Which type of code is used for sending characters in a PSK31 signal?

PSK31 uses a specialized character encoding system called Varicode. The primary feature of Varicode is that it uses variable-length binary codes for different characters based on how frequently they are used in standard text. Similar to how Morse code assigns the shortest symbol (a single dot) to the most common letter (E), Varicode assigns shorter digital bit sequences to frequently used characters (like 'e' or 't') and longer sequences to rare characters. This optimizes the transmission speed for live keyboard-to-keyboard chatting. Distractors like "Viterbi" refer to a decoding algorithm used in error correction, while "Binary" is too generic, as all digital modes inherently use binary data.