Originally Posted by cavalierlwt
Out of curiosity, since they weave the frames together after storing one field in memory--in effect giving us progressive frames?
Weaving is only an effectively de-interlacing algorithm for material that started off as 24p or 30p (and is just being carried within a 60i signal), where the information in two (or some cases three) fields is from the same point in time. This is true of much HD drama.
It isn't true of 60i native video origination - like sport. In this case 60 images are captured each second, and each field is derived from a separate point in time. You therefore can't merge the two 60i fields to create a 60p frame, as there will be motion between the two fields, and you will get combing.
30 per second?? why go through the trouble of interlacing in the first place?
1. Because 60i video delivers 60 separate images (fields) taken at 60 different points in time, giving fluid motion. What it doesn't do is delivery full vertical resolution on moving objects, because the two fields are offset by a frame-line vertically.
2. Because HD video standards, even HD, pre-date LCD and Plasma, and are designed for display on monitors without framestores, so have to have a refresh rate a lot higher than 30Hz. The 60i interlacing solves this refresh rate problem, but by interlacing 1080/60i, rather than running 540/60p, you get significant spatial resolution improvements in the analogue domain.
If nothing is lost/gained, why not just flat out go with 1920x1080p@30fps?
Because not everything is shot 1080/30p (in fact not much is)
1080/60i is used for sports and entertainment shows - with the fluid motion that 60i native capture delivers - you have to remember that 60i can carry 60 different images per second, not the 30 images that some people assume. You have to remember that the two fields in an interlaced frame don't have to come from the same picture taken at the same point in time.
(One crude way of thinking about 60i origination is to imagine a 1080/60p camera shooting the image, but only outputting alternate lines for one frame, and then the outputting the other lines for the next frame. In reality some more processing is done than this to avoid flicker)
Similarly if you only broadcast 30p - you'd have problems showing 24p source material - like almost all drama and movies. 24p converts neatly to 60i by showing one 24p frame for 3 60i fields, but the next for only 2. If you tried this trick at the frame rate you'd have far more juddery repetition.
You have to remember that the 1080/60i standard is over 30 years old in heritage...
I have a tough time wrapping my head around 1080i not being a compromise of some sort. I'm told they use approximately the same bandwidth, so more bandwidth+more resolution? Something has to give.
Interlacing is a compromise - but one we've been working with since 1936 at least (when the BBC launched their 405/50i interlaced system - which ran until 1985 over here!)
You have to stop thinking of 60i interlaced systems just in terms of 30 frames per second, and think more of 60 fields per second.
What interlacing does is trade-off resolution for motion.
1080/60i signals can deliver high vertical resolution (not 1080 lines but more than 540 lines ) on static and slow moving images, (where the full vertical detail can be built up across two fields, with little movement between them), but not on fast moving images (where there is a lot of movement between fields), where it drops to closer to a 540/60p system.
However 1080/60i delivers twice as many images per second as 1080/30p, so delivers more fluid motion when using a 1080/60i source.
Effectively interlacing allows you to have some of the benefits of 1080/30p (vertical resolution) and some of the benefits of 540/60p (fluid motion) in the same system, however it does so at the expense of complexity of decoding.
1080/60i, 1080/30p and 540/60p are all effectively systems with the same line rate and thus the same bandwith.
In an ideal world, of course, we'd use 1080/60p - but this requires twice the bandwith of the above three formats.
720/60p is, of course, in use, but it manages to operate a 60p system only by reducing the horizontal and vertical resolution to reduce the bandwith.
Then again I haven't figured out the drive for 60 fps. I figured 48 would be a better goal, 48 fps for sports, and just frame doubling for 24fps movies. I'm sure there's a good reason, it's just one of the many things to learn about all this still.
Well in Europe we have 1080/50i (most) and 720/50p (not much) and shoot drama and film at 25p. We show 24p material as 25p and live with the speed change.
However 50Hz and even-more-so 48Hz refresh rates are much more noticably flickery than 60Hz displays, particularly in peripheral vision (i.e. the corner of your eye) In the 50Hz world many TVs double the refresh rate to 100Hz to combat this, but the processing required to do this is not good for picture quality.
Some have suggested running at 72Hz or 75Hz (allowing you to frame triple film, and capture fluid motion at even higher frame rates) - however the bandwith required for such an increase in production frame/field rates probably precludes it. (It would also not be a great format for display of existing 50i and 60i material)