[mythtv-users] What don't I get here? (Playback FPS)

Steve Smith st3v3.sm1th at gmail.com
Thu Jun 14 16:05:43 UTC 2007

System summary:

Dual P3 1Ghz
Nvidia 5200 ->    S-VIDEO to CRT PAL TV   (PAL)

Recordings are  DVB-T or PVR-150 sourced from Antenna or Composite video
(Telewest Cable box)

So basically I'm playing Interlaced PAL SD @ 50Hz recordings through the
TVOUT of my nvidia.OK?

So what I don't get here is:
    Why does the framerate look slow in comparison to
      a) actual live tv e.g. straight from cable box to TV
      b) One of my mpegs played back through my MediaMVP?
    (What I mean is movement looks just slightly blurred and jerky)

I've done some diagnostics using mythfrontend -v playback and I'm getting a
solid 25fps....

So that leads me to the following.....

1) "Normal"  TV is rated at 25 frames per second but as it's interlaced it
actually 50 FIELDS per second.
2) The two fields that make up one frame often contain some movement
(otherwise we wouldn't have the de-interlace problem to sort out would we?)
3) THEREFORE "Normal" Interlaced TV actually has a pseudo 50hz frame rate
(albiet they're not really full frames but sort of half resolution).
4) So if my player deinterlaces and plays at 25fps we actually get  a
reduction in perceived frame rate and hence the slight jerky/blurriness in
comparison to live TV.

So am I correct in my analysis? Is the mythfrontend actually playing 25 full
frames a second or 50  fields a second?

How can I get the "correct" output  of 50 fields a second where potentially
the two fields in each frame can contain movement?
Would using a CRT output (RGB to SCART) instead of S-VIDEO help? (in this
respect not just overall quality) i.e. is the NVidia TV-Out doing something
to the timing...e.g. insisting on using 60hz without telling me thus
throwing out the timings, or maybe even forcibly de-interlacing the picture?


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