[el Filou]

Originally Posted by Ilovetv9

About 50% of interlaced videos will properly deinterlace for me when using mpc-be+madvr+lav filters, but some (like the one I posted) does not deinterlace.

I've looked at that file with DXVA deinterlacing on both a GeForce and a Radeon and it seems like it's mixed material, i.e. some parts are sourced from film and others from video, so it cannot be deinterlaced all the time as it's not video all the time. What often happened in the old days was outside scenes were shot on film, and inside scenes were shot on video. If you look for example at around 12:00 when it transitions from a street scene to a studio scene, the scene in the street is using 2:2 film cadence, while the studio scene is using 50i video (if you pause in MPC and use Ctrl+right key you can frame-skip and see the individual frames). If you disable deinterlacing (Shift+Ctrl+Alt+D until you see it says off), you can see the scene shot outside still has clear motion (well, as clear as a composite video recording from that time can look anyway ), while the following scene in the studio suddenly has a load of combing artifacts on motion.

I thought I remember reading that you are not supposed to use more than 1 deinterlacer when using mpc-be+madvr+lav filters becuase that is redundant and will worsen the quality as you are only supposed to use 1 deinterlacer to keep maximum quality. Is that wrong and I need to always use lav filters deinterlacer and madvr deinterlacer at the same time?

As huhn explained, you can't deinterlace an image two times, so there's no risk. However, your quality (and performance) will be determined by the deinterlacer that runs first, so the one in the decoder if it can run. The best way is usually to let madVR handle it, so not enable deinterlacing in LAV.

Do I always have to use one of the hardware decoders in lav filters settings to deinterlace these types of videos properly?

Not necessarily, you can also use software decoding and madVR will deinterlace the image using hardware as long as you set the Deinterlacing Mode setting to Auto.
To check that madVR is deinterlacing, press Ctrl+J to bring up the OSD and find the line that says "deinterlacing" (it should be just above the queue stats): it should say "on (says upstream)".

Next to hardware decoder setting there are resolutions sd hd. Hd was checked automatically, should I also check sd for the best possible quality?

That doesn't control deinterlacing, only if hardware decode is used for specific resolutions. You can check SD if you want, it shouldn't cause problems but SD video is very easy to decode by the CPU especially MPEG2, so it wouldn't make a lot of difference in performance.