single letter signal code

例句：

1. 1.The single-letter code is used to encode information in a way that only authorized personnel can decipher.
   这个单字母代码用于以只有授权人员才能解码的方式编码信息。
2. 2. In the early days of radio, single-letter codes were widely used to send messages over long distances.
   在无线电的早期，单字母代码被广泛使用来远距离发送信息。
3. 3.The use of single-letter codes has become less common as more advanced encryption methods have been developed.
   随着先进加密方法的发展，使用单字母代码的使用已经变得不那么常见了。