RTTY Decoder

General description

The RTTY decoder supports BAUDOT, asynchronous ASCII, SHIP and SYNOP decoding. The decoder searches the transmission within the given frequency boundaries. Also you can set the lower FSK frequency by double clicking mouse on the wanted frequency position, and then SkySweeper allows you to set a second (=higher) frequency (can be set by the mouse click). The values of frequencies and the frequency offset are shown also on the screen.

When a transmission has been detected within the given range, the decoder will then lock itself to that frequency. When locked, the decoder draws a bold blue line on the spectrum display on the currently locked frequency. If the decoder is not locked, then the blue line is not shown as bold. After locking the frequency, the decoder starts to estimate the keying parameters of the transmission. When all the parameters have been estimated, then the decoder starts to print characters on the screen.

In the RTTY's "autodetect" mode, the transmission mode is automatically recognised. The estimation takes about 20-30 seconds.

In the non-locked state, the carrier frequencies are automatically monitored in the given frequency boundaries. If the carrier frequency changes, then the decoder moves to a new frequency. If 'Lock' is pressed in the user interface, then decoder will not automatically change the frequency. ‘Reset’ resets the decoder and the received text can be saved using ‘Save’. The parameters can be modified pressing the ‘Config’ button. Note that you can install several decoders in one configuration, if needed.

The received text can be saved with ‘Save’. ‘Reset’ resets the decoder and ‘Config’ is used to set the parameters.

Configuration

Pressing the Config button opens the following configuration dialog.

 

Parameters

Mode

This is the main operation mode of the decoder (ASCII or RTTY). If RTTY is selected, all the RTTY modes are activated for selection. If ASCII is selected, then all ASCII modes are activated.

Baudot Speed

With this parameter the RTTY mode to be received can be selected. If the baud rate auto detection is switched on, it will take about 20 seconds to detect the correct transmission mode and start the decoding. If the mode (like 75 bauds) is selected by the user, then the decoder starts to decode immediately after the frequencies has been locked. If the user defined mode is selected, Baud Rate and Stop Pulse Length can be defined.

User Defined

If the user defined baudot mode is selected, Baud Rate and Stop Pulse Length can be defined into the edit boxes.

ASCII Asynchronous

With this parameter the ASCII mode to be received can be selected. If ‘Auto detect’ is selected SkySweeper automatically detects the mode of the transmission and starts to detect it.

Tone scan area

The RTTY transmission has two tone marks (typically 2125 Hz from the carrier) and space (typically 2295 Hz from the carrier). The scanned frequency range for Mark and Space will be given with two values: ‘low’ and ‘high’. The Mark and Space tones are searched only inside this frequency window.

Fixed Offset

If the fixed offset check box is selected, the user can give the fixed frequency offset between the Mark and Space carrier frequencies. The fixed frequency offset is used in automatic scanning mode.

Negative Polarity

If selected, the polarity is negative.

Parity Bit

If selected, parity bit is used. The parity bit parameter can be used only in the ASCII mode.

Unshift On Space

When this parameter is set as active, the decoder always returns to the LETTER character set when the character SPACE is detected. This is quite useful feature when a noisy RTTY signal is received.

Synop Decoding

The SYNOP (FM-12) and SHIP message decoding can be started by selecting this flag as active. LITE version does not support synop decoding.

Character Set Definition

By clicking this button, the RTTY/BAUDOT character set can be changed. The RTTY control characters cannot be changed in the Character Set Configuration Dialog

 

FFT

The FFT button opens and closes the power spectrum window. You can for example close the power spectrum window when the decoder has locked into correct frequency in order to get more space for text window

 

Save

The save button opens the log file control dialog, which is used to open or close a log file. The detector text window can be also saved into file or sent into socket.

Lock

The lock button locks or unlocks the decoder frequency. Decoders have the automatic frequency scanning feature. Push the lock button when you want to lock decoder into selected frequency. Push Un-Lock button when you want to release the frequency locking.

Reset

The reset button resets the decoder and clears the text window.