MT312CSLASHCG datasheet(14/90 Pages) ZARLINK

14 / 90 page

MT312 Functional Overview

1.4.4 The Reed Solomon Decoder Block

DVB and DSS data are encoded using shortened

versions of the Reed-Solomon code of block length

255, containing 239 message bytes and 16 check

bytes, that is (255,239) with T = 8. Both encoders

use the same generator polynomial. The code block

size for DVB is 204 and that for DSS is 146. Hence

DVB code is (204, 188) and DSS code is (146, 130),

with both having T = 8. The block structure of the

DVB and DSS Reed-Solomon codes are as shown in

Figure 9 and Figure 10 below.

The Reed-Solomon decoder can correct up to eight

byte errors per packet. If there are more than 8 bytes

containing

errors,

the

packet

ﬂagged

uncorrectable using the pin BKERR. In the case of

DVB the transport error indicator (TEI) bit of the

MPEG packet is set to 1, if setting of TEI is enabled.

1.4.5 The Energy Dispersal (De-Scrambler)

Block, DVB only

Before

Reed

Solomon

encoding

the

DVB

transmission system, the MPEG2 data stream is

randomised using the conﬁguration shown in Figure

below. This

Pseudo

Random

Binary

Sequence (PRBS) generator, with the polynomial:

1 + X14 + X15

The PRBS registers are loaded with the initialisation

sequence as shown, at the start of the ﬁrst transport

packet in a group of eight packets. This point is

indicated by the inverted sync byte B8hex. The

normal DVB sync byte is 47hex. The data starting

with the ﬁrst byte after the sync byte is randomised

by exclusive-ORing data bits with the PRBS. (The

sync bytes themselves are not randomised).

In the decoder, the process of de-randomising or de-

scrambling the data is exactly the same as described

above. The de-scrambler also inverts the sync byte

B8hex so that all MPEG output packets have the

same synch byte 47hex.

Sync byte

187 bytes

MPEG transport packet

Sync byte

187 bytes

Reed Solomon encoded block

16 check bytes

Figure 9 - DVB block structure

Figure 10 - DSS block structure

130 bytes

DSS transport packet

130 bytes

Reed Solomon encoded block

16 check bytes

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