[M4IF Technotes] AC prediction

[Keigo Hirakawa]

Hi.  I have a tedious question about AC prediction.
In section 7.4.3.3 of the MPEG4 spec, a formula is given to compute the
quantized AC coefficients.  For the sake of discussion, let's restrict our
attention to the case when block 'A' is selected:
QF_X[0][i] = PQF_X[0][i]+(QF_A[0][i]*QP_A)//QP_X
In this formula, (QF_A[0][i]*QP_A) is effectively an inverse quantization
step.  However, it differs from the inverse quantization step outlined in
section 7.4.4.2.1 (or section 7.4.4.1.2).  So, in order to decode the MPEG4
binary file, the decoder must perform inverse quantization step twice, using
two different techniques.
My question is this: it seems to make more sense to me if (QF_A[0][i]*QP_A)
in section 7.4.3.3 were replaced with the inverse quantization step outlined
in section 7.4.4.2.1.  That way, the decoder would be simpler.  In addition,
I suspect that the formula in 7.4.4.2.1 is better than doing
(QF_A[0][i]*QP_A).  So, why was MPEG4 designed this way?  Is there an
advantage to the way it is written in MPEG4 spec?
Thank you!
Keigo Hirakawa