[mythtv] Perfect sync to interlaced TVs

Mark Kendall mark.kendall at gmail.com
Thu Sep 25 02:00:12 UTC 2008

2008/9/24 Paul Gardiner <lists at glidos.net>:
> I have an idea for an alternative form of interlacing to achieve
> perfect sync for interlaced content played on interlaced TVs. In
> theory the best picture is achieved if the interlaces are displayed
> as is, with no processing, but there are problems in doing so
> in MythTV.

> Is this of any use?

Firstly, I'll just say that I don't believe there's a mechanism for
doing this properly in linux. I've searched wide and far and browsed a
lot of code and I've found nothing to suggest that there is a way to
determine when the graphics card is displaying which field when using
an interlaced mode.

Secondly, given point one, we need a workaround (see below) but I'm
not sure that a single workaround will give consistent results on all
combinations of drivers/GPU vendors/TV models. I've experimented using
nvidia cards (hdmi hd and sd, vga to rgb-scart sd) and my ps3 (hdmi
and rgb-scart). Most approaches worked on the ps3 but getting
consistent results out nvidia is trickier.

What I think you're describing above is what I've previously called
'field order' - whereby on each refresh we display the two most recent
fields. This is a deinterlacer option in both trunk and 0.21 fixes
when using the opengl renderer (it's called the 'Interlaced'
deinterlacer!). Other opengl rendering issues aside, I later realised
that that 'deinterlacer' doesn't give consistent results.

A lot more testing later (and various discussions on the -users list),
I realised that bobdeint does actually work consistently though
picture quality can be degraded slightly due to the 'bobbing'. So I
implemented a bobdeint without the bobbing.

What is now in the mythtv-vid branch for the 'Interlaced' deinterlacer
is effectively bobdeint without the bob. It shows each field
consecutively, scaled to full frame size. This appears to work
consistently - both in my own experience and feedback I've received
from others -  and picture quality (including OSD) seems not to be

It should be easy enough to implement as a software deinterlacer as well.



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