AVSForum

...oh how I love thee.

I shall quote.

4) What is this "macroblocking" everybody is talking about?
Macroblocks are always present in compressed video, they are generally visible as the square blurs that show up in fast action scenes or square splats of junk when there is a signal problem, e.g. fingerprints on the DVD, satellite TV during a blizzard. Why they show up is a long story...
In the beginning the FFT begat the DCT which begat JPEG, which begat MPEG the 1st which begat MPEG the 2nd, which begat MPEG the 4th, which begat Ray, the Blue... And in all of these the DCT was computed within an 8 x 8 box, these being the third power of two, for the powers of 2 are sacred unto the hardware and also the most fertile radix of the FFT and all its descendants for all generations. These 8x8 boxes are the vessels of the spectral powers that compress the frothy ergodic image bits into a solid block, packed and full of bit-entropy. So great is the power of these blocks that they are called the MACRO-BLOCKS. Whenever the foolish attempt to starve their bits or err when serving them, their wroth cannot be concealed and their form shows itself blotting out all things - yea crushing dissenting detail within their bound. Woe be to them who starve them or serve their bits in error, for they shall see only the block.