User's Manual
4-Level FSK Modem Data Pump Page 13 of 47 MX919B PRELIMINARY INFORMATION
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4.3 Binary to Symbol Translation
Although the over-air signal, and therefore the signals at the modem TXOUT and RXIN pins, consists of 4-
level symbols, the raw data passing between the modem and the µC is in binary form. Translation between
binary data and the 4-level symbols is done in one of two ways, depending on the task being performed.
1. Direct way: (simplest form) - converts between 2 binary bits and a single symbol.
SYMBOL MSB LSB
+3 1 1
+1 1 0
-1 0 0
-3 0 1
Accordingly, 1 byte = 4 symbols = 8 bits, and one byte translates to four symbols for the T4S and R4S tasks
and six bytes translates to twenty-four symbols for the T24S task described in Section 4.5.2.
MSB LSB
Bits:
7 6 5 4 3 2 1 0
Symbols:
a b c d
send first send last
2. FEC way: (more complicated) - essentially translates groups of 3 binary bits to pairs of 4-level
symbols using a Forward Error Correcting coding scheme for the block oriented tasks THB, TIB, TLB,
RHB, and RILB described in Section 4.5.2.