JP5740252B2 - OFDM transmitter and receiver for wireless microphone - Google Patents

Description

  The present invention relates to an OFDM transmitter and receiver for wireless microphones that transmit and receive digital audio signals by OFDM modulation.

  Conventionally, there are an analog method and a digital method as a transmission method of a wireless microphone. Non-Patent Document 1 describes the standards established for “radio equipment for land mobile stations of specific radio microphones”, and Non-Patent Document 2 describes “radio equipment for specific low-power radio stations radio microphones”. Standards written are written.

  An analog wireless microphone has a short delay time and is currently widely used, but has a problem that it is easily interrupted by an obstacle, has a short transmission distance, and easily interferes. Therefore, it is necessary to use a digital wireless microphone to provide high-quality sound outdoors or at a concert hall.

  For example, Patent Literature 1 discloses a wireless microphone system that compresses and encodes and transmits audio in a digital manner. FIG. 15 is a block diagram showing the configuration of such a conventional wireless microphone system. The wireless microphone transmission device 3 includes a microphone 31, an A / D conversion unit 32, a compression encoding unit 33, an interleave / error correction unit 34, a modulation unit 35, a D / A conversion unit 36, and a transmission frequency conversion. A unit 37 and a transmission antenna 38 are provided. The wireless microphone transmission device 3 converts an analog audio signal input from the microphone 31 into a digital signal by the A / D conversion unit 32, compresses and encodes the digital signal by the compression encoding unit 33, and interleaves and error correction unit 34. Interleave and error correction. Subsequently, in the wireless microphone transmission device 3, the modulation unit 35 modulates, for example, with the π / 4 shift DQPSK modulation method, the D / A conversion unit 36 converts the modulation signal into an analog signal, and the transmission frequency conversion unit 37 transmits the transmission frequency. And output to the transmitting antenna 38.

  The wireless microphone receiver 4 includes a reception antenna 41, a reception frequency conversion unit 42, an A / D conversion unit 43, a demodulation unit 44, a deinterleave / error correction unit 45, a decompression decoding unit 46, a D / A A conversion unit 47 and a speaker 48 are provided. The wireless microphone receiver 4 frequency-converts the signal input from the reception antenna 41 by the reception frequency conversion unit 42, converts it to a digital signal by the A / D conversion unit 43, and modulates the signal on the transmission side by the demodulation unit 44. The modulated signal is demodulated, and deinterleave and error correction unit 45 performs deinterleave and error correction. Subsequently, in the wireless microphone receiver 4, the decompression decoding unit 46 decompresses the signal compressed on the transmission side, the D / A conversion unit 47 converts the decompressed signal into an analog signal, and outputs the analog signal to the speaker 48.

  However, in the conventional digital wireless microphone system, in order to save the frequency band, the compression encoding unit 33 performs compression processing, and the expansion decoding unit 46 performs expansion processing of the signal. There is a delay time due to the process. In the conventional digital wireless microphone system shown in FIG. 15, a delay time of about 3 ms occurs in the wireless microphone transmission device 3 and the wireless microphone reception device 4 in total. Among them, the delay time due to the compression processing of the compression encoding unit 33 and the expansion processing of the expansion decoding unit 46 is said to be about 1 ms in total. In addition, when using a wireless microphone outdoors or while moving, fading due to multipath occurs and the quality deteriorates. Therefore, it is conceivable that a digital audio signal is modulated and transmitted by an OFDM (Orthogonal Frequency Division Multiplexing) modulation method.

JP-A-10-150692

“Radio equipment of land mobile stations with specific radio microphones”, ARIB RCR STD-22, The Japan Radio Industry Association "Radio equipment for specified low-power radio stations radio microphones", ARIB RCR STD-15, Japan Radio Industry Association

  However, in the conventional digital wireless microphone system, in order to save the frequency band, the compression encoding unit 33 performs compression processing, and the expansion decoding unit 46 performs expansion processing of the signal. There is a delay time due to the process. In the conventional digital wireless microphone system shown in FIG. 15, a delay time of about 3 ms occurs in the wireless microphone transmission device 3 and the wireless microphone reception device 4 in total. Among them, the delay time due to the compression processing of the compression encoding unit 33 and the expansion processing of the expansion decoding unit 46 is said to be about 1 ms in total.

  In addition, when using a wireless microphone outdoors or while moving, fading due to multipath occurs and the quality deteriorates.

  In order to solve the above problems, an object of the present invention is to provide an OFDM transmitter and receiver for a wireless microphone that reduce the delay time due to the transmission and reception of audio signals and prevent the degradation of reception quality due to multipath fading. It is in.

In order to solve the above problems, an OFDM transmitter for a wireless microphone according to the present invention is an OFDM transmitter for a wireless microphone that transmits a digital audio signal by an OFDM modulation method, and inputs the digital audio signal in units of blocks. An inner code encoder that generates an inner code by performing inner encoding for each block, an OFDM modulator that generates an OFDM signal by modulating the inner code using an OFDM modulation method, and an inner code encoder. that No the number of bits of data in block units, when the coding rate of the inner code Ri, M a modulation level of the OFDM signal, the number of data carriers of the OFDM signal was Nd, symbol O FDM signal A transmission parameter for setting the No, the Ri, the M, and the Nd so that the length is an integral multiple of the sampling interval of the audio signal. And a meter setting unit.

In the wireless microphone OFDM transmitter according to the present invention, the transmission parameter setting unit sets the No, the Ri, the M, and the Nd so that No = Ri × M × Nd. Features. The transmission parameter setting unit may set the No, the Ri, the M, and the Nd so that an effective symbol length of the OFDM signal is 66.6 μs or more and 230 μs or less. .

  The wireless microphone OFDM transmitter according to the present invention further includes an outer code encoding unit that performs block encoding of a digital speech signal to generate an outer code, and the inner code encoding unit includes the outer code encoding unit. An inner code is generated by inner-coding the outer code generated by the unit, and the transmission parameter setting unit sets the number of bits of the code length of the outer code to No.

  Also, in the wireless microphone OFDM transmitter according to the present invention, the transmission parameter setting unit may be Ro × Ri × M × Nd regardless of the parameter setting mode when the encoding rate of the outer code is Ro. The Ro, the Ri, the M, and the Nd are set so that the values are constant.

  Further, in the wireless microphone OFDM transmitter according to the present invention, the transmission parameter setting unit sets the Ro and the No so that the Ro is constant and the No is constant. .

  In order to solve the above problems, an OFDM receiver for a wireless microphone according to the present invention receives an OFDM signal transmitted by the above-described OFDM transmitter for a wireless microphone and generates a digital audio signal. To do.

  According to the present invention, it is possible to reduce a delay time due to transmission / reception of an audio signal and to prevent a reduction in reception quality due to multipath fading.

It is a block diagram which shows the structure of the OFDM transmitter for wireless microphones which concerns on Example 1 of this invention. It is a block diagram which shows the structure of the OFDM receiver for wireless microphones based on Example 1 of this invention. It is a figure which shows the example which samples an audio | voice signal. It is a block diagram which shows the structure of the OFDM transmitter for wireless microphones which concerns on Example 2 of this invention. It is a block diagram which shows the structure of the OFDM receiver for wireless microphones based on Example 2 of this invention. It is a figure which shows the 1st parameter example used with the wireless microphone system which concerns on this invention. It is a figure which shows the example of arrangement | positioning of SP signal and TMCC signal of the carrier symbol of 1 segment in the 1st parameter example shown in FIG. It is a figure which shows the 2nd parameter example used with the wireless microphone system which concerns on this invention. It is a figure which shows the example of arrangement | positioning of SP signal and TMCC signal of the carrier symbol of 1 segment in the 2nd parameter example shown in FIG. It is a figure which shows the 3rd parameter example used with the wireless microphone system which concerns on this invention. It is a figure which shows the example of arrangement | positioning of SP signal and TMCC signal of the carrier symbol of 1 segment in the 3rd parameter example shown in FIG. It is a figure which shows the 4th example of a parameter used with the wireless microphone system which concerns on this invention. It is a figure which shows the example of arrangement | positioning of SP signal and TMCC signal of the carrier symbol of 1 segment in the 4th parameter example shown in FIG. It is a figure which shows the parameter example in the case of performing the compression process set by the OFDM transmitter for wireless microphones based on Example 2 of invention. It is a block diagram which shows the structure of the conventional wireless microphone system of a digital system.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[OFDM transmitter for wireless microphone]
FIG. 1 is a block diagram illustrating a configuration of a wireless microphone OFDM transmitter according to a first embodiment of the present invention. As shown in FIG. 1, an OFDM transmitter 1a for a wireless microphone includes a microphone 11, an A / D converter 12, an interleave / error corrector 13, an OFDM modulator 14, a D / A converter 15, A transmission frequency conversion unit 16, a transmission antenna 17, a transmission parameter setting unit 18, a crystal oscillator 19 (19-1 to 19 -n), and a clock supply unit 20 are provided. The interleave / error correction unit 13 includes an outer code encoding unit 131, an interleaving unit 132, and an inner code encoding unit 133. The OFDM modulation unit 14 includes an S / P conversion unit 141, a carrier modulation unit 142, and the like. OFDM frame configuration section 143, IFFT section 144, and GI addition section 145.

  The A / D conversion unit 12 converts an analog audio signal input from the microphone 11 into a digital signal and outputs the digital signal to the outer code encoding unit 131.

  Outer code encoding section 131 adds an RS code, BCH code, difference set cyclic code, or CRC, and outputs the result to interleaving section 132. This is because error correction is performed on the receiving side, or error detection is performed and concealment is applied to an erroneous block. In particular, the delay time can be reduced by using the BCH code. For example, in the case of the RS (204, 188) code, the number of bits included in one code block is 204 bytes = 1632 bits, and the delay time due to encoding and decoding is 1310 μs. On the other hand, in the case of the BCH (144, 128) code, the number of bits included in one code block is 144 bits, and the delay time due to encoding and decoding is 115 μs. When a code having a code length No is generated from data having an information length Ko by block coding, this code is represented as a (No, Ko) code, and Ro = Ko / No is referred to as a coding rate. The code length No means the block length after outer coding. In order to distinguish from the coding rate of the inner code described later, the coding rate Ro of the outer code is referred to as the outer coding rate.

  Interleaving section 132 rearranges the order of outer codes input from outer code encoding section 131 and outputs the result to inner code encoding section 133 in order to increase the efficiency of error correction.

  The inner code encoding unit 133 performs inner encoding (for example, convolutional encoding) on the signal input from the interleaving unit 132 and outputs the signal to the S / P conversion unit 141. In general, when a code having a code length Ni is generated from data having an information length Ki by inner coding, Ri = Ki / Ni is referred to as a coding rate. In order to distinguish from the above-described outer coding rate (outer coding rate), the coding rate Ri of the inner code is referred to as an inner coding rate.

  The S / P converter 141 temporarily stores the inner code input from the inner code encoder 133 in a storage area such as an internal memory, and converts it into parallel data when a predetermined number of data is reached. And output to the carrier modulation unit 142. For example, when the number of data carriers is Nd and the modulation level of the modulation scheme of each carrier is M, the data is converted into Nd signals by M bits.

  The carrier modulation unit 142 performs mapping to the IQ plane according to a predetermined modulation method (modulation multilevel number M) for each carrier for a signal input in parallel from the S / P conversion unit 141 every M bits, A carrier modulation signal is generated and output to OFDM frame configuration section 143.

  The OFDM frame configuration unit 143 generates an OFDM segment frame by inserting and arranging a pilot signal with respect to the carrier modulation signal input from the carrier modulation unit 142 and outputs the OFDM segment frame to the IFFT unit 144. Since the pilot signal has a known amplitude and phase at the time of signal generation, the transmission path characteristics can be estimated on the receiving side. The OFDM frame configuration unit 143 may insert a CP (Continual Pilot) signal continuously arranged in the symbol direction in addition to SP (Scattered Pilot) signals arranged in a distributed manner as a pilot signal. Further, the OFDM frame configuration unit 143 inserts a TMCC (Transmission and Multiplexing Configuration Control) signal that is a signal for transmitting control information and an AC (Auxiliary Channel) signal that is a signal for transmitting additional information. Also good.

  The IFFT unit 144 performs an IFFT (Inverse Fast Fourier Transform) process on the OFDM segment frame input from the OFDM frame configuration unit 143 to generate an effective symbol signal, which is output to the GI adding unit 145 To do.

  The GI adding unit 145 inserts a guard interval obtained by copying the latter half of the effective symbol signal at the head of the effective symbol signal input from the IFFT unit 144, and outputs the guard interval to the transmission rate adjustment buffer memory 145. The guard interval is inserted in order to reduce intersymbol interference when receiving an OFDM signal, and is set so that the delay time of the multipath delay wave does not exceed the guard interval length.

  The D / A converter 15 converts the digital signal input from the transmission rate adjustment buffer memory 145 into an analog signal. The transmission frequency conversion unit 16 modulates the analog signal input from the D / A conversion unit 15 to a transmission frequency, amplifies the power, and outputs the modulated signal to the reception antenna via the transmission antenna 17. Send.

  The clock supply unit 20 selects the crystal oscillator 19 according to the symbol rate of the carrier of the OFDM signal, generates a clock, and supplies the clock to the interleave / error correction unit 13 and the OFDM modulation unit 14.

  The transmission parameter setting unit 18 sets the information length Ko and the code length No for the outer code encoding unit 131 as parameters, sets the interleave parameter for the interleaving unit 132, and sets the interleave parameter for the inner code encoding unit 133. The inner coding rate Ri is set, the modulation multi-level number M and the data carrier number Nd are set for the S / P converter 141, and the modulation scheme type and the data carrier number Nd are set for the carrier modulation unit 142. The IFFT unit 144 sets the number of FFT points, the modulation multi-level number M, and the number of data carriers Nd, and the GI adding unit 145 sets the guard interval ratio. Of course, if the modulation multi-level number M is different for each type of modulation scheme, the modulation multi-level number M may be set in the carrier modulation unit 142.

  If the code length of the outer code by the outer code encoding unit 131 is No, the data amount a per block output from the outer code encoding unit 131 is equal to No (the unit is bits). Further, if the inner coding rate by the inner code coding unit 133 is Ri, the modulation multi-level number of the modulation scheme in the carrier modulation unit 142 is M, and the number of data carriers of the OFDM signal is Nd, the data amount per symbol of the OFDM signal b is represented by Ri × M × Nd (the unit is bits).

  In the case of a = b, a bit of the code length output every time processing is performed from the outer code encoding unit 131 and the interleaving unit 132 in units of blocks, and the inner code encoding unit 133 and the OFDM modulation unit 14 in units of symbols. Since the b bits of the amount of data to be processed are equal, the inner code encoding unit 133 and the OFDM modulation unit 14 buffer the a bit (= b bits) data output from the outer code encoding unit 131 and the interleaving unit 132. Then, the inner code encoding process and the OFDM modulation process can be immediately executed. Therefore, the transmission parameter setting unit 18 controls and sets parameters so that the code length No satisfies the following expression (1).

  No = Ri × M × Nd (1)

  It is also possible to set the outer coding rate Ro = 1. In this case, the outer code coding unit 131 is unnecessary, and the inner code coding unit 133 inputs a digital audio signal in units of blocks. The inner code is generated by performing inner coding for each block. In this case, the parameters are set so as to satisfy Expression (1), where the bit length of the block unit data input to the inner code encoding unit is No.

  By setting parameters such that No = Ri × M × Nd, OFDM signals can be generated continuously, so that a buffer memory for transmission rate adjustment is not required, and the outer code encoding unit 131, interleaver The delay of the data output from the inner code encoding unit 133 and the OFDM modulation unit 14 with respect to the data input to the unit 132 can be reduced.

  Further, the transmission parameter setting unit 18 defines a relationship between the symbol length Ts of the OFDM signal and the sampling frequency fs of the audio signal by setting parameters. If the number of audio samples m to be transmitted per symbol is not an integer, there will be sampling data that cannot be decoded unless the next symbol data is also received on the receiving side. FIG. 3 is a diagram illustrating an example of sampling an audio signal at a sampling frequency fs (sampling interval 1 / fs). As shown in FIG. 3, when the number m of audio samples transmitted per symbol is 4.3, the sampling data m1 to m4 at time t1 to t4 are decoded and reproduced when the first symbol data is received. Can do. However, since the sampling data m5 at time t5 can be reproduced only after the data of the second symbol is received and decoded, a delay of one symbol occurs. Therefore, the transmission parameter setting unit 18 sets the parameters so that the symbol length Ts satisfies the following equation (2), that is, the symbol length Ts of the OFDM signal is an integral multiple of the sampling interval 1 / fs of the audio signal. Control and set.

  Ts = m × 1 / fs (m = integer) (2)

  Further, the symbol length Ts is obtained by the following equation (3). Here, Vsc is a symbol rate of one carrier, Vs is a symbol rate of the whole OFDM signal, and Vo is a rate after outer coding. Therefore, the symbol length Ts can be defined by the values of the parameters Vo, Ri, M, and Nd.

Ts = 1 / Vsc
= Nd / Vs
= Nd / (Vo × (1 / Ri) × (1 / M)) (3)

[OFDM receiver for wireless microphone]
Next, an OFDM receiver for a wireless microphone according to the present invention will be described. FIG. 2 is a block diagram illustrating a configuration of the wireless microphone OFDM receiver according to the first embodiment of the present invention. As shown in FIG. 2, the wireless microphone OFDM receiving apparatus 2a includes a receiving antenna 21, a receiving frequency converting unit 22, an A / D converting unit 23, an OFDM demodulating unit 24, a deinterleaving / error correcting unit 25, , A D / A conversion unit 26, a speaker 27, a reception parameter setting unit 28, a crystal oscillator 29 (29-1 to 29-n), and a clock supply unit 30. The OFDM demodulation unit 24 includes a GI removal unit 241, an FFT unit 242, a carrier demodulation unit 243, and a P / S conversion unit 244. The deinterleave / error correction unit 25 includes an inner code decoding unit 251 and a deinterleave unit. Unit 252 and outer code decoding unit 253.

  The reception frequency conversion unit 22 amplifies the power of the modulation signal received by the reception antenna 21, converts the frequency of the data, and outputs the data to the A / D conversion unit 23. The A / D conversion unit 23 converts the analog signal input from the reception frequency conversion unit 22 into a digital signal and outputs the digital signal to the GI removal unit 241.

  The GI removal unit 241 removes the guard interval from the digital signal input from the A / D conversion unit 23, extracts a valid symbol, and outputs the effective symbol to the FFT unit 242.

  The FFT unit 242 performs FFT (Fast Fourier Transform) processing on the effective symbol input from the GI removal unit 241 and outputs the result to the carrier demodulation unit 243.

  The carrier demodulator 243 demodulates the signal input from the FFT unit 242 for each carrier and outputs the demodulated signal to the P / S converter 244. When demodulating, the SP signal is extracted and compared with a reference value (known amplitude and phase) to calculate the channel characteristic of the carrier in which the SP signal exists, and the calculated channel characteristic is calculated in the time direction and Interpolate in the frequency direction to calculate the estimated values of the transmission path characteristics of all OFDM carriers.

  The P / S converter 244 converts the signal input in parallel from the carrier demodulator 243 into a serial signal.

  The inner code decoding unit 251 performs inner code decoding on the inner code input from the P / S conversion unit 244 to generate an outer code, and outputs the outer code to the deinterleaving unit 252. In addition, when the transmission side performs the inner coding by the convolutional coding, the inner code decoding unit 251 performs Viterbi decoding to correct the error, and outputs the error to the deinterleaving unit 252.

  The deinterleaving unit 252 rearranges the data order with respect to the outer code input from the inner code decoding unit 251, and outputs the rearranged data to the reception rate adjustment buffer memory 254.

  Outer code decoding section 253 decodes a code encoded by outer code encoding section 131 of wireless microphone OFDM transmitter 1a using an outer code such as a BCH code. If the wireless microphone OFDM transmitter 1a does not have the outer code encoder 131 with the outer code rate Ro = 1, the outer code decoder 253 is also unnecessary.

  The D / A conversion unit 26 converts the digital signal input from the outer code decoding unit 253 into an analog signal and outputs the analog signal to the speaker 27.

  The reception parameter setting unit 28 sets the same parameters as those set in the wireless microphone OFDM transmitter 1a in each block. For example, the guard interval ratio is set for the GI removal unit 241, the number of FFT points, the modulation multilevel number M, and the number of data carriers Nd are set for the FFT unit 242, and the modulation scheme is set for the carrier demodulation unit 243. Type (modulation multi-level number M) and data carrier number Nd are set, the inner coding rate Ri is set for the inner code decoding unit 251, and the interleave parameter is set for the deinterleaving unit 252. An information length Ko and a code length No are set for the outer code decoding unit 253. The parameter may be received from the wireless microphone OFDM transmitter 1a, or the parameter information may be acquired from the TMCC signal.

  Here, since the parameter set by the reception parameter setting unit 28 satisfies the condition of No = Ri × M × Nd in the equation (1), the reception rate adjustment is performed in the same manner as the wireless microphone OFDM transmitter 1a. No buffer memory is required.

  The clock supply unit 30 generates a clock by selecting the crystal oscillator 29 according to the symbol rate (parameter setting mode) of the carrier of the OFDM signal, and supplies the clock to the OFDM demodulation unit 24 and the deinterleave / error correction unit 25. Supply.

[Effective symbol length]
Next, regarding the effective symbol length Tu of the OFDM signal in the wireless microphone system according to the present invention, the delay time between transmission and reception, that is, the voice input to the microphone 11 of the wireless microphone OFDM transmitter 1a is the wireless microphone OFDM receiver 2a. The optimum value is examined from the viewpoint of the delay time until output from the speaker 27 and the delay time of the reflected wave with respect to the fundamental wave due to multipath.

First, consider the delay time between transmission and reception. According to the subjective evaluation, it is said that when the delay time between transmission and reception is about 2 ms or less, it becomes difficult to detect the delay, and the delay becomes almost unnoticeable. Therefore, in this embodiment, the effective symbol length Tu is determined so that the delay time between transmission and reception is 2 ms or less. The delay time by the A / D converter 12 is about 400 μs, and the delay time by the D / A converter 26 is about 400 μs. The total delay time by the outer code encoding unit 131 and the outer code decoding unit 253 is about 115 μs when the BCH code is used. The total delay time by the interleaving unit 132 and the deinterleaving unit 252 is about 125 μs. The total delay time of the inner code encoder 133 and the inner code decoder 251 is about 270 μs. Then, in order to set the delay time in the entire transmission and reception to 2 ms or less, the total delay time T _OFDM of the OFDM modulation unit 14 and the OFDM demodulation unit 24 needs to be set to 690 μs or less. Since the delay time of about 3 symbol length is generated in the processing of the OFDM modulation unit 14 and the OFDM demodulation unit 24, the effective symbol length Tu needs to satisfy the condition of the following equation (4).

Tu ≦ T _OFDM / 3 (4)

  Next, consider the maximum delay time due to multipath. When the maximum value of the propagation distance difference is L, the maximum delay time τ of the reflected wave is expressed by τ = L / c using the speed of light c. Under the use environment of the wireless microphone, even when an omnidirectional antenna is used, the maximum propagation distance difference L between the direct wave and the reflected wave is about 2000 m. In order to prevent carrier waveform distortion due to multipath fading, the effective symbol length Tu needs to be about 10 times the delay dispersion or more. Therefore, the effective symbol length Tu needs to satisfy the condition of the following equation (5). Regarding the point that the effective symbol length Tu needs to be about 10 times the delay dispersion, see, for example, Takashi Shono, “Impress Standard Textbook Series WiMAX Textbook”, Impress R & D, July 16, 2008, P71. See description.

  Tu ≧ τ × 10 = 10 L / c (5)

If T _OFDM ≦ 690 [μs] in equation (3) and L = 2000 [m] in equation (5), the effective symbol length Tu needs to satisfy the following equation (6). Therefore, the transmission parameter setting unit 18 sets parameters so that the effective symbol length Tu is within the range of the following equation (6). A specific parameter example will be described in the second embodiment.

  66.6 [μs] ≦ Tu ≦ 230 [μs] (6)

  As described above, according to the wireless microphone OFDM transmitter 1a and the wireless microphone OFDM receiver 2a according to the first embodiment, the OFDM modulation scheme is adopted and the guard interval is added, so that the reception quality is deteriorated due to multipath fading. Can be prevented. In addition, since the parameter is set to No = Ri × M × Nd, the rate adjustment buffer memory can be eliminated, and the delay of the audio signal due to transmission / reception can be reduced. Also, by setting the OFDM signal symbol length Ts to an integer multiple of the audio signal sampling interval, the wireless microphone OFDM receiving apparatus 2a can decode and reproduce the sampling data without waiting for the next symbol.

  In particular, by setting the parameters so that the effective symbol length Tu is within the range of Expression (6), the delay time between transmission and reception can be set to 2 ms or less.

  Next, a wireless microphone OFDM transmitter and receiver according to a second embodiment of the present invention will be described. The same components as those of the wireless microphone OFDM transmitter 1a and the wireless microphone OFDM receiver 2a according to the first embodiment illustrated in FIGS. 1 and 2 are denoted by the same reference numerals, and description thereof is omitted. FIG. 4 is a block diagram illustrating a configuration of an OFDM transmitter for wireless microphone according to a second embodiment of the present invention. Compared to the wireless microphone OFDM transmitter 1a according to the first embodiment shown in FIG. 1, the wireless microphone OFDM transmitter 1b only requires one crystal oscillator 19 and the operation of the transmission parameter setting unit 18 is improved. Is different.

  Similar to the first embodiment, the transmission parameter setting unit 18 sets No = Ri × M × Nd, and sets the symbol length Ts of the OFDM signal to an integer multiple of the sampling interval 1 / fs of the audio signal. Furthermore, the transmission parameter setting unit 18 of the second embodiment controls the parameters so that the symbol rates of the subcarriers are equal regardless of the parameter setting mode. As a result, as long as the guard interval ratio does not change, only one crystal oscillator 29 is required. The subcarrier symbol rate Sc is Sc = Vi × (1 / Ro) × using the input information rate Vi, the outer coding rate Ro, the inner coding rate Ri, the modulation multilevel number M, and the data carrier number Nd. It is expressed as (1 / Ri) × (1 / M) × (1 / Nd). Therefore, the transmission parameter setting unit 18 controls and sets parameters so as to satisfy the following expression (7).

  Ro × Ri × M × Nd = constant (7)

  Further, the transmission parameter setting unit 18 may control the parameters so that No (= Ri × M × Nd) is constant and Ro is constant regardless of the parameter setting mode. At this time, Expression (7) is always satisfied. When the code length No of the outer code and the outer code code rate Ro are constant, the information length Ko is also constant, so that the circuit of the outer code encoder 131 can be shared, and the circuit scale and circuit cost can be reduced. .

  FIG. 5 is a block diagram illustrating an OFDM receiver for a wireless microphone according to a second embodiment of the present invention. The wireless microphone OFDM receiver 2b according to the second embodiment is different from the wireless microphone OFDM receiver 2a according to the first embodiment illustrated in FIG. 2 in that only one crystal oscillator 29 is required, and reception parameter setting is performed. Only the operation of the unit 28 is different.

  The reception parameter setting unit 28 sets the same parameter as that of the wireless microphone OFDM transmitter 1b in each block. Therefore, as with the wireless microphone OFDM transmitter 1a, only one crystal oscillator 29 is required as long as the guard interval ratio does not change.

  In addition, when the transmission parameter setting unit 18 sets the code length No to be constant and the outer coding rate Ro to be constant, the circuit of the outer code decoding unit 253 can be shared, and the circuit scale and circuit Cost can be reduced.

  As described above, the wireless microphone OFDM transmitter 1b and the wireless microphone OFDM receiver 2b according to the second embodiment are configured such that the code length No, the outer coding rate Ro, and the Ro × Ri × M × Nd are constant. The inner coding rate Ri, the modulation multilevel number M, and the data carrier number Nd are controlled. For this reason, as long as the symbol rate Sc of the subcarrier is constant regardless of the parameter setting mode and the guard interval ratio does not change, only one crystal oscillator 19 is required.

  Further, by setting the outer coding rate Ro and the code length No so that the outer code rate Ro is constant and the code length No is constant, the outer code coding unit 131 and the outer code decoding unit 253 further The circuit scale can be reduced.

[Parameter example]
Next, specific parameter examples used in the wireless microphone system according to the present invention will be shown. As described above, the parameters satisfy the following conditions.
・ No = Ri × M × Nd (Formula (1))
Ts = m × 1 / fs (m = integer) (formula (2))
Ts = 1 / Vsc (Formula (3))
66.6 [μs] ≦ Tu ≦ 230 [μs] (formula (6))
-Regardless of the parameter setting mode, the code length No and the outer coding rate Ro are constant.

  In the following example, the A / D conversion unit 12 will be described assuming that an analog audio signal is converted into a digital signal with a quantization bit length N of 24 bits and a sampling frequency fs of 48 KHz. At this time, the input information rate Vi is Vi = N × fs = 24 × 48 = 1115 [kbps]. When the sampling frequency fs is 48 KHz, the sampling interval 1 / fs is about 20.8 us. If the guard interval ratio is about 1/16 or 1/32, the effective symbol length Tu satisfies the condition of Equation (6) when the number m of audio samples transmitted per symbol is 4 ≦ m ≦ 11.

[First parameter example]
FIG. 6 is a diagram showing a first parameter example used in the wireless microphone system according to the present invention. Here, an example of parameters when the number of audio samples m transmitted per symbol is 4 will be described.

  When the number of samples m is 4, the information length Ko is Ko = 24 × 4 = 96 [bits]. In the parameter example shown in FIG. 6, the code length No is constant at 104 bits regardless of the parameter setting mode, and Ro = Ko / No = 12/13 is constant regardless of the parameter setting mode. The rate Vo after the outer coding is Vo = Vi × 1 / Ro = 11152 × 13/12 = 1248 [kbps]. The inner coding rate Ri and the modulation multi-level number M are different values depending on the parameter setting mode.

  The case where the parameter setting mode is mode 1 will be described. The inner coding rate Ri = 1/2 and the modulation multi-level number M = 2. Therefore, the symbol rate Vs of the entire OFDM signal is Vs = Vo × (1 / Ri) × (1 / M) = 1248 × 2 × 1/2 = 1248 [ksps]. The number Nd of data carriers in the entire band is Nd = No × (1 / Ri) × (1 / M) = 104 × 2 × 1/2 = 104 according to Equation (1). Therefore, the symbol rate Vsc of one carrier is Vsc = Vs / Nd = 1248/104 = 12 [kbps]. The symbol length Ts is about 83.3 μs from the equations (2) and (3). When the guard interval ratio GIR is 1/16, the effective symbol length Tu = Ts × (16/17) ≈78.4 [μs], which satisfies the condition of Expression (6).

  When transmitting a carrier symbol by dividing it into a plurality of segments, in order to prevent the generation of null carriers, the number of data carriers Ndseg per segment, the number of data carriers Nd in the entire band, and the number of segments S Must be set so as to satisfy the following equation (8).

  Ndseg = Nd / S (8)

  In the parameter example shown in FIG. 6, the number of data carriers Ndseg per segment is 13, the number of SP signal carriers inserted per segment is 1, and the number of TMCC signal carriers inserted per segment is 1. As a result, the number of carriers per segment is 15.

  When the parameter setting mode is mode 1, the number of segments S is 8, and for each segment, the number of SP signal carriers is 1 and the number of TMCC signal carriers is 1. The carrier number Nsp is 8 and the TMCC signal carrier number Ntmcc is 8. Further, the number Ncp of CP signal carriers is 1 in the entire band. Therefore, the total number of carriers Ncarr is 104 + 8 + 8 + 1 = 121. The CP signal is arranged on the carrier with the highest frequency, or the CP signal is arranged on the carrier with the lowest frequency of each channel by inverting the spectrum of the segment. The transmission bandwidth BWt is expressed as BWt = Ncarr × 1 / Tu using the total number of carriers Ncarr and the effective symbol length Tu.

  FIG. 7 is a diagram illustrating an arrangement example of SP signals and TMCC signals of one segment of carrier symbols in the first parameter example shown in FIG. In the example shown in FIG. 7A, SP signals are arranged at carrier symbol positions that satisfy the following equation (9). In the example shown in FIG. 7B, SP signals are arranged at carrier symbol positions that satisfy the following equation (10). Is arranged. Here, p is a non-negative integer, i is a symbol number, k is a carrier symbol position where an SP signal is arranged, and mod means a remainder. Note that the arrangement of the SP signal and the TMCC signal is not limited to this.

k = 3 × (imod5) + 15p (9)
k = 5 × (imod3) + 15p (10)

[Second parameter example]
FIG. 8 is a diagram showing a second parameter example used in the wireless microphone system according to the present invention. The second parameter example shown in FIG. 8 differs from the first parameter example shown in FIG. 6 in that the number of TMCC signal carriers inserted per segment is two. Note that one of the two TMCC signals may be an AC signal or a CP signal.

  When the parameter setting mode is mode 1, the number of segments S is 8, and the SP signal carrier number is 1 and the TMCC signal carrier number is 2 for each segment. The number of carriers Nsp is 8, and the number of carriers Ntmcc of the TMCC signal is 16. Further, the number Ncp of CP signal carriers is 1 in the entire band. Therefore, the total number of carriers Ncarr is 104 + 8 + 16 + 1 = 129.

  FIG. 9 is a diagram showing an arrangement example of SP signals and TMCC signals of one-segment carrier symbols in the second parameter example shown in FIG. In the example shown in FIG. 9A, SP signals are arranged at carrier symbol positions that satisfy the following equation (11). In the example shown in FIG. 9B, SP signals are arranged at carrier symbol positions that satisfy the following equation (12). Is arranged.

k = 4 × (imod4) + 16p (11)
k = 2 × (imod8) + 16p (12)

[Third parameter example]
FIG. 10 is a diagram showing a third parameter example used in the wireless microphone system according to the present invention. In the parameter example shown in FIG. 10, the number m of audio samples transmitted per symbol is 4 (information length Ko is 96 bits), as in the first parameter example and the second parameter example. In the third parameter, the code length No is constant at 112 bits regardless of the parameter setting mode, and Ro = Ko / No = 6/7 is constant regardless of the parameter setting mode. Since the outer coding rate is lower than in the first and second parameter examples, the error correction capability is increased. The rate Vo after the outer coding is Vo = Vi × 1 / Ro = 11152 × 7/6 = 1344 [kbps]. The inner coding rate Ri and the modulation multi-level number M are different values depending on the parameter setting mode.

  The case where the parameter setting mode is mode 1 will be described. The inner coding rate Ri = 1/2 and the modulation multi-level number M = 2. Therefore, the symbol rate Vs of the entire OFDM signal is Vs = Vo × (1 / Ri) × (1 / M) = 1344 × 2 × 1/2 = 1344 [ksps]. The number Nd of data carriers in the entire band is Nd = No × (1 / Ri) × (1 / M) = 112 × 2 × 1/2 = 112 according to Equation (1). Therefore, the symbol rate Vsc of one carrier is Vsc = Vs / Nd = 1344/112 = 12 [kbps]. The symbol length Ts is about 83.3 μs from the equations (2) and (3). When the guard interval ratio GIR is 1/16, the effective symbol length Tu = Ts × (16/17) ≈78.4 [μs], which satisfies the condition of Expression (6).

  In the parameter example shown in FIG. 10, the number of data carriers Ndseg per segment is 8, the number of SP signal carriers inserted per segment is 1, and the number of TMCC signal carriers inserted per segment is 1. As a result, the number of carriers per segment is 14. In mode 1, the number of segments S is 8, and for each segment, the number of SP signal carriers is 1 and the number of TMCC signal carriers is 1. Therefore, the SP signal carrier number Nsp is 8 within the entire band. Thus, the carrier number Ntmcc of the TMCC signal is 8. Further, the number Ncp of CP signal carriers is 1 in the entire band. Therefore, the total number of carriers Ncarr is 112 + 8 + 8 + 1 = 129.

  FIG. 11 is a diagram illustrating an arrangement example of SP signals and TMCC signals of one-segment carrier symbols in the third parameter example illustrated in FIG. In the example shown in FIG. 11A, the SP signal is arranged at the carrier symbol position satisfying Expression (11), and in the example shown in FIG. 11B, the SP signal is arranged at the carrier symbol position satisfying Expression (12). doing.

[Fourth parameter example]
FIG. 12 is a diagram showing a fourth parameter example used in the wireless microphone system according to the present invention. Here, an example of parameters when the number of audio samples m transmitted per symbol is 8 will be described.

  When the number of audio samples m is 8, the information length Ko is Ko = 24 × 8 = 192 [bits]. In the fourth parameter example shown in FIG. 12, the code length No is constant at 216 bits regardless of the parameter setting mode, and Ro = Ko / No = 8/9 is constant regardless of the parameter setting mode. The rate Vo after the outer coding is Vo = Vi × 1 / Ro = 11152 × 9/8 = 1296 [kbps]. The inner coding rate Ri and the modulation multi-level number M are different values depending on the parameter setting mode.

  The case where the parameter setting mode is mode 1 will be described. The inner coding rate Ri = 1/2 and the modulation multi-level number M = 2. Therefore, the symbol rate Vs of the entire OFDM signal is Vs = Vo × (1 / Ri) × (1 / M) = 1296 × 2 × 1/2 = 1296 [ksps]. The number Nd of data carriers in the entire band is Nd = No × (1 / Ri) × (1 / M) = 216 × 2 × 1/2 = 216 according to Equation (1). Therefore, the symbol rate Vsc of one carrier is Vsc = Vs / Nd = 1296/216 = 6 [kbps]. The symbol length Ts is about 166.7 μs from the equations (2) and (3). When the guard interval ratio GIR is 1/16, the effective symbol length Tu = Ts × (16/17) ≈156.9 [μs], which satisfies the condition of Expression (6).

  In the fourth parameter example, the number of data carriers Ndseg per segment is 27, and the number of SP signal carriers inserted per segment is 1. If the number of carriers of a TMCC signal is 1, the number of carriers per segment is 29, and SP signals cannot be arranged at equal intervals. Therefore, the number of carriers of a TMCC signal inserted per segment is 2, and 1 segment The number of per carrier is 30. Note that one of the two TMCC signals may be an AC signal or a CP signal.

  When the parameter setting mode is mode 1, the number of segments S is 8, and the SP signal carrier number is 1 and the TMCC signal carrier number is 2 for each segment. The number of carriers Nsp is 8, and the number of carriers Ntmcc of the TMCC signal is 16. Further, the number Ncp of CP signal carriers is 1 in the entire band. Therefore, the total number of carriers Ncarr is 216 + 8 + 16 + 1 = 241.

  FIG. 13 is a diagram illustrating an arrangement example of SP signals and TMCC signals of one-segment carrier symbols in the fourth parameter example. In the example shown in FIG. 13, the SP signal is arranged at a carrier symbol position that satisfies the following equation (13).

  k = 6 × (mod5) + 30p (13)

  Although the above embodiments have been described as representative examples, it will be apparent to those skilled in the art that many changes and substitutions can be made within the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited by the above-described embodiments, and various modifications and changes can be made without departing from the scope of the claims. For example, the wireless microphone OFDM transmitter 1 may not include the interleave unit 132, and the wireless microphone OFDM receiver 2 may not include the deinterleave unit 252.

  In order to reduce the delay time due to the transmission / reception of the audio signal, it is preferable not to perform compression with an information compression rate of 1, but the wireless microphone OFDM transmitters 1a and 1b according to the present invention are The compressed audio signal is transmitted, and the wireless microphone OFDM receivers 2a and 2b can expand the received audio signal. FIG. 14 is a diagram illustrating an example of parameters when the information compression rate is 0.25. Modes 1 to 3 correspond to the first parameter example. An example in which the information length Ko is 96 bits, the code length No is 104 bits, and the number of carriers of the TMCC signal inserted per segment is 1. Show. Modes 4 to 6 correspond to the second parameter example, and an example in which the information length Ko is 96 bits, the code length No is 104 bits, and the number of carriers of the TMCC signal inserted per segment is two. Show. Mode 7 to Mode 9 correspond to the third parameter example, and an example in which the information length Ko is 96 bits, the code length No is 112 bits, and the number of carriers of the TMCC signal inserted per segment is 1. Show. Mode 10 to Mode 12 correspond to the fourth parameter example, and an example in which the information length Ko is 192 bits, the code length No is 216 bits, and the number of TMCC signal carriers inserted per segment is two. Show.

  As described above, according to the present invention, the delay time due to the transmission / reception of the audio signal can be reduced, and the deterioration of the reception quality due to the multipath fading can be prevented, so that the digital audio signal is transmitted / received by the OFDM modulation method. Useful for any application.

DESCRIPTION OF SYMBOLS 1a, 1b OFDM transmitter for wireless microphones 2a, 2b OFDM receiver for wireless microphones 11 Microphone 12 A / D conversion unit 13 Interleave / error correction unit 14 OFDM modulation unit 15 D / A conversion unit 16 Transmission frequency conversion unit 17 Transmitting antenna DESCRIPTION OF SYMBOLS 18 Transmission parameter setting part 19 Crystal oscillator 20 Clock supply part 131 Outer code encoding part 132 Interleaving part 133 Inner code encoding part 141 S / P conversion part 142 Carrier modulation part 143 OFDM frame structure part 144 IFFT part 145 GI addition part 21 Reception antenna 22 Reception frequency conversion unit 23 A / D conversion unit 24 OFDM demodulation unit 25 Deinterleave / error correction unit 26 D / A conversion unit 27 Speaker 28 Reception parameter setting unit 29 Crystal oscillator 30 Clock supply unit 241 GI Removal unit 242 FFT unit 243 Carrier demodulation unit 244 P / S conversion unit 251 Inner code decoding unit 252 Deinterleave unit 253 Outer code decoding unit

Claims (7)

An OFDM transmitter for a wireless microphone that transmits a digital audio signal by an OFDM modulation method,
An inner code encoder that inputs a digital audio signal in units of blocks and performs inner encoding for each block to generate an inner code;
An OFDM modulator that modulates the inner code using an OFDM modulation scheme to generate an OFDM signal;
The number of bits of block unit data input to the inner code encoder is No, the coding rate of the inner code is Ri, the modulation multi-level number of the OFDM signal is M, and the number of data carriers of the OFDM signal is Nd. and when the O symbol length FDM signal to be an integral multiple of the sampling interval of the audio signal, the No, the Ri, transmission parameter setting the M, and the Nd setting unit,
An OFDM transmitter for a wireless microphone, comprising:   2. The wireless microphone OFDM according to claim 1, wherein the transmission parameter setting unit sets the No, the Ri, the M, and the Nd such that No = Ri × M × Nd. Transmitter device. The transmission parameter setting unit sets the No, the Ri, the M, and the Nd so that an effective symbol length of the OFDM signal is 66.6 μs or more and 230 μs or less. Item 3. The wireless microphone OFDM transmitter according to Item 1 or 2 . And further comprising an outer code encoder that performs block encoding on the digital audio signal to generate an outer code,
The inner code encoding unit generates an inner code by inner encoding the outer code generated by the outer code encoding unit, and the transmission parameter setting unit sets the number of bits of the code length of the outer code as No. The OFDM transmitter for a wireless microphone according to any one of claims 1 to 3, wherein: The transmission parameter setting unit is configured such that when the encoding rate of the outer code is Ro, the values of Ro, Ri, M, and Nd are constant regardless of the parameter setting mode. The OFDM transmitter for wireless microphone according to claim 4 , wherein the M and the Nd are set. 6. The wireless microphone OFDM transmission apparatus according to claim 5 , wherein the transmission parameter setting unit sets the Ro and the No so that the Ro is constant and the No is constant. An OFDM receiver for a wireless microphone, which receives the OFDM signal transmitted by the OFDM transmitter for a wireless microphone according to any one of claims 1 to 6 and generates a digital audio signal.

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