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。-Transmission of digital signals into base band transmission and band-pass transmission, in practice, most of the channel with band-pass characteristics can not directly transmit base-band signal. In order to make the digital signal is transmitted with a communication channel, you must use the digital baseband signal to modulate the carrier to make the signal to match the characteristics of the channel. The digital baseband signal control with a carrier, the digital baseband signal conversion to digital bandpass signal is called digital modulation. Keying method is commonly used digital modulation, such as on the carrier' s amplitude, frequency and phase shift keying.