Re: Why interlaced HDTV?

Roderick Stewart wrote:

> I think it is the use of the word "progressive" which is used because it
> "sounds better", because all it really means is "non-interlaced", and without
> increasing the available bandwidth, the only "progress" it offers is towards
> a less realistic portrayal of moving objects.

Don't you remember the long and tedious thread we had discussing 720p50
(50 1280x720 progressive frames per second) vs 1080i25 (25 1920x1080
interlaced frames, i.e. 50 1920x540 fields per second) vs 1080p50 (50
1920x1080 progressive frames per second)?

What I got from that discussion was that, if codecs were genuinely
intelligent, then interlacing would be redundant, but current codecs
are _not_ so intelligent as to render interlacing redundant.

1080i25 has a slightly higher _raw_ datarate than 720p50. 1080p50 has
roughly double the _raw_ datarate of either.

Going from 1080p50 to 1080i25 is trivial, while "deinterlacing" 1080i25
to 1080p50 (e.g. for viewing via a progressive display) is impossible
to perfect, but possible to do very well. It's often done very poorly!

Currently 720p50 requires a slightly lower bitrate (MPEG-2) than
1080i25 to encode at a given quality wrt the original, while 1080p50
requires nearly double the bitrate. However, at a given (high-ish)
bitrate, 1080i25 looks better than 720p50 for much content.

When the source is film or progressive 25 frames per second material,
1080i25 will actually carry a 1080p25 signal; this is trivial to
deinterlace and will look better than 720p50 (which will be carrying a
720p25 signal!) because the resolution is double. However, the bitrate
required to encode 720p25 well will be about half that for 1080p25.

It would be ideal to dump interlacing, and to allow video codecs to use
it internally on all or part of the image if it were beneficial - but
video codecs don't yet do this, so for now interlacing buys a useful
bandwidth advantage, but comes with some disadvantages, not least that
most progressive displays don't deinterlace very well!

FWIW 50Hz progressive content displayed on a CRT can flicker badly
(depending on the phosphors) unless it's interlaced to 100Hz, and
there's an argument for using more than 50fps anyway to reduce flicker
on CRTs and motion blur on LCDs.

Cheers,
David.

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