PMR Signalling Processor
CMX881
1.5.3
Receive Mode
The CMX881 can receive voice and various signal formats: CTCSS tone, DCS code, Selcall and FFSK/
MSK data at 1200 and 2400bps. Reception of each of these signal types can be independently
enabled/disabled by C-BUS command. If enabled, an interrupt will be issued to notify the host µC of the
presence and type of the incoming signal.
In receive mode the CMX881 performs signal type identification in 2 frequency bands, sub-audio (60 -
260Hz) and voice band (300 - 3kHz), to determine what type of signal is being received. When an
enabled signal is detected this will be indicated to the host over the C-BUS and the CMX881 will continue
to process the received signal in its band. Identification / process mode will continue in the other band.
The CMX881 can process voice and simultaneously identify and process at least 2 other signal types
(one in the sub-audio in parallel with one in the voice band). See Table 1 for valid combinations.
The receive gain and audio output amplifier gain can be adjusted by the host µC, via C-BUS command,
to provide receive signal level adjustment and output volume control or muting.
Table 1 Concurrent Rx Signalling Modes Supported by the CMX881
Sub-Audio
Voice band signalling
Any one of A - E:
None
All combinations of:
A
B
C
D
E
With Rx Voice
Processing1 or
Audio Tone
DCS
1200bps FFSK/MSK
2400bps FFSK/MSK
1200 & 2400 bps FFSK/MSK
Selcall
Inverted DCS
CTCSS
generation
Sub-Audio
Voice band signalling
All combinations of:
All combinations of:
No Voice
Processing or
Audio Tone
generation
DCS
Selcall
Inverted DCS
CTCSS
1200bps FFSK/MSK
2400bps FFSK/MSK
1 Including optional de-emphasis
By disabling all the decoding modes, the device can be configured to receive voice only signals with no
decoding of the voice band, CTCSS or DCS signalling. This will result in reception of all signals as if
they are voice. In this case it is up to the user/host µC to respond appropriately to incoming signals.
The CMX881 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/Power-save 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.
2004 CML Microsystems Plc
15
D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
