progressive segmented frame

[Piotr]

I must say this has been bothering me for a while:
Is picture captured as progressive, recorded as progressive segmented frame (PsF) and transmitted/displayed over an interlaced system visually indiscernible from the truly progressive pictured ? eg. PsF over 1080i, is it any worse than say truly progressive 1080/25p? Does it matter what kind of display it is? CRT vs LCD vs DLp vs Plasma? What happenes with resolution? Has anybody done research on all of that ?
In light of that what does it mean that blue ray discs (BD) will have a 1080/24p capacity?
Will it be 1080/25p ini Europe?

[Alan Roberts]

There's been a great deal of work done on this, in many countries. I spent a large part of my R&D career working on HD (starting around 1976), and problems like this have been very well investigated.

PSF is the basis of all film presentation on conventional television, where each frame is effectively scanned twice, interlacing the lines, to form the two fields of the interlaced transmitted signal. PSF is not the same as Progressive, because in P each frame is captured, transmitted, displayed in it's full form, so there's never any "interlace twitter", the visual result of the transmitted signal carrying vertical frequency content near the vertical Nyquist frequency limit (where alternate lines of the frame may be black/white, such that the alternate fields are black/white) and detail can be confused with motion.

All this is true for crt displays, where there is no image processing. But, for lcd or plasma or dlp, the rules change. This is because they (mostly) de-interlace the incoming sugnal such that all the displays pixels are addressed in each field period. De-interlacing is not trivial, it's the one thing that marks out good and expensive displays from cheap and bad ones. Poor de-interlacing results in aliased frequency components which cause confusion in the eye, making the display seem "restless" and worrying to watch.

1080psf/25 is not the same as 1080p/25. In psf, the image rate is not 25Hz but 50Hz, since each although frames are captured progressively at 25Hz, they are transmitted and displayed at 50Hz, but with only half the lines in each viewed image, the process of interlacing. In progressive, the capture is the same, at 25Hz, but the transmission and display is also at 25Hz unless other steps are taken. So, on a crt display, there wo
uld be only 25 images per second and you'd get very noticeable and disturbing flicker, the result of the display flashing at 25Hz.

Fortunately, there's an alternative for 25p that makes sense; this is to send each captured frame twice, so that the display image rate is now 50Hz but the content rate is still only 25Hz. This is excatly the same as projected film, where a shutter is rotated in the beam to interrupt the light path and create 48 flashes per second instead of 24. The JVC HD100 series cameras do this, but they've not created a name for it, so I have done so and called it Progressive with Repeated Frames (PRF). I was going to call it Progressive with Dupliated Frames but that would have been pdf, which ASdobe already use.

PRF is already used at 23.98 in NTSC countries, where they use irregular "pull-down" to get up to the transmitted rate of 59.94 (traditionally called "2:3 pulldown", althoughb there's a more recent version call "2:3:3:2 pulldown" where the resulting irregular motion is less of a problem).

There is a decent case to be made for coding and storing film in its native form, at 24Hz, and this is what the HD discs attempt to do. It means that the decision on how to read it is made at replay, rather than at recording. So one disc fits all. Play it at 25Hz and you get psf/25, play it at 23.98 and you get prf/29.97 with either 2:3 or 2:3:3:2 pulldown. That means a single world market for recordings, with the regional differences taken up in the playing, not the coding.

One point to make though, 24Hz does not always mean 24Hz. In film, it really does mean 24, but in tv it's far more likely to mean 23.98 (actually 24/1.001) so that it makes sense for NTSC countries. Even the camera manufacturers make this mistake, the Panasonic HVX200 has 24Hz as a rate setting in its menus, but it actually does 23.98 not 24. This might sound a bit picky, but the difference is profound once you start making programmes because 24Hz has non-drop-frame timecode, whereas 23.98 has drop-frame; get this wrong when you shoot and your editing suddenly gets far more complicated.

Hope this helps.

[Piotr]

Allan,
Thanks, that helps a lot. But in a simple test, two flat displays, one showing 1080i/25psf/50hz , the other 1080/25p/50Hz:
would one see the difference? And on CRT?
About interlace, i read a true story (?) account of one ITU/EBU meetings on DTV standards. Progressive vs interlace was debated. One gentleman got up and said: I spent my whole life on deinterlacing and I am wondering why you even consider an interlaced standard. Now that I sold my company I can tell you: deinterlacing does not work. Who is that? somebody called. My name is Ives Faroudja.
All the best
Piotr
PS. We finally introduced your full BBC setups for sdx900 here in Warsaw and it is great!
Thanks for the help here too.

[SimonMW]

So, Alan, with Blu-Ray etc on the horizon, does it make more sense to shoot HD video at 23.98 fps even for the UK market?

I have been wondering about this recently and figured that it is much easier to convert 24p to 25p then the other way around (slight speed up is more desirable to me than slight slow down which makes things seem more sluggish).

[StevenBagley]

There's a big problem at the moment, in that all the HD players on the market in the UK at the moment can only handle 24fps playback, and so releasing UK shot material is going to be hugely problematically. Especially as a lot of BBC shows (Torchwood, Robin Hood for example) are not true 25psf shows since they contain 25i sections

Ideally, the disks should contain the material at native frame rate (23.98p, 24p, 25p) and output it at the correct rate for the display and handle the sound transposition themselves -- just like a HDCam deck does. Unfortunately, it is going to require a firmware update to enable this in UK players, which is crazy.

Steven

[SimonMW]

There is a good case to be made for shooting 24p. For example, televisions in the US are disabled from displaying signals such as 50i, 25p etc, because they want to reduce competition over there.

Standardising a framerate would make things a lot easier for someone like me who doesn't have access to Snell & Wilcox format converters, but who would still like to release stuff for sale in the States. It would save me from having to make two seperate HD-DVD's, Blu-Ray etc with all the work that needs to go into that conversion (menus, sound), which on a complicated disc is quite an effort.

I would prefer faster framerates to become normal (50p 60p etc), but for the sake of standardisation for the time being, 24p might make things easier, even if it has the silliness of pulldown to cope with.

[Alan Roberts]

Piotr, on a crt you'd see interlace twitter on the psf display but not on the p display. But the p display would have to run it's line scans at twice the speed of the i display (and that costs a lot more). On a flat panel, you would probably not see any difference, depending on how good the de-interlacer is (the best ones can detect when the signal is psf and not do any processing, which is best, the others just get it wrong). Interlace is a good way of getting low-flicker pictures over a transmission system that can't handle the full bandwidth of p, it's worked well for us for over 60 years but the time's getting close when we can drop it, but not before full p pictures (and I mean at 1080p/50, not 720p/50) can be recorded, edited, delivered, and dsiplayed well, and that's still several years away.

I understand Faroudja's views, and he's probably right for compressed transmission, but interlace still has a place in television, until proper 1080p/50 systems become available. At present, there's no kit, so there's no point debating it, but 1080p/50 is the EBU's stated aim; the only real issue is how we get there, some in the EBU want to eliminate interlace althogether and that means all transmission would be 720p, others (like myself) say that material shot in 1080psf is already progressive and the down-scaling of it to 720p is a fat idea, since the transmission would contain repeated frames (unless the coder were bright enough to work it out). Currently in the UK, drama is now being shot in 1080psf, and some documentaries as well, so we've already got 1080p as far as trhe image is concerned, so why not transmit it? Provided the decoders don't mess about with it, we get the big sharp pictures that look like film. Only news, light entertainment and sport is likely to want to shoot interlaced in order to get smooth motion, so they can use 720p instead of 1080. Provided both 1080psf and 720p can be transmitted, we get the best of all worlds.

Simon, I'm with you on that, shoot at 24 if you must (that's genuine 24, not 23.98) and speed up to 25. That's exactly the way that film has been handled on tv since 1936, and we're well used to it. Slowing 25 down to 24 gives a dullness to it all, so is a bad idea (but has been accepted in the US for the same period). The real problems come when you start involving drop-frame shooting, because the editing can be a nightmare especially when you record sound on an external box and tgry to lock them together.

Steven, many of the BBC HD productions (all except Jools and Sport so far, plus the odd CBBC bits) are shot 720p or 1080psf, but all are edited in 1080psf, so all have film motion. In the edit, titles are always added interlaced, so that rollers don't jerk too much (there are guidelines for what speeds can be used, and interlace allows a wider range). I don't think there's much i/25 shooting from the cameras, unless they're playing the trick of shooting 1080i/25 and then slowing down 50% (when each field has to make a frame, and so makes soft and twittery pictures). But I'm with you 100% on delivery media handling source material at native rates, it's no business of the delivery media to decide how we're going to see the stuff, that sghould be left to the display designer, even when he has no idea how to do it properly since the crap kit will die when faced with the good stuff.

Hope that helps.

[StevenBagley]

In the edit, titles are always added interlaced, so that rollers don't jerk too much (there are guidelines for what speeds can be used, and interlace allows a wider range). I don't think there's much i/25 shooting from the cameras, unless they're playing the trick of shooting 1080i/25 and then slowing down 50% (when each field has to make a frame, and so makes soft and twittery pictures).

It was the titles and textual overlays in Robin Hood that I was thinking of and several varispeed elements in Torchwood, which move from 25p to 25i and back again as the speed changes. And of course, there is Sinchronicity where the camera was set to interlaced by mistake and so it can literally change from shot to shot -- which is actually quite interesting because the image texture doesn't change just the fluidity of motion.

Speaking of Sinchoricity, I've a strong suspicion it was shot on the Panasonic HDX400 since some shots had colour fringing on ventilation grille, which I'm assuming is the frequency of the grille beating with the pixel offset techniques used on the camera to interpolate to 1080 from the 720p CCDs. As in this shot:

http://www.eprg.org/tmp/sinch-fringe.png

Steven

[Alan Roberts]

I don't know Sinchronicity, but material shot on the HDX400 may well have problems like that; Panasonic have replaced it with the HDX900 which is a whole lot better in many ways (and cheap). As far as I can tell from analysis, the HDX400 doesn't have proper precision offset, while the HDX900 seems to have it both horizontally and vertically, just like the HVX200, this means you don't get the full 50% extra res, only about 30%, but you get it horizontally and vertically.

[infocus]

I'm noticing that an increasing number of displays are now making much of the claim that they are "True 1080" (so presumably 1920x1080 resolution) and that they are also 1080p capable. I assume that means they can handle 24p material (and 25p?), but does anybody know if they will be able to cope with the 50p Holy Grail if it ever becomes a reality?

[Alan Roberts]

I think that "true 1080" means that they have 1080 lines of pixels; it might not mean they're square though (and that's not really all that important since the UK HD transmissions of 1080 are 1440x1080). I'd need a really hefty dose of realism before I believed that any of them could cope with 1080p/50, largely because that standard doesn't yet exist internationally, any more than 720p/50 does.

I'd not assume anything about any display unless I saw it written in the spec.

[infocus]

I think that "true 1080" means that they have 1080 lines of pixels; it might not mean they're square though ............
I'd not assume anything about any display unless I saw it written in the spec.
Certainly in the case of Sony TVs they make a big play of saying in the spec that they are 1920x1080. (Which I understand Blu-Ray is capable of, if not broadcast transmission.) They also state 1080p capable - just don't say anything about the frame rate.