PMR Signalling Processor
CMX881
1.5.4.1 Processing Voice Signals for Transmission over Analogue Channels
The microphone input(s), with programmable gain, can be selected as the voice input source. Pre-
emphasis is selectable with either version of the 2 analogue Tx audio filters (for 12.5kHz and 25kHz
channel spacing).
These are designed for use in ETS-300-086 and/or TIA/EIA-603 compliant
applications. Both filters attenuate sub-audio frequencies below 250Hz by more than 33dB wrt the signal
level at 1kHz. These filters together with a built in limiter help ensure compliance with ETS-300-086
(25kHz and 12.5kHz channel spacing) when levels and gain settings are set up correctly in the target
system.
10
reference 0dB at 1kHz
+0.5dB/-2dB wrt ref.
3kHz
0
-10
-20
-30
-40
-50
-60
-14dB/octave
-33dB
10
100
1000
10000
100000
Frequency (Hz)
Figure 8 25kHz Channel Audio Filter Response Template
The filter characteristics of the 12.5kHz channel filter fits the filter template shown in Figure 9 (solid
outline). This filter also facilitates implementation of systems compliant with TIA/EIA-603 ‘A’ and ‘B’
bands. To achieve attenuation above 3kHz of better than −100dB/decade for TIA/EIA-603 ‘C’ bands
(dashed outline), additional external circuitry is required, such as suggested in section 1.4.2.
10
reference 0dB at 1kHz
+0.5dB/-2dB wrt ref.
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
3kHz
-60dB/decade wrt ref.
-33dB
(-50dB)
20kHz
-100dB/decade
(-82.5dB)
20kHz
10
100
1000
10000
100000
Frequency (Hz)
Figure 9 12.5kHz Channel Audio Filter Response Template
2004 CML Microsystems Plc
23
D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
