I've split this off as I might send the other topic off topic.
Why doesn't FFDShow or MPC - MPEG-2 Video Decoder (Gabest) show up under the source filter box in MC?
Which source filter does MC default to when set to automatic?
I guess source filter isn't the same as video decoder, in which case, where is video decoder selected?
Source filters are the filters that read the file and possibly parse the content. An example of source filter is Haali Media Splitter. You need exactly one source filter, often just Microsoft's "File Source (Async.)", but may be other filters depending on file type. Because you can only have one source filter, they are separated from "Other Filters" which allow multiple filter selection. The "Other Filters" include all filters between the source filter and the renderer filter. Decoders such as FFDShow are included in "Other Filters". MC does not specifically choose a source filter in general (except for a few file types), unless the user specifically chooses one. DirectShow automatically choose the right source filter.
With mpg files, DirectShow automatically loads "File Source (Async.)" (which is a source filter) and "MPEG-2 Demultiplexer" (which is a parser filter selectable from "Other Filters" group), and a suitable video decoder and a audio decoder if the user does not specify them. For this file type, the only meaningful selections for users are the video and audio decoders since the other two filters ("File Source (Async.)" and "MPEG-2 Demultiplexer") do not have many alternatives and there is really no need to find any alternatives for them.