JP3957367B2 - FFT signal processing apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an FFT (Fast Fourier Transform) signal processing apparatus, and more particularly to an apparatus for forming a threshold value for reliably separating a frequency to be obtained from noise.
[0002]
[Prior art]
FIG. 5 is a diagram for explaining a conventional example in which a frequency to be obtained in a frequency spectrum is separated from noise. As shown in this figure, in the frequency spectrum obtained by frequency analysis, level peak values a, b, c, d, e, f, g, level bottom values a ′, b ′, c ′, d ′, e ′, The peak frequencies fb, fd, and fe corresponding to the level peak values b, d, and e that have f ′ and exceed the fixed threshold are identified as the frequencies to be obtained. This fixed threshold is determined from the noise level in the frequency spectrum. This fixed threshold value may be determined based on the maximum level peak value. However, even if the level peak value is smaller than the threshold value, if the noise level is small, there is a possibility that the peak frequency should be significantly obtained. There is a problem that it cannot be identified.
[0003]
Furthermore, when the secular change and the environmental change are large, the S / N deteriorates and the noise level increases, and the problem is that the peak frequency corresponding to the noise level peak value is erroneously identified as the frequency to be obtained. There is.
[0004]
[Problems to be solved by the invention]
Therefore, in view of the above problems, an object of the present invention is to provide an FFT signal processing apparatus that can identify a frequency that should be reliably obtained regardless of how the noise level in the frequency spectrum changes.
[0005]
[Means for Solving the Problems]
The present invention, in order to solve the above problems, in the FFT signal processing apparatus for identifying the frequency to be from the obtained frequency spectrum to remove the following peak level threshold as noise by fast Fourier transform, the reference value determination An FFT signal processing apparatus comprising a threshold value forming unit that forms a threshold value based on a level bottom value of a frequency spectrum in a range. The FFT signal processing unit updates the reference value determination range based on the level trough in the reference value determination range. Further, FFT signal processing device, the reference value determination range data, further comprising a reference value determination range storage unit for storing data of the correction value for the offset the fluctuation band of the noise, the threshold value forming unit, the reference value determination An average value is obtained for the bottom level of the frequency spectrum in the range, and a correction value is added to the average value to form a threshold value. In this case, the offset of the correction value can be increased as the average value of the level bottom values increases. By this means, even if the noise level is small, even if the noise level becomes large due to secular change or environmental change, it is possible to form a threshold according to the change in the noise level. Therefore, the frequency to be obtained can be reliably separated from the noise and identified.
[0006]
The correction value may be added to the offset by adding a filter characteristic that decreases in level as the frequency increases. This means makes it possible to apply continuous FM modulation radar.
The reference value determination range storage unit may be a backup memory. By this means, the present invention can be applied even when the system using the apparatus is turned from OFF to ON.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram for explaining an example of an FFT signal processing apparatus according to the present invention. As shown in the figure, a fast Fourier transform unit (FFT) 1 performs frequency analysis on an input signal to obtain a frequency spectrum. The frequency identification unit 2 identifies the peak frequency corresponding to the level peak value exceeding the threshold value as the frequency to be obtained in the frequency spectrum.
[0008]
The threshold value forming unit 3 averages the level bottom value when the level bottom value is within the reference value determination range in the frequency spectrum obtained by the fast Fourier transform unit 1, and adds a correction value to the average value (Ab) to set the threshold value. And this threshold value is given to the frequency identification unit 2. The reference value determination range storage unit 4 is a backup memory, stores a reference value determination range and a correction value to be given to the threshold value forming unit 3, and each time the threshold value forming unit 3 forms a threshold value, the reference value The judgment range is updated.
[0009]
FIG. 2 is a diagram for specifically explaining the threshold value forming unit 3 and the reference value determination range storage unit 4 of FIG. As shown in the figure, the reference value determination range storage unit 4 stores level X and level Y (X> Y) as the reference value determination range. The threshold value forming unit 3 applies the level peak values a, b, c, d, e, f, g and the level bottom values a ′, b ′, c ′, d ′, e ′, f ′ in the frequency spectrum. Thus, an average value (Ab) of the level bottom values a ′, c ′, e ′ (a ′ <c ′ <e ′) between the threshold level X and the level Y is obtained. The reason why the level bottom values b ′, d ′, and f ′ are removed from the reference value determination range in this manner is that they suddenly become smaller or larger and give rise to fluctuations in the average value. The correction value is a fluctuation range of the noise level, and is added as an offset to the average value Ab of the level bottom value to form a threshold value. There is a certain relationship that when the level bottom value increases, the correction value increases accordingly.
[0010]
The reference value determination range storage unit 4 updates the level X to the level bottom value e ′ and the level Y to the level bottom value a ′ as the reference value determination range. By updating in this way, it becomes possible to form a threshold according to this change even if the noise level is subject to aging and environmental changes. Since the updated reference value determination range is stored in the backup memory, the value of this memory can be used as the reference value determination range when the system using the FFT signal processing device is turned on next time. .
[0011]
Thus, since the threshold value is formed according to the noise level fluctuation, it is possible to reliably identify the frequency to be obtained in the frequency spectrum.
Next, in the frequency spectrum analyzed by the radar of the continuous FM modulation system, the frequency to be obtained may become larger and the power level may be lowered as the target is further away. The correction in this case will be described below.
[0012]
FIG. 3 is a diagram for explaining an example of forming correction values in consideration of filter characteristics. As shown in FIG. 5A, the filter characteristic whose power level decreases as the frequency increases is stored in the reference value determination range storage unit 4. In this filter characteristic, when the frequency is increased to some extent, the power level has a certain level due to white noise when there is no signal. As described above, the threshold value forming unit 3 calculates the offset correction value and further adds this filter characteristic to form a threshold value.
[0013]
FIG. 4 is a flowchart for explaining a series of operations of the FFT signal processing apparatus according to the present invention. In step S1, frequency analysis of the input signal is performed by a fast Fourier transform unit (FFT) to obtain a frequency spectrum. In step S <b> 2, the threshold value forming unit 3 searches for a valley point (level bottom) of the frequency spectrum. In step S3, using the reference determination range read from the reference value determination range storage unit 4, a point (level bottom) in the reference value determination range (X to Y) is extracted from the points searched in step S2. The In step S4, the extracted level bottom values are averaged and the average value is set to Ab. In step S5, using the correction value read from the reference value determination range storage unit 4,
A threshold value is formed by performing an operation of threshold value = average value Ab + correction value. In step S6, the frequency identification unit 2 extracts a peak point (level peak) exceeding the threshold value. The peak frequency corresponding to the peak point (level peak) extracted in step S7 is identified as the frequency to be obtained. In step S8, the next reference value determination range (X to Y) is calculated. In step S9, the calculated result is saved in the reference value determination range storage unit 4 composed of a backup memory.
[0014]
【The invention's effect】
From the above description, according to the present invention, even if the noise level is small, and even if the noise level increases due to secular change and environmental change, the threshold value is formed according to the change in the noise level. The frequency to be obtained can be reliably separated and identified from the noise.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an example of an FFT signal processing apparatus according to the present invention.
FIG. 2 is a diagram for specifically explaining a threshold value forming unit 3 and a reference value determination range storage unit 4 in FIG. 1;
FIG. 3 is a diagram illustrating an example of forming a correction value considering filter characteristics.
FIG. 4 is a flowchart illustrating a series of operations of the FFT signal processing apparatus according to the present invention.
FIG. 5 is a diagram for explaining a conventional example in which a frequency to be obtained in a frequency spectrum is separated from noise.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Fast Fourier transform part 2 ... Frequency identification part 3 ... Threshold value formation part 4 ... Reference value determination range storage part 4

Claims (4)

In an FFT signal processing apparatus for identifying a frequency to be obtained by removing a peak level below a threshold value as noise from a frequency spectrum obtained by fast Fourier transform,
And a threshold-shaped forming portion for forming a threshold based on the level trough of the frequency spectrum in the reference value determination range,
A reference value determination range storage unit that stores data of the reference value determination range and correction value data having the noise fluctuation range as an offset;
The reference value determination range is updated based on the level bottom value in the reference value determination range,
The threshold value forming unit obtains an average value for the level bottom value of the frequency spectrum in the reference value determination range, and adds the correction value to the average value to form the threshold value. Signal processing device. The FFT signal processing apparatus according to claim 1 , wherein the offset of the correction value increases as the average value of the level bottom value increases. 2. The FFT signal processing apparatus according to claim 1 , wherein the correction value is obtained by adding a filter characteristic having a level that decreases as the frequency increases to the offset. 3. The FFT signal processing apparatus according to claim 1 , wherein the reference value determination range storage unit is a backup memory.

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