实现整数倍采样率转换可以采用时不变滤波器，然而实现分数倍采样率转换一般需要采用较复杂的时变滤波器。
For integer ratio conversion, the filters would be time-invariant, but for fractional ratio conversion, the filters would be time-variant whose structure is more complex.
麻省理工学院之成绩计算并非以原始分数及其分布函数情形而一成不变，也就是说，成绩不是仅以既定的百分比来核定。
Grades at MIT are not rigidly related to any numerical scores or distribution functions, that is, grades are not awarded solely according to predetermined percentages.
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