Marine VHF Audio and Signalling Processor
CMX885
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 CMX885 provides selectable pre-emphasis filtering of +6dB per octave from 300Hz to 3000Hz,
matching the template shown in Figure 10.
pre-emp & 12.5k & hpf -
$3400
pre-emp & 25k & hpf -
$2C00
template
15
10
5
0
-5
-10
-15
-20
-25
-30
-35
-40
Frequency (Hz)
Figure 10 Audio Frequency Pre-emphasis
Modulator Output Routing
The Tx signal can be routed to both MOD1 and MOD2 outputs in accordance with the settings of:
o
o
AuxADC and Tx MOD Mode – $A7 write
Input Gain and Output Signal Routing – $B1 write
Input AGC
An Automatic Gain Control system can be enabled by setting the relevant bits of the Program Block 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 the:
•
•
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.
o
o
Input Gain and Output Signal Routing – $B1 write
Program Block 4 – Gain and Offset Setup
© 2010 CML Microsystems Plc
22
D/885/3
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