Low Power V.22 bis Modem
CMX868
1.5.6 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 of four separate
detectors:
The DTMF detector 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 programmable tone pair detector includes two separate tone detectors (see Figure 12). 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 Call Progress detector measures the amplitude of the signal at the output of a 275 - 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 7a Response of Call Progress Filter
The Answer Tone detector measures both amplitude and frequency of the received signal and sets bit 6
or bit 7 of the Status Register when a valid 2225Hz or 2100Hz signal is received.
ã 2004 CML Microsystems Plc
14
D/868/9
CMLMICRO [ CML MICROCIRCUITS ]
