Picture Quality Analysis of Digital TV Signals - Digital Video and Audio Broadcasting Technology - page 202

Since the DCT-related artefacts of a compressed video signal are always
associated with blocking, an SSCQE type digital picture analyzer will at-
tempt to verify the existence of this blocking in the picture. To be able to
do this, the macroblocks and blocks must be analyzed in detail.

Raster position i

0 8

AD i
AD i

=
=

0
1

⎛ ⎞

Block borders

⎜
⎜
⎜
⎜
⎜
⎜
⎜

⎟
⎟
⎟
⎟
⎟
⎟
⎟

⋅

AD i = 8

AD i () =

⋅
⋅

Macroblock borders

⎝ AD i = 15 ⎠

Fig. 12.4. Pixel difference at the block and macroblock boundaries

In a test procedure developed by the Technical University of Braun-
schweig (Germany) and Rohde&Schwarz, the differences between adjoin-
ing pixels within a macroblock are formed. Pixel difference means that
simply the amplitude values of adjacent pixels of the Y signal within a
macroblock, and also separately those of the Cb and Cr signals are sub-
tracted. For each macroblock, 16 pixel differences are then obtained per
line, e.g. for the Y signal. Then all 16 lines are analyzed. The same is also
done vertically which also provides 16 pixel differences per column for the
macroblock of the Y signal. This analysis is performed for all columns
within the macroblock. The pixel differences at the block borders are of
special significance here and will be particularly large in the case of block-
ing.
The pixel differences of all macroblocks within a line are then combined
by adding them together in such a way that 16 individual values are ob-
tained per line (Fig. 12.4.). The 16 pixel difference values of the individual
lines are then also added together within a frame, resulting in 16 values per
frame as pixel difference values. This, finally. provides information about
the mean pixel difference 0 ... 15 in the horizontal and vertical direction
within all macroblocks. The same process is repeated for Cb and Cr, i.e.
the color difference signals.
Considering then the pixel differences of a video sequence with good
picture quality and one with poor picture quality, it can be seen quite