User Guide

MPEG Overview 311
Motion Compensation Within a scene, much of the display information remains the same
from frame to frame. The only difference between Frame 1 and Frame 2 in Figure 8 is the
counter that changes from :01 to :02. Much of the information needed to draw the pixels for
Frame 2, therefore, can be eliminated. It can simply refer to Frame 1 and contain the
information needed to draw the single pixel that changed between the two frames. Frames
that reference other frames are called P-pictures and B-pictures.
Intraframe Compression
While interframe compression reduces redundancy between pictures, intraframe
compression reduces redundancy with one picture. Intraframe encoding includes the
following three steps:
Discrete Cosine Transformation (DCT) Both image blocks and prediction-error blocks have
high spatial redundancy. To reduce this redundancy, the MPEG algorithm transforms 8 × 8
blocks of pixels or 8 × 8 blocks of error terms to the frequency domain with the Discrete
Cosine Transform (DCT).
Quantization The MPEG algorithm quantizes frequency coefficients. Quantization is the
process of approximating each frequency coefficient as one of a limited number of allowed
values. The encoder chooses a quantization matrix that determines how each frequency
coefficient in the 8 × 8 block is quantized. Human perception of quantization error is lower
for high spatial frequencies, so high frequencies are typically quantized more coarsely (with
fewer allowed values) than low frequencies.
Figure 8: Frame-by-frame changes
10:01:02:02
10:01:02:01
Frame 1
Frame 2