6 February 2015

PACTOR-I FEC

PACTOR-I FEC, used for broadcast transmission, is a bit synchronous simplex system  in a fixed timing cycle and is based on PACTOR. The change-over time and signal propagation delay limits the system’s range to approximately 20.000 km.
PACTOR operates adaptively so the baud rate can be either 100 or 200 Baud. During day time 200 Baud may be successfully used. In the evening, however, strong propagation distortion occurs which necessitates a reduction of the baud rate to 100 Baud.


A short recording may be heard here

The PACTOR-FEC data block consists of three sections: Header, data and control (status and 16 bit CRC).  At 100 baud the data field is 64 bits and at 200 baud it increases to 160 bits.
Block coding is performed according to the CCITT standard starting with the data section. In contrast to PACTOR, all data blocks are in consecutive order with no or little space between them.
There are several variants on the market.

PACTOR includes HUFFMAN data compression by design. This scheme relies on the fact that frequently occurring characters e.g. space, e, n or i, c an be represented with shorter bit combinations than characters which are rarely used. A compression factor of approximately 1.7 is achieved in comparison to uncompressed ASCII.


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