FRS Signalling Processor
CMX882
1.5.3.3
Receiving and Decoding DCS Codes
DCS Code is in NRZ format and transmitted at 134.4 ±0.4bps. The CMX882 is able to decode any 23 or
24 bit pattern in either of the two DCS modulation modes defined by TIA/EIA-603 and described in Table
4. The CMX882 can detect a valid DCS Code quickly enough to avoid losing the beginning of voice
transmissions.
Table 4 DCS Modulation Modes
Modulation Type:
A
Data Bit:
FM Frequency Change:
Minus frequency shift
Plus frequency shift
Plus frequency shift
Minus frequency shift
0
1
0
1
B
The CMX882 detects the DCS code that matches the programmed code defined in the ‘DCS Code’ words
(P2.2-3) in the Programming register ($C8).
To detect the pre-programmed DCS code the signal is low pass filtered to suppress all but the sub-audio
band using the filter shown in Figure 7. Further equalisation filtering, signal slicing and level detection
are done to extract the code being received. The extracted code is then matched with the programmed
23 or 24-bit DCS code to be recognised, in the order least significant first through to most significant DCS
code bit last. Table 5 shows a selection of valid 23-bit DCS codes, this does not preclude other codes
being programmed. Recognition of a valid DCS Code will be flagged if the decode is successful (3 or
less errors). A failure to decode is indicated by a '0' flag. This flag is updated after the decoding of every
4th bit of the incoming signal.
Once a valid DCS Code has been detected, the voice band signal can be passed to the AUDIO output
under the control of the host µC. The voice signal is extracted from the received input signal by band
pass filtering; see Figure 5. More details for programming DCS Codes are provided in section 1.6.20.3.
The end of DCS transmissions is indicated by a 134.4 ±0.5Hz tone for 150-200ms. To detect the DCS
turn off tone while receiving DCS, the DCS turn off tone option must be selected in the Audio and CTCSS
Control ($C2) register and CTCSS receive must also be enabled. When a DCS turn off tone is detected
it will cause a DCS interrupt; the receiver audio output can then be muted by the host.
2004 CML Microsystems Plc
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D/882/7
CMLMICRO [ CML MICROCIRCUITS ]
