CMX138A datasheet(21/73 Pages) CMLMICRO

21 / 73 page

Audio Scrambler and Sub-Audio Signalling Processor

CMX138A

 2014 CML Microsystems Plc

Page 21

D/138A/4

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.

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:

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:

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.

CMX138A Datasheet(PDF) 21 Page - CML Microcircuits