CMX138 CML Microcircuits, CMX138 Datasheet - Page 21

• Manufacturer Part Number: CMX138
• Description: Audio Scrambler and Sub-Audio Signalling Processor
• Manufacturer: CML Microcircuits

Audio Scrambler and Sub-Audio Signalling Processor
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 CMX138 provides selectable pre-emphasis filtering of +6dB per octave from 300Hz to 3000Hz,
matching the template shown in Figure 10.
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:
Tx Companding (Compressing)
The CMX138 incorporates an optional syllabic compandor in both transmit and receive mode. This
compresses audio band signals before transmission to enhance dynamic range. See section 7.4.3 and:
Audio Scrambling
The CMX138 incorporates an optional frequency inversion scrambler in transmit and receive modes. This
scrambles transmitted audio band signals, which can then be de-scrambled in the receiver. The inversion
frequency can be programmed using the Scramble Frequency register, $CB. The default value is 3300Hz.
The Scrambler frequency may be changed while the device is in an active Rx or Tx mode.
See:
© 2008 CML Microsystems Plc
o
o
o
o
o
-10
-15
-20
-25
-30
-35
-40
15
10
-5
5
0
AuxADC and TX MOD mode - $A7 write
Analogue Output Gain - $B1 write
Audio Control – $C2 write
Audio Control – $C2 write
Scrambler Inversion Frequency – $CB write
pre-emp & 12.5k & hpf -
$3400
pre-emp & 25k & hpf -
$2C00
template
Figure 10 Audio Frequency Pre-emphasis
frequency (Hz)
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CMX138