The MPEG-1 Codec - A Practical Guide to Video and Audio Compression - page 199

When creating MPEG-2 video content for DVDs, the length of the GOP structures
should be set according to the guidelines established for PAL and NTSC DVD formats. For
PAL you should not exceed 15 frames per GOP. For NTSC the value is 18 frames max per
GOP.

If you exceed these values, then it is possible (likely even) that your content won't
play correctly on a domestic DVD player.

10.5.5 GOP Transmission Sequence

Knowing about the different types of frames and the grouping rules, let's now examine the
coding order and how the frames are transmitted.
The I-frame is compressed by itself, with no reference to any other frame. It is
processed and placed in the first physical slot in the GOP. The GOP structure is defined
beforehand so the encoder knows that it must jump forward a few frames to compute the
forward differences required to make the next P-frame. The compressed P-frame is placed
next in the physical GOP structure. Then the B-frames between the I-frame and that
P-frame are computed using the bi-directional technique.
The frames are transmitted in a very different order from the playback sequence as
you will see in Figure 10-6.
So the player must reorganize the received frames into the correct order so that the
playback sequence is reconstructed properly.

10.6 Conversion from RGB to Luma and Chroma

If the video is not already in a luma plus chroma form, the RGB color values must be con-
verted into the Y'CbCr components before the compression process starts in earnest. This
can be done in memory if you have enough, or by converting from one file to another. The
file-based approach costs you more in disk space consumed but is inherently more robust
and is easily undone.

Viewing order

I B B B B B B B B B P

I P B B B B B B B B B

Transmission order

Figure 10-6 GOP transmission sequence.