MPEG Coding GOP

Question

I am trying to figure out how GOP works

Consider the uncoded frame sequence:

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

I understand what The N(GOP) is, it's 12 if I am counting from Iframe to Iframe. I know what the predicted span is, it's 2.

1. What is the reordered sequence for decoding for these frames?
2. How would you increase random access?
3. How would you increase compression?
4. How do errors creep in?

could someone explain it to me the 4 questions. thank you

Answer 1

Check out this MPEG compression FAQ, particularly the section "Q. So is each frame predicted from the last frame?" for a more detailed discussion of these exact questions.

Straight answers:

1. Since both I and P need to be decoded before either of the intervening B frames can be decoded, P is necessarily transmitted before either of the first 2 B frames. Therefore, the reordered sequence would look like: I P B B P B B P B B B B I ..
2. Eschew B frames in favor of all P frames for interframes, or drop interframes entirely and go for all I frames.
3. Use more B frames and longer GOPs (I frames are the largest frames).
4. If a frame's data is corrupted, errors can creep in. If other frames depend on the corrupted frames, errors can propagate through other frames.