PMR Signalling Processor
CMX881
1.5.3.1
Receiving Voice Band Signals
When a voice based signal is being received, it is up to the µC, in response to signal status information
provided by the CMX881, to control muting/enabling of the voice band signal to the AUDIO output.
The discriminator path through the device has a programmable gain stage. Whilst in receive mode this
should normally be set to 0dB (the default) gain.
Receive Filtering
The incoming signal is filtered, as shown in Figure 5, to remove sub-audio components and to minimise
high frequency noise. When appropriate the voice signal can then be routed to the AUDIO output.
10
250Hz
0
-10
Filter response
-20
-30
-40
-50
-60
Template
300Hz
3kHz
10Hz
100Hz
1000Hz
Frequency (Hz)
10000Hz
100000Hz
Figure 5 Rx Audio Filter Frequency Response
De-emphasis
Optional de-emphasis at -6dB per octave from 300Hz to 3000Hz (shown in Figure 6) can be selected to
facilitate compliance with TIA/EIA-603.
16
12
8
4
0dB/octave
0
-4
-6dB/octave
dB
+1dB
0dB
-8
-3dB
-12
-16
-20
-18dB/octave
100
1000
Frequency (Hz)
10000
Figure 6 De-emphasis Curve for TIA/EIA-603 Compliance
2004 CML Microsystems Plc
16
D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
