Audio Scrambler and Sub-Audio Signalling Processor
CMX138A
The characteristics of the 12.5kHz channel filter fit the template shown in Figure 8 and Figure 9. This filter
also facilitates implementation of systems compliant with TIA/EIA-603 ‘A’ , ‘B’ and ‘C’ bands .
The CMX138A provides selectable pre-emphasis filtering of +6dB per octave from 300Hz to 3000Hz,
matching the template shown in Figure 10.
20
Template ((TIA/EIA)
10
+6dB/octave
25kHz & HPF Enabled ($C2=2C00)
12.5kHz & HPF Enabled ($C2=3400)
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
10.0
100.0
1000.0
10000.0
100000.0
Frequency (Hz)
Figure 10 Audio Frequency Pre-emphasis
Modulator Output Routing
The sub-audio component is combined with the audio band signal and this composite signal routed to the
MOD output in accordance with the settings of:
o
o
AuxADC and TX MOD Mode - $A7 write
Analogue Output Gain - $B1 write
Input AGC
An Automatic Gain Control system can be enabled by setting the relevant bits of the Program register
P4.9. The setting of the Input 1 Gain stage is recorded when the device enters Tx mode and if the signal
exceeds the pre-set threshold, the Input 1 Gain is automatically reduced in 3.2dB steps until it falls within
the operational levels or the range of the gain stage is exhausted. When the signal level drops, the gain
will be automatically increased in 3.2dB steps at the rate set in P4.9 until the initial value has been
reached. For maximum effect the system should be designed such that the +22.4dB setting of the Input 1
Gain stage achieves the nominal levels. To ensure consistent operation, it is recommended that the
Input 1 Gain stage value be re-initialised before entering Tx mode. The signal that is used as an input to
this process can be selected to be either:
o
o
Output of Input1 gain stage
Output of the Pre-emphasis filter.
by selecting the relevant bit in P4.9. The Pre-emphasis option should only be chosen if this block is
actually in use.
© 2010 CML Microsystems Plc
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