[mythtv] Ticket #2903 is necessary to achieve perfect playback with an interleaved display mode.

[Paul Gardiner]

I've been fiddling with mythtv for six months now, just wishing
to play 720x576i video to a PAL TV. I've built several systems,
never being quite content, but now the 4th, which uses VGA to
Scart, is near perfect.

I've just started working on the one remaining problem, which
is that there is nothing keeping the interlaces in sync between
content and display. Half the time I get perfection, half the time
I get terrible combs. I believe this is a well known problem. I can
hit "back" a few times when this happens and get it back in sync.
I've been using it like that for some time. I'd rather put up
with having to do so, than distroy the sharpness and smoothness
with processing.

My intention to correct the problem is to write a software,
field-order deinterlacer (there's already an opengl one, but
I can't use that).

Before I could even start on that work, I ran into another problem:
my system wont allow me to use a doublerate deinterlacer. I traced
that to the refresh rate calculation ignoring the fact that the
screen is interlacing and returning 25, rather than 50. (25 is
the true framerate, but we need 50 to acknowledge that there
are 50 vsyncs per second, and allow the use of the doublerate
deinterlacer).

I created a patch, to fix the problem, and now I can use
yadifx2 which does give really good results, and only loses a
little quality (it depends on the sync, now having the wrong
sync, rather than giving combs, gives a little loss of quality
because I then get all the lines calculated by yadif rather
than all the original lines).

I thought, before continuing the work, I'd open a ticket and
submit my patch so that others can at least use yadifx2 and
get the really very good results I'm getting, but I found that
someone had solved the problem 3 years ago, just to have their
ticket closed as invalid. The explanation given says to use
kernel or linear blend. That's madness. Both are going to
blur the image horribly, although kernel not so bad for
still images I guess.

Please can this decision be reconsidered. I guess it doesn't
matter to me so much: I now build my own copies of minimyth,
but there must be lots of other users who could benefit.

As an aside, I think some drivers (openchrome) get around this
problem by allowing a dummy mode line in xorg.conf that isn't
used in setting up the screen, but will be read by myth. The
dummy modeline has a doubled dotclock rate, which make the pal
mode come out as 50fps. I've certainly seen mode lines with
openchrome that have 27 where you'd expect 13.5

P.S. If we do reopen Ticket #2903 then we should test for
framerate 25 or 30, and double only in that case.

Cheers,
	Paul.