NTSC video -> PAL: just curious

Discussion in 'Projectors, Screens & Video Processors' started by irg, Apr 18, 2007.

  1. irg

    irg
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    Hello,

    I've been listening to one of the recent AV Podcasts with Gordon's section on video processing, and a question occurred to me. Wondered if Gordon or anyone can answer...

    Purely out of curiousity, I was wondering what happens to American 60i video material when it is transferred to PAL 50i for either broadcast over here or SD DVD? This presumably includes most American TV shows (a few of which my gf and I are partial to).

    I guess there must be a loss of information, unlike with film material, which can be reconstructed exactly by a smart processor. So how does a good processor cope with 60i material that has been shoehorned down to 50i?

    Apologies if this has been answered elsewhere!

    Iain
     
  2. tryingtimes

    tryingtimes
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    Well with film information done properly (like on DVD or on TV programmes that people actually care about), the timing isn't really an issue as the 24 frames per second are sped up to 25 frames per second and doubled to get your PAL 50 (i.e. each frame is actually the same in both territories). If you're lucky the audio will be passed through a filter to keep the pitch correct during this speedup process. But often you get a semitone higher pitch.
    Check out dvd film cases and you'll see that there is a 4% running time difference between NTSC and PAL versions of the movie.

    Also on DVD and really good TV programming, the scaling is done again from the film master, so there is no loss in resolution.

    However this isn't the case with all material broadcast on UK tv. A lot of is is simply run through a standards converter which results in a wide variety of quality available. Some look great and you wouldn't notice/care. Others look terrible.

    With recordings done on video (60 differently timed fields a second). Again it can be a mixed bag. See older or minority American sports broadcasts and you'll see colour problems and softness in the majority of cases.

    These days with most sports shot in HD, these problems are getting easier to move across the channel as HD colour info and resolutions are the same in both territories.

    Hope that helps
     
  3. VirusKiller

    VirusKiller
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    Worth pointing out that most popular US shows are shot 24fps film, not 60i video.
     
  4. StooMonster

    StooMonster
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    As mentioned above mostly 'film sources' are sped up from 24fps to 25fps, 4% speed increase and resulting higher pitch in audio and shorter running time. 'Film sources' include movies, scripted drama (e.g. Lost, CSI, 24, BSG, Nip/Tuck, etc.)

    'Video sources' will be processed to get 60 fields per second into 50 fields per second. Typically either fields will be dropped or blended together, blending is like transparency in Photoshop where one field has 50% transparency overlaid on another. 'Film sources' include sports, news, studio based programmes (e.g. chat shows, soaps, etc.).

    Sometime, albeit rarely, 'film sources' are processed as per 'video sources' using field blending -- I believe 'Friends' did this, as does 'The Simpsons' -- so there is no speed up, but it doesn't look as good.

    StooMonster
     
  5. choddo2006

    choddo2006
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    You can quite often see the worst excesses of this on shows like Megastructures where the frame rate conversion gives a really horrible weird shimmering on any vertical edges during pans.
     
  6. irg

    irg
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    That's good to know. Actually it looks from StooMonster's reply that most of the stuff we would watch (Lost, CSI etc.) is film not video. (But of course Freeview still manages to make it look awful...) I must be one of the few plasma owners who doesn't care about watching sport!

    If we like a show we'll buy the DVD box-set, but our player is region-2 only. I know you can unlock it but I had a problem with a previous multi-regioned player which my local Pioneer service centre refused to repair under warranty because of the multi-regioning!
     
  7. StooMonster

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    For 'film sources' to look their best you need good cadence detection, i.e. devices need to know they are handling 'film' and can weave the video field together to reconstruct the film's full resolution frames.

    In Region 1 / American it's a process called 3:2 pulldown, and in Region 2 / Europe it's called 2:2 pulldown.

    Now, it could be that if you plug Freeview straight into your display, the display doesn't have 2:2 pulldown detection (this is quite common) so 'Lost', 'CSI' etc won't look as good as DVD.

    If you have a good quality progressive-scan / upscaling DVD player it should be able to handle 2:2 pulldown and output full resolution 'film source' picture; if you don't, the display with process the signal and will have same problems as Freeview.

    3:2 pulldown can be fairly easy to do, so lots of displays and players handle it, but 2:2 is more difficult and many don't.

    This is one of the reasons why there is a market for external video processors -- such as the ones discussed on this forum.

    StooMonster
     
  8. Barcoing Mad

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    I think 'Friends' was shot at 30 fps native, which makes conversion to 25fps harder.

    Some of the HD channels look nasty during pans - the dropped frames are obvious.
     
  9. irg

    irg
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    Cheers Stoo. Yep I'd pretty much got my head around film sources and 2:2 and 3:2 pulldown, much helped by Gordon's explanations when he ISF'd my display. We found that the screen has fairly unreliable film detection (though I remember it was much vaunted in Pioneer's brochure :rolleyes: ) but the DVD player's is better. (And pulldown can be manually set on or off as appropriate.)

    I am planning a screen upgrade this summer at which time I hope to get a video processor too if funds allow :thumbsup:

    Iain
     
  10. Quickbeam

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    It was actually shot at 24fps, the standard frame rate for US filmed TV drama. The US web site HomeTheaterHifi claims that it was shot at 30fps progressive. However I have checked an NTSC DVD of one the later Friends seasons and it's definitely 24fps. The PAL broadcast version of the early seasons look quite juddery but I think this is due to a poor standards conversion. Even the first season has high-pitched sound (PAL speed-up) despite the judder and softness.

    Which channels or programmes on SkyHD are you referring to?
     
  11. StooMonster

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    I would assume he's talking about Discovery HD and National Geographic HD which carry mostly 1080i 'video' material. Quite often on these 'video' heavy channels you can see motion artefacts in pans, one would assume caused by 60Hz to 50Hz conversion.

    StooMonster
     
  12. nidi

    nidi
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    I don't understand why they have to do framerate conversion on HD. all displays, which can display HD , can do 60 Hz perfectly.
    Yes, Discovery HD here in Europe judders horribly.

    Michael
     
  13. VirusKiller

    VirusKiller
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    I imagine simulcasting of SD and HD presents "challenges".
     
  14. NicolasB

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    Yeah, just imagine how many people would ring up to say their Sky HD box was faulty because they didn't get any signal over SCART on the HD channels. You have to be able to easily convert HD to SD on the fly, and so Sky wanted to avoid 60Hz/50Hz issues. (Depressingly I'm not being sarcastic).
     
  15. nidi

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    there is a picture coming out of SCART ? :confused:

    that shouldn't be.

    too many compromises

    Michael


    I will never subscribe to an HD service that converts 60 to 50 Hz


    I thought with HD that should be history
     
  16. Quickbeam

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    Sometimes compromise can be a good thing.

    If European HD broadcasts were 60Hz think how much worse the picture quality of Sky Sports would be for the 95% of subscribers watching the 50Hz SD downconversion. Imagine the result if the Sky One simulcast channel were upconverted to 60Hz for the HD transmission then downconverted again to 50Hz for all the SD viewers. It would be unwatchable.

    Why not support both formats in the HD broadcast chain, producing our own HD material in 50Hz while allowing 60Hz material to be shown in its native format? This is of course theoretically possible and it would not degrade the picture quality for SD viewers, but it involves running 50Hz and 60Hz timecodes in the same transmission area, which isn't a practical option in a broadcast environment. In addition, on some TVs there is a delay when switching between different resolutions (e.g. AUTO mode on SkyHD), which some viewers may find objectionable.

    SCART shouldn't in itself be a major barrier to 60Hz broadcasts because most if not all modern PAL TVs can lock onto a 60Hz signal (hence the popularity of region 1 DVDs in this country). The SD output of many Xbox 360 games is 525/60 for this very reason. Not many VHS recorders can record 525/60 though.

    While obviously it isn't ideal that 60Hz native HD material has to be converted (badly) to 50Hz for transmission, the vast majority of US TV imports are 24Hz, not 60Hz. 24 frame material is not standards converted but is played back slightly fast, at 25p, so there is no loss of picture quality whatsoever.

    Few people expected European HD-DVD and Blu-ray movies releases to be encoded at refresh rates other than 50Hz, and the fact that they are available here in their native frame rate is a major step forward!
     
  17. choddo2006

    choddo2006
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    Good post.

    One question though: 525/60? I thought PAL60 was 480 lines, just like NTSC? Or is that what you mean by 525, including blanking?
     
  18. StooMonster

    StooMonster
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    Isn't BBC HD content sourced in 50Hz? 25fps for 'film sources'?

    Slowed down 4% for 60Hz telecine. ;)

    StooMonster
     
  19. nidi

    nidi
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    I'm talking about Video, not Film.

    don't you mean sped up by 4% (24fps to 25 fps) ?

    aren't those BBC documentaries still filmed in Video at 50 Hz ?

    BBC HD is free (at least for me , I live in Switzerland)


    Michael
     
  20. NicolasB

    NicolasB
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    RGB SCART offers the best-quality SD picture for most Sky HD subscribers.

    But, if you can't stand to watch 50Hz conversions, I guess you'll never get Sky HD, so either you'll be using up an awful lot of Internet bandwidth doing illegal downloads, or you'll be stuck watching Lost in standard-definition, either of which would be a shame. (It looks absolutely stunning in HD on a 1080p screen. :) )
     
  21. nidi

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    what illegal?

    in Switzerland, it's not illegal to download movies, only upload is.

    we have a very liberal law here. and bandwidth comes very cheap here too

    10+ Mbit/s doubled almost every half year !

    btw, lost isn't Video it's FILM, so it's 'only' a 4% speedup ;)

    but those Discovery docs are very anoying.

    we finally have 24748 Hz and what do those jerks do, go back to the

    middle ages with conversions


    Michael
     
  22. Quickbeam

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    Yes. I only referred to it as 525/60 because the OP mentioned NTSC, which is usually referred to in terms of the total number of lines, rather than the active number.

    One thing I'm curious to know is how Americans get on with displays linked to video processors such as the VP50 that can convert film sources from 60i/p to 24p, and whether leads to noticeable lag when viewing broadcast TV. Ads for example are likely to consist of a mix of 24p and 60i/p material. How many current displays can change formats seamlessly?
     
  23. choddo2006

    choddo2006
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    The broadcast doesn't change though, it's 60i (or 720p/60) all the way. The vp50 just (if it's on Auto) changes how it's deinterlacing it in order to produce the requested output framerate. Of course, when it's video... if you've set it to 48Hz output, the results won't be good, so you probably never would for TV sources.
     
  24. NicolasB

    NicolasB
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    Ah, well, if you're not living in Britain, that moves the goalposts considerably. (This is a primarily british forum, hence my incorrect assumption).
     
  25. StooMonster

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    Depends on the programme -- Bleak House, Hotel Babylon, etc. -- shot on digital 'film' at 25 frames per second, not 50 fields per second video; just like adverts, UK music videos, etc.

    To convert 25fps to 24fps, you slow it down 4% -- the opposite of 24fps to 25fps speed up.

    StooMonster
     
  26. VirusKiller

    VirusKiller
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    One of the questions that I'm trying to get a definitive answer to is how Planet Earth has been produced for HDDVD in the States.
     
  27. StooMonster

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    What's the running time versus the UK DVDs?

    StooMonster
     
  28. bluevortex

    bluevortex
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    Sorry to jump in. I have a new 1080p screen and whilst watching Open season on blue ray via PS3 i noticed this judder on pan & scan scenes. from what i have read on Av forums this issue will only be resolved by

    1, Using a true 24p screen - due to be released later in year.

    2, Using a video scaler.

    Can anybody confirm this please? As i have just taken delivery of my 1080p screen option 2 seems the logical choice if it works? Can anybody point a complete scaler noob in the right direction to overcome this issue and give an approx cost of a piece of kit that would work?

    Thanks Blue
     
  29. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Blue: There are currently screens that accept and display 1080p24. So no need to wait for that.

    Judder can be one of two things though. If it is a uniform suttering motion error then it's likely caused by turning the orignal 24 film frames in to 60field video. If it is a uniform smooth judder then it's quite possibly that it's just due to the fact film has such a low temporal resoltuion..ie only 24 images recorded a second. This low frame rate means that in some fast pans you get what looks like judder, most noticeable if you have a nice tall vertical object on screen that acts as a reference point. There is nothing you are going to be able to do about the latter, just now.

    The PS3 doesn't currently output 1080p24Hz. There is a rumour it might with new firmware but I've only heard that third hand. In the absence of this ability you need two things. One is a scaler that can take the 1080i60 material and re-create 1080p24 (OR 1080p48) and then you need a display that can accept either of those res/refresh rates without frame rate converting it back to 60Hz. The most affordable video processor I can think of right now that has this ability is the Lumagen HDP with an SRP of £975.

    Gordon
     
  30. StooMonster

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    Additionally, the DVDO iScan VP50 can accept 1080p60 (from likes of PS3) and convert it to 1080p24 without having to go through any interlaced stages via its 'Progressive Cadence Detection' feature.

    StooMonster
     

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