Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
Decoded Voice Signal Processing for APCO-25
If the C4FM signal being received is digitally encoded voice, this must be decoded by the external DSP. The
decoded voice signal can be routed to the AUDIO output of the CMX880 to drive a loudspeaker. The decoded
16-bit voice data word is transferred to the CMX880 from the DSP over the FSB or “C-BUS” at 8000words/s.
The voice data is filtered and interpolated to 24kS/s, then D-to-A converted to drive the audio output port. The
filter fits the output filter mask recommended in ‘Speech Input/Output Requirements’ of the ‘APCO Project 25
Vocoder Description, Standard IS102BABA, 1/11/95’, shown in Figure 10.
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Frequency (Hz)
Figure 10 Filter Characteristics for Decoded C4FM Voice Signals
Data Quality
The CMX880 decodes C4FM signals at each symbol sample point. The decoded value corresponds to the valid
symbol level that the sampled signal level is closest to. ‘Loss of data’ can be associated with the number of
decoded symbol errors, or some other measure of data quality, exceeding a threshold, which may be detected
by the external [DSP] error detection and correction function. ‘Loss of data’ may also occur if the RSSI falls
below the minimum signal strength threshold; to detect this the appropriate RSSI interrupt flag must be enabled.
Associated Control and Status
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Reset.
Enable C4FM signal reception.
Received data.
Data quality.
Interrupt masks - C4FM data word present; frame sync detected; FS tolerance data ready.
Interrupt flag - C4FM data word present; frame sync detected; FS tolerance data ready. Flags set on change
of associated status and interrupt issued if appropriate mask bit is set.
Interrupt status - C4FM data word present; frame sync detected; FS tolerance and bit error count.
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ã 2001 Consumer Microcircuits Limited
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D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
