The MPEG-1 Codec - A Practical Guide to Video and Audio Compression - page 198

GOP 1 GOP 3

B I B I B I B I B

GOP 2

Figure 10-4 An open GOP sequence.

The GOP structures are quite short and there are no P-frames; GOP structures are all joined
by the differences. This is called an open GOP structure, and there is no boundary between
frames that does not have some kind of dependency joining that frame to another frame
in the sequence. The I-frames are shared by two GOP sets and it is impossible to find a
suitable point at which to break a sequence up.
Avoid this situation by using a P-frame at the end of each GOP so it becomes a closed
GOP, or by eliminating B-frames altogether.
You may have all sorts of reasons for choosing GOP structures of one kind or another
and you must determine what the appropriate format is for your target player. But keep
in mind that a service provider may impose this structure on you.

10.5.4 Long and Short GOPs

Keeping the GOP small implies that the compression will not be as efficient. Increasing the
number of difference frames makes the GOP bigger and the compression ratio is better.
These two approaches are called short GOP and long GOP, respectively. A long GOP may
accumulate more errors as the differences cascade. Sometimes using a key frame once every
second will be fine. That would yield a GOP somewhere between 24 and 30 frames in
length. For other content, a GOP every 6 frames may be called for. It depends on the picture
content and the quality of the source, as well as the target platform on which you are put-
ting the content. There are defaults in some compression tools that are set at 7, 12, and 18
frames for GOP length. It is a good idea to run some tests to exercise each setting and see if
it makes a difference to your output. A selection of GOP structures is shown in Figure 10-5.

N orma l GOP

I B B B B P

S hort GOP

I B P

L ong G O P

I B B B B B B B B B B P

Figure 10-5 A simple GOP.