1080i

[Chris Longley]

I honestly thought I'd got my head around high-def. Been using an HD panel and mucking about for more than a month now.

From what I've read I've concluded that all LCD TVs and Plasma TVs display images progressively.

When fed an interlaced source, they need to deinterlace the signal.

Is that correct?

If so, why do broadcasters sometimes use 1080i (I think I see how this would help when capturing fast sports for example?)

Does it still make sense if displays have to deinterlace that material?

[tom hardwick]

Broadcasters originally inverted interlace so that half the picture could be transmitted, then the other half a bit later. The bandwidth available made this technique imperative.

I'm guessing (though I don't know) that this situation still exists.

[StevenBagley]

From what I've read I've concluded that all LCD TVs and Plasma TVs display images progressively.

Most, but not all. I believe some Hitachi ALIs panels where natively interlaced...


When fed an interlaced source, they need to deinterlace the signal. Is that correct?

Yes.

If so, why do broadcasters sometimes use 1080i (I think I see how this would help when capturing fast sports for example?)

Because it is the only high-resolution HD standard about the alternative would be 720p50, which carries the same amount of temporal data but, on average, less spatial information than 1080i.

As for why 1080i exists, simply because it predates LCD and Plasma panels and was designed to drive CRTs at high-resolution. As Tom points out, interlace is a compression mechanism and while it would now be (just about) possible to compress and distribute 1080p50 video, it certainly wasn't the case in 1997 or even 2006.

Steven

[Alan Roberts]

Just remember that 1080i has a clear ancestry all the way back to 1964, when NHK started their work on HDTV. They came to the conclusion that 1035 lines were needed, and 30 MHz luma for a video signal carried in an 1125/60 interlaced format. 1080 is carried in the same format but with square pixels. Interlace was the only possible way of getting close then, so that's what we've got. It works well on crt displays, and was initially intended to be transmitted in analogue by the HD-MUSE system.

When digits came along, and pixel-based displays, the need for interlace receded, but is still there as a legacy. It will eventually go away altogether. A p;ixel-based display needs to show all the pixels all the time, so it has to generate a progressive image at the field rate, by de-interlacing. That's hard filtering to get right, and it costs a lot to do a decent job. Some of the latest panels are pretty good, but there's still room for improvement.

I have some off-screen photos of the results of imperfect de-interlacing, it makes for an interesting analysis session (still don't understand some of it).

[tom hardwick]

NHK started their work on HDTV. They came to the conclusion that 1035 lines were needed, and 30 MHz luma for a video signal carried in an 1125/60 interlaced format.

I remember seeing this analogue HDTV on the Sony stand at a video show - must've been in about 1990 I think. They used rather static shots in their demo I seem to remember but we were all mightily impressed by the on-screen pictures.

Then we went home to out Lo-8 and VHSc camcorders :(

tom.

[Alan Roberts]

By 1988 we were working on Eureka 1250 stuff in Europe. I was at IBC several years, keeping the BBC kit going. All our pictures were interlaced because we couldn't do progressive (at 50fps), using Saticon-tubed cameras until about 1992 when we traded our lovely Bosch KCH1000s for the rather better 3-ccd LDK9000s. There were no flat panel displays then, or at least nothing bigger than 20", and they were far from acceptable for tv.

[Dave R Smith]

Are Sky HD transmissions therefore all/mainly 1080i?

If so, in choosing a new telly we need to see that that the interlaced to progressive(constant) display conversion is handled well - no herring bones on panning diagonal objects etc.
So, if we ask the shop to use a sky source (rather than blu-ray), is it likely to be interlaced, so we know it's a good test?

Do you know if tellys have menu options to change the conversion method or firmware to update, given that it's a 5-10 year (plus) purchase?

[SimonMW]

Is there any particular reason why an LCD display can't be made to handle interlacing without deinterlacing? The MXO on the Mac handles it pretty well and the results are superb.

[harlequin]

Are Sky HD transmissions therefore all/mainly 1080i?



as far as i can tell from info available.

[Dave R Smith]

as far as i can tell from info available.

Thank-you Gary.

[Alan Roberts]

Sky and BBC HD transmissions are all 1080. There's a mixture of interlace and psf, mostly the sport and LE is interlaced, the drama and documentaries are psf. There are fundamental reasons why panels have to de-interlace, there's not really a practical way round it. Some panels (the ALIS types) drive the pixels in a sort of interlaced way, but they aren't popular and are an ever diminishing portion of the market.

[Dave R Smith]

...mostly the sport and LE is interlaced, the drama and documentaries are psf. ....

Thank-you Alan. Useful to know when judging telly purchase in shop.
Quality of upconversion of inerlaced/progressive from whatever source (dvd/sky) will be another consideration, but that's a different kettle of fish.

[Chris Longley]

Just caught up with this thread. Cheers for the answers on the original question.

It raises an interesting point about Sky.

Based on the HD Ready sets that most consumers are using. Might there be an argument for them to broadcast in 720p?

People without full HD panels wouldn't be downscaling, nobody would be deinterlacing?

(Sorry if this is a duplicate post, earlier today I managed to post my response in entirely the wrong thread!

Mods please move, edit or delete anything that would help future readers follow the threads properly)

[Alan Roberts]

And I'll duplicate my reply as well :)

Freeview could well end up with 720p as HD, 1080 on Freesat.

And Sky News is already 1080 on P2.

[StevenBagley]

It raises an interesting point about Sky.

Based on the HD Ready sets that most consumers are using. Might there be an argument for them to broadcast in 720p?

People without full HD panels wouldn't be downscaling, nobody would be deinterlacing?

Except most HD Ready panels haven't been 1280x720 but rather 1366x768 or 1920x1080, so everyone would be upscaling the image... SkyHD is completely 1080i and they are looking towards 1080p50 for the future...

Steven

[Alan Roberts]

Absolutely right, but there's more.

We all want to end up with progressive, and to junk interlace because it's not needed in digital coding. The only issue is how to get there. We can have 720p/50 right now, and 1080p/50 in about 5 years time. BUT, we already have 1080p/25 for much (if not most) of the broadcast HD in this country. And the producers and programme-makers or the existing 1080p/25 stuff are all saying that they don't want 1080p/50 because they want the jerky motion of content-repetition, film-style. So the only incentive for 1080p/50 is sport.

[Chris Longley]

Ah I thought there'd be the 1388 x 768 issue mentioned. I had to ask about why that existed once before. I'll have to revisit the reply.

Alan, forgive my ignorance, but what's P2? As in "Sky News is already 1080 on P2"

[Alan Roberts]

P2 is Panasonic's solid-state acquisition card.

1366x768 comes from the computer display world, being the 16:9 expansion of 1024x768 (XGA) and is, unsurprisngly, called WXGA.

[infocus]

And Sky News is already 1080 on P2.
Are you absolutely, 100%, sure about that, Alan?

A few months ago I certainly had it comfirmed that Sky News had gone for P2, but I was pretty sure that the intention for the foreseeable future was for the P2 and field acquisition side to continue SD for the foreseeable future.

That's Sky News, the rest of Sky is a different matter, and I believe is 1080.

[Alan Roberts]

The Sky News chap at the Guild Awards do said at the NFT. Perhaps I extrapolated that to HD. Could be.