[vdr] Re: More thoughts on Matrox TV out with interlaced programmes

[Tony Houghton <h@realh.co.uk>]

In <200502032112.47331.linuxtv@foft.fsnet.co.uk>, Mark Watson wrote:

> I was just quickly replying to Tony's post, but this kind of grew. Guess some 
> people may be interested... 
> 
> With live TV VDR isn't recording at all I believe (I wish it did, but thats 
> another post...). Presuming the graphics card clock is not exact PAL/NTSC 
> frequency you'll either skip or have to repeat fields. Running slightly slow 
> and skipping the odd frame is probably the easiest option. If running fast 
> you wouldn't want to repeat a complete frame (i.e. 2 fields). The odd field 
> is before the even field in time, so any motion will appear to flicker 
> backwards.

To keep the field parity correct you'd have to skip pairs of frames or
show one frame three times rather than twice; the latter might look a
little odd, but not as bad as showing the same pair of interlaced frames
twice, but showing them as 1-1-2-2 rather than 1-2-1-2 might be OK.

And then you have to resync the sound as quickly as possible, but
preferably without stutters or obvious pitch bends.

> That said if it doesn't happen often that might not look so bad. 
> Thoughts? Might be easiest to change the hardware clock on the card to a PAL 
> frequency one!

The problem isn't whether the card can generate the correct frequency,
but whether it can be kept in sync with the broadcast, or if not,
whether both the graphics card's and broadcaster's clocks are close
enough to an exact 50Hz for frame skips and repeats to be tolerably
rare. I don't believe the former is possible, but having played with
mplayer dvb:// for a little while it looks like the latter is the case,
because I didn't notice any jumps in a few minutes.

-- 
TH * http://www.realh.co.uk