Butterworth带通滤波Fotran子程序，严格按照原文献在对事件序列正滤一遍后又倒滤一遍，以合理避免了位相偏移。-Butterworth band-pass filter Fotran subroutine, in strict accordance with the original documents in the sequence of events is filtered again and then filter down again in order to avoid a reasonable phase shift.