Given that LCDs and Plasmas are progressive, by their nature they cannot show interlaced video, so they must deinterlace the content (or the STB box does it)- what I am unclear of is whether they deinterlace to 25 frames or 50 frames per second- if it is only 25 why do they not just transmit at 1080p 25?

LCD and Plasma are neither Interlaced nor Progressive, they don't work in that way as they aren't scanned.

The 1080i picture is de-interlaced (built up in memory), and then transferred directly to the screen. The original is 25 frames per second, so the result is 25 frames per second. A 100Hz set creates frames in between the existing ones, giving 50 frames per second. A 200Hz set does the process again, givng 100 frames per second.

The 'claimed' 600Hz Plasma sets (Panasonic and LG) are just an advertising con, and refer to something completely different - but are only 100Hz (or even 50Hz) sets.

Now seeing as the program is 25fps, whats the point in 100 or even 200hz sets, surely they are just showing the same frames again.

I can see the benefit for CRTs as low refresh rates make your eyes bleed but Ive seen no difference on LCD monitors for computers.

I think it is a tad pedantic to say it's not progressive just because the display isnt scanned.

Also I don't think after de-interlacing it is 25 fps.

If you are talking about 50i video then the original is not 25fps, it is 50 fields per second with motion between the fields.

My video processor certainly outputs 50 frames per second to the plasma.

Also, if you think about it, if it turned 50i fields per second into 25p frames per second wouldn't you get the motion judder on pans and fast motion that plagues film and 25 psf video?

That would certainly make the world cup look yuk!

Of course if the source is film or 25 psf video then the de-interlacing should recreate the original 25 progressive frames per second. A display may repeat those frames to reduce flicker, or as some do interpolate extra frames in between.

They interpolate the video data in adjacent frames and invent the extra frames. Personally I think the results look decidely odd and turning off this feature results in a better picture.For what it's worth which agree. Quote from current edition "It can make the pictures look so unnatural that our viewing experts quickly switch it off

Don't think you're right there Nigel, Wikipedia states:

"Progressive scan (also known as: P-Scan) is used for most cathode ray tube (CRT) computer monitors, all LCD computer monitors, and most HDTVs as the display resolutions are progressive by nature"


If that is the case why don't they just transmit at 1080p 25 in the first place? I'm sure I read somewhere that they don't do that as the motion would seem too jerky for sports, but I can't remember where I read this- I don't really understand why this could be though, if the STB or display just reduces it to 25p, hence my original question.

Yep, thats spot on, motion is the issue.

As I said above 50i has motion between the fields so it looks smoother. Reducing it to 25 p frames /sec would make it more jerky.

No, as I said above, they 'create' frames in between each of the two 'real' frames - interpolating from the previous and next.


It's not to reduce flicker (as there is none), it's supposedly to reduce motion blur - I've no idea if it helps or not. However, the 200Hz Sony sets have great pictures - but I suspect it's because it's a great TV, and not just because it's 200Hz.

As with everything, check them out yourself, and see what YOU personally prefer. Never buy just from numbers.

I suspect they just mean it's not interlaced, which isn't the same thing at all - there's no need for the display to be progressive, it would be adding limitations for no advantage.


Probably because the technology wasn't there when the standard was set?

However, transmitting 1080i50 from a 1080P source can easily be no different at all to a 1080P25 broadcast.

It IS only 25 fps for LCDs/plasmas (the fields are gone by then) and it IS jerky whenever the programme makers pan or zoom too fast or allow motion to be too fast, it sometimes drives me mad. This is in addition to any LCD-specific issues (but I have a plasma). Well made programmes will not allow movement/pan/scans to reach a speed where jerkiness (and temporarily lowered reolution) is obvious to most people - but some do, and people have different tolerances to it.

High spatial resolution is grossly overrated and high temporal resolution (greater than 25 fps) is grossly underrated. And all because a 1080 still or slow moving picture looks "pretty"; well for anyone who hasn't yet realised it (broadcasters and programme makers!!!) TV is also about moving pictures, pans and scans - for which 50 fps 720p is better suited than jerky 25fps 1080 (or even worse, if converted from US 30fps).

Interlacing issues are lower down the scale of importance in my book (compared to low frame rate jerkiness).

That quote clearly says LCDs and Plasmas are progressive, can you prove that is wrong? Surely the point is LCDs and Plasmas cannot display interlaced fields, only frames, hence they are progressive?


Well iplayer HD transmits at 720p 25, and the jerkiness can be quite apparent.


Are we absolutely certain about this- why do they even bother transmitting interlaced at all then if it is just converted to 25 frames?

OK, look at Stephen Neal's reply on avforums here, he states that when archiving 50i material (not from film) you should archive it at 50Hz progressive, and that DVD players with progressive outputs replay 576i as 576p/50p rather than 576/25p, so..

"as to preserve the motion in 576/50i interlaced video sourced content, you have to run at 50Hz progressive not 25Hz progressive"

Does the same not apply to 1080i?

Yes you are correct.

No its not. If it were the tennis I am watching at the moment would be significantly more jerky.....and I can assure you my video processor is utputting 50 fps.

Film or 25 PSF video source material does indeed suffer from the jerkiness and have to have strictly limited pan rates to avoid it being obvious.

I am correct in that LCDs/Plasmas convert 1080i to 1080p/50?

Depends what you understand by 'progressive'?.

As I understand it interlacing sends lines 1, 3, 5 etc, progressive sends lines 1, 2, 3, and so on. For a display to be 'progressive' it would need to display those lines progressively, 1, 2, 3, 4 etc.

LCD and Plasma build up the image in memeory (from whatever source) and then 'blast' the picture to the screen effectively in a single action, it's not built up progressively, and there's no reason (or advantage) to do so.

In the case of a Plasma even more so, as each image is 'blasted; to the screen 256 times (in order to give brightness variations), or perhaps more on modern Plasma's?

Fairly obviously, the more fps the better.


Interlacing is a hangover from analogue TV, transmitting as 576i50 (as it would be called now) - it was crucial to CRT analogue TV. Presumably it's used because the technology wasn't there when HD started for 1080P25?.

No, of course it doesn't

It converts whatever is input to fit the screen, 1080P50 (or 1080i50) doesn't apply to a screen, it's just a source standard.

A 25 frame source will still be a 25 frame result, UNLESS it's a 100Hz TV (which doubles it to 50), or a 200Hz TV which quadruples it.

As far as I'm aware you need a Progressive DVD in order to provide a true P50 output?, although it's quite possible they can provide a 100Hz type interpolated output from only a P25 or i50 DVD.

but only film (sped up to 25fps) and 25 PSF video are 25 frames/sec.

50i video is 50 fields per second with the potential for motion between fields.

If 50i fields were combined into one frame 25 fps you would lose the temporal resolution of the 50i.

...and you don't!

So are you saying LCDs/Plasmas don't deinterlace, if they do, how do you get any benefit from the interlacing?

100Hz sets will add an extra frame between existing frames.

200Hz sets, especially Sony, will add an extra frame between existing frames, but then it flashes the backlight giving the effect of 200Hz, as far as I know they don't quadruple.

600Hz is poorly explained - not a con.

600Hz sub field driving should not be confused/compared with 100Hz/200Hz refresh rates or features like truemotion.

In the case of LG, the plasmas are 50Hz - the TV splits the fields into 12 sub fields (12x50 = 600) and shows these individually, no extra frames are inserted. The end result is better colour reproduction, a reduction in colour banding/dithering and less picture noise.

erm no, I'm saying the deinterlacing techniques used in modern TVs doesn't reduce the picture to 25 fps. If it did you lose the temporal resolution of 50i video sources.

OK, thanks. So do you get 50fps, something in between 25 or 50, or do you not get any real progressive frames at all?

Without any electronic jiggery pokery 1080i gives you 25fps from UK Digital transmissions just as 576i does. If 576i did not you could not watch it on a 625 line crt display. Neither could you watch a 1080i source connected to a UHF modulator using downscaled cvbs and view it using an analogue tuner on a uhf crt TV. Which is what is on my kitchen TV now with my hdr receiving BBC-HD

You could not record it either using a PAL vcr or dvdr

No, it gives you 50 fields per second, the same as 576i does. With the potental for motion between fields.

A crt dipslays the fields alternatively. It's just downscaled to 625 (576i).

Persistence of the crt phosphers and your persistance of vision give the impression its one frame.