Sony DTV-01, Manual

Page: 11 / 132

14
I, P and B picture generation process
To clearly understand the relationship between the I, P and B pictures
we need to understand how they are generated.
1. The start of an entirely new scene would require an I-Picture or a
reference for other pictures to follow until the next I-Picture.
- The information between the I-Picture and the next reference I-
Picture is called a GOP (Group of Pictures) which consist of one I
and many P and B Pictures.
- I-Pictures typically re-occur at 15 picture intervals.
1. Next, the first P-Pictures in the GOP is generated based on the I-
Picture.
2. In between the I and first P-picture, several B-Pictures are gener-
ated as necessary to convey motion information from the I to the
first P-Picture. For this reason B-Pictures are dependent on past
and future pictures.
3. Then the process repeats with the generation of second P-Picture
which is now based on the first P-picture. And so on…
I
B
B
P
B
B
P
GOP
Time
I
Hoffman Encoding
The last step in the MPEG process uses a statistical approach to com-
press the data further.
The last process is called Hoffman Encoding. Basically, this process
takes a look at the string of MPEG data and replaces it with information
which allows regeneration.
The best example is a string of eight “1’s” {11111111} replaced by {1x8}
which represents; repeat the 1 eight times.