[M4IF Technotes] mismatch control

[William J. Fulco]

Erik,
What's going on here is that the arithmetic of all the MPEG standards is,
well standardized - except the inverse-DCT. There are Lots-and-Lots of ways
to do IDCT - with all kinds of precision and all kinds of multiplications
and additions and all sorts of icky arithmetic stuff. It's really up to the
decoder-maker to decide how they're going to decide a valid bit-stream
(remember, MPEG tells you what a valid-bit-stream looks like - you're in
charge of turning that standard-bit-stream into pretty pictures).
What "mismatch control" does is "control mismatches" (sorry) - mismatches
between the way DCTs on the encoding side and IDCTs on the decoding side for
8x8 blocks. Theoretically, you should be able to DCT and 8x8 block and feed
it to an IDCT and get the same answer back. When you start hacking up the
coefficients with quantizers and start using different ways to IDCT than to
DCT, well - you have mismatch possibilities.
 As it turns out, some bit patterns have a higher statistical probability of
screwing up (depending on arithmetic precision) over time. Really smart
people with way too much time on their hands figure out how to prevent these
things from happening! So... in MPEG-1 this "mismatch control" process is
called "Oddification" (I wish I'd thought up THAT word!). The encoder forces
all the DCT coefs to have high-order bits on (negative) and this prevents
(well, reduces) mismatch problems between standardized-DCTs and
non-standardized-IDCTs for some reason only the guys that actually stayed
awake in their college math classes can tell you.
In MPEG-2 and MPEG-4 they did away with Oddification (just LOVE that word)
and do what's called LSB-Toggling to the 63rd coef Again, people that
understand this more than me (plenty of them out there) found that this
worked better... the other difference if I remember is that Oddification in
MPEG-1 happens before IDCT and LSB-Toggling happens after.
Anyone want to fix my possibly lame explanation?
++Bill
William J. Fulco
wjf   NetworkXXIII.com
310-927-4263 Cell
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