RTTY is a program that listen to your sound-card audio input and try to decode FSK modulated signals. These signals can be found in HF bands and are commonly known as Radio-Tele-TYpe (RTTY).
In RTTY signals a given frequency is used to transmit a logic ``1'' and a different frequency is used for the logic ``0''. We call these frequencies MARK & SPACE frequencies respectively. The data format is those found in asynchronous serial communications: Every character starts with a START bit with ``0'' logic value, then follows the data bits, LSB first, and finally one or two STOP bits with a logic level ``1'' are added to the character. Inter-character time is filled with a logic ``1''. The character is often 5 bits long, and uses an old encoding called BAUDOT, but sometimes a plain ASCII encoding is used. in this case the character is 7 or 8 bits long, and one optional parity bit can be added in order to check the received data integrity. The data rate is slow. Typical values range from 42 to 300 baud.