General description
The QPSK decoder supports soft bit Viterbi decoding. The decoder uses the most modern digital PLL synchronization technology for bit synchronization and a neural network for detection. It prints QPSK varicode characters on the screen. It searches the QPSK transmission within the given frequency boundaries. Also you can set the decoded frequency by double clicking the mouse on the required frequency position.
When QPSK transmission has been detected at some frequency 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, at 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 locked state, the carrier frequency is automatically monitored within the given frequency boundaries. If the carrier frequency changes, then the decoder moves to the new frequency. If 'Lock' is pressed in the user interface, then the decoder will not automatically change its frequency. When ‘UnLock’ is pressed the lock will be released and the automatic tuning starts again to work. ‘Reset’ resets the decoder and the received text can be saved using ‘Save’.
SkySweeper supports both FFT and waterfall display modes for decoder tuning. The Decoder Spectrum Dialog chapter gives more information about spectrum display options.
Configuration
Pressing the ‘Config’ button opens the following Configuration dialog box.
Parameters
Tone Scan Area
The scanned frequency range is given with two values - ‘low’ and ‘high’. The QPSK transmission is searched for only within this frequency window. In most cases it is useful to first search for the transmission visually using the spectrogram, FFT and 3DFFT displays and then define the scan area for the QPSK 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 been locked onto the correct frequency in order to get more space for the 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 also be saved into a file or sent into a socket.
Lock
The ‘Lock’ button locks or unlocks the decoder frequency. Decoders have the ‘automatic frequency tracking’ feature. Push the ‘Lock’ button when you want to lock decoder to a selected frequency. Push the ‘Unlock’ button when you want to release frequency locking.
Reset
The ‘Reset’ button resets the decoder and clears the text window.
