Marine VHF Audio and Signalling Processor
CMX885
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.
Audio De-scrambling
The CMX885 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 defaults to 3300Hz, but may be modified by writing to P4.8.
See:
o
Audio Control – $C2 write
7.5.2 Audio Transmit Mode
The device operates in half duplex, so when in transmit mode the receive path (discriminator and audio
output amplifiers) should be disabled, and can be powered down, by the host µC.
Two modulator outputs with independently programmable gains are provided to facilitate single or two-
point modulation. If one of the modulator outputs is not used it can be disabled to conserve power.
To avoid spurious transmissions when changing from Rx to Tx, the MOD1 and MOD2 outputs are ramped
to the quiescent modulator output level, VBIAS before switching (if enabled by b7 of the Analogue Gain
register, $B0). Similarly, when starting a transmission, the transmitted signal is ramped up from the
quiescent VBIAS level and when ending a transmission the transmitted signal is ramped down to the
quiescent VBIAS level. The ramp rates are set in the Program Block P4.6. When the modulator outputs
are disabled, their outputs will be set to VBIAS. When the modulator output drivers are powered down,
their outputs will be floating (high impedance), so the RF modulator will need to be turned off.
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 CMX885 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,
or an internal AGC function may be used.
See:
o
o
o
Audio Control – $C2 write
AuxADC and Tx MOD Mode – $A7 write
Input Gain and Output Signal Routing – $B1 write
Processing Audio Signals for Transmission over Analogue Channels
The microphone input(s), with programmable gain, can be selected as the audio input source. Pre-
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 frequencies below 250Hz by more than 33dB with
respect to the signal level at 1kHz. In Figure 8, Figure 9 and Figure 10, the template shows the +1 and
-3dB limits.
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.
© 2010 CML Microsystems Plc
20
D/885/3
CMLMICRO [ CML MICROCIRCUITS ]
