Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
Receiving and Decoding CTCSS Tones
The CMX880 is able to accurately detect valid CTCSS tones quickly to avoid losing the beginning of voice
transmissions, and is also able to continuously monitor the detected tone with minimal probability of falsely
dropping out. The 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.
10
300Hz
0
260Hz
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-50dB
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400Hz
-60
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200
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Frequency (Hz)
Figure 7 Low Pass Sub-Audio Band Filter for CTCSS and DCS
Once a valid CTCSS tone has been detected, the digitised voice band data is passed either to the µC/DSP or to
the audio output. The destination is set by the µC. The voice band is extracted from the received input signal by
band pass filtering as shown in Figure 5.
To decode received CTCSS tones, the CTCSS decoder should first be set up according to the desired
characteristics. This entails setting the TONE decoder bandwidth and level in the ‘CTCSS TONE BW AND
LEVEL’ word (B2.4) of the ‘PROGRAMMING REGISTER’ ($C8) and programming the centre frequencies of the
desired tones in the ‘RX CTCSS TONE’ and ‘RX CTCSS POLE COEFF’ words (B2.6 to B2.37) of the
‘PROGRAMMING REGISTER’, which holds up to 16 different tones. Any tone can be in any location. When the
device is decoding, the tones are scanned in the sequence of their location, i.e. tone 0 first and tone 15 last.
Once a tone is detected the remaining tones are not checked. Therefore if two tones are close enough in
frequency for their bandwidths to overlap then the one programmed in the lowest location will be detected.
The facility to decode any of up to 16 programmed tones allows the use of tones for various signalling functions
such as masking a free channel or identifying sub groups within a user's groups.
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.
Associated Control and Status
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Reset.
Enable CTCSS Tone Detection - enables the tone detection and identification.
1-16 programmed tones.
Set decoder bandwidth.
Set tone detection threshold level.
Interrupt masks - Fast Tone Detected; Tone Detected, Lost Tone.
Interrupt flag - Fast Tone Detected; Tone Detected, Lost Tone. Flags set on change of associated status
and interrupt issued if appropriate mask bit is set.
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Interrupt status - CTCSS Tone detected; Identification of tone frequency (one of the 15 pre-programmed
tones); valid CTCSS (sub-audio) tone but not one of the preset frequencies; mode of tone detection -
standard, fast or fast predictive, Lost Tone.
ã 2001 Consumer Microcircuits Limited
15
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
