RALCWI Vocoder
CMX608/CMX618/CMX638
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.
7
6
5
4
3
2
1
0
D0
D8
D1
D9
D2
D10
D3
D11
D4
D5
D6
D7
D12
D13
D14
D15
Intervening bits follow the same pattern
D200
D208
D201
D209
D202
D210
D203
D211
D204
D212
D205
D213
D206
D214
D207
D215
For 4 frame packets, the sequence continues thus...
D272
D280
D273
D281
D274
D282
D275
D283
D276
D284
D277
D285
D278
D286
D279
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.
7
6
5
4
3
2
1
0
n3
D0
D2
n2
n1
n0
n3
D1
D3
n2
n1
n0
Intervening bits follow the same pattern
D212
D214
D213
D215
For 4 frame packets, the sequence continues thus...
D284
D286
D285
D287
2012 CML Microsystems Plc
23
D/608_18_38/10
CMLMICRO [ CML MICROCIRCUITS ]
