VP310

PRELIMINARY DATA

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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

randomised using the configuration shown in Figure 9 below. This is a Pseudo Random Binary

Sequence (PRBS) generator, with the polynomial:

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

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.

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010

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010

0000

00

Initialisation sequence

XOR

Figure 9. DVB Energy dispersal conceptual diagram.

1.4.6 Output stage

Transport stream can be output in a byte-serial or bit-serial mode. The output interface consists

of an 8-bit output, output clock, a packet validation level, a packet start pulse and a block error

indicator.

The output clock rate depends on the Symbol rate, QPSK/BPSK choice, convolutional (Viterbi)

coding rate, DVB/DSS choice and byte-parallel or bit-serial output mode. This rate is computed

by VP310 to be very close to the minimum required to output packet data without packet overlap.

Furthermore, the packets at the output of VP310 are as evenly spaced as possible to minimise

packet position movement in the transport layer. The maximum movement in the packet

synchronisation byte position is limited to