JP5504727B2 - Modulation and demodulation method and modulation and demodulation system - Google Patents

Description

The present invention relates to a modulation and demodulation method and a modulation and demodulation system for transmitting a code mainly using sound.

Examples of code transmission techniques that use sound as a transmission medium include those shown in Patent Documents 1 and 2. The method of Patent Document 1 is a method in which a carrier wave in the audible sound band is modulated with a baseband signal, and a masker sound is added to the modulated signal to transmit it with difficulty in hearing. The method of Patent Document 2 is a method of embedding a digital watermark in an audio signal using amplitude modulation.

JP 2007-104598 A

JP 2006-251676 A

  Since both methods transmit information using sound (sound waves) propagating in the air, Doppler shift due to movement of the transmitting device (speaker) and receiving device (microphone), and clocks on the transmitting side and the receiving side This is a problem. In particular, since the propagation speed of sound waves is about 340 m / sec, which is extremely low compared to radio waves, for example, a large Doppler shift occurs even when a person carrying the receiving device walks or swings his arm.

  In the method of Patent Document 1, as a countermeasure against Doppler shift, a resampling process is performed by detecting a frequency transition of a pilot signal added on the transmission side on the reception side. This has the disadvantage that not only it is essential to add a pilot signal to the transmission signal, but also that a resampling process is required on the receiving side. In particular, since resampling processing is heavy, it has been a factor in increasing the cost of the receiving apparatus.

  In addition, since the method of Patent Document 2 is amplitude modulation, there is a problem that frequency information is not added to a transmitted sound wave and it is impossible to cope with a case where a Doppler shift occurs.

An object of the present invention is to provide a modulation and demodulation method and a modulation and demodulation system that can transmit information with a simple configuration mainly using sound waves as a medium.

The modulation and demodulation method of the present invention includes a spreading processing procedure for spreading the data code by modulating the spreading code with the data code, a differential coding procedure for differentially coding the spreading code modulated with the data code , Up-sampling procedure for up-sampling the differential code to extend the chip rate of one chip of the spreading code to a plurality of samples, multiplying the up-sampled differential code by a carrier signal, frequency shifting, and using as a modulation signal A modulation procedure for outputting, a delay detection procedure for inputting the modulation signal and delay-detecting the modulation signal with a delay time corresponding to one chip of the up-sampled differential code, the signal waveform subjected to the delay detection, and the spreading synchronization detection for detecting a synchronization point of the code in a time unit shorter than the time of one chip of the upsampled differential codes Order, and performs.

In the above invention, after the modulation procedure, a synthesis procedure for mixing and outputting the modulation signal output in the modulation procedure with an acoustic signal in an audible frequency band is further performed, and the delay detection procedure is mixed in the synthesis procedure. The modulated signal extracted from the signal may be input.

In the above invention, prior to the synthesis procedure, a low-pass filter procedure for cutting a high frequency band higher than a predetermined frequency band from an audio signal in an audible frequency band in the synthesis procedure is further performed, and the modulation procedure is upsampled A differential code may be multiplied by the carrier signal to shift the frequency to a band equal to or higher than the predetermined frequency band and output as the modulated signal.

The modulation and demodulation system according to the present invention includes a differential encoding unit that differentially encodes a spreading code, and an upsampling unit that upsamples the differential code and extends the chip rate of one chip of the spreading code to a plurality of samples. And a modulator that multiplies the up-sampled differential code by a carrier signal to shift the frequency, and outputs the modulated signal as a modulated signal, and inputs the modulated signal, A delay detection unit that performs delay detection with a delay time of one chip of the sampled differential code, and a synchronization point between the delay detected signal waveform and the spread code is 1 of the upsampled differential code And a demodulator including a synchronization detection unit that detects in units of time shorter than the time for the chip .

In the above invention, a synthesizing unit that mixes and outputs the modulation signal output from the modulation unit with an acoustic signal in an audible frequency band is provided after the modulation unit of the modulation device, and the delay of the demodulation device A signal separation unit that inputs the signal mixed by the synthesis unit and extracts the modulation signal from the signal may be further provided in the preceding stage of the detection unit.

In the above invention, a low-pass filter unit that inputs a signal obtained by cutting a high frequency band of a predetermined frequency band or higher from the audio signal in the audible frequency band to the synthesizing unit is provided, and the modulation unit includes the upsampled differential signal. The code may be multiplied by the carrier signal to shift the frequency to a band equal to or higher than the predetermined frequency band and output as the modulated signal.

  In the present invention, the data code to be transmitted is spread by a spreading code. As the spreading code, for example, an M-sequence pseudo noise code or the like may be used. By spreading, it is possible to transmit a highly reliable data code even in an environment where an environmental sound or other acoustic signal exists and the SN ratio is poor. Further, a differential code string is generated by differentially encoding the spreading code. By performing differential encoding, it is possible to demodulate the original spreading code using the presence or absence of inversion of the code of each chip of the code string even if the receiving side does not have a clock accurately synchronized with the transmitting side. . Further, the frequency is shifted by modulating the differential code. By shifting the frequency, the band of the differential code is shifted from the baseband to a frequency band where sound can be emitted and transmitted.

  In addition, by shifting the differential code to a higher range than the audible band, it is possible to mix and emit sound signals such as musical sounds. The acoustic signal to be mixed may be cut as appropriate so as not to overlap the modulation signal.

  Further, by up-sampling the differential code, the synchronization shift can be finely absorbed in the chip unit of the up-sampled signal on the receiving side, and the entire differential code for one chip is not shifted, It becomes possible to absorb a frequency shift such as a Doppler shift with high accuracy.

  With the above method, both modulation and demodulation are time domain processing that does not require frequency domain processing, that is, robust information that is highly resistant to frequency shifts and disturbances such as Doppler shift with a small processing load. Transmission is possible.

  In the present invention, since the data code is transmitted after being spread with a white noise-like spreading code, a single carrier method using a sine wave that is easily attached to the ear, or because the phase and amplitude change discontinuously. Compared with the multi-carrier method in which noise is generated, the sense of incongruity in hearing is greatly reduced. Furthermore, the sense of incongruity is further improved by shifting the modulation signal to a high range where the human auditory sensitivity becomes dull and mixing the acoustic signal in the mid-low range.

  According to the present invention, even if a Doppler shift occurs in a data code transmitted as sound and the frequency of the signal fluctuates, stable demodulation can be performed without being affected by the fluctuation.

  In addition, by mixing the data code with the acoustic signal, it is possible to transmit information with less sense of discomfort even in the case of spatial sound emission.

The figure which shows the structure of the transmitter of an acoustic communication system with which this invention is applied, and a receiver The figure which shows the structure of the data superimposition part of the said transmitter. Diagram showing examples of spreading and differential encoding waveforms The figure which shows the example of a spectrum of each part of a data superimposition part The figure which shows the structure of the demodulation part of the said receiver The figure which shows the example of an output waveform of HPF and delay detection The figure which shows the output waveform example of LPF and a matched filter

  An acoustic communication system and an acoustic communication system according to an embodiment of the present invention will be described with reference to the drawings.

≪Acoustic communication system≫
FIG. 1 is a diagram showing a configuration of an acoustic communication system according to an embodiment of the present invention. This acoustic communication system includes a transmission device 1 and a reception device 2.

  The transmission device 1 includes a data superimposing unit 10, an analog circuit unit 11, and a speaker 12. The data superimposing unit 10 is a circuit unit that performs a diffusion process on the data code D to superimpose the data code D on the high frequency range of the acoustic signal S. Details of the configuration and operation of the data superimposing unit 10 will be described later.

  The analog circuit unit 11 includes a D / A converter and an audio amplifier. The analog circuit unit 11 converts the digital composite signal output from the data superimposing unit 10 into an analog signal, amplifies the signal, and supplies the analog signal to the speaker 12. The speaker 12 emits the synthesized signal input from the analog circuit unit 11 as sound. The emitted synthesized signal sound propagates through the space and reaches the microphone 22 of the receiving device 2.

  The receiving device 2 includes a microphone 22, an analog circuit unit 23, and a demodulation unit 21. The analog circuit unit 23 includes an amplifier that amplifies the audio signal picked up by the microphone 22 and an A / D converter that converts the audio signal into a digital signal. The demodulation unit 21 is a circuit unit that detects a spread signal included in the collected audio signal and demodulates the data code D superimposed on the spread signal. Details of the configuration and operation of the demodulator 21 will be described later.

≪Data superimposition part≫
FIG. 2A is a diagram illustrating a configuration example of the data superimposing unit 10 of the transmission device 1. The acoustic signal S (music, voice, etc.) input from the acoustic signal input unit 31 is cut off at high frequencies by the LPF 32. The cut-off frequency of the LPF 32 is determined based on the audibility and the bandwidth allocated to the modulation signal. If the cut-off frequency is too low, the sound quality of the acoustic signal S deteriorates. At the same time, when the band frequency of the modulation signal is lowered in accordance with the low cut-off frequency, the modulation signal is likely to be attached to the ear for the listener's sense of hearing (loudness increases). Conversely, if the cut-off frequency of the LPF 32 is too high, the modulation signal band cannot be widened, and the transmission quality of the data code is lowered. Therefore, the cutoff frequency of the LPF 32 is determined in consideration of the audibility evaluation of the acoustic signal that has passed through the LPF 32, the required bandwidth of the modulation signal, and the like.

  The gain of the acoustic signal S whose high frequency is cut by the LPF 38 is adjusted by the gain adjusting unit 33. The acoustic signal S whose gain has been adjusted is input to the adder 34. Note that the LPF 32 may be omitted when the input acoustic signal has a frequency component only in the mid-low range and no component exists in the high range.

  The data code D is input from the data code input unit 35. The spread code generator 36 generates a spread code. As the spreading code, a pseudo-random code string having a fixed cyclic period such as an M series is used. The period of the data code D input from the data code input unit 35 is adjusted so that one symbol period coincides with one cyclic period of the spread code.

  The multiplier 37 multiplies the data code D and the spread code PN. This process is a process generally called diffusion. By this spreading process, the spread code PN is phase-modulated for each cyclic period by the value (1/0) of the data code D, and the frequency spectrum of the data code D is spread.

  The spread code MPN modulated by the data code D by the multiplier 37 is converted into a differential code DMPN by the differential encoding unit 38. The differential encoding process is a process of replacing the value of each chip of the spread code with a value representing a change from the previous chip from its absolute value. By this differential encoding, the symbol can be demodulated with high accuracy using delay detection on the receiving side (detailed later) without a clock that is accurately synchronized with the transmitting side.

  FIG. 2B is a diagram illustrating an example of the differential encoding unit 38. The differential encoding unit 38 includes an XOR circuit 45 to which the spread code MPN is input to one input terminal, and a one-chip delay circuit that delays the output of the XOR circuit 45 by one chip and returns it to the other input terminal of the XOR circuit 45 46. By feeding back the output of the XOR circuit 45 with a delay of one chip, the XOR circuit 45 outputs the comparison result between the input spread code MPN and the output of the XOR circuit 45 one clock before as a differential code DMPN. That is, in the differential code DMPN, the absolute value of each chip of the spread code MPN is replaced with the presence or absence of a phase change from the chip of the immediately preceding differential code DMPN. As a result, the spread code MPN can be restored on the receiving side by comparing two consecutive chips.

  FIG. 3 shows waveform examples of the data code D, the spread codes PN, MPN, and DMPN. FIG. 4A shows the spread code PN generated by the spread code generator 36. FIG. 5B shows the data code D input by the data code input unit 35. FIG. 5C shows a spread code MPN that is phase-modulated by the data code D for each cyclic period. Since the data code string D shown in the figure is “10”, the phase is normal in the first period of the spread code MPN, and the phase is inverted in the second period. FIG. 4D shows a code string (differential code) DMPN obtained by differentially encoding the modulated spread code MPN. This code string is a value based on a comparison result (exclusive OR) between the value of each chip of the spreading code MPN and the value of the differential code DMPN of the immediately preceding chip. The differential code DMPN is converted into a binary signal of -1,1.

  The differential code DMPN converted into a binary signal is input to the upsampling unit 39. The upsampling unit 39 upsamples the input code string. Based on the chip rate of the spread code PN generated by the spread code generation unit 36 and the upsampling rate in the upsampling unit 39, the chip rate and bandwidth of the spread code to be transmitted (sound emission) are determined.

  Returning to FIG. 2B, the upsampled signal (differential code DMPN) is input to the LPF 40. The LPF 40 is a filter that limits the band of the baseband signal and restricts the band of the baseband signal while suppressing inter-chip interference, and is called a Nyquist filter. The Nyquist filter is a filter having a characteristic that an impulse response rings at a symbol rate (passes through 0), and is configured by an FIR filter generally called a cosine roll-off filter. The order of the filter, the roll-off rate α, etc. are determined according to the conditions to be applied.

  In this embodiment, since filtering is also performed by the LPF 54 on the receiving side, each of the LPFs 40 and the LPF 54 on the receiving side is configured by a root raised cosine roll-off filter so that a complete Nyquist filter is obtained. The

  The signal subjected to band limitation and waveform shaping by the LPF 40 is multiplied by a carrier signal in the multiplier 42 and is frequency-shifted to a high frequency. The frequency of the carrier signal generated by the carrier signal generation unit 41 is arbitrary, but the band of the spread code whose frequency is shifted is equal to or higher than the cutoff frequency of the LPF 32, the movable frequency band of an acoustic device such as a speaker and a microphone, and the signal The digital signal processing unit (CODEC) including compression is set to fall within the range of the coding frequency band.

  That is, when the frequency of the carrier signal is lowered, the modulated signal component is likely to be attached to the ear for hearing, and the transmission signal may be deteriorated due to the acoustic signal mixed into the modulated signal. If the frequency of the carrier signal is too high, the waveform may be distorted due to deterioration of the high frequency characteristics of speakers, microphones, etc. or from the CODEC coding frequency band, and the transmission quality may be lowered. When the frequency exceeds the Nyquist frequency, aliasing distortion may be mixed.

  That is, the bandwidth (chip rate) of the spread signal and the frequency of the carrier signal satisfy the following conditions. If the bandwidth of the modulated signal after upsampling is fBW, the sampling frequency is fs, the cutoff frequency of the LPF 32 is fc, and the frequency of the carrier signal is fa, the following equation must be satisfied.

  The gain adjustment unit 43 adjusts the gain of the modulation signal MDMPN shifted to the high frequency range. The gain-modulated modulation signal MDMPN is added and synthesized by the adder 34 with the acoustic signal S. This synthesized signal is output to the outside. The gain of the gain adjusting unit 43 is determined based on the sound output sound pressure level allowed in the environment to be applied and the system, the required propagation distance, the audibility evaluation, and the like. Note that the gain of the gain adjusting unit 43 may be adaptively controlled according to the level of the acoustic signal S output from the LPF 32. For example, since the masking effect can be expected when the level of the acoustic signal S is high, the level of the modulation signal MDMPN is also increased to increase the gain against noise, and when the level of the acoustic signal S is small, the acoustic signal S is audible. It is also possible to control to lower the level of the modulation signal MDMPN so as not to deteriorate.

  FIG. 4 is a diagram illustrating an outline of the frequency spectrum in each block of the data superimposing unit 10. FIG. 2A is a diagram showing a frequency spectrum of the acoustic signal S input to the acoustic signal input unit 31. FIG. FIG. 5B is a diagram showing the frequency spectrum of the acoustic signal S whose high sound range is cut by the LPF 32. The cut-off frequency fc of the LPF 32 is set to, for example, about a dozen kHz in accordance with the auditory characteristics of the target audience.

  FIG. 6C is a diagram showing the frequency spectrum of the differential code DMPN (band limited) and the carrier signal (sine wave of frequency fa) output from the LPF (Nyquist filter) 40. FIG. 4D shows a modulation signal MDMPN obtained by multiplying the carrier signal and the differential code DMPN. Since this example is an example in which real numbers are multiplied, bands (sidebands) are formed on both sides of the carrier signal.

  FIG. 5E shows a composite signal output from the adder 34. FIG. This synthesized signal is a signal obtained by adding and synthesizing the acoustic signal S output from the gain adjusting unit 33 and the modulation signal MDMPN output from the gain adjusting unit 43. The synthesized signal is converted into an audio signal by the analog circuit unit 11 and is spatially emitted from the speaker 12. Further, it can be transmitted as an analog signal through a wired or wireless audio signal transmission path.

≪Demodulation part≫
FIG. 5 is a diagram illustrating a configuration example of the demodulation unit 21 of the reception device 2. The demodulating unit 21 receives a composite signal collected by the microphone 22 and A / D converted by the analog circuit unit 23. The input composite signal is input to the HPF 51. The HPF 51 is a filter for removing the acoustic signal component from the synthesized signal and extracting the spread signal component MDMPN frequency-shifted by the carrier signal. The cutoff frequency of the HPF 51 is set to the lower limit frequency (fa−fBW / 2 (see FIG. 4E)) of the modulation signal band. The modulation signal MDMPN extracted by the HPF 51 is input to the delay unit 52 and the multiplier 53. The delay time of the delay unit 52 is set to the time of one chip of the spread code upsampled on the transmission side. For example, when up-sampling is performed N times, the delay amount of the delay unit 52 is also N samples. The multiplier 53 multiplies the sample for one chip of the HPF 53 by the sample for one chip of the delay unit 52. This process is the above-described delay detection process. By this delay detection processing, the differentially encoded signal MDMPN is converted into a signal including the original spreading code MPN.

  FIG. 6A shows an output waveform example of the HPF 51, and FIG. 6B shows an output waveform example of the multiplier 53. In the waveform of FIG. 9A, the envelope of the carrier signal has the shape of a differential code DMPN that is band-limited by the LPF 40 (transformed into a smooth waveform). On the other hand, in the waveform of FIG. 5B, the envelope of the carrier signal has the shape of the spread code MPN modulated by the data code D.

  It should be noted that the code waveform decoded by the delay detection by the delay unit 52 and the multiplier 53 shown in this figure is inverted in sign from the code waveform before being differentially encoded by the differential encoder 38 on the transmission side. ing. There is no problem if it is handled as a signal in which positive and negative are inverted, but an inverter or the like may be inserted if necessary.

  The feature of this delay detection processing is that it does not require the reproduction of a carrier signal at the time of demodulation. Thus, by adopting differential encoding on the transmission side and delay detection on the reception side, it is possible to construct a communication system that is robust against frequency fluctuations and has a low processing load.

  The multiplication output of the multiplier 53 is input to the LPF 54. The LPF 54 is a filter for filtering a carrier component to extract a baseband signal and filtering extra noise to improve an S / N ratio, and has the same characteristics as the LPF (Nyquist filter) 40 used on the transmission side. belongs to. As described above, the LPF 40 of the modulation unit and the LPF 54 are combined with each other to have a complete Nyquist filter characteristic.

  FIG. 7A is a diagram illustrating an example of an output waveform of the LPF 54. Note that this output waveform and the waveform shown in FIG.

  The output of the LPF 54 is input to the matched filter 55. The matched filter 55 is configured by an FIR filter having a coefficient of a spread code PN used for spreading data codes on the transmission side. The chip rate of the spreading code used for the coefficient is the same as the chip rate after upsampling on the transmission side. That is, the same code of the same spreading code PN is repeated in the matched filter 55 by the upsampling rate.

  The matched filter 55 performs a convolution operation between the output waveform of the LPF 54 and the spread code PN shown in FIG. 7A, and outputs a correlation value between the output waveform of the LPF 54 and the spread code PN. FIG. 7B is a diagram illustrating an example of an output waveform of the matched filter 55. Since the interference and noise received on the transmission line have a low correlation with the spread code, the correlation value output from the matched filter is not greatly affected. Thus, transmission that is resistant to disturbance can be achieved by the diffusion processing.

  The correlation value indicates a strong correlation peak in the period of the spreading code PN, and the phase of the peak is phase-modulated by the transmission symbol, so that a positive peak and a negative peak correspond to 1 and −1 of the transmission symbol. Appears. The output of the matched filter 55 is input to the peak detector 56. The peak detector 56 detects a large peak near the cycle of the spread code PN and sets it as a correlation peak. The detected correlation peak is input to the code determination unit 57. The code determination unit 57 decodes the symbol from the peak phase and outputs it as a data code D.

  With the above configuration, an acoustic transmission system with high robustness against frequency fluctuations and interference can be processed relatively lightly even when spatially transmitted by superimposing a code-modulated signal on the acoustic signal with a little sense of incongruity. It can be realized with a load.

≪Read more≫
In the above embodiment, addition of an error correction code or the like is not described, but when error correction or interleaving is used on the transmission device side, those processing is added to the received symbol on the reception device side. do it.

  In the above-described embodiment, multiplication of the carrier signal and the differential code DMPN is performed by arithmetic in the real number domain. However, the carrier signal is converted to a complex number by Hilbert transform, and the differential code DMPN is multiplied by multiplication in the complex domain. Band shifting may be performed. In this case, since the modulated signal band after the shift is a single sideband, the condition shown in [Expression 1] changes to [Expression 2] below.

DESCRIPTION OF SYMBOLS 1 Reception apparatus 2 Transmission apparatus 10 Data superimposition part 21 Demodulation part 32 LPF
37 Multiplier (Diffusion processing)
38 Differential Encoding Unit 42 Multiplier (Modulation Unit)
52 Delay Unit 53 Multiplier

Claims (6)

A spreading procedure for spreading the data code by modulating the spreading code with the data code;
A differential encoding procedure for differentially encoding a spreading code modulated with a data code ;
An upsampling procedure for upsampling the differential code to extend the chip rate of one chip of the spreading code to a plurality of samples;
A modulation procedure for multiplying the upsampled differential code by a carrier signal, shifting the frequency, and outputting it as a modulation signal;
A delay detection procedure for inputting the modulation signal and delay-detecting the modulation signal with a delay time of one chip of the up-sampled differential code;
A synchronization detection procedure for detecting a synchronization point between the delayed detected signal waveform and the spread code in a unit of time shorter than the time of one chip of the upsampled differential code ;
A demodulation procedure for demodulating the data code based on a peak polarity of the detected synchronization point;
And a modulation and demodulation method. After the modulation procedure, further performs a synthesis procedure for mixing and outputting the modulation signal output in the modulation procedure with an acoustic signal in an audible frequency band,
The modulation and demodulation method according to claim 1 , wherein the delay detection procedure inputs the modulation signal extracted from the signal mixed by the synthesis procedure. Prior to the synthesis procedure, further performs a low-pass filter procedure for cutting a high frequency above a predetermined frequency band from an acoustic signal in an audible frequency band in the synthesis procedure
The modulation procedure multiplies the up-sampled differential code by the carrier signal to shift the frequency to a band equal to or higher than the predetermined frequency band, and outputs the modulated signal.
The modulation and demodulation method according to claim 2 . A spreading processor that spreads the data code by modulating the spreading code with the data code;
A differential encoding unit that differentially encodes a spreading code modulated by a data code ;
An upsampling unit for upsampling the differential code to extend a chip rate of one chip of the spreading code to a plurality of samples; and
A modulation device comprising a modulation unit that multiplies the up-sampled differential code by a carrier signal and shifts the frequency and outputs it as a modulation signal;
A delay detection unit that inputs the modulation signal and delay-detects the modulation signal with a delay time of one chip of the upsampled differential code ;
A synchronization detection unit that detects a synchronization point between the signal waveform subjected to delay detection and the spread code in a unit of time shorter than the time of one chip of the up-sampled differential code ; and
A demodulator comprising a demodulator that demodulates the data code based on the detected peak polarity of the synchronization point ;
A modulation and demodulation system. In the subsequent stage of the modulation unit of the modulation device, a synthesis unit that mixes and outputs the modulation signal output from the modulation unit with an acoustic signal in an audible frequency band is provided.
5. The modulation and demodulation system according to claim 4 , further comprising: a signal separation unit that inputs a signal mixed by the synthesis unit and extracts the modulation signal from the signal before the delay detection unit of the demodulation device. A low-pass filter unit that inputs a signal obtained by cutting a high frequency band of a predetermined frequency band or higher from the audio signal in the audible frequency band to the synthesis unit;
The modulation unit multiplies the up-sampled differential code by the carrier signal to shift the frequency to a band equal to or higher than the predetermined frequency band, and outputs the modulated signal.
6. A modulation and demodulation system according to claim 5 .

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