where there is no lower limit, that is, (to 3 kHz) may be used.

  FIG. 3 is an explanatory diagram of the operation of the speaker device 3 of the present embodiment. As shown in FIG. 3, when the speaker device 3 of the present embodiment receives an audio signal transmitted from the wireless microphone 2 (see FIG. 3A), the frequency of the audio signal is inverted (re-inverted). A balanced modulation process (inverse balanced modulation process) is performed (see FIG. 3B), and the sound is output from the speaker 19.

  According to the communication system 1 of the first embodiment of the present invention, the sound level of leaked sound output from the speaker device 3 can be reduced in a broadband system (high sound quality system). Improves secrecy.

  In the present embodiment, a wideband (for example, 100 Hz to 15 kHz) audio signal including a frequency band higher than the audio band (for example, 300 Hz to 3 kHz) is transmitted and received. In this case, the wireless microphone 2 performs balanced modulation processing for inverting the frequency of the wideband audio signal, and the signal in the low frequency band (for example, 3 kHz or less) is cut from the audio signal whose frequency is inverted. The Therefore, even if the audio signal after balanced modulation (after frequency inversion) includes the audio signal before balanced modulation (before frequency inversion) as “leakage sound”, the frequency band (the lower frequency band) It is possible to suppress the leakage signal from being output from the speaker device 3. In this way, in a broadband system (high sound quality system), the level of leaked sound output from the speaker device 3 can be reduced, and the secrecy can be enhanced.

  In the present embodiment, the tone signal is superimposed on the audio signal transmitted from the wireless microphone 2, and the code key data is carried by the tone signal. The speaker device 3 outputs an audio signal only when the verification of the code key data is successful. As a result, the speaker device 3 can output only the audio signal transmitted from the specific wireless microphone 2 (wireless microphone 2 with the matching code key data).

  In the present embodiment, the wireless microphone 2 switches whether or not to cut a signal in a low frequency band (for example, 3 kHz or less) out of the audio signal whose frequency is inverted. This switching is appropriately performed based on the audio level of the audio signal, the frequency component of the audio band included in the wideband audio signal, and the like. That is, if the sound level of the original sound signal is high, it is considered that the leaked sound component becomes large even after balanced modulation. Further, since it is considered that the leakage component after the balanced modulation is increased even when the frequency component of the audio band is large in the wideband audio signal, control for cutting the low frequency is performed.

(Second Embodiment)
Next, the communication system 1 according to the second embodiment of the present invention will be described. Here, the communication system 1 according to the second embodiment will be described with a focus on differences from the first embodiment. Unless otherwise specified, the configuration and operation of the present embodiment are the same as those of the first embodiment.

  FIG. 4 is a block diagram showing a configuration of the communication system 1 according to the present embodiment. As shown in FIG. 4, the wireless microphone 2 of the present embodiment does not include the high-pass filter 8, and instead, noise is generated in a band below a predetermined reference frequency (for example, 3 kHz) of the audio signal after balanced modulation. A noise superimposing unit 20 that superimposes a signal is provided. As the noise signal, for example, in addition to general white noise or colored noise for noise, a melody or another voice different from the voice to be transmitted can be used. On the other hand, the speaker device 3 cuts a frequency component (a portion on which a noise signal is superimposed) having a cutoff frequency (for example, 3 kHz) or less as a second predetermined frequency in the audio signal received from the wireless microphone 2. A filter 21 (HPF) is provided. The high-pass filter 21 is operated when noise is superimposed by the noise superimposing unit 20 in the wireless microphone 2, but may not be operated when noise is not superimposed by the noise superimposing unit 20. In this case, whether the high-pass filter 21 of the speaker device 3 is operating / not operating is transmitted by a tone signal or the like from the wireless microphone 2 side as in the first embodiment. In addition, although the case where the frequency of the noise signal is 3 kHz or less is illustrated here, it is needless to say that the frequency of the noise signal is not limited to this.

  About the communication system 1 comprised as mentioned above, the operation | movement is demonstrated using drawing.

  FIG. 5 is an explanatory diagram of the operation of the wireless microphone 2 of the present embodiment. As shown in FIG. 5, in the wireless microphone 2 of the present embodiment, when a broadband audio signal including an audio band (300 kHz to 3 kHz) is input from the microphone 4 (see FIG. 5A), A balanced modulation process for inverting the frequency of the audio signal is performed (see FIG. 5B). The low-frequency part (voice band part) of the audio signal after balanced modulation may contain leaked voice, but in this embodiment, the noise signal is included in this part (part containing the leaked voice). Is superimposed, and a voice signal in which the leaked voice is masked is transmitted (see FIG. 5C).

  FIG. 6 is an explanatory diagram of the operation of the speaker device 3 of the present embodiment. As shown in FIG. 6, in the speaker device 3 of the present embodiment, when an audio signal transmitted from the wireless microphone 2 is received (see FIG. 6A), the second predetermined frequency is included in the audio signal. A frequency component (a portion on which a noise signal is superimposed) having a cutoff frequency (for example, 3 kHz) or less is cut (see FIG. 6B). Thereafter, balanced modulation processing (inverse balanced modulation processing) for inverting (re-inverting) the frequency of the audio signal is performed (see FIG. 6C), and the sound is output from the speaker 19.

  Such a communication system 1 according to the second embodiment of the present invention also achieves the same operational effects as the first embodiment. That is, in a broadband system (high sound quality system), the level of leaked voice output from the speaker device 3 can be reduced, and the secrecy can be improved.

  In the present embodiment, the wireless microphone 2 superimposes a noise signal in a low frequency band (for example, 3 kHz or less) of the audio signal whose frequency is inverted. Therefore, even if the audio signal after balanced modulation (after frequency inversion) includes the audio signal before balanced modulation (before frequency inversion) as “leakage sound”, the frequency band (the lower frequency band) This signal is masked on the perception by the noise signal, so that the leaked sound output from the speaker device 3 can be made difficult to hear. In this manner, in a broadband system (high sound quality system), the leaked sound output from the speaker device 3 can be masked, and the secrecy can be improved.

  The embodiments of the present invention have been described above by way of example, but the scope of the present invention is not limited to these embodiments, and can be changed or modified according to the purpose within the scope of the claims. is there.

  For example, FIG. 7 shows another embodiment of the present invention. As shown in FIG. 7, in this embodiment, the first embodiment and the second embodiment are combined. Such a communication system 1 also provides the operational effects of the first embodiment and the second embodiment. The order of the tone signal superimposing unit and the noise superimposing unit may be the reverse order shown in the figure.

  As described above, the communication system according to the present invention can reduce the sound level of leaked sound output from the receiving device in a wideband system (high sound quality system), and can improve the secrecy. It has an effect and is useful as a wireless microphone system or the like.

DESCRIPTION OF SYMBOLS 1 Communication system 2 Wireless microphone 3 Speaker apparatus 4 Microphone 5 AF amplifier 6 Balance modulation circuit 7 Low pass filter 8 High pass filter 9 Filter control part 10 Tone signal superposition part 11 FM modulation circuit 12 Transmission part 13 Reception part 14 FM demodulation circuit 15 Tone signal Extraction unit 16 Balance modulation circuit 17 Low-pass filter 18 AF amplifier 19 Speaker 20 Noise superimposition unit 21 High-pass filter

Claims (8)

A communication system including a transmission device and a reception device for transmitting and receiving a wideband audio signal including a frequency band higher than an audio band,
The transmitter is
An audio input unit for inputting a wideband audio signal;
A balanced modulation unit for performing a balanced modulation process for inverting the frequency of the audio signal;
A filter unit that cuts a signal in a band equal to or lower than a first predetermined frequency out of the audio signal having the inverted frequency;
A transmitter that transmits an audio signal in which a band equal to or lower than the first predetermined frequency is cut;
With
The receiving device is:
A receiver that receives the audio signal transmitted from the transmitter;
An inverse balanced modulation unit for performing an inverse balanced modulation process to reinvert the frequency of the received audio signal;
An audio output unit that outputs an audio signal having the frequency re-inverted;
A communication system comprising: The transmission device includes a noise superimposing unit that superimposes a noise signal in a band equal to or lower than a second predetermined frequency among the audio signals in which the frequency is inverted,
The communication system according to claim 1, wherein the reception device includes a noise cut unit that cuts a signal in a band of the second predetermined frequency or less from the received audio signal. The transmitting device includes a tone signal superimposing unit that superimposes a tone signal including a control signal for controlling the receiving device on the audio signal transmitted by the transmitting unit;
3. The communication system according to claim 1, wherein the receiving apparatus controls the operation of the inverse balanced modulation unit or the noise cut unit based on the control signal transmitted by the tone signal. 4. . The transmitting device superimposes code key data on the tone signal as the control signal,
4. The receiving device includes a code key data collating unit that outputs the audio signal from the audio output unit when the code key data carried by the tone signal is successfully collated. The communication system according to 1.   The transmission device uses the filter unit to perform the second based on the operation / non-operation of the balanced modulation unit, the level of the audio band of the audio signal, or the size of the frequency component of the audio band included in the wideband audio signal. The communication system according to any one of claims 1 to 3, wherein switching of whether or not to cut a signal in a band of a predetermined frequency or less is performed. A transmission device used in a communication system for transmitting and receiving a wideband audio signal including a frequency band higher than an audio band,
The transmitter is
An audio input unit for inputting a wideband audio signal;
A balanced modulation unit for performing a balanced modulation process for inverting the frequency of the audio signal;
A filter unit that cuts a signal in a band equal to or lower than a predetermined frequency among the audio signals having the inverted frequency;
A transmitter that transmits an audio signal in which a band equal to or lower than the predetermined frequency is cut;
With
In the receiving device provided in the communication system, the audio signal transmitted from the transmitting device is received, the inverse balanced modulation process is performed to reinvert the frequency of the received audio signal, and the audio in which the frequency is reinverted A transmitter characterized in that a signal is output. A receiving device used in a communication system for transmitting and receiving a wideband audio signal including a frequency band higher than an audio band,
In the transmission device provided in the communication system, a wideband audio signal is input, balanced modulation processing for inverting the frequency of the audio signal is performed, and the audio signal having a frequency equal to or lower than a predetermined frequency among the audio signals having the inverted frequency. The signal is cut, and an audio signal in which the band below the predetermined frequency is cut is transmitted,
The receiving device is:
A receiver that receives the audio signal transmitted from the transmitter;
An inverse balanced modulation unit for performing an inverse balanced modulation process to reinvert the frequency of the received audio signal;
An audio output unit that outputs an audio signal having the frequency re-inverted;
A receiving apparatus comprising: A communication method used in a communication system including a transmission device and a reception device for transmitting and receiving a wideband audio signal including a frequency band higher than an audio band,
In the transmitter, a broadband audio signal is input, balanced modulation processing is performed to invert the frequency of the audio signal, and a signal in a band equal to or lower than a predetermined frequency is cut out of the audio signal in which the frequency is inverted, An audio signal in which a band below the predetermined frequency is cut is transmitted,
The reception device receives the audio signal transmitted from the transmission device, performs an inverse balanced modulation process that reinverts the frequency of the received audio signal, and outputs the audio signal with the frequency reinverted A communication method characterized by the above.

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