Video starts too late

Have searched for hours on this, sorry if I missed it.

I found a third party song that had a video in it. The background image fades to the video (the fade-in is not part of the video file) and the video is in perfect sync with the song.

I have created a background video for a song. I have
1. carefully synchronized the video with the MP3 of the song that goes into StepMania
2. copied the sm data of the third party song - should start at 0.000 and fade in

When I play the song in StepMania, my video:
1. does not fade in - it just cuts from the background to the video
2. (most important) The video starts too late, by about 0.25-0.5 second.

My suspicion is that StepMania is not reading the video correctly/quickly enough (30MB, xvid), but would have thought that SM should ensure that it is synced up with the song. I tried changing the start beat of the video to negative but it had no effect. I am loathed to chop half a second off the video, in case this is a performance/syncing issue, and this wouldn't solve the missing fade-in in any case.

Any suggestions greatly appreciated.

Rather than cut the video, why not add the necessary silence to the beginning of the song and resync it with the video?

I am going to investigate what is going on and make negative start times at least start the video in the desired place, though even if I make negative start times work, if you put a negative start time in your simfile, it will only work on the next release and later.

Good idea, I'll certainly give that a go for now.

I am right though, aren't I - that I should make my video file so that the video is synchronized with the song file, and not some other timing?
There's not a huge amount of documentation concerning adding videos.
Perhaps there's something not quite right about my video format. I'll try to find time to play with other formats/smaller videos to see if that helps.

Btw, StepMania is awesome. Only just discovered it, managed to grab myself some pads and am having so much fun with it so many thanks.

Tried adding silence to the beginning of the mp3 and this helped a bit, but by adding silence it appears to have changed the video load again. This makes me think the video isn't being loaded fast enough, or some problem with the video format.

I'm also no closer to understanding why there is no fade in. Is there any detailed documentation about the BGCHANGES field, specifically with relation to fade in?

You may be interested to know that it suddenly started working correctly.

I'm guessing the song data cache was not getting updated. I recently added some extra songs so maybe that caused enough disk churn to trigger a reload...

The song data cache is only updated when the cache file for a song doesn't exist, or something in the song folder is renamed (if fast load is off), or a new version of stepmania bumps the cache version.
Having fast load turned off will double or triple the amount of time stepmania takes to start up, so it's usually better to have fast load on and just manually delete the cache file for a song on the rare occasions when you have to edit a song outside of the built in edit mode.