X - CONVOLUTIONAL DE-INTERLEAVER

This is a 204 x 12 convolutional interleaver in

Mode A ; the periodicity of 204 bytes for sync byte

is preserved.

The de-interleaver may be skipped (see RS register).

XI - REED-SOLOMON DECODER

AND DESCRAMBLER

The input blocks are 204 byte long with 16 parity

bytes in Mode A; the synchro byte is the first byte

of the block. Up to 8 byte errors may be fixed.

Code Generator polynom:

g(x) = (x -

over the Galois Field generated by :

X

Energy dispersal descrambler :

Output energy dispersal descrambler generator :

X

The polynom is initialised every eight blocks with

the sequence 100101010000000. The synchro

words are unscrambled.

Control register : RS register

Internal Address : Hex0A

The reset value is written in each register cell

RS7

RS6

RS5

RS4

RS3

RS2

RS1

RS0

10111000

RS7 : De-interleaver Enable

If 1, the input flow is deinterleaved.

If 0, the flow is not affected.

RS6 : If 0, Outp ut data are corrected bytes

(normal operating mode).

If1, OutputdataareReed-Solomoncorrection

bytes (error count mode) (see Note 1).

RS5 : Reed-Solomon Enable

If 1, the input code is corrected.

If 0, no correction happens; all the data are

fed to the descrambler.

The error signal remains inactive.

RS4 : Descrambler Enable

If 1, the output flow from Reed-Solomon

decoder is descrambled.

If 0, the descrambler is desactived.

RS3 : Write Error Bit

If RS3=1, and uncorrectible error happens,

the MSB of the first byte following the sync

byte is forced to 1after descrambling.

RS2 : Super Synchro Suppression

If RS2=1, all synchro bytes are Hex47 in

mode A.

If RS2=0, the synchro is complemented

every 8th packet. It allows, when scrambler

is off, to provide RS coded signals for use

in low-cost SMATV interface.

RS1 : Output Clock Polarity

If RS1=0, data and control signals change

during high to low transition of CK_OUT.

If RS1=1, they change during the low to

high transition.

RS0 : Output Clock Configuration

If RS0=0, CK_OUT is continuous.

If RS0=1, CK_OUT remains low during the

parity bits.

Note 1 :

When RS6 = 1, the output data are the correction bytes

applied to data incoming the Reed-Solomon block.

The number of bits at 1 in these output data represent

therefore the number of errors remaining at the output of

VITERBI decoder.

All null output data mean no error left after VITERBI

decoding.

Remark : Output datas are meaningless when error flag (Pin 34) is

set to high level.

FUNCTIONAL DESCRIPTION (continued)

Da ta

P a rity

No Error

Uncorre cte d P a cke t

No Error

CK_OUT

D/P

STR_OUT

ERROR

RS 0=0, RS 1=0

RS 0=1, RS 1=0

RS 0=0, RS 1=1

RS 0=1, RS 1 =1

Da ta

P a rity

Da ta

P a rity

Figure 3

STV0196B

13/23