MPEG 4 part 2 basics

[stevep98]

Hi there. I am having a problem understanding something about MPEG 4 part 2. I think the descriptions of video codecs and formats are so complicated - and I'm not even trying to understand the encoding and decoding technicalities! And, I'm not sure my question is that relevant any more since it has been overtaken with MPEG 4 part 10.

Anyway, here goes my question.

I had heard when MPEG 4 pt 2 came out that it would offer better compression than MPEG 2, so I was eager to try it out. What surprised me is that there were multiple incompatible implementations, for example from divx and xvid.

What was it about MPEG 4 pt 2 which led to incompatible implementations?

How did the standard leave enough lee-way that files produced by one software couldn't be played by another?

I can understand how a vendor might claim that their software was able to beat a competitor in quality tests, because they ran a more intelligent, say, motion prediction model. But surely the files they create must be compliant to MPEG 4 pt2, and thus be read by other MPEG 4 pt2 software.

This question has been troubling me for years! Perhaps I have just been unlucky with buggy implementations which create bad files.

Steve