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RFX144V24-S23 and RFX96V24-S23 Modem Designer's Guide
9.3 SIGNAL RECOGNITION ALGORITHM
9.3.1 FSK vs. High Speed
A method of determining whether a high speed message or FSK handshaking is being received by the modem is necessary
when implementing the T.30 recommendation. When the calling unit transmitter and called unit receiver configure for V.17,
V.29, or V.27 ter, sometimes the high speed message may not be received (typically due to a noisy line). In this case, the
calling unit transmitter will try to send the message up to three times before re-negotiating in FSK signaling. The called unit
receiver must, therefore, be able to distinguish between a high speed message and FSK handshaking. This algorithm is
shown in Figure 9-26.
9.3.2 FAX vs. Voice
A method for determining whether a fax or voice message is being received is necessary for combined fax answering
machines.
When configured for Voice/Audio Codec mode decoding of an outgoing message, three tone detectors are available. These
detectors can be used to detect the fax 1100 Hz calling tone or the 1650 Hz fundamental energy of the FSK flags that begin
some T.30 handshakes. The coefficients for FSK detection are shown in Table 9-1. The 1100 Hz calling tone detector
coefficients are shown in Table 6-3 and Table 6-4. The algorithm for FSK detection in Voice Mode is shown in Figure 9-27.
The algorithm for CNG detection (1100 Hz) is shown in Figure 9-28.
α
0
α
1
α
2
α'
0
α'
1
α'
2
β
1
β
2
β'
1
β'
2
α''
β''
1.
AREXx = 0, BRx = 0, and CRx = 1.
2.
4000 Hz sample rate.
3.
8000 Hz sample rate.
1070
Table 9-1. FR3 Coefficient for 1650 Hz Detection
RAM Access Code (Hex)
31
32
33
34
35
36
B2
B3
B5
B6
B4
B1
1
Coefficient Value(Hex)
0097
0000
FF69
01D0
0000
FE30
97A8
C12D
9588
C120
0145
7EBB
2
Coefficient Value(Hex)
0113
FE7B
0113
0113
FE7B
0113
25C4
C12D
23F8
C120
00A3
7F5C
3
9-29
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