evertz 7881TSM-IP, Руководство пользователя

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7881TSM-IP
User Manual
Page - 38
Version 1.0
7.5.1. P1 NECESSARY FOR DECODING
•
TS_sync_loss:
The most important function for the evaluation of data from the MPEG-2 TS is the
sync acquisition. The actual synchronization of the TS depends on the number of correct sync bytes
necessary for the device to synchronize; two or more consecutive corrupted sync bytes indicate sync
loss. After synchronization has been achieved the evaluation of the other parameters is carried out.
•
Sync_byte_error: The indicator "Sync_byte_error" is set as soon as the correct sync byte (0x47) does
not appear after 188 or 204 bytes. This is fundamental because this structure is used throughout the
channel encoder and decoder chains for synchronization. It is also important that every sync byte is
checked for correctness since encoders do not necessarily check the sync byte.
•
PAT_error:
The Program Association Table (PAT), which only appears in PID 0x0000 packets, tells the
decoder what programs are in the TS and points to the Program Map Tables (PMT) which in turn point
to the component video, audio and data streams that make up the program. If the PAT is missing then
the decoder can do nothing, no program is decodable. Nothing other than a PAT should be contained in
a PID 0x0000.
• Continuity_count_error: For this indicator three checks are combined. The preconditions "Incorrect
packet order" and "Lost packet" could cause problems for receivers that are not equipped with
additional buffer storage and intelligence. It is not necessary for the test equipment to distinguish
between these two preconditions as they are logically OR-ed, together with the third precondition, "a
packet occurs more than twice" into one indicator.
•
PMT_error:
The Program Association Table (PAT) tells the decoder how many programs there are in
the stream and points to the PMTs that contain the information where the elementary streams for any
given event program can be found. Elementary streams in this context are the video stream (normally
one) and the audio streams and the data stream (e.g. Teletext). Without a PMT the corresponding
program is not decodable.
• PID_error: It is checked whether there exists a data stream for each PID that occur. This error often
occurs where TS are multiplexed, or demultiplexed and again remultiplexed.