FSK Modem and DTMF Codec
CMX865
5.6
Rx DTMF/Tones Detectors
In Rx Tones Detect mode the received signal, after passing through the Rx Gain Control block, is fed to
the DTMF / Tones / Call Progress / Answer Tone detector. The user may select any one of four separate
detectors:
The DTMF detector detects standard DTMF signals. A valid DTMF signal will set b5 of the Status Register
to 1 for as long as the signal is detected.
The programmable tone pair detector includes two separate tone detectors (see Figure 10). The first
detector will set b6 of the Status Register for as long as a valid signal is detected, the second detector
sets b7, and b10 of the Status Register will be set when both tones are detected.
The call progress detector measures the amplitude of the signal at the output of a 275 - 665 Hz bandpass
filter and sets b10 of the Status Register to 1 when the signal level exceeds the measurement threshold.
10
0
-10
-20
dB
-30
-40
-50
-60
0
0.5
1
1.5
2
2.5
3
3.5
4
kHz
Figure 6a Response of Call Progress Filter
The Answer Tone detector measures both amplitude and frequency of the received signal and sets b6 or
b7 of the Status Register when a valid 2225Hz or 2100Hz signal is received.
© 2005 CML Microsystems Plc
13
D/865/3
CMLMICRO [ CML MICROCIRCUITS ]
