MT312 Functional Overview
1.4.4 The Reed Solomon Decoder Block
randomised using the configuration shown in Figure
11 below. This is a Pseudo Random Binary
Sequence (PRBS) generator, with the polynomial:
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.
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1 + X + X
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).
The Reed-Solomon decoder can correct up to eight
byte errors per packet. If there are more than 8 bytes
containing errors, the packet is flagged as
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.
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.
1.4.5 The Energy Dispersal (De-Scrambler)
Block, DVB only
Before Reed Solomon encoding in the DVB
transmission system, the MPEG2 data stream is
Sync byte
Sync byte
187 bytes
Reed Solomon encoded block
16 check bytes
187 bytes
MPEG transport packet
Figure 9 - DVB block structure
130 bytes
16 check bytes
Reed Solomon encoded block
130 bytes
DSS transport packet
Figure 10 - DSS block structure
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