FM broadcast system, FM transmitter, and FM relay device

Abstract

This FM transmitter generates a monaural composite signal by mixing a transmission signal modulated by a monaural sound signal and a broadcast signal obtained by delaying a monaural sound signal by an adjustment delay time. The FM relay device generates a monaural composite signal by mixing a transmission signal extracted from a received host station wave and a broadcast signal obtained by delaying a monaural sound signal demodulated from the transmission signal by a relay time delay time. The adjustment delay time is set to a time longer than a transmission delay time from the host station to the relay station. The relay time delay time is set to a length obtained by subtracting the transmission delay time from the adjustment delay time.

Description

FM broadcasting system, FM transmitter, and FM repeater

[0001] The present disclosure relates to a technique for relaying broadcast waves of an FM broadcasting system.

[0002] FM synchronized broadcasting is known, in which frequency-modulated audio broadcast waves of the same frequency and program are transmitted from multiple transmitting stations (hereinafter referred to as "host stations"). FM stands for Frequency Modulation. In FM synchronized broadcasting, in areas where broadcast waves from multiple host stations are received overlapping with approximately the same intensity, the host station must adjust the delay time so that the broadcast waves are received simultaneously. In addition, broadcasting systems generally include relay transmitting stations (hereinafter referred to as "relay stations") to expand the reception area.

[0003] A relay station receives a higher-level station wave, which is a broadcast wave from a higher-level station, and retransmits the relayed wave. If a relay station receives a relayed wave that is stronger than the higher-level station wave at the same frequency, loop oscillation occurs, making the relay impossible. In response to this, Patent Document 1 listed below describes a technology for canceling unwanted waves, such as loop oscillation, contained in the wave received by the relay station.

[0004] Japanese Patent Application Laid-Open No. 2020-157756

[0005] However, since the relayed wave is delayed compared to the higher-order station wave, there is a problem that in areas where the higher-order station wave and the relayed wave are received at the same time, it is not possible to match the delay times.

[0006] One aspect of the present disclosure is to provide a technology that enables adjustment of the delay time between a higher-level station wave transmitted from a higher-level station and a relayed wave transmitted from a relay station in an area where the higher-level station wave transmitted from a higher-level station and the relayed wave transmitted from a relay station are received overlappingly in an FM broadcasting system.

[0007] One aspect of the present disclosure is an FM broadcasting system. The FM broadcasting system includes an FM transmitter and an FM repeater. The FM transmitter is configured to transmit a higher-level station wave FM-modulated by a monaural composite signal. The FM repeater is configured to receive the higher-level station wave transmitted from the FM transmitter and transmit a repeater wave FM-modulated by a monaural composite signal reproduced from the higher-level station wave. The monaural composite signal has a structure in which a broadcast signal, which is a monaural audio signal, is superimposed on the band of the L+R signal in the stereo composite signal, and a transmission signal modulated by the broadcast signal is superimposed on the band of the L-R signal in the stereo composite signal. The monaural composite signal also has a structure in which the pilot signal in the stereo composite signal is omitted. The FM transmitter is configured to generate a monaural composite signal by mixing the transmission signal modulated by the monaural audio signal with a broadcast signal obtained by delaying the monaural audio signal by an adjustment delay time. The FM repeater is configured to generate a monaural composite signal by mixing a transmission signal extracted from a received higher-level station wave with a broadcast signal obtained by delaying a monaural audio signal demodulated from the transmission signal by a relay delay time. The adjustment delay time is set to a time longer than the transmission delay time from the FM transmitter to the FM repeater. The relay delay time is set to a length obtained by subtracting the transmission delay time from the adjustment delay time.

[0008] With this configuration, the delay time of the broadcast signal contained in the relay wave can be adjusted so that the broadcast signal contained in the relay wave and the broadcast signal contained in the higher-level station wave are received at the same time near the FM relay device.

[0009] Another aspect of the present disclosure is an FM transmission device. The FM transmission device includes a modulation unit, a delay unit, a mixing unit, an FM modulation unit, and a higher-level station transmission unit. The modulation unit is configured to generate a transmission signal modulated by an input monaural audio signal. The delay unit is configured to generate a broadcast signal by delaying the monaural audio signal by an adjustment delay time. The mixing unit is configured to generate a monaural composite signal by mixing the transmission signal generated by the modulation unit and the broadcast signal generated by the delay unit. The FM modulation unit is configured to generate a transmission signal that is FM-modulated by the monaural composite signal generated by the mixing unit. The higher-level station transmission unit is configured to transmit a higher-level station wave based on the transmission signal generated by the FM modulation unit. The monaural composite signal has a structure in which a broadcast signal is superimposed on the L+R signal band of the stereo composite signal, and a transmission signal is superimposed on the L-R signal band of the stereo composite signal. The mono composite signal has a structure in which the pilot signal in the stereo composite signal is omitted. The adjustment delay time is set to a time longer than the transmission delay time from the transmission of the higher-order station wave to the FM repeater that relays the higher-order station wave.

[0010] Such a configuration can be used as an FM transmitter that constitutes the above-mentioned FM broadcasting system.

[0011] Another aspect of the present disclosure is an FM repeater device. The FM repeater device includes a repeater receiving unit, a signal extracting unit, a repeater demodulating unit, a repeater delay unit, a repeater mixing unit, a repeater FM modulating unit, and a repeater transmitting unit. The repeater receiving unit is configured to receive a higher-level station wave that has been FM-modulated by a monaural composite signal. The signal extracting unit is configured to extract a transmission signal from the received signal received by the repeater receiving unit. The repeater demodulating unit is configured to demodulate the transmission signal to generate a monaural audio signal. The repeater delay unit is configured to generate a broadcast signal by delaying the monaural audio signal generated by the repeater demodulating unit by a relay delay time. The repeater mixing unit is configured to generate a monaural composite signal by mixing the transmission signal extracted by the signal extracting unit with the broadcast signal generated by the repeater delay unit. The repeater FM modulating unit is configured to generate a relay signal that has been FM-modulated by the monaural composite signal generated by the repeater mixing unit. The relay transmitting unit is configured to transmit relay waves based on the relay signal generated by the relay FM modulating unit. The mono composite signal has a structure in which a broadcast signal is superimposed on the band of the L+R signal in the stereo composite signal, and a transmission signal modulated by the broadcast signal is superimposed on the band of the L-R signal in the stereo composite signal. The mono composite signal also has a structure in which the pilot signal in the stereo composite signal is omitted. The mono composite signal included in the higher station wave is set so that the broadcast signal is delayed from the transmission signal by an adjustment delay time. The adjustment delay time is set to a time longer than the transmission delay time from the FM transmitting device transmitting the higher station wave to the FM repeater device. The relay delay time is set to a length obtained by subtracting the transmission delay time from the adjustment delay time.

[0012] Such a configuration can be used as an FM repeater that constitutes the above-mentioned FM broadcasting system.

[0013] FIG. 1 is an explanatory diagram showing an overview of an FM broadcasting system. FIG. 1 is an explanatory diagram showing the spectrum of a mono composite signal used in the first embodiment in comparison with the spectrum of a stereo composite signal. FIG. 2 is a block diagram of an FM transmitting device constituting a higher station of the FM broadcasting system in the first embodiment. FIG. 3 is a block diagram of an FM repeater device constituting a repeater station of the FM broadcasting system in the first embodiment. FIG. 4 is an explanatory diagram showing the relationship between higher station waves, repeater waves, and loop waves in the FM broadcasting system. FIG. 5 is an explanatory diagram showing waveforms and delay adjustment methods for broadcast signals and repeater signals. FIG. 6 is an explanatory diagram showing other formats of the spectrum of broadcast signals and repeater signals. FIG. 7 is an explanatory diagram illustrating the spectrum of a mono composite signal used in a second embodiment. FIG. 8 is a block diagram of a signal processing unit of an FM transmitting device in the second embodiment. FIG. 9 is a block diagram of a repeater processing unit of an FM repeater device in the second embodiment.

[0014] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0015] 1. First Embodiment 1-1. System Configuration As shown in FIG. 1, an FM broadcasting system 100 of the first embodiment includes a host station 101 and one or more relay stations 102.

[0016] The upper station 101 converts audio signals distributed via a predetermined transmission network into signals of a predetermined format and transmits them to the broadcast area A1. The upper station 101 and other upper stations (not shown) form an SFN that transmits broadcast waves obtained by frequency-modulating (hereinafter referred to as FM modulation) a carrier wave of the same frequency using the same distributed audio signals, thereby realizing FM synchronized broadcasting (hereinafter simply referred to as synchronized broadcasting). SFN is an abbreviation for Single Frequency Network.

[0017] The FM broadcasting system 100 uses a mono composite signal, which is a new composite signal created using the stereo composite signal mechanism used in FM stereo broadcasting. As shown in FIG. 2 , a mono composite signal for one channel includes frequency components in the 50 Hz to 53 kHz frequency band used in the stereo composite signal. Specifically, the mono composite signal includes a broadcast signal superimposed on the 50 Hz to 15 kHz band of the L+R signal in the stereo composite signal, and a transmission signal superimposed on the 23 kHz to 53 kHz band of the L-R signal in the stereo composite signal. Furthermore, unlike the stereo composite signal, the mono composite signal omits the 19 kHz pilot signal.

[0018] The broadcast signal is a mono audio signal used for playback on an FM radio receiver.

[0019] The transmission signal is a signal with a lower sideband of 23 kHz to 38 kHz and an upper sideband of 38 kHz to 53 kHz, with the 38 kHz carrier suppressed, obtained by carrier suppression amplitude modulation of a 38 kHz carrier using a broadcast signal.

[0020] An FM radio receiver that receives a monaural composite signal reproduces the broadcast signal in the 50 Hz to 15 kHz band as a monaural audio signal because the monaural composite signal does not contain a pilot signal. In other words, the monaural composite signal can be received by existing FM radio receivers.

[0021] Furthermore, the host stations 101 use a clock synchronized with a one-second pulse signal (hereinafter referred to as 1 pps) acquired using GPS or QZSS to control the FM modulation characteristics, including the timing of the FM modulation, so that they are the same among the multiple host stations 101. GPS is an abbreviation for Global Positioning System. QZSS is an abbreviation for Quasi-Zenith Satellite System, a Japanese quasi-zenith satellite system.

[0022] The relay station 102 is provided to enable listening to programs in areas (hereinafter referred to as hearing-impaired areas) that cannot be covered by the upper station 101 due to geographical or other reasons. The relay station 102 is located within the broadcast area A1 of the upper station 101, receives broadcast signals from the upper station 101, and retransmits them at the same frequency to the broadcast area A2, which includes the hearing-impaired areas, thereby realizing relay broadcasting.

[0023] Hereinafter, the area where the broadcast area A1 of the upper station 101 and the broadcast area A2 of the relay station 102 overlap each other will be referred to as an overlapping area Ad.

[0024] [1-2. Configuration of Transmitter] The FM transmitter 1 constituting the upper station 101 will now be described.

[0025] As shown in FIG. 3, the FM transmitter 1 includes a signal processing unit 2, a transmitting unit 3, a power amplifier 4, and a transmitting antenna 5.

[0026] The signal processing unit 2 may be realized entirely by hardware, or at least a part thereof may be realized by processing executed by a microcomputer having a processor 201 and a memory 202, which is a non-transitory tangible recording medium. In this case, various functions realized by the microcomputer are realized by the processor 201 executing a program stored in the memory 202, which is a non-transitory tangible recording medium.

[0027] The signal processing unit 2 includes an audio signal input unit 21 , a pre-emphasis unit 22 , a delay adjustment unit 23 , a transmission signal generation unit 24 , a signal addition unit 26 , and an FM modulation unit 27 .

[0028] The audio signal input unit 21 inputs, via the connector 20, an audio signal for FM broadcasting that is distributed to each upper station 101 via a predetermined transmission network. The audio signal input unit 21 adjusts the timing of the audio signal based on a clock synchronized to 1 pps so that the synchronization points added to the audio signal are synchronized among the multiple upper stations 101. The timing adjusted here is called inter-station synchronization timing.

[0029] The pre-emphasis 22 amplifies the high frequency components of the audio signal in order to correct noise that increases as the frequency increases during FM reception.

[0030] The delay adjustment unit 23 adjusts the delay amount so that the synchronization point added to the audio signal is delayed from the inter-station synchronization timing by a preset adjustment delay time, and outputs the signal. This adjustment delay time is set to be sufficiently longer (for example, 10 ms) than the worst-case transmission delay from the upper station 101 to the relay station 102. The audio signal whose delay has been adjusted by the delay adjustment unit 23 becomes the broadcast signal.

[0031] The transmission signal generating unit 24 amplitude-modulates a 38 kHz carrier wave with the audio signal output from the pre-emphasis 22, and removes the carrier wave component from the modulated signal using a filter, thereby generating a transmission signal.

[0032] The signal addition unit 26 mixes the broadcast signal output from the delay adjustment unit 23 with the transmission signal output from the transmission signal generation unit 24 to generate a mono composite signal having a spectrum as shown in Figure 2.

[0033] 6, the broadcast signal and the transmission signal included in the higher station wave, which is the broadcast wave transmitted by the FM transmitter 1, are mixed at a timing where the broadcast signal is delayed from the transmission signal by an adjustment delay time. Note that in FIG. 6, the transmission signal is shown not as the waveform of the transmission signal itself, but as the waveform of the audio signal demodulated from the transmission signal, i.e., the same waveform as the audio signal used as the broadcast signal.

[0034] 3 , the FM modulator 27 calculates an FM modulation index Δf for each unit period Δt for the transmission signal output from the signal adder 26, and calculates an instantaneous phase change Δθ from the FM modulation index Δf. From this instantaneous phase change Δθ, the FM modulator 27 generates an I signal representing the in-phase component of the transmission signal and a Q signal representing the quadrature component, i.e., a transmission IQ signal.

[0035] The transmitter 3 includes a quadrature converter (hereinafter referred to as QMOD) 31 , a digital-to-analog converter (hereinafter referred to as D / A converter) 32 , a local signal generator 33 , a mixer 34 , and an amplifier 35 .

[0036] The QMOD 31 multiplies the transmission IQ signal generated by the signal processing unit 2 by two carrier signals that are orthogonal to each other and adds the multiplied signals together to generate an FM modulated transmission signal.

[0037] The D / A converter 32 converts the transmission signal, which is represented as a series of digital values, into an analog signal. In the description of the processing downstream of the D / A converter 32, "signal" means an analog signal.

[0038] The local signal generator 33 generates a local signal LO for up-converting the frequency of the transmission signal.

[0039] The mixer 34 generates a broadcast signal RF by mixing the transmission signal generated by the D / A converter 32 with the local signal LO generated by the local signal generator 33 and up-converting the frequency of the transmission signal. The broadcast signal RF is a signal in the frequency band used for FM radio broadcasting (for example, 76 MHz to 108 MHz).

[0040] The amplifier 35 amplifies the broadcasting signal RF generated by the mixer 34 and supplies the amplified signal to the power amplifier 4 .

[0041] The power amplifier 4 further amplifies the broadcast signal generated by the transmitter 3 and supplies it to the transmitting antenna 5. The power amplifier 4 is an amplifier that is set according to the size of the broadcast area A1 covered by the upper station 101, and may be connected in multiple stages or may be omitted.

[0042] The transmitting antenna 5 transmits a broadcast wave corresponding to the broadcast signal RF toward the broadcast area A1.

[0043] [1-3. Configuration of FM Repeater] The FM repeater 10 constituting the repeater station 102 will now be described.

[0044] As shown in FIG. 4, the FM repeater 10 includes a receiving antenna 11 , a receiving section 12 , a relay processing section 13 , a transmitting section 14 , a power amplifier 15 , and a transmitting antenna 16 .

[0045] Hereinafter, as shown in FIG. 5, the broadcast wave that arrives directly from the transmitting antenna 5 of the FM transmitter 1 (i.e., the upstream station 101) to the receiving antenna 11 of the FM repeater 10 (i.e., the relay station 102) is referred to as the upstream station wave D. The reproduced broadcast wave transmitted from the transmitting antenna 16 of the FM repeater 10 is referred to as the relay wave Dr. Furthermore, the relay wave Dr that detouring from the transmitting antenna 16 to the receiving antenna 11 is referred to as the detouring waves U0 to Un. The detouring waves U0 to Un are numbered in order of the strength of reception at the receiving antenna 11. Typically, the detouring wave U0, which has the greatest reception strength, is a direct wave that arrives directly from the transmitting antenna 16 to the receiving antenna 11. The other detouring waves U1 to Un are reflected waves that arrive indirectly after being reflected by some object. Each of the detouring waves U0 to Un has a waveform that is attenuated and delayed from the relay wave Dr.

[0046] Furthermore, the FM repeater 10 transmits the broadcast signal reproduced from the transmission signal contained in the higher station wave D at the same timing as when the broadcast signal contained in the higher station wave D is received.

[0047] 4, the receiving antenna 11 is positioned to receive the higher-order station wave D. The receiving antenna 11 may be an omnidirectional antenna or a directional antenna. The higher-order station wave D is an FM-modulated wave that has been FM-modulated by a monaural composite signal.

[0048] The receiving unit 12 includes an amplifier 41 , a local signal generator 42 , a mixer 43 , an A / D converter 44 , and a quadrature demodulator (hereinafter, referred to as QDEM) 45 .

[0049] The amplifier 41 amplifies the signal received from the receiving antenna 11 .

[0050] The local signal generator 42 generates a local signal LO for down-converting the frequency of the received signal supplied from the receiving antenna 11 .

[0051] The mixer 43 mixes the received signal amplified by the amplifier 41 with a local signal LO supplied from the local signal generator 42, thereby down-converting the frequency of the received signal.

[0052] The A / D converter 44 samples the received signal downconverted by the mixer 43 at a preset sampling frequency. In the description of the processing upstream of the A / D converter 44 in the FM repeater 10, the term "signal" refers to an analog signal, whereas in the description of the processing downstream of the A / D converter 44, the term "signal" refers to a series of digital values. The sampling frequency of the A / D converter 44 is set to approximately several tens of megahertz in order to remove wideband noise components (or broadcast waves of other channels) other than the desired channel (i.e., the band assigned to the broadcast wave D). Specifically, the sampling frequency is set to an integer multiple of the sampling frequency for signal processing, e.g., 49.152 MHz. Furthermore, setting the sampling frequency to an integer multiple of the sampling frequency for signal processing simplifies downsampling, as described below.

[0053] The QDEM 45 performs a Hilbert transform on the received signal to obtain the in-phase and quadrature components for each sample value of the received signal, thereby complexifying the received signal. Specifically, the received signal is multiplied by two carrier signals that are orthogonal to each other (i.e., have a 90° phase difference) to generate an I signal representing the in-phase component and a Q signal representing the quadrature component. The I signal and Q signal are baseband signals. The I signal and Q signal may be downsampled to a frequency that covers a bandwidth at least twice that of the FM-modulated signal, for example, 768 kHz, to reduce the number of data items and reduce the computational load of the subsequent relay processing unit 13. The sampling frequency for this signal processing may be set to a frequency other than 768 kHz, as long as it sufficiently covers the bandwidth of the FM-modulated signal.

[0054] Hereinafter, the I signal and Q signal generated by the receiving unit 12 will be collectively referred to as a received IQ signal.

[0055] The relay processing unit 13 extracts a transmission signal from the received IQ signal generated by the receiving unit 12 and demodulates the extracted transmission signal into an audio signal. Furthermore, the relay processing unit 13 generates a broadcast signal by adding a predetermined delay to the demodulated audio signal, and generates FM-modulated I and Q signals from a monaural composite signal generated by combining the transmission signal, the broadcast signal, and the transmission wave identification signal. Details of the relay processing unit 13 will be described later. Hereinafter, the I and Q signals generated by the relay processing unit 13 will be collectively referred to as a transmission IQ signal.

[0056] The transmitter 14 includes a QMOD 71, a D / A converter 72, a mixer 73, and an amplifier 74. The QMOD 71, the D / A converter 72, the mixer 73, and the amplifier 74 are similar to the QMOD 31, the D / A converter 32, the mixer 34, and the amplifier 35 in the FM transmitter 1, and therefore description thereof will be omitted. However, the mixer 73 upconverts the relay signal using a local signal LO generated by the local signal generator 42 in the receiver 12. The local signal LO may be generated by a local signal generator provided separately from the local signal generator 42.

[0057] The power amplifier 15 further amplifies the relay signal generated by the transmitter 14 and supplies it to the transmitting antenna 16. The power amplifier 15 is an amplifier that is set according to the size of the broadcast area A2 covered by the relay station 102, and may be connected in multiple stages or may be omitted.

[0058] The transmitting antenna 16 transmits a relay wave Dr corresponding to the relay signal toward the broadcast area A2.

[0059] [1-3-1. Relay Processing Unit] The relay processing unit 13 includes a signal regenerating unit 50 and a loop wave removing unit 60.

[0060] The functions of relay processing unit 13 may be realized entirely by hardware, or at least some of them may be realized by processing executed by a microcomputer having processor 131 and memory 132, which is a non-transient physical recording medium. In this case, the various functions realized by the microcomputer are realized by processor 131 executing programs stored in memory 132.

[0061] [1-3-2. Signal Reproduction Unit] The signal reproduction unit 50 includes a channel filter (hereinafter referred to as CH filter) 51, an FM linear detection unit 52, a filter 53, an audio signal demodulation unit 55, a delay adjustment unit 56, a signal addition unit 57, and an FM modulation unit 58.

[0062] The CH filter 51 extracts signals within the frequency range that an FM modulated carrier wave can take from the received IQ signals supplied from the receiver 12 via the loop wave remover 60 .

[0063] The FM linear detector 52 uses the received IQ signal from which unnecessary components have been removed by the CH filter 51 to calculate the phase of the received signal for each preset unit period Δt, and calculates an instantaneous phase change Δθ, which is the difference from the phase calculated in the immediately preceding unit period Δt. The unit period Δt is set to a sampling period Ts, which is the reciprocal of the sampling frequency used for signal processing, or an integral multiple thereof. The calculated instantaneous phase change Δθ is then converted into an FM modulation index using a pre-prepared conversion table or conversion formula to perform Δf detection, thereby extracting a monaural composite signal containing the broadcast signal, transmission signal, and transmission wave identification signal.

[0064] The filter 53 is configured by, for example, a band-pass filter, and removes the broadcast signal from the monaural composite signal supplied from the FM linear detection unit 52 to extract the transmission signal.

[0065] The audio signal demodulation unit 55 generates a monaural audio signal by demodulating the amplitude-modulated transmission signal. Specifically, the transmission signal is mixed with a signal of 38 kHz (i.e., the frequency of the carrier wave used for amplitude modulation), and the difference signal between the two signals is extracted as an audio signal using a filter.

[0066] The delay adjustment unit 56 adjusts the delay of the audio signal so that the audio signal demodulated by the audio signal demodulation unit 55 is delayed by the relay delay time from the reception timing of the transmission signal. As shown in Fig. 6, the relay delay time is the adjustment delay time minus the transmission delay time from the higher station 101 to the relay station 102. In other words, the relay delay time is set so that the broadcast signal contained in the higher station wave D and the broadcast signal contained in the relay wave Dr are received at the same timing near the relay station 102, which is in the overlap area Ad.

[0067] The signal adder 57 mixes the transmission signal extracted by the filter 53 with the broadcast signal, which is a delay-adjusted audio signal, to generate a monaural composite signal.

[0068] The FM modulator 58 generates a transmission IQ signal that is FM modulated by the monaural composite signal generated by the signal adder 57. The specific operation is the same as that of the FM modulator 27 in the FM transmitter 1, and therefore a description thereof will be omitted here.

[0069] [1-3-3. Loop Rejection Unit] The loop rejection unit 60 includes a CH filter 61 , a correlation analysis unit 62 , a profile storage unit 63 , an adaptive filter 64 , and a subtractor 65 .

[0070] The CH filter 61 extracts a signal in the frequency range that an FM modulated carrier wave can take from the transmission IQ signal generated by the signal regeneration unit 50. The CH filter 61 is a filter similar to the CH filter 51 described above.

[0071] The correlation analysis unit 62 calculates the time axis correlation between the received IQ signal output from the echo removal unit 60 during the echo detection period and the transmitted IQ signal supplied from the CH filter 61. The echo detection period is a period during which the delay time for the transmission signal included in the transmitted IQ signal is between 0 and ΔTr. ΔTr is an echo setting time that is set to a value greater than the maximum time required for the relay wave Dr transmitted from the transmitting antenna 16 to be received by the receiving antenna 11 as an echo wave U. The echo setting time ΔTr is set by adjusting the number of taps of the adaptive filter 64, and is set to, for example, approximately 100 μs.

[0072] The correlation analysis unit 62 extracts the received IQ signal and the transmitted IQ signal every preset convolution calculation time To (<ΔTr). Then, during the echo wave detection period 0 to ΔTr, the transmitted IQ signal is sequentially delayed by the time of the sampling period Ts, and the convolution calculation is performed by multiplying the received IQ signal by the complex conjugate signal of the transmitted IQ signal for the convolution calculation time To. The convolution calculation time To is set to a time that allows the signal waveform to be sufficiently identified, for example, approximately 10 ms to identify an audio signal whose main frequency is 100 Hz to several kHz.

[0073] Based on the time-axis correlation that is the result of the convolution operation, the correlation analysis unit 62 extracts a maximum correlation value, which is the maximum value of the correlation coefficient, and a delay time DL (where DL ≠ 0) at which this maximum correlation value is obtained.The correlation analysis unit 62 then stores the signal strength A, phase θ, and delay time DL of the delayed wave estimated from the extraction result as a delay profile.The delay profile is stored in the profile storage unit 63, which is a memory whose contents can be rewritten.Hereinafter, the delay profile generated by the correlation analysis unit 62 and stored in the profile storage unit 63 will be referred to as the delay profile.

[0074] A delay profile generated for each convolution calculation time To is referred to as a generated profile, and a delay profile already stored in the profile storage unit 63 is referred to as an existing profile.

[0075] If there is no existing profile whose delay time DL matches the generated profile, the correlation analysis unit 62 additionally stores the generated profile in the profile storage unit 63. If there is an existing profile whose delay time DL matches the generated profile, the correlation analysis unit 62 updates the content of the existing profile by adding the signal strength A and phase θ of the generated profile to the signal strength A and phase θ of the existing profile.

[0076] As a result of processing by the correlation analysis unit 62, a plurality of delay profiles with different delay times DL are stored in the profile storage unit 63. The delay profile is information that represents the state of the loop signals U0 to Un received by the receiving antenna 11.

[0077] The adaptive filter 64 generates replica IQ signals based on each of the delay profiles stored in the profile storage unit 63. The replica IQ signals are signals obtained by delaying the transmission IQ signals by the delay time DL based on the delay time DL, signal strength A, and phase θ indicated in the delay profile, adjusting the amplitude based on the signal strength A, and adjusting the phase based on the phase θ. The replica IQ signals are a general term for the replica I signals and the replica Q signals that are 90° out of phase with the replica I signals. Hereinafter, the replica IQ signals generated by the adaptive filter 64 will be referred to as replica IQ signals. The adaptive filter 64 generates the same number of replica IQ signals as the number of delay profiles.

[0078] The subtractor 65 subtracts the replica IQ signal generated by the adaptive filter 64 from the received IQ signal supplied from the receiving unit 12 , and supplies the result to the signal regenerating unit 50 .

[0079] [1-4. Operation] The operation of the system will now be described.

[0080] The FM transmitter 1 transmits a monaural composite signal including a broadcast signal and a transmission signal. The monaural composite signal uses the same frequency band as the stereo composite signal, but because it does not include a pilot signal, existing FM radio receivers receive the broadcast wave as an FM mono broadcast and reproduce the broadcast signal, which is a mono audio signal. In other words, the transmission signal and transmission wave identification signal are ignored by FM radio receivers.

[0081] The signal regenerator 50 of the FM repeater 10 extracts a transmission signal from the received monaural composite signal and demodulates the extracted transmission signal to generate an audio signal. The signal regenerator 50 also delays the generated audio signal by a relay delay time to generate a broadcast signal, and mixes this broadcast signal with the previously extracted transmission signal to generate a monaural composite signal. The signal regenerator 50 then transmits a signal FM-modulated by the generated monaural composite signal as a relay wave Dr.

[0082] The transmission signal included in the relay wave Dr transmitted from the FM repeater device 10 is delayed from the transmission signal included in the higher station wave D transmitted from the FM transmitter 1 by the transmission delay from the higher station 101 to the relay station 102. Note that if the processing delay in the FM repeater device 10 cannot be ignored, the transmission delay may include the processing delay in the FM repeater device 10. Furthermore, the broadcast signal included in the relay wave Dr transmitted from the FM repeater device 10 has the same timing as the broadcast signal included in the higher station wave D received by the FM repeater device 10.

[0083] In the loop wave removal unit 60 of the FM repeater 10, the correlation analysis unit 62 generates a delay profile (i.e., a delay profile) for the loop wave Ui (i = 0, 1, ..., n) that is the strongest included in the received IQ signal. Therefore, first, a delay profile for the direct wave U0 is generated and stored in the profile storage unit 63. The adaptive filter 64 generates a replica IQ signal for the direct wave U0 according to the delay profile stored in the profile storage unit 63. The subtractor 65 subtracts the replica IQ signal from the received IQ signal, thereby removing the signal component based on the direct wave U0 from the received IQ signal.

[0084] Subsequently, the loop wave elimination unit 60 performs the same processing on the received IQ signals from which the influence of the direct wave U0 has been removed, thereby generating a new delay profile for the reflected wave U1, which has the maximum intensity excluding the direct wave U0. The contents of the delay profile stored in the profile storage unit 63 are updated with this newly generated delay profile. The adaptive filter 64 generates replica IQ signals for the direct wave U0 and the reflected wave U1 in accordance with the delay profiles stored in memory. The subtractor 65 subtracts the replica IQ signal from the received IQ signals, thereby removing signal components based on the direct wave U0 and the reflected wave U1 from the received IQ signals.

[0085] By repeating the same process thereafter, the signal components based on the feedback signals U0 to Un are sequentially removed from the received IQ signals in descending order of reception strength.

[0086] [1-5. Correspondence of Terms] In this embodiment, the delay adjustment unit 23 corresponds to an example of a delay unit of the present disclosure, and the delay adjustment unit 56 corresponds to an example of a relay delay unit of the present disclosure. In this embodiment, the signal addition unit 26 corresponds to an example of a mixer of the present disclosure, and the signal addition unit 57 corresponds to an example of a relay mixer of the present disclosure. In this embodiment, the transmission unit 3 corresponds to an example of a higher-level station transmission unit of the present disclosure, and the transmission unit 14 corresponds to an example of a relay transmission unit of the present disclosure. In this embodiment, the reception unit 12 corresponds to an example of a relay reception unit of the present disclosure. In this embodiment, the FM linear detection unit 52 corresponds to an example of a signal extraction unit of the present disclosure, and the audio signal demodulation unit 55 corresponds to an example of a relay demodulation unit of the present disclosure. In this embodiment, the correlation analysis unit 62 corresponds to an example of a profile generation unit of the present disclosure, and the adaptive filter 64 and the subtractor 65 correspond to an example of a suppression unit of the present disclosure. In this embodiment, the replica IQ signal corresponds to an example of a replica signal of the present disclosure.

[0087] [1-6. Effects] According to the embodiment described above in detail, the following effects are achieved.

[0088] (1a) In the FM broadcasting system 100, the FM transmitter 1 constituting the upper station 101 transmits an upper station wave D that is FM-modulated using a monaural composite signal that utilizes the FM stereo broadcasting mechanism. The monaural composite signal includes a broadcast signal, which is a monaural audio signal, and a transmission signal, which is an amplitude-modulated version of the broadcast signal and is transmitted earlier than the broadcast signal. Therefore, the FM repeater 10 constituting the relay station 102 can adjust the transmission timing of the broadcast signal contained in the relay wave Dr to coincide with the reception timing of the broadcast signal contained in the upper station wave D by adjusting the delay time of the audio signal demodulated from the transmission signal contained in the upper station wave D. Moreover, this adjustment of the delay time of the broadcast signal can be achieved without modifying existing FM radio receivers or introducing a frequency deviation between the upper station wave D and the relay wave Dr.

[0089] (1b) In the FM repeater 10, the replica IQ signal used to remove signal components based on the loop waves U0 to Un from the received IQ signal is generated from the transmission IQ signal used to generate the repeat wave Dr, i.e., the signal that is the source of the loop waves U0 to Un. Therefore, the FM repeater 10 can accurately suppress the components based on the loop waves U0 to Un that cause oscillation.

[0090] [1-7. Modifications] In this embodiment, the monaural composite signal includes both a lower sideband signal and an upper sideband signal obtained by amplitude modulating the broadcast signal. Because the monaural composite signal does not include a pilot signal, the signal in the L-R signal band is not demodulated by an FM radio receiver. Therefore, the broadcast signal superimposed on this band does not need to be carrier suppression modulated. For example, as shown in Modifications 1 and 2 of FIG. 7 , an SSB (Single Side Band Amplitude Modulation) signal with only the upper sideband or only the lower sideband may be used. When an SSB modulated signal is used, it is sufficient that either the upper sideband or the lower sideband is within the 23 kHz to 53 kHz range. As shown in Modification 3 of FIG. 7 , the carrier frequency may be other than 38 kHz. Furthermore, the transmission signal is not limited to amplitude modulation, and other modulation methods, such as digital modulation, may also be used.

[0091] [2. Second Embodiment] [2-1. Differences from the First Embodiment] The second embodiment has the same basic configuration as the first embodiment, so differences will be described below. Note that the same reference numerals as in the first embodiment indicate the same configuration, and the preceding description will be referred to.

[0092] In the first embodiment described above, the monaural composite signal is configured to include a broadcast signal and a transmission signal. In the second embodiment, an SSB-modulated broadcast signal is used as the transmission signal, and a signal for identifying the transmitted wave is superimposed on an unused band of the band corresponding to the L-R signals of the stereo composite signal, which is different from the first embodiment.

[0093] Furthermore, the configuration of the signal processing unit 2a belonging to the FM transmitter 1 and the configuration of the signal regenerating unit 50a belonging to the FM repeater 10 are partially different from those in the first embodiment.

[0094] [2-2. Monaural Composite Signal] In the monoaural composite signal of this embodiment, a transmission signal and a transmission wave identification signal are superimposed on the band in which the L-R signals are superimposed in the stereo composite signal, as shown in Configuration Example 1 of Fig. 8. For example, the lower sideband is used as the transmission signal, and the transmission wave identification signal is superimposed on the 38 kHz to 53 kHz range that was assigned to the upper sideband.

[0095] The transmitted wave identification signal is a signal for identifying which upper station 101 or which relay station 102 is the generator of the monaural composite signal, and therefore the transmitter of the broadcast wave FM-modulated by the monaural composite signal. The transmitted wave identification signal is assigned a frequency belonging to an unused frequency band that is not used for broadcast signals or transmission signals, within the 0 to 53 kHz band of the monaural composite signal. Different frequencies may be assigned to the upper station 101 and the relay station 102. Alternatively, the transmitted wave identification signal may be a signal obtained by modulating a carrier wave of a frequency belonging to an unused frequency band with a unique identification number assigned to each of the upper station 101 and the relay station 102.

[0096] As shown in configuration example 2 of Figure 8, the mono composite signal may have the transmission signal centered at 38 kHz, and the transmission wave identification signal may be placed in a band between the broadcast signal and the transmission signal.

[0097] [2-3. Signal Processing Unit of FM Transmitter] As shown in FIG. 9, the signal processing unit 2 a includes an identification signal assigning unit 25 between a transmission signal generating unit 24 and a signal adding unit 26 .

[0098] The identification signal assigning section 25 assigns a transmission wave identification signal assigned to the FM transmitter 1 to the transmission signal generated by the transmission signal generating section 24 .

[0099] 2-4. Relay Processing Section of FM Relay Device As shown in FIG. 10, the relay processing section 13 a includes a signal regenerating section 50 a and a loop wave removing section 60 .

[0100] The signal regenerator 50 a includes an identification signal assigner 54 between a filter 53 and a signal adder 57 .

[0101] The identification signal assigning section 54 is configured similarly to the identification signal assigning section 25 and assigns a transmission wave identification signal assigned to the FM repeater 10 to the transmission signal supplied from the filter 53 .

[0102] [2-5. Correspondence of Terminology] In this embodiment, the identification signal assigning unit 25 corresponds to an example of a higher-level station information assigning unit of the present disclosure, and the identification signal assigning unit 54 corresponds to an example of a relay station information assigning unit of the present disclosure.

[0103] [2-6. Effects] According to the second embodiment described above in detail, in addition to the effects (1a) to (1g) of the first embodiment described above, the following effects are also achieved.

[0104] (2a) The monochrome composite signal is superimposed with transmission wave identification information that distinguishes between the higher-order station wave D and the relay wave Dr (and thus the return wave U). The transmission wave identification signal exists even during silence when the broadcast signal and transmission signal are at noise level, making it easier to analyze the return wave U and suppressing malfunctions during silence.

[0105] 3. Other Embodiments Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments and can be implemented in various modifications.

[0106] (3a) In the above-described embodiment, the technology of the present disclosure is applied to the FM broadcasting system 100 that constitutes an SFN, but it may also be applied to an FM broadcasting system that constitutes an MFN. MNF is an abbreviation for Multi-Frequency Network.

[0107] (3b) Multiple functions of one component in the above embodiments may be realized by multiple components, or one function of one component may be realized by multiple components. Also, multiple functions of multiple components may be realized by one component, or one function realized by multiple components may be realized by one component. Also, part of the configuration of the above embodiments may be omitted. Also, at least part of the configuration of the above embodiments may be added to or substituted for the configuration of another of the above embodiments.

[0108] (3c) The present disclosure can be realized in various forms, including the above-mentioned FM transmitter 1, FM repeater 10, and FM broadcasting system 100, as well as a program for causing a computer to function as the FM transmitter 1 and FM repeater 10, a non-transient physical recording medium such as a semiconductor memory on which this program is recorded, and an FM wave relay method.

Claims

1. An FM transmitter configured to transmit a higher-level station wave FM-modulated by a monaural composite signal; and an FM repeater configured to receive the higher-level station wave transmitted from the FM transmitter and transmit a relay wave FM-modulated by the monaural composite signal reproduced from the higher-level station wave, wherein the monaural composite signal has a structure in which a broadcast signal which is a monaural audio signal is superimposed on the band of the L+R signal in the stereo composite signal, a transmission signal modulated by the broadcast signal is superimposed on the band of the L-R signal in the stereo composite signal, and the pilot signal in the stereo composite signal is omitted, and the FM transmitter is configured to generate the monaural composite signal by mixing the transmission signal modulated by the monaural audio signal with the broadcast signal obtained by delaying the monaural audio signal by an adjustment delay time, The FM repeater is configured to generate the mono composite signal by mixing the transmission signal extracted from the received higher station wave and the broadcast signal obtained by delaying the mono audio signal demodulated from the transmission signal by a relay delay time, the adjustment delay time being set to a time longer than the transmission delay time from the FM transmitter to the FM repeater, and the relay delay time being set to a length obtained by subtracting the transmission delay time from the adjustment delay time.

2. An FM broadcasting system according to claim 1, wherein the monaural composite signal has a structure in which a transmission wave identification signal indicating the source of the monaural composite signal is further superimposed on the band of the L-R signal in the stereo composite signal.

3. An FM transmitting device comprising: a modulation section configured to generate a transmission signal modulated by an input monaural audio signal; a delay section configured to generate a broadcast signal by delaying the monaural audio signal by an adjustment delay time; a mixing section configured to generate a monaural composite signal by mixing the transmission signal generated by the modulation section with the broadcast signal generated by the delay section; an FM modulation section configured to generate a transmission signal FM modulated by the monaural composite signal generated by the mixing section; and a higher station transmitting section configured to transmit a higher station wave based on the transmission signal generated by the FM modulation section, wherein the monaural composite signal has a structure in which the broadcast signal is superimposed on the band of the L+R signal in a stereo composite signal, the transmission signal is superimposed on the band of the L-R signal in the stereo composite signal, and the pilot signal in the stereo composite signal is omitted, and the adjustment delay time is set to a time longer than the transmission delay time from the transmission of the higher station wave to an FM repeater that relays the higher station wave. FM transmitter.

4. An FM transmitter according to claim 3, further comprising a higher station information assigning section that assigns a transmission wave identification signal for identifying the source of the higher station wave to the monaural composite signal.

5. An FM repeater device comprising: a repeater receiving unit configured to receive higher-level station waves FM modulated by a monaural composite signal; a signal extracting unit configured to extract a transmission signal from the received signal received by the repeater receiving unit; a repeater demodulating unit configured to demodulate the transmission signal to generate a monaural audio signal; a repeater delaying unit configured to generate a broadcast signal by delaying the monaural audio signal generated by the repeater demodulating unit by a relay delay time; a repeater mixing unit configured to generate the monaural composite signal by mixing the transmission signal extracted by the signal extracting unit with the broadcast signal generated by the repeater delaying unit; a repeater FM modulating unit configured to generate a repeater signal FM modulated by the monaural composite signal generated by the repeater mixing unit; and a repeater transmitting unit configured to transmit a repeater wave based on the repeater signal generated by the repeater FM modulating unit, wherein the monaural composite signal is An FM repeater having a structure in which the broadcast signal is superimposed on the band of the L+R signal in a stereo composite signal, and the transmission signal modulated by the broadcast signal is superimposed on the band of the L-R signal in the stereo composite signal, and the pilot signal in the stereo composite signal is omitted; the monaural composite signal included in the higher station wave is set so that the broadcast signal is delayed from the transmission signal by an adjustment delay time, and the adjustment delay time is set to a time longer than the transmission delay time from an FM transmitting device that transmits the higher station wave to the FM repeater; and the relay delay time is set to a length obtained by subtracting the transmission delay time from the adjustment delay time.

6. An FM repeater according to claim 5, further comprising a repeater station information adding section that adds a transmission wave identification signal for identifying the source of the repeater wave to the monaural composite signal.

7. An FM repeater device according to claim 5 or 6, comprising: a profile generation unit configured to calculate a time axis correlation using the relay signal generated by the relay FM modulation unit and the received signal received during a loop wave detection period, and to generate a delay profile which is information including a maximum correlation value which is the maximum value of the time axis correlation, and the delay time of the relay signal relative to the received signal when the maximum correlation value is obtained; a suppression unit configured to delay the relay signal generated by the relay FM modulation unit by the delay time in accordance with the delay profile and adjust the strength and phase according to the maximum correlation value to generate a replica signal, and to subtract the replica signal from the received signal input to the signal extraction unit; and the loop wave detection period is set to the period from when the relay signal is generated by the relay FM modulation unit until the loop wave setting time has elapsed, using a loop setting time which is set based on the time required for the relay wave transmitted by the relay transmitting unit to be received by the relay receiving unit as a loop wave.

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