Re: Why interlaced HDTV?

On Wed, 24 Aug 2005 12:47:34 +0100, Roderick Stewart
<rjfs@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>To display such a signal with the whole picture changing 50 times per second
>would require twice the transmission bandwidth of the equivalent interlaced
>signal, which is why interlace has been used by all broadcasters since the
>invention of television.

In the analogue world yes. With the move to digital standards it does
require more bandwidth but not double.

>Updating only one field's worth of the picture, or half the number of picture
>lines, 50 times per second (instead of all of them half as often) is quite
>sufficient to present the illusion of smooth movement as if the whole picture
>were updated this often, whereas scanning the image "progressively" only
>updates the picture information 25 times per second, similar to the update rate
>of film, which is either 24 or 25 frames per second.

Wrong. 720p is 50 FULL FRAMES per second. That's the same refresh as
the equivalent 50 Hz interlace, but with TWICE the vertical
resolution.

>In standard unadulterated television signals from television cameras the
>picture information is slightly different between the two successive *fields*
>that make up each *frame*, because the information is sampled at different
>times, this being an inevitable consequence of the way scanned tube cameras

Exactly, which is why progressive display of interlace leads to a
reduction in visible resolution.

>worked. Modern chip cameras *can* be made to integrate the light over 1/25th
>second and then make two interlaced fileds with identical pictorial
>imformation, but the result looks intolerably jerky if you do this. If you then
>also set the electronic shutter time to 1/50th second or less, it only looks as
>jerky as film, because half the action is missing, but more jerky than if you
>just left it alone.

Exactly, which is why there is a move to 1080p/720p 50 FRAME
progressive production and transmission, removing this problem and in
the process helping solve all the other problems that interlace
causes. A 1080p production can also be easily converted to interlaced
576i for legacy transmission.

>Picture update rate is not the same as the brightness "flicker rate", which can
>be anything you want it to be without requiring any change in the banddwidth
>needed to transmit the signal or the storage capacity needed to record it.
>Flicker only depends on how you design the display mechanism.

As I said above. Changing the refresh rate on any interlaced system
will always be a compromise as two time different half FIELDS have to
be merged in memory and effectively redisplayed multiple times.

Imagine taking two separate photographs at half resolution, a short
time apart of something that's moving and then trying to merge them to
get double resolution. The moving object will be in a different
position on the second field to the first. Now imagine rapidly
flicking between these two images or showing them merged, before
moving on to the next two.

With a progressive system you just reshow the same FRAME multiple
times or in the case of a memory type display such as LCD, just change
the pixels as required.

>You can eliminate flicker by displaying the image a thousand times a second or
>using a method that illuminates it continuously, and you won't see any
>variations in brightness, but if the picture information is only being updated
>every 25th of a second, moving objects will move in 25 jerky little steps every
>second instead of smoothly. 50 jerky little steps every second isn't perfect
>either, but it looks a lot better and it's what we've had for the last 70
>years, so it seems madness to throw this away for ever.

We're moving from 50 half fields per second (as you say, 25 FRAMES) to
50 full FRAMES per second. I'd hardly call a doubling of the FRAME
rate throwing something away.

Rgds
Jonathan

.

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