CMX138 CML Microcircuits, CMX138 Datasheet - Page 19

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

Audio Scrambler and Sub-Audio Signalling Processor
De-emphasis
Optional de-emphasis at -6dB per octave from 300Hz to 3000Hz (shown in Figure 7) can be selected, to
facilitate compliance with TIA/EIA-603, EN 300 086, EN 301 025 etc. The template shows the +1, -3dB
limits.
Rx Companding (Expanding)
The CMX138 incorporates an optional syllabic compandor in both transmit and receive modes. This
expands received audio band signals that have been similarly compressed in the transmitter to enhance
dynamic range. See section 7.4.3 and:
Audio De-scrambling
The CMX138 incorporates an optional frequency inversion de-scrambler in receive mode. This de-
scrambles received audio band signals that have been scrambled in the transmitter. The inversion
frequency can be programmed using the Scramble Frequency register, $CB. The default value is 3300Hz.
See:
7.4.2
The device operates in half duplex, so when the device is in transmit mode the receive path (discriminator
and audio output amplifiers) should be disabled, and can be powered down, by the host µC.
A single modulator output with programmable gain is provided which combines both the audio and sub-
audio signals to facilitate single or two-point modulation.
To avoid spurious transmissions when changing from Rx to Tx the MOD output is ramped to the
quiescent modulator output level, V
transmitted signal is ramped up from the quiescent V
transmitted signal is ramped down to the quiescent V
Programming register P4.6 and enabled by bits 0,1 of the Analogue Input Gain register. When the
modulator output is disabled, their outputs will be set to V
powered down, its output will enter a hi-Z state (high impedance), so the external RF modulator should be
disabled to avoid unwanted transmissions.
For all transmissions, the host µC must only enable signals after the appropriate data and settings for
those signals are loaded into the C-BUS registers. As soon as any signalling is enabled the CMX138 will
use the settings to control the way information is transmitted.
A programmable gain stage in the microphone input path facilitates a host controlled VOGAD capability.
See:
Processing Audio Signals for Transmission over Analogue Channels
The microphone input, with programmable gain, can be selected as the audio input source.
emphasis is selectable with either of the two analogue Tx audio filters (for 12.5kHz and 25kHz channel
spacing). These are designed for use in EN 300 086, TIA/EIA-603 or EN 301 025 compliant applications.
When the 300Hz HPF is enabled, it will attenuate sub-audio frequencies below 250Hz by more than 33dB
with respect to the signal level at 1kHz.
These filters, together with a built in limiter, help ensure compliance with EN 300 086 and EN 301 025
(25kHz and 12.5kHz channel spacing) when levels and gain settings are set up correctly in the target
system. The channel filters incorporate a soft-limiter function by default, however, should a hard-limiter be
required, this can be enabled by setting bit 13 of Program Register P4.9 (see section 9.2.5). The level at
which the limiter starts to operate can also be adjusted using Program Register P4.7 (see section 9.2.5).
© 2008 CML Microsystems Plc
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Audio Control – $C2 write
Audio Control – $C2 write
Scrambler Inversion Frequency – $CB write
Audio Transmit Mode
Audio Control – $C2 write
Analogue Input Gain - $B0 write
BIAS
before switching. Similarly, when starting a transmission, the
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BIAS
BIAS
level and when ending a transmission the
BIAS
level. The ramp rates are set in the
. When the modulator output driver is
D/138_FI1.0/5
CMX138
Pre-