FX929B Datasheet, PDF (11/48 Pages) CML Microcircuits – Flexible Operating Modes

4-Level FSK Modem Data Pump
FX929B
1.5.3 Binary to Symbol Translation
Although the over-air signal, and hence the signals at the modem TXOP 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.
Direct: the simplest form, which converts between 2 binary bits and a single symbol, such as the 'S' Channel
Status symbol.
symbol
+3
+1
-1
-3
ms bit
1
1
0
0
ls bit
1
0
0
1
This is expanded so that an 8-bit byte translates to four symbols for the T4S, T24S and R4S tasks described
in Section 1.5.5.2.
Bits:
Symbols:
msb
7
6
a
sent first
5
4
b
3
2
c
lsb
1
0
d
sent last
With FEC: This is more complicated, but 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, TSID,
RHB, RILB and RSID described in Section 1.5.5.2.
© 1997 Consumer Microcircuits Limited
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D/929B/1