把消息转换成电信号。信源可分为模拟信源和数字信源。区分模拟信号和数字信号的准绳是看其取值是连续的还是离散的，而不是看时间。

信源编码：主要是利用信源的统计特性，解决信源的相关性，去掉信源冗余信息，提高系统有效性的目的。

信道编码：保证通信系统的传输可靠性，克服信道中的噪声和干扰。

调制：其主要目的是使经过编码的信号特性与-The message into electrical signals. Source can be divided into analog source and digital source. Distinguish between analog and digital signals yardstick is to look at its value is continuous or discrete, rather than of time. Source code: The main source is the use of the statistical properties to address the source of the relevance, to remove redundant information source to enhance effectiveness of the system purpose. Channel coding: to ensure the transmission reliability of communication systems to overcome the channel noise and interference. Modulation: Its main purpose is to make the signal characteristics and encoded