With PACTOR-II, a modern, narrow-band and reliable two-channel PSK mode is available, which covers the needs of both amateur and professional users for a secure and fast data transmission system. The good performance during disturbances and a small bandwidth of 500Hz are achieved by the use of a two channel DPSK modulation. As propagation conditions change, PACTOR-II is able to select the most suitable modulation (DBPSK, DQPSK, D8PSK, D16PSK and 16PSK) and the length of the frame (SHORT or LONG). Under very poor conditions PACTOR-II changes to DBPSK short block, which has a frame duration of 1.25 seconds and carries only a small amount of data. For very good conditions, D16PSK with a long frame is used, which transfers a large amount of data and needs 3.75 seconds.

To achieve a data transmission which is robust against all kind of disturbances, a decision must be made, whether a short or a long frame-length should be used. The resulting input bit stream is then terminated with a convolutional encoded and interleaved CRC-sum. After that, the symbol mapping for the two channels is done, and then this signal is applied to the DPSK or PSK demodulator.

To extract the data from a PACTOR-II signal, the signal is received and demodulated with a two channel demodulator. The center frequency must be adjusted to the center between the two channels. To achieve the best performance, the AFC (automatic frequency control) should be enabled. After the demodulation and the bit synchronization, the symbols of the two channels are combined to one bit stream. After the modulation type has been determined, the resulting bit stream is de-interleaved, passed through a Viterbi-decoder. In a next step the CRC is checked.

PACTOR-II is suited to the transfer of large files rather than hand-typed text. The data may be transmitted as ASCII without compression or can be compressed with HUFFMAN or PSEUDO-MARKOV compression. In addition, run length encoding (RLE) is included.

Tuning a PACTOR-II Signal

After launching the mode, the first step is to look for a valid PACTOR-II signal in the spectrum:

The bandwidth of the spectrum can be switched between 500 Hz, 1000 Hz, 4000 Hz and 24000 Hz. The upper and lower frequency boundaries of the signal are the selected with the right and left cursors. With the center cursor, the center frequency may be set more precisely. The precision of the center frequency is very important and directly influences the performance of the decoding.

To ensure, that the system can handle drifting signals or a coarsely adjusted center frequency, AFC may be enabled in the Demodulator menu. The carrier tracking function is now activated. If the deviation between the tracked and the adjusted center frequency is too large, the center frequency should be corrected. The carrier tracking only works over a range of the center frequency ± 10 Hz.

The deviation of the adjusted center frequency and the tracked frequency is shown in the tuning display. If a PACTOR-II signal was not found, the bar of the tuning display moves randomly. If a valid PACTOR-II signal was detected, the bar is frozen during the entire frame and shows the deviation. Now the center frequency may be manually adjusted to fit the correct frequency if necessary.

If a valid PACTOR-II signal was found the detected modulation type, frame length and the polarity is displayed. If it was an error free frame, the detected compression type, the status and the decoded text is displayed.

Options | CRC Recognition The detected CRC is displayed in the decoder status bar, see CRC Recognition.

Using Options | CRC Table a certain CRC mask can be defined, see CRC Table.

Options | Auto Decrypt If enabled, the mode tries to calculate the key of encrypted connections. It may take a few seconds until the key has been determined correctly. This function can not be used together with CRC Recognition.