The 19th Century Telecommunications

Telegraph and Telephone

The Chappe Telegraph

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• 每个塔都有一个articulated arms和一个观察平台。观察员通过观察前一个塔的状态，摆出预定的姿势，传向下一个塔。
• 每个短臂有8个states,长臂有4个states，所以一共有256（4 * 8 * 8）个positions。有92个positions表示characters。
• 这种传递方式可以通过修正前一个的状态到预定状态来减少传递误差。

The Morse Telegraph

• It consisted in a electrical circuit with a switch on one end, an electromagnet on the other end and a battery in between. By closing the switch, the electromagnet get excited and, for instance, pushes a pencil on a moving strip of paper.

• 摩尔斯密码根据字母使用频率来编写。

  
  
  
• Morse telegraph也是可以通过开关来进行中继（relay），这样每段产生的小误差可以被修正。

The Bell Telephone

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• 输入端通过振动，由磁信号变成电信号。输出端通过电磁铁由电信号变成磁信号，并引起振膜运动，从而产生声音。
• Bell Telephone是模拟信号不是数字信号。

Analog and Digital Transmission

模拟信号和数字信号的区别

• Almost all digital electronic systems represent information by means of binary variables, that are variables that can only have two different values, often represented by 0 and 1 or also by TRUE ands FALSE.
• n位二进制数可以表示2ⁿ个状态。