SITOR Decoder

General description

The SITOR decoder supports both SITOR-A (also called AMTOR) and SITOR-B (also called NAVTEX). The decoder uses the most modern digital PLL synchronization technology for bit synchronization and a neural network for detection, and it prints SITOR characters on the screen. It searches the SITOR transmission in the range of the given frequency boundaries. Also you can set the first FSK frequency by doubleclicking mouse on the wanted frequency position and then SkySweeper allows you to set the second frequency (can be set by the mouse click). The numeral values of frequencies and the frequency offset are shown also on the screen.

When SITOR transmission has been detected in the given range, the decoder will then lock itself to that frequency. When locked, the decoder draws the bolded 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 bolded. 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 locked state, the carrier frequencies are automatically monitored in the given frequency boundaries and 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 it's frequency. ‘Reset’ resets the decoder and received text can be saved using ‘Save’. The parameters can be modified by pressing the ‘Config’ button. Note, that you can install several decoders in one configuration if needed.

 

Configuration

Pressing the Config button opens the following configuration dialog.

 

 

Parameters

Sitor Mode

You can choose whether SITOR-A or SITOR-B will be detected.

Selective Call Mode

By this switch, you can select how the SITOR-A selective call is decoded. Both four and seven letter modes are supported. The four letter call can also be presented as a four or five digit number. If the letter to digit conversion is not successful, string '****' will be printed.

Tone scan area

The SITOR 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.

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.

In most cases it is useful first to search transmission visually using spectrogram, FFT and 3DFFT displays and then define scan areas for the SITOR decoder.

 

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.