调制，是用基带信号对载波波形的某参量进行控制，使该参量随基带信号的规律变化从而携带消息。对数字信号进行调制可以便于信号的传输；实现信道复用；改变信号占据的带宽；改善系统的性能。(Modulation is the use of baseband signals to control a certain parameter of the carrier waveform, so that the parameter changes with the baseband signal, so as to carry the message. Modulation of digital signals can facilitate the transmission of signals, realize multiplexing channels, change the bandwidth occupied by signals, and improve the performance of the system.)