“But a De-Interlacer is just a function that has to be “programmed” with some Lines of Program-Code…”
Sorry, but do you really know how interlaced formats worked ? , a quick and dirty some lines of code is totally waste of time !
you have to handle tv-shows and movies different, because in the past they use mostly two different half pictures in tv-shows, and in movies they used two exactly the same half pictures (thats the reason why in the past the tv-shows looked more fluid in movement and movies not), when you deinterlace it, you have two handle both different, also you have to add an additional filter for deinterlaced movies otherwise it will look stuttering.
Then you have to handle PAL and NTSC different, best deinterlaced effect is the combination of PAL and movie (with exactly the same half pictures)
so a quick and dirty deinterlacing is totally garbage.
maybe your tv has a hardware deinterlacer ?