[mythtv] Please help me here: How to obtain a stable full frame rate?

[Jeroen Brosens]

People,

Please read the following: http://www.100fps.com/why_bobbing.htm
I am seriously doubting the correct implementation of the 'Field Bob' 
filter (as this website calls it) in MythTV's bobdeint, because this 
occurs in every single show I watch up till now, be it a recording or 
Live TV. I have had discussions with several people on the mailing list 
already to find the origin of this behaviour but never got a satisfying 
answer or even a solution.

THE PROBLEM: TV output at full frame rate (50 Hz) is only possible with 
'bobdeint' but never stable. Here are my thoughts:

0. I am in Europe (The Netherlands), so we use PAL @ 50 Hz.

1. I use an ASUS Pundit barebone with a SiS graphics chip; which can't 
feed interlaced material to its video out (see here: 
http://www.winischhofer.at/linuxsispart3.shtml#faq, look for the 14th 
question).

2. To bypass this problem, I built a VGA-to-SCART converter, that 
connects my Pundit directly to the TV set; hence no interlacing 
incompatibilities - you would think. The output still doesn't look 
smooth as 'normal' TV. The video is captured at 25 fps (2 fields per 
frame!) and displays it at 25 fps, both fields interlaced and at the 
same time. Using a modeline set to 720x576i (interlaced) the output 
*should* be fluid, 50 Hz. But it isn't, I don't understand why. By the 
way, the GUI graphics look horrible due to no flicker filtering.

3. I concluded that the S-Video TV-out image looks way better than 
direct RGB, which doesn't even solve my primary problem.

4. Using the Bob deinterlacer one *can* achieve full 50 Hz frame rate 
(actually: field rate!), but all fields must be played back in the 
correct order. Explanation: the recorded NuppelVideo files are 25 fps 
and contain 2 fields per frame, the playback order is like this: [ a+b, 
a+b, a+b ] where a and b are the even and odd fields and every comma 
marks a frame.
Bobdeint *should* split the frames into a an b an d display them at 
double frame rate to achieve: [ a, b, a, b, a, b ]; the result is 50 Hz 
and fluid. <-- I want this!

5. I experience the fact that every now and then, the result of bobdeint 
gets out of (vertical) sync for a while (!), like this: [ a, b, b, a, a, 
b ]. It loses its sequence and starts swapping fields or even frames, I 
am not 100% sure about this. Some pointed out this is due to the video 
driver not knowing the monitor's Vsync. But reading the website I linked 
at the top, a faulty 'Field Bob' filter implementation could also be the 
cause.

6. NVidia users for example have hardware OpenGL acceleration that can 
take care of Vsyncing, I do not. Unfortunately, the SiS X driver does 
not support hardware DRI and/or OpenGL. They can also benefit from Xv 
Motion Compensation (XvMC), which presumably also gives good quality 
(does it output full framte rate?). I guess I am running out of options.

7. MythTV's output is decoded by the IVTV driver, correct me if I'm 
wrong. I was told to blame the IVTV developers for bobdeint not doing 
its job well without knowing vsync. Can anybody confirm this?

8. Other software like tvtime, MPlayer, Xine deinterlace very well. For 
instance, playing recordings through MPlayer delivers excellent image 
quality. Why can't MythTV? For Windows, there is DScaler 
(http://deinterlace.sourceforge.net), it does deinterlacing very well too.

Sigh. Must I really go and buy a relatively expensive PCI based nVidia 
card to get proper 50 Hz image? I am open to all of your ideas! Thanks a 
lot in advance,

-- Jeroen