Re: Why interlaced HDTV?

On Tue, 23 Aug 2005 09:23:44 +0100, Kennedy McEwen
<rkm@xxxxxxxxxxxxxxxxxx> wrote:

>No it isn't, it is something you filter out very quickly and the size of
>the screen has nothing to do with it - how many kids from the 60's and
>70's sat a couple of feet from their 26" CRTs watching TV while their
>parents told them "don't sit too close, johnny, your eyes will go
>square!"? None of them saw any flicker, yet the angular screen size was
>far bigger than anything viewed at normal distance.

Well I was a kid from the 70's with exactly this experience but I must
say that I've always noticed mild flicker on colour TVs since we got
our first in around 1980. Flicker is definitely more noticeable as
screen sizes get larger, 32" 50Hz sets are unwatchable on bright
scenes for me which I guess is why most are 100 Hz. We had a 21" 50Hz
4:3 set for years which was OK but when I upgraded to a 28" 16:9 50 Hz
set I ended up watching most TV on the portable in the bedroom and
ended up replacing it with an LCD almost immediately.

>Ever had american visitors to your home for a week or two? At first
>they complain that TV flickers in this country, but by the time they go
>home they are marvelling about the picture quality.

After watching an LCD i'm now like this even with some 100 Hz TVs. Yes
you adjust after a while with smaller screens but I've never been able
to cope with anything over 28".

>>Hence the number of 100 Hz TVs on the market and all the
>>artifacts that their frame stores cause.
>
>Marketing. Looks good in the showroom when you have a bank of TVs
>stretching out to the extreme periphery of your vision where you have
>most sensitivity to flicker because you haven't regularly watched it and
>learned to filter it out.

I don't think it's just marketing. Some of the 100 Hz TVs have
horrible frame store artifacts (a friend has a Phillips set where you
can actually see the interlace "jaggies" on any moving image), but the
reduced flicker is noticeable.

>What difference does that make - it is just manipulation of digital data
>so it doesn't make a ha'penny difference if it is implemented in a
>£50,000 Quantel box or in a £5 chip.

The real point is that with a move to progressive production and
progressive display, we don't need interlace in the middle ruining
perfectly good pictures. Any legacy conversion that needs to be done
can be done as well or better by the broadcaster that inside the TV.

>>However the adoption of a progressive
>>broadcast system allows the migration to all progressive production
>>over time eliminating this problem completely.
>>
>But the problem doesn't need to be present - there is no reason why a
>flat panel progressive screen cannot display an interlace signal
>accurately. There is nothing intrinsically superior about a progressive
>source which has the same overall bandwidth as the alternate interlace
>system, and generally it has inferior resolution, as demonstrated by the
>1080i/720p debate.

A progressive source will in general digitally compress better than an
equivalent interlaced source in a given bandwidth. In addition, the
move to progressive production allows more fluid use of features such
as high speed/slow motion, frame accurate editing etc. Coupled with
the fact that 720p has double the temporal resolution of 1080i making
it the only real choice for any material with movement (sports, "arty"
camerawork, action movies, pop videos etc) this in my mind is a pretty
convincing argument in favor of 720p (..and of course I must add that
1080p would be better still).


>>But nobodies going to be using an interlaced screen for HD. Even an HD
>>CRT should be capable of 50 or 100 Hz progressive refresh without
>>interlace and I'd be very surprised to see an HD CRT set in Dixons etc
>>in a years time.
>>
>That is exactly the point: nobody would need them - if high quality
>backwards compatibility were delivered. Whilst that is certainly
>possible, indeed just as simple to achieve, it isn't what most flat
>panels provide. Consequently the push for progressive standards
>alienates about half a century of existing video heritage.

I don't follow this. The push for progressive standards is as much
about moving forward with technology as the move to high definition.

This is a new system based on higher resolution displays and more
efficient codecs. Since the existing systems are sadly lacking in the
temporal resolution department (25 fps really is very poor) it seems
logical to fix this at the same time. This no more alienates existing
video heritage than any other part of HD. HD may consign much of what
you and I are familiar with to a museum, but that's progress for you.

>>Progressive production and display is the future. To tie our HD
>>broadcast standards to the legacy interlace is even more crazy than
>>tieing DAB to Layer 2.
>>
>I would agree if we were discussing a comparison of 1080p versus 1080i,
>but the option is 720p versus 1080i, so what you are calling for is to
>tie our HD broadcast standard to little more than the legacy static
>resolution of the interlaced system we have had for the past half
>century, when we could be quadrupling it! So much for the term
>"progressive" - "marginally incremental" would be more appropriate!

I really agree that we should be looking to 1080p but as production is
moving in that area anyway it would appear logical to adopt the
broadcast standard that's the closest technical match.

With 720p you have a down sampling of the source to from 1080 to 720
at the broadcaster and then this is maintained all the way through the
transmission system to the progressive TV.

With 1080i you first have to throw away half the temporal information,
transmit this at 1080i and then the domestic TV has to reconstruct a
1080 line progressive frame in memory from two 540 line halves of the
interlace each taken 1/50 of a second apart. Since these frames may
have completely different information (depending on the amount of
movement etc) this then leads to an apparent quality reduction. Add to
that the inefficiencies of digitally compressing this system and for
any given bit rate 1080i will look on average the same or even worse
than 720p, except on medium to fast movement where 1080i will look
significantly worse.

>As I say, that isn't my experience. 1080i is superb when displayed
>correctly - which is often not even attempted because it requires a much
>higher resolution screen to do it properly.

The HD I've seen has been almost exclusively on high (and low) end
domestic equipment - native 1080 and 768 (why??) progressive LCD and
Plasma screens. On this equipment my experience has been that
progressive material appears to have the advantage.

It's interesting what you say about the progressive DMD system and I'm
sure you're right that it and LCD/Plasma could be made to display a
native interlace cleanly. But with all the above advantages of a
progressive source would this not be the equivalent of making a wonky
road for a square wheeled car? ;-)

Rgds
Jonathan

.

Relevant Pages

  • Re: Why interlaced HDTV?
    ... if you ignore the fact that interlace is already 50% data ... >>the fact that 720p has double the temporal resolution of 1080i ... >additional information in that frame time, ... >>you and I are familiar with to a museum, but that's progress for you. ...
    (uk.tech.digital-tv)
  • Re: Why interlaced HDTV?
    ... The fact is that 1080i provides additional information in that frame time, in the form of significantly enhanced horizontal resolution and additional vertical resolution through the interlace. ... temporal resolution department it seems logical to fix this at the same time. ... If it means that it cannot be rebroadcast without introduction of artefacts than it is not progress, ...
    (uk.tech.digital-tv)
  • Re: Displaying pdfs and size
    ... resolution independence, 300ppi screens and beyond. ... it still a sort of work in progress. ... There's a screeenshot of window title bars not scaling right at but you can see that SL is generally doing better than L. ... that extra screen space looks extremely useful for a lot of the ...
    (uk.comp.sys.mac)
  • Re: Why interlaced HDTV?
    ... elimination of these screens. ... Again, the latency is consistent on all pixels, however the hold time on each pixel is now doubled, which is the analogous to a very long persistence CRT phosphor. ... The normal interlace display on a CRT, where each pixel is only present for a very short period of the frame time is actually temporally undersampling - which artificially exaggerates the limitations of interlace. ... There is nothing intrinsically superior about a progressive source which has the same overall bandwidth as the alternate interlace system, and generally it has inferior resolution, as demonstrated by the ...
    (uk.tech.digital-tv)
  • Re: Why interlaced HDTV?
    ... A progressive display can be configured to display interlaced material perfectly correctly. ... That particular trade-off results in 50Hz flicker, but no worse than on a conventional interlaced CRT. ... However there is a third trade-off which maintains the flicker elimination of these screens. ... You can display interlaced images on a progressive screen correctly, and I suspect that the reason it isn't done has more to do with manufacturers agendas than a limitation of interlace per se. ...
    (uk.tech.digital-tv)