In an earlier post I noted that the newly released Blu-ray of Miss Potter runs at 25 frames per second, rather than the usual 24. So it appears to have been sped up, for no good reason. I made reference to it being ‘the first Blu-ray movie I had seen which was not recorded at 1080p24, but at 1080p25.’ A couple of days later Craig Mecak, a tech wiz at Channel 9 in Sydney, corrected me on this point:
From my research, 1080/25p is not a valid blu-ray resolution/format.Allowable combinations are:
1080/50i & 60i (actually 59.94i)
720/50p & 60p (actually 59.94p)
Suspiciously absent are 1080/25p or 1080/30p. Not really a big issue, as I’m sure you’re aware that 1080/25p can losslessly be conveyed as 1080/50i, whereby each field contains half the number of lines from each original film frame. In effect, it’s 1080/25Psf (progressive segmented frame).
Deinterlace back to 25p by weaving the fields. Simple.
I’m prepared to take Craig’s word for this; he really does know a lot about this stuff. At the time I replied to that effect, and I agree that it is odd that 1080p25 and 1080p30 weren’t provided for in the Blu-ray specification. Still, perhaps they were worried about chipset availability or some such.
I was tied up for a while so it wasn’t until last night that I actually watched this movie. I must say it is a charming little movie. But of principal interest here is the picture quality. Watching fairly closely and using an actual TV for once instead of a front projector (it was the forthcoming Pioneer PDP-C509A 50 inch plasma, $5,499), at several points in the movie two things were obvious: Craig was right, and the Blu-ray player wasn’t doing a very good job.
In those scenes in which almost horizontal diagonal lines were moving across the screen, the jaggies were obvious. Which means that, first, even though the material originated on a progressive source (it was shot on film), it was encoded on the disc at 1080i50, not 1080p25. That meant that the Blu-ray player had to deinterlace it.
As Craig says, it should be a simple matter of weaving the two fields back together to restore perfect 1080p25 picture quality. But the Blu-ray player was doing a shockingly poor job at deinterlacing, not just those jaggie bits but also, I think, the whole movie. It certainly seemed a lot softer on screen than I expected. At this stage, I am thinking that the player was doing drop field deinterlacing. But I shall explore further.
Meanwhile, I now have a Blu-ray disc I can use for testing 1080i50 deinterlacing of progressive-sourced material, just too late for a set of five low cost Blu-ray player reviews I completed a few weeks ago, but just in time for set of five expensive Blu-ray player reviews I shall soon be starting.