精度：gettimeofday 精度是固定在微秒级别上的。
Precision: gettimeofday precision is fixed to the microsecond.
由于它们应用于机床制造、两种设计制作仅在高精度的级别。
Due to their use in machine tool manufacture, both designs are produced only in high-precision grades.
实验结果表明，算法更合理地将水印嵌入在MPEG-4视频流中，水印容量得到较大提高，对恶意攻击的认证精度定位在块级别。
Through experiments the method can hide watermark in MPEG-4 video stream more rationally, the capacity of watermark increases greatly and the authentication precision locates in block level.
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