4-Level FSK Modem Data Pump
Page 14 of 50
MX929B PRELIMINARY INFORMATION
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 two binary bits and a single symbol, such as the 'S'
Channel Status symbol.
SYMBOL
MSB
LSB
+3
+1
-1
1
1
0
0
1
0
0
1
-3
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
7
LSB
0
6
5
4
3
2
1
Bits:
a
b
c
d
Symbols:
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, RILB,
and RSID described in Section 4.5.2.
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