Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
Receiving and Decoding DCS Codes
DCS Code is in NRZ format and transmitted at 134.4 ±0.4bps. The CMX880 is able to decode either of the two
DCS modulation modes defined by TIA/EIA-603 and described in Table 4. The CMX880 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
Frequency Change
Minus frequency shift
Plus frequency shift
Plus frequency shift
Minus frequency shift
0
1
0
1
B
The CMX 880 detects the DCS code that matches the pre-programmed code defined in the ‘DCS CODE
(UPPER)/(LOWER)’ words (B2.2, B2.3) 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.
Further equalisation filtering, signal slicing and level detection are done to extract the code being received. The
extracted code is then matched with the pre-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 valid 23-bit DCS codes and
the programming data needed to select the desired code. This does not preclude other codes being
programmed. Recognition of a valid DCS Code will be flagged if the decode is successful (3 or fewer errors). A
failure to decode is indicated by a ''0" flag. This flag is updated after every 4 bits of the incoming signal is
decoded.
Once a valid DCS Code has been detected, the digitised voice band data is passed, either to the µC/DSP or to
the AUDIO loudspeaker output. The destination is set by the µC. The voice band is extracted from the received
input signal by band pass filtering; see Figure 5.
The end of a DCS transmission is indicated by a continuous 134.4 ±0.5Hz tone for 150-200ms. In order to
detect the DCS turn-off code, the CTCSS Tone Decoder should also be enabled and programmed with the
corresponding value. Once detected this will cause a CTCSS tone decode interrupt; the receiver audio output
should then be muted by the µC.
More details for programming DCS Codes are provided in section 1.5.2.
Associated Control and Status
·
·
·
·
·
·
·
Reset.
Enable DCS (Modulation type A) Code detection [0 -> -ve freq shift; 1 -> +ve freq shift].
Enable DCS (Modulation type B) Code detection [0 -> +ve freq shift; 1 -> -ve freq shift].
Select 23 or 24-bit DCS Code.
DCS Code to be recognised.
Interrupt mask - DCS Code Detected.
Interrupt flag - DCS Code Detected (£3 bit errors). Flag set on change of associated status and interrupt
issued if mask bit is set.
·
Interrupt status - DCS Code detected.
ã 2001 Consumer Microcircuits Limited
19
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
