Question

4. (10 marks) A digital video is to be compressed using MPEG-1 standard assuming frame sequence of: I B B PBBPBBI..., average compression ratios of I frame, P frame, and B frame are:10:1l (I) , 20: 1 (P), 50:1 (B); The duration of this video is 2 seconds, the frame size is 400 x 200, each pixel represented by 8 bits, the color space used is YUV, and the sampling format is NTSC 4:1:1 (1) Calculate the size of this video before compression. (5 marks) (2) Calculate the size of this video after compression. (5 marks)

Answer 1

Solution:

Frame sequence = IBBPBBPBBI...

Hence: 1/9 of frames are I-frames, 2/9 are P-frames, and 6/9 are B-frames.

Average compression ratio = (1 × 0.1 + 2 × 0.05 + 6 × 0.02)/12

= (0.1 + 0.1 + 0.12)/12

= 0.32/12

= 0.02666

NTSC frame size: Without compression= 400 × 200 × 8 + 2 (200 × 100 × 8)

= 640,000 + 320,000

= 960,000 bits

NTSC frame size: With compression = 960,000 * 0.02666

= 25600 bits