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Appendix E - DVB-S Description
DVB-S uses phase shift keying to digitally modulate a carrier. Quadrature phase
shift keying is used in both and the digital data is encoded as a 90 degree phase shift
in the signal. This gives 4 possible states for each sampling interval. The sampling
interval is called the Symbol Rate and each state of 2 bits is a Symbol. During
transmission, this data is interleaved to allow recovery during noise bursts and
redundant data is added called FEC (forward error correction). During reception, the
data is re-shuffled to restore the order and the FEC data is used to correct the
bitstream as necessary.
This bitstream is called a “transport stream”. A transport stream is made up of
packets. All packets are the same length of 188 bytes and they all start with the sync
byte 0x47 so that when the data is read, a starting point can be located. Each packet
also contains a PID (packet identification). The information in the transport stream
consists of several video and audio streams and also SI tables (Service Information)
to allow the receiver to decode and display the correct data. There are several types
of tables in the SI called PSI data. This table data is generally longer than one
packet so several packets are assembled together to make a “section” which can be
up to 4096 bytes.
1) Program Association Table (PAT): for each service in the multiplex, the PAT
indicates the PID of the corresponding Program Map Table (PMT). It also gives the
location of the Network Information Table (NIT).
2) Program Map Table (PMT): the PMT identifies and indicates the PIDs of the
video, audio, and other streams that make up each service.
3) Network Information Table (NIT): the NIT gives the Network Number, Name and
Satellite Position of the satellite. It also lists all the other transponders on the
satellite.
4) Service Description Table (SDT): the SDT gives information about each service in
this transport stream.
Once these tables are decoded by the receiver, the correct PID for the audio and
video streams can be found and presented to the video and audio decoders. In
DVB-S, the video streams are presented in MPEG-1 or MPEG-2 encoding (usually
MPEG-2) but can be presented in the newer HD format MPEG-4.
(For a more complete understanding, see ISO 13818-1 “Information technology,
Generic coding of moving pictures and associated audio information: Systems”)
Summary of Contents for satlook Lite
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