How TTYs Work: Transmission Codes
TTYs generate and receive text through tones similar to those of touch-tone phones. There are two standard codes for transmitting text: Baudot and ASCII. This is an important consideration when buying or specifying a TTY. TTY technology was standardized on Baudot (5-bit) codes in the 1960s while computer applications require an ACSXI (8- bit) format. Baudot code has fewer characters and a slower transmission rate but will likely remain the standard for TTYs in the years ahead as ASCII dominates the market. ASCII, while faster than Baudot, provides more capability than necessary for TTYs and is susceptible to interruptions such as call waiting, transfers, and line noise. For this reason, telephone emergency services, such as 911 and similar fast-dial lines, must be compatible with Baudot. Many TTYs have an ASCII option. Also available are TTYs with Turbo Code, an enhanced Baudot code with a faster transmission speed employed automatically when used with other TTYs with this capability.
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