These are stand-alone UART devices, similar to computer serial port peripherals. The 5 data bits allow for only 32 different codes, which cannot accommodate the 26 letters, 10 figures, space, a few punctuation marks and the required control codes, such as carriage return, new line, bell, etc. To overcome this limitation, the teleprinter has two "states", the "unshifted" or "letters" state and the "shifted" or "numbers" or "figures" state. The change from one state to the other takes place when the special control codes "LETTERS" and "FIGURES" are sent from the keyboard or received from the line. In the "letters" state the teleprinter prints the letters and space while in the shifted state it prints the numerals and punctuation marks. Teleprinters for languages using other alphabets also use an additional "third shift" state, in which they print letters in the alternative alphabet. The modem is sometimes called the terminal unit and is an electronic device which is connected between the teleprinter and the radio transceiver. The transmitting part of the modem converts the digital signal transmitted by the teleprinter or tape reader to one or the other of a pair of audio frequency tones, traditionally 2295/2125 Hz (US) or 2125/1955 Hz (Europe). One of the tones corresponds to the "mark" condition and the other to the "space" condition. These audio tones, then, modulate an SSB transmitter to produce the final audio-frequency shift keying (AFSK) radio frequency signal. Some transmitters are capable of direct frequency-shift keying (FSK) as they can directly accept the digital signal and change their transmitting frequency according to the "mark" or "space" input state. In this case the transmitting part of the modem is bypassed. On reception, the FSK signal is converted to the original tones by mixing the FSK signal with a local oscillator called the BFO or "beat frequency oscillator". These tones are fed to the demodulator part of the modem, which processes them through a series of filters and detectors to recreate the original digital signal. The FSK signals are audible on a communications radio receiver equipped with a BFO, and have a distinctive "beedle-eeeedle-eedle-eee" sound, usually starting and ending on one of the two tones ("idle on mark"). The transmission speed is a characteristic of the teleprinter while the shift (the difference between the tones representing mark and space) is a characteristic of the modem.