User's Manual

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GOP Structure
GOP STRUCTURE
I IP IBP IBBP 422IBBP
This parameter controls the structure of picture groups used in the MPEG-2 encoding process. These groups are based on three
possible picture types:
• I: Intra-frames (completely encoded).
• P: Predicted-frames (using motion estimation).
• B: Bi-directionally estimated frames (using motion estimation).
This parameter has a direct affect on video encoding delay. For a given Bit rate:
• IBBP and IBP frame modes offer good video quality in difficult transmission conditions.
• IP frame mode offers a good compromise between video quality and “near-line-of-sight” conditions.
• I-frame mode offers the same encoding delay as IP mode, with high Bit rates to ensure the greatest video quality.
GOP Length
GOP LENGTH
6 12 18 24
This is a secondary parameter involving a trade-off between video encoding efficiency and transmission error tolerance. A high
value slightly improves the video quality at the expense of an increased worst-case recovery time required by the MPEG2 decoder
when a transmission error occurs.
The value is expressed in terms of full frame pictures. The typical value is 12.
Expert COFDM Parameters
You can find some information about COFDM modes characteristics in Annex A.
The COFDM mode is controlled by adjusting certain COFDM parameters, described below.
Constellation
CONSTELLATION
QPSK 16QAM 64QAM
This represents the constellation scheme that is used to individually modulate each sub-carrier of the COFDM signal. The
Constellation can be set to optimize robustness but also affects the data transmission rates. QPSK will provide the greatest
robustness but lowest data rates. 16QAM is somewhat robust with a faster data rate. 64QAM will provide the least robust signal,
but at the fastest data rate.
The characteristics corresponding to the 3 possible choices are summarized in this table:
Constellation
# of points # of bits
per Sub-carrier
Relative
Bit rate
QPSK 4 2 x 1
16QAM 16 4 x 2
64QAM 64 6 x 3