MPEG video compression - The MPEG - page 156

Figure 5.31: Weighting is used to make the noise caused by requantizing different at each frequency.
P and B pictures are decoded by adding a prediction error image to a reference image. That reference image will
contain weighted noise. One purpose of the prediction error is to cancel that noise to prevent tolerance build-up. If
the prediction error were also to contain weighted noise this result would not be obtained. Consequently prediction
error coefficients are flat weighted.
When forward prediction fails, such as in the case of new material introduced in a P picture by a pan, P coding
would set the vectors to zero and encode the new data entirely as an unweighted prediction error. In this case it is
better to encode that material as an I macroblock because then weighting can be used and this will require fewer
bits.
Requantizing increases the step size of the coefficients, but the inverse weighting in the decoder results in step
sizes which increase with frequency. The larger step size increases the quantizing noise at high frequencies where
it is less visible. Effectively the noise floor is shaped to match the sensitivity of the eye. The quantizing table in use
at the encoder can be transmitted to the decoder periodically in the bitstream.
5.12 Intra-coding in MPEG-1 and MPEG-2
Study of the signal statistics gained from extensive analysis of real material is used to measure the probability of a
given coefficient having a given value. This probability turns out to be highly non-uniform suggesting the possibility
of a variable-length encoding for the coefficient values. On average, the higher the spatial frequency, the lower the
value of a coefficient will be. This means that the value of a coefficient tends to fall as a function of its radius from
the DC coefficient. DC coefficients also have certain characteristics. As they represent the average brightness of a
block, in many picture areas, adjacent blocks will have similar DC coefficient values. This can be exploited using
differential coding which will be dealt with in section 5.14 . However, errors in the magnitude of DC coefficients
cause wide area flicker in the picture and so the coding must be accurate.
Typical material often has many coefficients which are zero valued, especially after requantizing. The distribution of
these also follows a pattern. The non-zero values tend to be found in the top-left-hand corner of the DCT block, but
as the radius increases, not only do the coefficient values fall, but it becomes increasingly likely that these small
coefficients will be interspersed with zero-valued coefficients. As the radius increases further it is probable that a
region where all coefficients are zero will be entered.
MPEG uses all these attributes of DCT coefficients when encoding a coefficient block. By sending the coefficients
in an optimum order, by describing their values with Huffman coding and by using run-length encoding for the zero-
valued coefficients it is possible to achieve a significant reduction in coefficient data which remains entirely
lossless. Despite the complexity of this process, it does contribute to improved picture quality because for a given
bit rate lossless coding of the coefficients must be better than requantizing, which is lossy. Of course, for lower bit
rates both will be required.