CMX608 datasheet(22/70 Pages) CMLMICRO

22 / 70 page

RALCWI Vocoder

CMX608/CMX618/CMX638

 2014 CML Microsystems Plc

D/608_18_38/11

5.5.2.

Packets of FEC Protected Frames

The device has a built-in FEC system to protect the Vocoder frames when transported over an error prone

channel. The data interleaving has been organised to provide good performance over a channel with burst

errors, typical in wireless applications. Only the bits that are more sensitive to corruption are protected

FEC can be applied over 3 or 4 frames resulting in a packet of 216 or 288 bits, regardless of the Vocoder

bit rate. The bits in the packet are named D0 through D215 (or D287). The bits should be transmitted in

order, starting with D0 and ending with D215 (or D287).

Hard-bits

Hard bits are represented by one bit per bit. These are packed into 27 bytes for 3-frame FEC and 36 bytes

for 4-frame FEC.

The following table shows how these bits are packed and transferred between the

Vocoder and the host microprocessor. The byte at the top is the first out of the C-BUS for the encoder and

the first into the C-BUS for the decoder.

D10

D11

D12

D13

D14

D15

Intervening bits follow the same pattern

D200

D201

D202

D203

D204

D205

D206

D207

D208

D209

D210

D211

D212

D213

D214

D215

For 4 frame packets, the sequence continues thus...

D272

D273

D274

D275

D276

D277

D278

D279

D280

D281

D282

D283

D284

D285

D286

D287

Soft-bits

Soft bits are represented by one nibble per bit. These should be packed into 108 bytes for a 3-frame FEC

and 144 bytes for a 4-frame FEC. The following table shows how these bits should be packed and sent to

the decoder over the C-BUS. The byte at the top should be the first one sent. The bits of a nibble are

shown as n3 to n0, with n3 being the most significant and n0 being the least significant.

Intervening bits follow the same pattern

D212

D213

D214

D215

For 4 frame packets, the sequence continues thus...

D284

D285

D286

D287

CMX608 数据表(PDF) 22 Page - CML Microcircuits

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