[M4IF Technotes] Motion estimation + SAD

[the_ether]

I have some basic questions:
1- What is the formula for calculating the sum of absolute differences (SAD)
for a block, for which the motion vector (MV) has been determined to be
(0,0)?
I have seen:
    SAD = SAD(0,0) - (Nb / 2 + 1)
but with two interpretations of Nb:
a) Nb = the number of pixels inside the VOP
b) same as (a) but multiplied by 2^(bits per pixel - 8) and number of pixels
inside the macroblock instead of VOP
I have a feeling that MPEG-4 only supports 8 bits per pixel, or at least
only 8 bits each pixel in each of the Y, Cb and Cr components. In which case
2^0 = 1 so (a) = (b). (Except for no. of pixels  inside VOP vs MB which I
think is a typo)
2- Shouldn't Nb be the number of pixels in a *block* (ie Nb = 64) instead of
a macroblock since DCT is calculated on each of the 6 blocks within a
macroblock?
3- Why have this special case for MV(0,0) anyway? What if we have no
movement between two frames (ie SAD(0,0) = 0), why subtract (Nb / 2 + 1)
prior to performing DCT on that block ?
Regards
graham