PMR Signalling Processor
CMX881
1.5.3.2
Receiving and Decoding CTCSS Tones
The CMX881 is able to accurately detect valid CTCSS tones quickly to avoid losing the beginning of
voice or possibly data transmissions, and is able to continuously monitor the detected tone with minimal
probability of falsely dropping out. The received signal is filtered in accordance with the template shown
in Figure 7, to prevent signals outside the sub-audio range from interfering with the sub-audio tone
detection.
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Figure 7 Low Pass Sub-Audio Band Filter for CTCSS and DCS
Once a valid CTCSS tone has been detected, the voice band signal can be passed to the audio output.
The voice band signal is extracted from the received signal by band pass filtering as shown in Figure 5.
To help decode received CTCSS tones adjustable decoder bandwidths and threshold levels permit
decode certainty and signal to noise performance to be traded when congestion or range limits the
system performance. This entails setting the tone decoder bandwidth and threshold level in P2.1 of the
Programming register ($C8) and programming the Audio & CTCSS Control register with the desired tone.
Tone Cloning :
Tone CloningTM facilitates the detection of CTCSS tones 1 to 39 in receive mode. This allows the device
to non-predictively detect any tone in this range. The received tone number will be reported in the Tones
Status register. This tone code can then be programmed into the ‘Audio and Device Address Control’
register, by the host µC. The cloned tone will only be active when CTCSS is enabled in the Mode
register.
Tone Cloning is a trademark of CML Microsystems Plc.
2004 CML Microsystems Plc
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D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
