The MPEG-1 Codec - A Practical Guide to Video and Audio Compression - page 206

DC
Figure 10-12 Frequency coefficients.
By arranging the coefficients into a 2-dimensional matrix, Figure 10-13 shows how
the detail on the horizontal and vertical axes increases toward the lower right.
To compress the macroblock further, truncate the data working from the lower right
toward the upper-left corner. This sacrifices fine detail in the decoded image but retains
the overall average (nominal) appearance of that macroblock. This is why bit-starved
transmission streams yield a very blocky appearance. They reveal the blocky structure
because the fine detail is lost and that prevents the edges of the macroblocks from merg-
ing into one another perfectly.
The value at the extreme top left is a DC coefficient of the image, while all the other
values represent AC (frequency) coefficients. Reconstructing the pixel values starts with
Figure 10-13 Frequency coefficient 2-dimensional matrix.