Two signals from the mass flowmeter are filtered by no change of phase, and the phase difference is given by zero passage arithmetic.

两路信号数据采集后使用了无相位变化的滤波方法，通过过零点算法计算出得出相位差。

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Then, the filtered image was subtracted from the raw image and a difference image with flat background and outstanding defects was obtained.

将原始图像与背景图像相减，获得背景平坦、缺陷突出的差值图像；

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