V.22 bis Modem with AT Commands
CMX866
©
2008 CML Microsystems Plc
32
D/866/5
1 + x-14 + x-17
1.5.6
FSK and QAM/DPSK Modulators
Serial data from the USART is fed via the optional scrambler to the FSK modulator if V.21, V.23, Bell 103
or Bell 202 mode has been selected or to the QAM/DPSK modulator for V.22, V.22 bis and Bell 212A
modes.
The FSK modulator generates one of two frequencies according to the transmit mode and the value of
current transmit data bit.
The QAM/DPSK modulator generates a carrier of 1200Hz (Low Band, Calling modem) or 2400Hz (High
Band, Answering modem) which is modulated at 600 symbols/sec as described below:
For V.22 and Bell 212A 1200bps DPSK the transmit data stream is divided into groups of two
consecutive bits (dibits) which are encoded as a carrier phase change:
Dibit
(left-hand bit is the
first of the pair)
Phase change
00
+90
°
01
0
°
11
+270
°
10
+180
°