50fps capture and how to display it.

[madhatter300871]

At any given moment in time is a PAL TV only ever showing a video field of 288 lines ? 2 such fields of course make up the 576 line frame.

To put it another way, if it was possible to count the lines of video on a paused TV would I only be able to count 288 lines as the TV can only ever display 1 field at a time ? (This is a theoretical question..... I know I cant count them !!)

The reason I ask is to help me with my continual de interlacing troubles. If I capture 50fps at only 288 lines (1 field) then surely this doesnt do any field eliminating, it simply captures each field but doesnt attempt to make them into a complete 576 line frame.

I then play back the video on the PC but can not acheive the same result as seen on the TV. The picture "bobs", the edges shimmer if you like.

I have read alot of info on PAL TV (the analogue TV section here is great) but can someone please clarify if the TV is infact only ever showing one 288 line field at any given time, or wether it is infact showing 2 fields at a time making up a 576 line frame.

If its only showing one filed, then why doesnt it bob up and down like it does on the PC monitor ? Cany I play it back on the PC monitor 1 field at a time, 50 fields per second, but have each feild display with a gap between each line (presumbaly just how the TV does it.)

I have a strong suspicion I havnt really made this question very clear ........ but I hope you will be able to see what Im getting at and offer your thoughts.

Thanks.

Chris.

[foxyshadis]

A TV plays them like this:

Field 1:
ooooooooooooooo
xxxxxxxxxxxxxxxxxxx
ooooooooooooooo
xxxxxxxxxxxxxxxxxxx

Field 2:
ooooooooooooooooo
vvvvvvvvvvvvvvv
ooooooooooooooooo
vvvvvvvvvvvvvvv

Field 3:
uuuuuuuuuuuuuuu
vvvvvvvvvvvvvvvvv
uuuuuuuuuuuuuuu
vvvvvvvvvvvvvvvvv

Field 4:
uuuuuuuuuuuuuuuuu
xxxxxxxxxxxxxxx
uuuuuuuuuuuuuuuuu
xxxxxxxxxxxxxxx

So you can see where the bobbing comes from when you try to play every field at the same line. Televisions have enough softness, fade, and bloom that they can mask the effect of the shimmer, whereas PC monitors are so much sharper & faster and consequently it's unavoidable. There are many ways to make it play back properly, but it depends on how you're playing it now. The effect of avisynth's bob on a woven stream (fields mixed into a frame) will have the correct offsets applied, as will the other dumb and smart bobs available. Edges will always shimmer to some degree, though, because each field is in a different spatial and temporal location than the neighbors, so filling in the gaps is educated guesswork. That's why deinterlacing so hard when IVTC is so simple.

Or you can just doubleweave, it doesn't look as good as bobbing but at least it looks a little smoother and it's very fast.

[Guest]

At any given moment in time is a PAL TV only ever showing a video field of 288 lines ? 2 such fields of course make up the 576 line frame. If you have a CRT then you are showing only one pixel "at a time" (or approximately so), because the electron gun is a spot that scans across horizontally and vertically. All the even lines are done and then all the odd lines are done.

I have read alot of info on PAL TV (the analogue TV section here is great) but can someone please clarify if the TV is infact only ever showing one 288 line field at any given time, or wether it is infact showing 2 fields at a time making up a 576 line frame. A digital TV in progressive mode shows both fields at once. A CRT shows a spot at once; it alternates scanning the fields. As foxyshadis says, the phosphor has a delay that nicely integrates things over time.

Can I play it back on the PC monitor 1 field at a time, 50 fields per second, but have each fied display with a gap between each line (presumbaly just how the TV does it.) Long ago I made a filter to do that and it worked when displayed as uncompressed RGB, but the problem was that to encode it in any well-known encoder caused a nightmare. The bitrate went through the roof and only one specific DV encoder I had made anything playable out of it. Of course, the "gaps" are actually black lines. You might as well just bob it with a good bobber.

[LoRd_MuldeR]

There is one thing about interlacing I still don't understand. I understand very well why displaying interlaced material on a progressive screen doesn't work well and produces the well known "combing" artifacts (assuming the fields are simply weaved). What makes me wonder is: Why does progressive video look 100% fine when displayed on an interlaced display, e.g. a CRT TV screen??? I'd expect that to cause similar problems as throwing interlaced material on a progressive screen. But that is obviously not the case! A lot of stuff I capture from analog TV turns out to be progressive (progressive with segmented frames), although it looks absolutely "normal" on my CRT TV. Or in other words: While the human eye can easily distinguish progressive form interlaced/weaved material on a progressive display (PC screen), there seems to be no visible difference on an interlaced display (CRT TV screen). This leads to the most important question: Why is interlaced material broadcasted at all, when progressive material looks fine on both types of display, progressive and interlaced screens???

[Guest]

I'd expect that to cause similar problems as throwing interlaced material on a progressive screen. Why would you expect that? Suppose you have a vertical line that moves between two fields. When weaved you have:

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But for a progressive frame there is no movement between the fields so you see this:

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Interlaced material has twice as much temporal resolution as progressive material, i.e., it is "smoother", because you can send twice as many different pictures per second. This is good for fast action such as sports.

[LoRd_MuldeR]

Why would you expect that? Suppose you have a vertical line that moves between two fields.
Hmm yes. But a progressive frame won't be displayed as a "complete" frame on an interlaced screen. Instead first the odd lines and then the even lines (or vice versa) will be displayed. So you never see the whole progressive frame, instead you see two fields (displayed one after another) that actually refer to the same frame! While the first part of the frame is displayed, the second one isn't there yet. And while the second part is displayed, the first part is already fading. Or in other words: There is no motion between the first and the second field (as they refer to the same frame), but there is motion between the second and the third field (the third field refers to a new frame) and again no motion between the third and the forth frame. And so on. So you get: Motion -> No Motion -> Motion -> No Motion -> Motion.... Doesn't that necessarily lead to horribly stuttering video? Well, obviously it doesn't, but I don't really understand why...
Interlaced material has twice as much temporal resolution as progressive material, i.e., it is "smoother", because you can send twice as many different pictures per second. This is good for fast action such as sports.
Good point. But wouldn't that mean that progressive video displayed on an interlaced screen looks significant less smooth than interlaced content, as half of the temporal information is missing? For some reason that difference seems to be negligible in practice, as the progressive stuff I captured from analog TV doesn't look noticeable less smooth than the interlaced stuff (when displayed on the CRT TV screen). Maybe the quality of the CRT TV screen (flicker, etc.) is simply so low that this difference becomes negligible?

[foxyshadis]

Because the first field is fading to black at the time the second sweeps by, your mind attempts to reconstruct the area by combining the old with the surrounding new. Some psychological trick of bright vs dark, like lumimasking. Plus your head can adjust to anticipating forward movement vs stationary for progressive. I'll look up the research, but I think some of it is only accessible from school.

There are new LCDs that have flashing backlights to create the same phenomenon, but there it's to make you ignore ghosting from lagging response times.

[2Bdecided]

Hmm yes. But a progressive frame won't be displayed as a "complete" frame on an interlaced screen. Instead first the odd lines and then the even lines (or vice versa) will be displayed. So you never see the whole progressive frame, instead you see two fields (displayed one after another) that actually refer to the same frame! While the first part of the frame is displayed, the second one isn't there yet. And while the second part is displayed, the first part is already fading. Or in other words: There is no motion between the first and the second field (as they refer to the same frame), but there is motion between the second and the third field (the third field refers to a new frame) and again no motion between the third and the forth frame. And so on. So you get: Motion -> No Motion -> Motion -> No Motion -> Motion.... Doesn't that necessarily lead to horribly stuttering video? Well, obviously it doesn't, but I don't really understand why...
It does! It leads to horrible stuttering, double images, and combing. You just have to have a high enough quality source, sit close enough, and have enough movement between frames that your eye can track.

The stuttering is inevitable because the frame rate is so low (25p) - the double images are inevitable if you double 25p (to 50i) because you show (part of) each image twice - the combing can be avoided if you use 50p instead of 50i and isn't very visible from far enough away anyway.

Good point. But wouldn't that mean that progressive video displayed on an interlaced screen looks significant less smooth than interlaced content, as half of the temporal information is missing? For some reason that difference seems to be negligible in practice, as the progressive stuff I captured from analog TV doesn't look noticeable less smooth than the interlaced stuff (when displayed on the CRT TV screen). Maybe the quality of the CRT TV screen (flicker, etc.) is simply so low that this difference becomes negligible?

The 25p / 50i difference is obvious!

My Wife can't hear if one of the speakers is disconnected from the stereo. She has measurably excellent hearing - but she just doesn't notice.

Maybe your eyes are like her ears?

(Not trying to be insulting - just suggesting one reason you can't see something that's obvious to many people - there could be other reasons, e.g .equipment)

Cheers,
David.

[madhatter300871]

Thanks for all the info.

Just my two pence worth. If you ploay a progressive DVD source (i.e. a DVD) into a PAL CRT TV in the UK, then DVD player itself does the interlacing. PAL TVs can ONLY display 50 fields per second interlaced, I would think trying to input anything else into it would cause all manner of unsightly consequences.

When I capture film to be re-edited, i simply cap[ture 768x576, huffyuv. Then I check wether its truly interlaced, or wether its just progressive split into 2 frames (interlaced) for UK transmission. Then I apply my filters, resize, deinterlace... etc etc.

Its for when I want to record material for instand watching..... using my PC as a VCR if you like, and dont want to do anything other than watch it. Nopt too bothered wether its strictly speaking the best picture I can acheive, just that i deal with deinterlacing - bobbing - weaving correctly so I can at least enjoy watching the programme.

So.....whats a good way to bob or double qweave on playback....would love to hear your suggestions, think I'm very close to putting this one to bed.

Many thanks guys.

Chris.

[2Bdecided]

Many software players will bob content for you.

e.g. in VLC go to video, deinterlace, bob. Easy as that!

Not sure about weaving 2x288-line fields back together though - a simple AVIsynth script? Don't know which standard players will play them though - I don't think you want to watch everything through virtualdub!

Cheers,
David.

[foxyshadis]

You can watch an avisynth script in any directshow player, actually, as well as having a filter in ffdshow where you can customize your own. There's a few bobbers built into ffdshow as well (along with dscaler's if you have that), but tdeint with tryweave=true is better/slower than all of those. Otherwise yadif built into mplayer/vlc is quite decent and fast.