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Baudot code is a 5-bit character encoding invented by Émile Baudot in the 1870s. It was the predecessor to ASCII and was widely used in telegraphy and early telecommunications.

Baudot uses two modes: Letters (LTRS) and Figures (FIGS). The same 5-bit code represents different characters in each mode. Special shift codes (11111 for LTRS, 11011 for FIGS) switch between modes.

ITA2 (International Telegraph Alphabet No. 2) is the international standard. US-TTY is the American variant with some different symbol assignments in figures mode.

In the 1870s, mechanical limitations and the need for simple, reliable transmission made 5 bits optimal. It provided enough combinations (32) while keeping hardware simple.

博多码，又称 Baudot code 或 ITA2（International Telegraph Alphabet No.2，国际电报字母表第 2 号），由法国工程师 Émile Baudot 于 1870 年代发明，是最早的数字化字符编码。

与 ASCII 的区别：
• 位数：博多码用 5 位（32 个码位），ASCII 用 7 位（128 个码位）。
• 字符集：博多码通过 字母/数字换档（LTRS/FIGS） 复用 32 个码位表示 58+ 字符，而 ASCII 一个码位对应一个字符。
• 年代：博多码诞生于 1870 年代电报时代，ASCII 出现于 1963 年。
• 场景：博多码曾用于 电传机（Teletype/TTY）、Telex 网络、海事通信、新闻电传；ASCII 则是现代计算机文本的基础。
今天博多码仍活在 业余无线电 RTTY、TDD 聋人文字电话、以及复古通信协议中。

博多码在线解码：把 5 位二进制串（如 00011 11001 01110 00001）贴入输入框，工具自动按 ITA2 或 US-TTY 表查找对应字符，并处理 11111（LTRS 换档）和 11011（FIGS 换档）状态切换。

博多码在线编码：输入英文字母、数字或符号，点击 [ENCODE]，得到 5 位二进制序列及必要的换档码。

常见来源：解码 RTTY 无线电信号、老式电传纸带 数字化数据、复古计算机项目、密码学 CTF 题目。全部解码在浏览器本地完成，无服务器上传。

Because 5 bits only give 32 codes, Baudot reuses each code for two characters — one in Letters mode, one in Figures mode. Two special codes switch modes:
• 11111 → LTRS (Letters Shift): next codes are A-Z and space.
• 11011 → FIGS (Figures Shift): next codes are digits and punctuation.

So to send HELLO 123:
[LTRS] H E L L O space [FIGS] 1 2 3

The receiver tracks which mode it's in. This is why early TTY operators started every transmission with "RYRY RY RY" — alternating letters and figures to force the receiver into a known state and confirm synchronization before the real message.

Baudot's shift mechanism is the first example of what we'd now call stateful encoding — a pattern later used by UTF-7, ISO-2022-JP, and MIME quoted-printable.

A Baudot code translator (also searched as baudot code converter, ita2 encoder, ita2 生成, baudot 無線電電傳打字機) has four typical use cases:
1. Decoding RTTY transmissions — amateur radio operators capture 45.45-baud FSK audio, convert to bitstream, then to Baudot/ITA2.
2. Restoring vintage teleprinter tape — museums and archives digitize 5-hole paper tape and need to convert the punches back to readable text.
3. CTF challenges — cipher challenges that present a binary stream hinting at a 5-bit encoding.
4. Education — teaching information theory and how pre-computer communication used stateful codes.
Our tool supports both ITA2 (international) and US-TTY (American variant with different FIGS assignments).

The letters mode is identical (A-Z are the same), but figures mode diverges:
• ITA2 (international): includes +, =, ', £, and control characters used in European Telex networks.
• US-TTY: replaces some international symbols with &, #, $, %, @ — characters more familiar in American business and news-wire use.
The divergence happened because Western Union and American Telex deployed US-TTY in the 1930s-40s based on local needs, while CCITT (now ITU-T) standardized ITA2 internationally. Choose ITA2 for historical European messages and standard amateur radio RTTY; choose US-TTY for American news-wire captures and early computer teleprinters like the Teletype Model 33.

Baudot 无线电电传打字机（RTTY, Radio Teletype）是 20 世纪中叶电报技术的巅峰，将博多码（ITA2）与无线电 FSK（频移键控）结合，实现跨洋文字通信。

关键场景：
• 二战时期：盟军和轴心国都使用 RTTY 传输军事指令，德军 Enigma 的明文常以 Baudot 输入。
• 新闻社：合众社（UPI）、路透社通过电传网络 24/7 分发稿件。
• 海事通信：SOS 求救信号的文字版——NAVTEX、INMARSAT-C 系统至今仍用类 Baudot 协议。
• 业余无线电：今天 QRP 爱好者仍在 14.080 MHz、7.040 MHz 等频段用 45.45 波特 RTTY 通联。

博多码催生了后来的 ASCII、EBCDIC 以及现代 Unicode 的字符编码思想。每次你按下键盘，某种意义上都在继承 1874 年 Émile Baudot 的设计。