说明：  数字信号的传输方式分为基带传输和带通传输，在实际应用中，大多数信道具有带通特性而不能直接传输基带信号。为了使数字信号在带通信道中传输，必须使用数字基带信号对载波进行调制，以使信号与信道的特性相匹配。这种用数字基带信号控制载波，把数字基带信号变换为数字带通信号的过程称为数字调制。通常使用键控法来实现数字调制，比如对载波的振幅、频率和相位进行键控
(The transmission mode of the digital signal is divided into a base-band transmission and band-pass transmission, in practical applications, most of the channel having a band pass characteristic but can not directly transmit baseband signal. In order to make the digital signal transmitted in band communication channel, the digital baseband signal modulates the carrier must be used, so that the signal with the characteristics of the channel to match. This control carrier of the digital baseband signal, the digital baseband signal is converted into digital the Bandpass process called digital modulation. Keying method generally used to implement the digital modulation, such as the carrier' s amplitude, frequency and phase shift keying)