Page "Radioteletype" Paragraph 17

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.