3471-ddc.ib.rev2.doc
page 10 of 20
17/10/2007
Operation
Front indicators:
Input loss alarm:
This LED lights when no data transmission is detected at the input for a given
time. MPEG-2 TS always contain a sync byte every 204 or 188 bytes irrespective
of data content. Therefore, if 2040 or more, consecutive 1’s or 0’s are been
detected; then the input is deemed lost.
Sync loss alarm:
This LED lights for at least 300 ms when two or more MPEG-2 TS sync bytes are absent. The LED extinguishes
when five or more correct SYNC bytes are detected.
188 byte indicator:
This LED lights when a valid MPEG-2 TS stream containing 188 bytes between sync bytes is detected.
204 byte indicator:
This LED lights when a valid MPEG-2 TS stream containing 204 bytes between sync bytes is detected.
Scrambling presence indicator:
This LED lights when a valid MPEG-2 TS stream containing a byte sequence that corresponds to scrambling. A
scrambling byte sequence uses an inverted 47H sync byte (B8H) every eighth sync to signify the start of the
scrambling sequence.
RS (Reed Solomon) presence indicator:
This LED lights when Reed Solomon error correction bytes are present in place of the 16 dummy bytes of a 204
Byte MPEG-2 TS. The DDC-3471 considers any data content other than all 0’s during the 16 dummy bytes to be
RS correction bytes.
Processing controls:
This module is capable of performing scrambling, de-scrambling, RS encoding, RS decoding, interleaving and de-
interleaving. It can encode or decode different MPEG TS formats.
In this context, the word scrambling refers to the process of Sync1 inversion and randomisation for the purpose of
energy dispersal of the signal. It does not refer to the encryption applied to Pay TV signals to control access to
particular channels or programs.
For a description of the processes involved see
Application examples - Cable Systems
and
Technical specifications -
Characteristics of signal types - MPEG-2 transport layer coding
.
The module is normally set up as either a decoder or an encoder. Combinations of both functions simultaneously
should be avoided.
The processing functions are selected using three switches (interleaving, RS coding
and scrambling) on the front panel. Each switch has three positions (up, centre or
down).
The DDC-3471 is internally divided into a decoder followed by an encoder. See
Block diagrams
.
A switch set to the UP position applies processing to the decoding section;
a switch set to the DOWN position applies processing to the encoding section;
and a switch set to the CENTRE position does not perform that function to either the
decoding or encoding section.
Applying a function to both the decoder and encoder section simultaneously is prevented by means of each switch
having only three positions. Full encoding and decoding is possible by using two DDC-3471 modules connected
together, if this is ever required.
In most instances, the DDC-3471 would be set to decoder mode with the de-interleaver and RS decoder functions
enabled.
INPUT
SYNC
188
204
R-S
SCRAM
R-S
UP - DECODE
CENTRE - OFF
DOWN - ENCODE
SCRAM
REED
SOLOMON
INTER
LEAVE
IRT
Communications
www.irtcommunications.com