A form of frequency modulation in which the carrier frequency is shifted between a markfrequency and a space frequency in response to the impressed intelligence signal (frequency-shift keying).
频率调制的一种方式，其中载波频率在标志频率和场频率之间移动，以响应外加的情报(数据)信号。
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By analysing frequency spectrum in this paper, we can understand that when NRZ code be transferred into RZ code of 1/2 mark-to-space ratio, the timing clock can be obtained at largest power.