JP5375027B2 - Demodulator and modem system - Google Patents

Description

  The present invention relates to a demodulator that demodulates a modulated signal and a modulation / demodulation system that modulates and demodulates an input signal such as voice.

When signals are transmitted and received by modulating electromagnetic waves, a Doppler shift may occur due to fluctuations in relative speed. In order to cope with such a phenomenon, conventionally, in a digital modulation method such as OFDM, a pilot signal having a predetermined frequency is inserted into a transmission signal by a modulation device during modulation, and a signal received by the demodulation device is demodulated. In doing so, the frequency deviation of the pilot signal is analyzed, and the Doppler shift is corrected according to the result (see Patent Document 1).
JP 2008-5541 A

  However, in a digital modulation method such as OFDM, in order to perform Doppler shift correction using a pilot signal in the demodulator, it is necessary to analyze the frequency of the carrier signal and calculate the amount of Doppler shift based on the result of the frequency analysis. There was a problem that the algorithm was complicated.

  Accordingly, an object of the present invention is to provide a demodulation device and a modulation / demodulation system that can perform Doppler shift correction without using a complicated algorithm for the demodulation itself.

In the demodulating device of the present invention, a passband signal superimposed with a pilot signal having a frequency higher than the sampling Nyquist frequency , which is a signal input from the outside, is demodulated by the demodulating means at the sampling sampling frequency . The demodulator downsamples the passband signal on which the pilot signal is superimposed by the downsampling means to ½ times the sampling so as to return at a frequency equal to or lower than the frequency of the pilot signal. Then, to extract the pilot signal from the down-sampled signal extracting means, in accordance with the shift of the frequency of the pilot signal by the correction means, said demodulating means for correcting the shift of the frequency of the demodulated signal demodulated. If the relative speed of the signal source and the demodulator changes when a passband signal on which the pilot signal is superimposed is input from the outside, the frequency of this signal changes due to the influence of Doppler shift. However, since the pilot signal is turned back at a frequency equal to or lower than the frequency of the pilot signal by the downsampling means, the pilot signal shifts in a direction opposite to the shift direction by the Doppler shift. Therefore, even if the pitch of the demodulated signal demodulated by the demodulating means is fluctuated due to Doppler shift, the demodulated signal pitch fluctuation is canceled by correcting the demodulated signal frequency according to the frequency of the pilot signal. Can do. For example, when the pitch of the demodulated signal is increased by a certain frequency due to Doppler shift, the pitch of the pilot signal extracted by the extracting means is decreased by the same frequency. Therefore, the pitch fluctuation can be canceled by correcting the demodulated signal at the frequency of the pilot signal. As described above, since the Doppler shift is corrected through a path different from the demodulation operation, the passband signal can be demodulated without depending on any demodulation algorithm (without using a complicated algorithm).

  In the demodulating device of the present invention, the control signal generating means generates a voltage proportional to the frequency of the pilot signal extracted by the extracting means, and the voltage controlled oscillating means is a clock proportional to the voltage generated by the control signal generating means. Output a signal. Further, the correcting means corrects the frequency of the demodulated signal in accordance with the clock signal.

In this configuration, since the clock signal is generated using the pilot signal extracted from the signal downsampled by the downsampling means, the demodulated signal is reproduced by this clock signal, thereby performing the arithmetic processing. Correction can be performed without any problem.

  Further, in the modulation / demodulation system of the present invention, the modulation device modulates the input signal into a passband signal by the modulation means, and superimposes the pilot signal on the passband signal and outputs it. The demodulator processes the signal output from the modulator. Therefore, it is only necessary to newly add a configuration for adding a pilot signal during modulation to the modulation apparatus, and a large design change is not required in the existing modulation algorithm.

  According to the present invention, the Doppler shift can be corrected without using a complicated algorithm for the demodulation process itself.

  FIG. 1 is a block diagram showing a schematic configuration of a modulation apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram showing a schematic configuration of the demodulation device according to the embodiment of the present invention. FIG. 3 is a diagram for explaining changes in frequency characteristics due to signal processing in the demodulation system according to the embodiment of the present invention. The alphabets with parentheses shown in FIGS. 1 and 2 correspond to the respective drawings in FIG. Further, in the following description, a case will be described as an example where a voice band is modulated into an ultrasonic band and transmitted and demodulated in a modem system.

  The modulation / demodulation system 1 includes a modulation device 2 and a demodulation device 3. At least one of the modulation device 2 and the demodulation device 3 is movable, and the relative speed of both devices may change. If the passband signal is emitted and collected when the relative speed of the modulation device 2 and the demodulation device 3 is changing, the passband signal is affected by the Doppler shift. However, the present invention is configured to cancel the influence of this Doppler shift. Details will be described below.

  The modulation device 2 includes a modulator 21 (corresponding to modulation means), a sine wave generator 22, and an adder 23 (corresponding to superimposing means).

  When an audio signal in the audible band (see FIG. 3A) is input to the modulator 21, the modulator 21 modulates the signal into an ultrasonic band signal (modulated signal) and outputs the signal to the adder 23. The adder 23 superimposes the sine wave (pilot signal) generated by the sine wave generator 22 on the modulation signal output by the modulator 21 (see FIG. 3B). The adder 23 outputs a passband signal that is a signal obtained by superimposing a pilot signal on the modulated signal. The passband signal output from the adder 23 is amplified by an amplifier (not shown) and emitted as an ultrasonic wave from a speaker or sensor (not shown) (not shown).

  The modulation method performed by the modulator 21 is not particularly limited as long as the audio signal in the audible band is modulated into the signal in the ultrasonic band. Further, it is desirable that the modulation band (ultrasonic band) and the frequency band of the pilot signal should be set as far as possible without being overlapped so that the pilot signal can be easily extracted at the time of demodulation. Further, the modulation device 2 may be configured to modulate the passband after adding the pilot signal in the baseband band inside the modulator 21.

  Here, the sampling frequency (fs) shown in FIG. 3 is 40 kHz as an example, and the pilot signal is 30 kHz as an example. The frequency band of the input signal of the modulator 21 is an audible band, and the frequency band of the signal modulated by the modulator 21 is an ultrasonic band.

  In the modulation device 2, a microphone, a data input device, an amplifier, or the like may be provided before the modulator 21. When digital processing is performed by inputting an analog signal to the modulation device 2, it is preferable to provide an ADC before the analog modulator 21 and a DAC after the adder 23. Further, when the input signal is a digital signal, a DAC may be provided after the adder 23.

  As shown in FIG. 2, the demodulator 3 includes a downsampler 31 (corresponding to the folding means), a BPF (bandpass filter) 32 (corresponding to the extracting means), a control signal generator 33 (corresponding to the control signal generating means), and A VCO (voltage controlled oscillator) 34 (corresponding to voltage controlled oscillation means) is provided. The demodulator 3 also includes a demodulator 35 (corresponding to demodulating means), a buffer 36, and a DAC (digital-analog converting circuit) 37 (corresponding to correcting means).

  The demodulator 3 picks up the ultrasonic wave (passband signal) emitted by the modulator 2 with a microphone or sensor (not shown), amplifies it with an amplifier (not shown), and generates a passband signal with an ADC (not shown). Digitized and output to down sampler 31 and demodulator 35.

  As described above, when the relative speed of the modulation device 2 and the demodulation device 3 is changing, the frequency of the ultrasonic wave (passband signal) emitted by the modulation device 2 is shifted by the influence of the Doppler shift. For example, when both devices are approaching, the frequency of the passband signal is increased according to the relative speed as shown in FIG.

  The demodulator 3 downsamples the digitized passband signal by a downsampler 31 by a factor of 1/2. As a result, as shown in FIG. 3D, the passband signal is folded around the frequency fs / 2 having a frequency lower than that of the pilot signal to become a signal in the audible band. The downsampler 31 may have any frequency as long as the downsampler 31 is set to be turned back at a frequency equal to or lower than the frequency of the pilot signal.

  Next, as shown in FIG. 3E, the demodulator 3 extracts a pilot signal from the signal after downsampling by the BPF 32. The band of the BPF 32 is set in advance so as to extract only the pilot signal.

  The extracted pilot signal is input to the control signal generator 33, and the control signal generator 33 outputs a voltage proportional to the frequency variation of the input sine wave. For example, the control signal generator 33 counts the zero cross period of the sine wave, detects the frequency fluctuation of the sine wave, and outputs a voltage corresponding to the frequency as a control signal.

  The VCO 34 outputs a clock signal proportional to the change in the control signal. The clock signal follows in the range of several percent of the reference sampling frequency. The follow-up range may be determined according to the assumed Doppler shift range.

  The clock signal output from the VCO 34 is a sampling period clock generally called a word clock. The DAC 37 is operated using this clock.

  On the other hand, the demodulator 35 demodulates the modulation signal included in the ultrasonic wave (passband signal) input from the outside into an audio signal (demodulation signal). The demodulation method performed by the demodulator 35 is not particularly limited as long as the signal in the ultrasonic band is demodulated into the audio signal in the audible band. The demodulator 35 may be configured not to demodulate the pilot signal during demodulation. Further, if the demodulating device is constituted by an analog circuit, D / A conversion may be performed once at the reference sampling frequency before demodulation.

  The DAC 37 converts the digital audio signal (demodulated signal) output from the demodulator 35 and stored in the buffer 36 into an analog signal using the clock signal output from the VCO 34.

  The buffer 36 is for absorbing a delay in pitch variation due to Doppler shift when the DAC 37 converts a demodulated signal (digital signal) into an analog signal.

  As described above, the modulation device 2 emits ultrasonic waves while approaching the demodulation device 3. For this reason, the modulation signal and the pilot signal included in the ultrasonic wave (passband signal) collected by the microphone 31 are subjected to the relative speed of the modulation device 2 and the demodulation device 3 as shown in FIG. The frequency is shifted (increased) in response to. Further, the audio signal demodulated by the demodulator 35 is in a state where the pitch is increased as shown in FIG.

  On the other hand, since the pilot signal is turned back at the frequency fs / 2 by downsampling, the pilot signal shifts in a direction opposite to the shift direction by the Doppler shift. For example, when the pitch of the demodulated audio signal is shifted (increased in frequency) as described above, as shown in FIG. 3E, the clock signal of the DAC 37 is delayed (frequency is increased). Down). That is, the amount by which the frequency of the demodulated audio signal is increased and the amount by which the frequency is decreased by folding the pilot signal are the same value. For this reason, the DAC 37 reduces the speed at which the audio signal is analogized by an amount corresponding to the increase in the audio signal pitch, so that the audio signal has an appropriate pitch. That is, as shown in FIGS. 3G and 3A, the pitch variation is canceled and an audio signal having the same frequency characteristic as the original audio signal input from the input terminal 20 of the modulation device 2 is output.

  In the above description, as an example, the case where the modulation device 2 and the demodulation device 3 approach each other increases the frequency of the modulation signal and the pilot signal. However, when both devices move away, the modulation signal and the pilot signal are increased. The same applies to the case where the frequency decreases.

  As described above, by operating the DAC 37 with the clock signal generated from the pilot signal extracted as the aliasing component by the downsampling, it is possible to cancel the Doppler shift pitch change. In addition, even if communication is performed between the moving modulation device 2 and the demodulation device 3, it can be reproduced as sound with a well-tuned pitch that is easy to hear.

  Further, in the modulation / demodulation system 1, it is only necessary to newly add a pilot signal at the time of modulation, and a large design change is not required in the existing modulation algorithm. Furthermore, since the correction is performed through a path different from the demodulation operation, it does not depend on any demodulation algorithm. Therefore, both modulation and demodulation can be additionally implemented as post-processing in the existing processing configuration.

  As an example, the case of performing 1/2 downsampling has been described. However, the present invention is not limited to this, and other magnifications may be used.

  In the above description, an example in which a signal is folded back by digital processing has been shown. However, the present invention is not limited to digital processing, and may be analog processing. Although an example of controlling the DAC clock has been shown, other configurations may be used as long as the configuration can correct the frequency.

  Further, the present invention is not limited to the case where an audio signal (input signal) is modulated into an ultrasonic signal (modulation signal), and the input signal and the modulation signal include other electromagnetic waves, light, and other Doppler effects. You may apply to a medium.

It is a block diagram which shows schematic structure of the modulation apparatus which concerns on embodiment of this invention. It is a block diagram which shows schematic structure of the demodulation apparatus which concerns on embodiment of this invention. It is a figure for demonstrating the change of the frequency characteristic by signal processing in the demodulation system which concerns on embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Modulation / demodulation system 2 ... Modulator 3 ... Demodulator 21 ... Modulator 22 ... Sine wave generator 23 ... Adder 31 ... Down sampler 32 ... BPF 33 ... Control signal generator 34 ... VCO 35 ... Demodulator 36 ... Buffer 37 ... DAC

Claims (3)

A demodulating means for demodulating a passband signal superposed with a pilot signal having a frequency higher than the sampling Nyquist frequency , input from the outside, at the sampling frequency of the sampling ;
Down-sampling means for down-sampling the passband signal on which the pilot signal is superimposed to ½ times the sampling so as to return at a frequency equal to or lower than the frequency of the pilot signal;
An extraction means for the down-sampling means for extracting the pilot signal from the down-sampled signal,
Correction means in accordance with the shift of the frequency of the pilot signal extracted by the extracting unit, said demodulation means to correct the shift of the frequency of the demodulated signal demodulated,
Including a demodulator. Control signal generating means for generating a voltage proportional to the frequency of the pilot signal extracted by the extracting means;
Voltage controlled oscillation means for outputting a clock signal proportional to the voltage generated by the control signal generating means;
With
The demodulator according to claim 1, wherein the correction unit corrects a frequency shift of the demodulated signal in accordance with a clock signal output from the voltage controlled oscillation unit. Modulation means for modulating the input signal into a passband signal;
Superimposing means for superimposing a pilot signal on the passband signal;
Output means for outputting a passband signal on which the pilot signal is superimposed;
A modulator comprising:
The demodulator according to claim 1 or 2,
Modulation / demodulation system with

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