如何才能产生一个正负对称的正弦波
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http://w260021.s58-86.myverydz.com/forum.php?mod=viewthread&tid=887
http://www.openedv.com/posts/list/17738.htm
http://blog.sina.com.cn/s/blog_5e467f8a0101746a.html
http://blog.csdn.net/hongkangwl/article/details/14228677