Re: Why interlaced HDTV?

In article <fj8lg1h8uqb55cii1r7ukcs8m1jtjb12gq@xxxxxxx>, JC <nospam@xxxxxxxxxxx> writes 

But conventional 50 Hz CRT flicker is horrible, especially on larger
screens.

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.

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.

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.


However there is a third trade-off which maintains the flicker
elimination of these screens.  Instead of blanking the lines which are
not present in each field, just repeat the data from the previous field.
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.  Consequently this time you are trading some
motion blur to eliminate both flicker and motion tearing.


This is effectively what any good broadcast interlace to progressive
converter would do

Would do, but don't, hence the motion tearing on most progressive screens when fed an interlaced source - that is, to my mind, the worst option, but for some reason it is what most of them do.

and as I said, I'd rather it was done by the
broadcaster than a sub £ 5 chip in my TV.

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 blurring artifacts
within the picture reduce the visable resoloution to, it would appear,
similar to 720p levels.

No it doesn't - the blurring is simply the maximum temporal bandwidth capable of the interlaced structure, with minimal temporal aliasing. 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.

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.

manufacturers agendas than a limitation of interlace per se.  You can't
display progressive images on an interlaced screen though, without
artefacts or throwing half the information away, which defeats any
advantage the system has.


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. 

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 was looking at some 720p a couple of days ago, and wasn't overly
impressed.  Yes, better than 625i, but not dramatically so.  When I saw
1080i a while ago, I was completely bowled over by it.  That extra
horizontal resolution isn't worthless, you know.  Equipment was
different of course, and perhaps my expectations have changed with time,
but that was my impression viewing both options.  As always, YMMV.


Was the 1080i and 720p displayed on a progressive or interlaced
screen?

1080i was on a projection DMD system at the TI plant in Dallas where they are made. That used a progressive DMD but driven using an algorithm similar to those I was explaining. The main difference is that the brightness of each pixel in a DMD is achieved by time multiplexing throughout the available frame period, but the phasing of the interlaced fields was accurate - that was the point of the TI demo.

720p was on 720p compatible plasmas and LCDs - lots of different types.

I have to say some of the HD material I've seen has been a bit
of a disappointment but still a worthwhile improvement. One thing I
can say is that in my experience, 720p beats 1080i hands down for what
I would call "normal" TV material.

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.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
.

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