X6

MPEG glossary 375
How does it work?
The encoder has a precisely defined GOP, for example IBBPBBPBB. This sequence is
transmitted together with the encoder, which always knows exactly which kind of
frame comes next. I, P, and B frames are differentiated.
Hint: When we talk about pictures, we mean frames of the video output, and I, P an
dB frames are the frames of the encoded video. Just as in movement approximation,
blocks (8x8 pixels) are united into macroblocks (16x16 pixels) during prediction.
The first frame is always the I frame. It is completely encoded from the first picture.
Afterwards, the 4th picture is analyzed for the creation of the first P frame. (As
already said, the encoder, and later the decoder, will know that two B frames belong
between them.) This image will also be completely encoded, and afterwards all
macroblocks that haven't changed in comparison to the I frame will be deleted. They
will be replaced by corresponding references for the decoder that tell it "you already
know what should be shown here, and you can get it from the last I frame".
Now, the 2nd will be completely encoded, and all macroblocks identical to the first I
frame and the following P frame will be removed. References to previous frames are
called backward predictions, and references to following frames are called forward
predictions. The third picture will be edited in exactly the same fashion.
The fourth picture we have already explained, and now we need the next P frame, or
picture number 7. Pictures 5 and 6 are B frames again, which are compared to P
frames to both sides of them (picture 4 and 7); these are followed by the last two B
frames. These have a special place, since in closed GOPs, they may contain only
backward predictions, and no references to the next I frame, because it belongs to
the next GOP.
Something else: Since the decoder is no prophet, the P frames are always transmitted
before the B frames! The GOP explained above will be encoded and transmitted in the
order it is written.
Original
GOP
I
0
B
01
B
02
P
01
B
11
B
12
P
02
B
21
B
22 I1
Data stream I
0
P
01
B
01
B
02
P
02
B
11
B
12
B
21
B
22 I1
... for closed GOPs
I
0
P
01
B
01
B
02
P
02
B
11
B
12
I
1
B
21 B22
P
11
... For open GOPs
Due to this nested structure, it is easy to see that during direct editing of MPEG
material, complicated computations have to take place! These are made easier using
a frame table. A frame table contains a list, where the information of every frame in
the data stream is found, identifying the type of frame it is.
Using Movement prediction (view page 370) P and B frames are likewise reduced.