What is Avisynth's Equivalent IDCT Algorithm?

What is Avisynth's Equivalent IDCT Algorithm? 

When i frameserve avisynth to virtual dub using the command: DirectShowSource("file.vob") it is using the direct show filters that i already have installed. But what equivalent iDCT algorithm is being used? Other programs use IDCTs like : MMX optimized, or IEEE reference, ...etc... , but I see no mention of this from avisynth. Someone please explain this ... ?

Directshow decodes the video, not AviSynth, therefore iDCT is irrelevant. Whatever filter you can get to decode the vobs, decide what to use. Use Mpeg2Source or Nics filter instead.