Telephone Signalling Transceiver / Least Cost Router
CMX860
1.5.6 DTMF Decoder and Tone Detectors
In Rx Tones Detect mode the received signal, after passing through the Rx Gain Control block, is fed to
the DTMF decoder and Dual Tone/Call Progress/Answer Tone detector. The user may select one of four
separate operations:
The DTMF decoder detects standard DTMF signals. A valid DTMF signal will set bit 5 of the Status
Register to 1 for as long as the signal is detected. The DTMF signal is then decoded and output in bits 0
to 3 of the Status Register.
The programmable tone pair detector includes two separate tone detectors (see Figure 9a). The first
detector will set bit 6 of the Status Register for as long as a valid signal is detected, the second detector
sets bit 7, and bit 10 of the Status Register will be set when both tones are detected. The frequency and
bandwidth of each detector can be set in the Programming Register. Without programming, the default
values in the Programming Register are set for 2130 and 2750 Hz detection.
The Call Progress detector measures the amplitude of the signal at the output of a 275 Hz - 665 Hz
bandpass filter and sets bit 10 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 5a Response of the Call Progress Filter
The Answer Tone detector measures both amplitude and frequency of the received signal and sets bit 6
or bit 7 respectively of the Status Register when a valid 2225Hz or 2100Hz signal is received.
ã 2002 Consumer Microcircuits Limited
15
D/860/5
CMLMICRO [ CML MICROCIRCUITS ]
