Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
1.5.1.3
Receive Mode
The CMX880 is designed to receive and decode both digital and analogue types of coded signal format. This
facilitates meeting the basic requirement of the APCO Project 25 to provide an orderly migration to digital
communications, whilst maintaining backward compatibility with existing analogue systems. The CMX880 can
receive C4FM digitally encoded voice or Data signals or analogue voice or Data signal formats, covering CTCSS
tone, Selcall tone, DCS code and FFSK/MSK. Reception of each of these signal types can be independently
enabled/disabled by “C-BUS” command. The CMX880 continuously monitors the incoming signal for each
enabled signal type. When one is detected, the operating mode changes to track the detected signal type only.
By disabling all the decoding modes, the device can be configured to receive voice only signals with no decoding
of the Selcall or CTCSS tone or DCS code. This will result in reception of all signals as if they are voice,
including digital transmissions. In this case it is up to the user/host mC to respond appropriately to incoming
signals.
Following a receive mode enable from the RSSI level detector, if receive mode is enabled, the CMX880 performs
a sequence of signal type identification processes to determine what type of signal is being received. Once
identified, the CMX880 continues to process the received signal in accordance with the protocol appropriate to
that signal type.
If enabled, a “C-BUS” interrupt will be issued to notify the host mC of the presence and type of the incoming
signal. Signal type status information can be read by the mC over the “C-BUS”. The decoded voice or data can
be routed to the external DSP over the Fast Serial Bus, to the host mC over the “C-BUS” or to the audio output
port. The Fast Serial Bus protocols and timing are defined in section 1.5.1.7. The “C-BUS” protocols and timing
are defined in section 1.5.1.6. Control of the routing options is defined in section 1.5.1.5. Suggested routing is
shown in Table 1.
Table 1 Suggested Routing of Data
Data origin
Route
FSB
“C-BUS”
FSB
Destination
External DSP
Host mC
External DSP
Audio Output Port
C4FM Data
FFSK/MSK Data
FFSK/MSK Data
Analogue Channel
voice
Internal
The audio output amplifier gain can be adjusted by the mC, via “C-BUS” command, to provide output volume
control.
The output signal levels are all equalised (to VBIAS) before switching between the audio port and the modulator
ports, to prevent generation of frequency components outside the radio transmission band and to minimise
unwanted audible transients. The Off/Power-save level of the modulator outputs is VBIAS, so the audio output
level must also be at this level before switching.
The CMX880 operates in half duplex, so whilst in receive mode the transmit path (microphone input and
modulator output amplifiers) can be disabled and powered down.
Receiving Voice Band Signals on Analogue Channels
When a voice based signal is being received, it is up to the mC, in response to signal status information provided
by the CMX880, to control muting/enabling of the voice band signal to the AUDIO output, or to route the digitised
signal via the FSB or “C-BUS”. When the signal type has been identified as an analogue voice signal, the
digitised 16-bit samples are filtered and output at a sample rate of 8k samples/second (S/s). If data routed
across the “C-BUS” is not read within 125µs (the sample period) of the interrupt, it will become overwritten by
subsequent data.
If a voice plus CTCSS or DCS signal is detected, or the device is set to receive all signals as voice, the incoming
signal is filtered, as shown in Figure 5 to remove sub-audio components and to minimise high frequency noise.
The digitised voice data can then either be routed directly to the AUDIO output or read over the “C-BUS”.
ã 2001 Consumer Microcircuits Limited
13
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
