V.22 bis Modem with AT Commands
CMX866
1.5.10 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 detectors. The selection of the four separate detectors
is handled automatically by the on-chip µController:
The DTMF detector detects standard DTMF signals. A valid DTMF signal will set bit 5 of the DSP Status
Register to 1 for as long as the signal is detected. The output of this detector is not currently used by the
on-chip µController, however, this register can be accessed by the @R command.
The programmable tone pair detector includes two separate tone detectors. The first detector will set bit
6 of the DSP Status Register for as long as a valid signal is detected, the second detector sets bit 7, and
bit 10 of the DSP Status Register will be set when both tones are detected. By default, this
programmable tone pair detector is set to detect the dual alert tone pair (2130Hz and 2750Hz) used on
some Type 1 CLI systems. The output of this detector is not currently used by the on-chip µController,
however, this register can be accessed by the @R command.
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 DSP Status Register to 1 when the signal level exceeds the
measurement threshold. This is handled automatically by the on-chip µController. When the CMX866 is
on-hook, the Call Progress detector can be enabled so that it will detect voice activity arising from a
parallel phone connection which is off-hook. The output of the DSP Status Register will need to be
monitored by the external host µC, using an @R command, for several seconds and decision taken on
whether there is voice activity.
10
0
-10
-20
dB
-30
-40
-50
-60
0
0.5
1
1.5
2
2.5
3
3.5
4
kHz
Figure 8a 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 DSP Status Register respectively when a valid 2225Hz or 2100Hz signal is received. This
is handled automatically by the on-chip µController, which looks for either of the Answer Tones.
ã 2004 CML Microsystems Plc
33
D/866/4
CMLMICRO [ CML MICROCIRCUITS ]
