Marine VHF Audio and Signalling Processor
CMX885
7.5
Audio Functions
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 modes, 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 pre-
and de-emphasis, 300Hz HPF and frequency inversion scrambling, all of which may be individually
enabled1. The order in which these features are executed is selectable to ensure compatibility with
existing implementations and provide optimal performance (see section 8.2.5).
7.5.1 Audio Receive Mode
The CMX885 operates in half duplex, so whilst in receive mode the transmit path (microphone input and
modulator output amplifiers) can be disabled and powered down if required. The AUDIO output signal
level is equalised (to VBIAS) before switching between the audio port and the modulator ports, to minimise
unwanted audible transients. The Off/Powersave level of the modulator outputs is the same as the VBIAS
pin, so the audio output level must also be at this level before switching.
See:
o
Audio Control – $C2 write
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 CMX885, 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:
•
•
•
300Hz High Pass
2.55kHz Low Pass (for 12.5kHz channel operation)
3.0kHz Low Pass (for 25kHz channel operation)
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. In Figure 6 and Figure 7, the template shows the +1 and -3dB
limits.
1 The typical responses shown in Figure 6, Figure 7, Figure 8, Figure 9 were recorded using the PE0201
Evkit, DISC to AUDIO and MOD1 out.
© 2010 CML Microsystems Plc
18
D/885/3
CMLMICRO [ CML MICROCIRCUITS ]
