CMX138 CML Microcircuits, CMX138 Datasheet - Page 17

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

Audio Scrambler and Sub-Audio Signalling Processor
7.4
The audio signal can be processed in several ways, depending on the implementation required, by
selecting the relevant bits in the Audio Control – $C2 write register. In both Rx and Tx, a selectable
channel filter to suit either the 12.5kHz or 25kHz TIA / ETSI channel mask can be selected. This filter also
incorporates a soft limiter to reduce the effects of over-modulation. Other features include 300Hz HPF,
pre- and de-emphasis, companding and frequency inversion scrambling, all of which may be individually
enabled
existing implementations and provide optimal performance (see section 9.2.5).
7.4.1
The CMX138 operates in half duplex, so whilst in receive mode the transmit path (microphone input and
modulator output amplifiers) can be disabled and powered down. The AUDIO output signal level is
equalised (to V
unwanted audible transients. In the powersave state, the AUDIO output pin enters a hi-Z state, however, if
left enabled and the preceding stages powersaved, it will be driven to the V
See:
Receiving Audio Band Signals
When a voice-based signal is being received, it is up to the host µC, in response to signal status
information provided by the CMX138, to control muting/enabling of the audio signal to the AUDIO output.
The discriminator path through the device has a programmable gain stage. Whilst in receive mode this
should normally be set to 0dB (the default) gain.
Receive Filtering
The incoming signal is filtered, as shown in Figure 6 (with the 300Hz HPF also active), to remove sub-
audio components and to minimise high frequency noise. When appropriate, the audio signal can then be
routed to the AUDIO output. Separate selectable filters are available for:
Note that with no filters selected, the low frequency response extends to below 5Hz at the low end but still
rolls off above 3.3kHz at the top end.
1
PE0501 Evkit, DISC in to AUDIO out.
© 2008 CML Microsystems Plc
The typical responses shown in Figure 6, Figure 7, Figure 8 and Figure 9 were recorded using the
o
Audio Functions
1
Audio Receive Mode
Audio Control – $C2 write
. The order in which these features are executed is selectable to ensure compatibility with
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•
•
BIAS
300Hz High Pass (to reject sub-audible signalling)
2.55kHz Low Pass (for 12.5kHz channel operation)
3.0kHz Low Pass (for 25kHz channel operation)
) before switching between the audio port and the modulator ports, to minimise
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BIAS
level.
D/138_FI1.0/5
CMX138