[SOLVED] VSE title with transparency alpha over

I rendered an animated title (moving letters) as a video RGBA (ffmpeg h264) with premul key toggle to have the background transparent (black).
The I load another video clip (background clip) and I put the title video in the track above the first one. With the alpha over effect I want the letters of the title clip moving over the background video.
However the black of the title clip remains black, so I do not see the background video clip. Titles with PNG photo’s with alpha channel work fine with transparency.
Can anybody set me on the right track ?
Thanks in advance.

Found the solution. This video tutorial was very helpful.
The moving text is to be rendered as a png image sequence with alpha channel and imported a series of images. Common video formats do not have alpha support.