[M4IF Technotes] Mismatch of IDCT algorithms

[Christoph Stadler]

Dear MPEG-4 community,
I am currently concerned with the accumulation of mismatch errors
when repeatedly adding residual images during MPEG-4 video decoding.
The MPEG-4 Visual standard (e.g. document N4350 ISO/IEC 14496-2) says
that every macroblock is required to be 'intra' coded at least every
132 reference VOPs.
But this requirement is somewhat hidden as 'Note 1' of the (normative)
'Appendix A'.
And the MoMuSys reference codec seems to ignore this rule too.
I understand that IEEE 1180-1990 demands a limitation of the number of
residuals that may be added even with a conform IDCT. And our tests
show, that violation of this can cause a severe degradation in image
quality if encoder and decoder use different (but conform) IDCT
algorithms.
So there are my questions :
1.) To which extend is the intra refresh every 132 frames required ?
2.) Can an MPEG-4 bitstream exceeding the 132-refresh-rule actually be
    valid ?
3.) Does the MoMoSys reference codec really ignore this refresh,
    and if so, why ?
Thanks in advance and
Best regards,
        Christoph Stadler