CMX882 View Datasheet(PDF) - CML Microsystems Plc

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CMX882

FRS Signalling Processor

CML Microsystems Plc 

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.

Modulation Type:

A

B

Table 4 DCS Modulation Modes

Data Bit:

FM Frequency Change:

0

Minus frequency shift

1

Plus frequency shift

0

Plus frequency shift

1

Minus frequency shift

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

21

D/882/7

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