Fldigi

Fldigi is a modem application that features support for most digital modes used by radio amateurs. Its main functions are related to performing frequency measurement tests or calibrating a sound card to a signal.
The application relies on the system sound card to input and output audio-frequency signals. It works together with a standard HF SSB radio transceiver, while also controlling the radio using serial port connections.
All the popular modes are supported, including FM, AMS, LSB, RTTY, MFSK16, DominoEX, MFSK, Throb, Hell, Olivia, CW and PSK. Fldigi enables you to control the transmitter, read incoming signals and tune the high-frequency propagation.
The configuration wizard starts at the first initialization, ready to guide you through all the steps needed for choosing the functioning settings of Fldigi, from entering the operator information and the audio devices to choosing the tabular data sources. Alternatively, you can change the options at a later time using the designated menu.
Aside from the receiver and the transmitter pane where you can view decrypted and sent messages, respectively, the main interface also comprises a section dedicated to identifying and tuning the signal. Depending on the selected method (Waterfall, FFT or Signal), you can view a spectrogram display of signal strength versus frequency or an oscilloscope representation of the captured raw audio.
The application features a built-in signal browser, automatic mode detection and tuning, text capturing options, rig controlling capabilities, data logging and advanced appearance customization options. Common tasks can be automated using macros and complete text, CSV or Cabrillo reports can be generated with just a few clicks.
Overall, Fldigi is not difficult to use for those who know what all the buttons do, but a strong knowledge base is required for exploiting its full potential, especially since recognizing the different modes comes with experience. It is dedicated to those who are really passionate about radio signals and want to learn more.