HDTV 1980i are we being tricked?

[Jan Marijniszoon]

Hi, I have a question...

To have a good image on a DVD (720 x 576) an average bitrate of 8/9 Mbit/s is required.

1980 x 1080 is about 4 times bigger pixelwise, so I figured that the bitrate should also be 4 times higher than with a DVD, resulting in a bitrate of about 35 Mbit/s.

But with most TS-files the average bitrate ain't higher than 16 Mbit/s which is only double the amount of a DVD-bitrate.

So what I am thinking is this...

When they have a progessive film of 24 fps, aren't they actually encoding at a resolution of 1920 x 540 and then with some kind of pulldown-trick they are making a duplicate field (which does not require extra bits) and then weave them together so that you get a 'tricked' full resolution of 1980 x 1080?

Am I right or is there something else to it that they can get away with decent quality at only double the bitrate of an average DVD.

Thanks in advance for answering my question!

[Isochroma]

Yes, it's true! You & I are being lied to by the powerful and their rotten video conspiracies...

The video is only encoded at 1920x540 for each frame (called 'fields' because of the way they are captured and displayed). Also note that video compression gets more efficient at larger frame sizes, so datarate should alway increase slower than pixel-count.

Interlace is, of course, the devil's spawn. If you need convincing, just search this forum for all the poor souls trying using various techniques to fix the evils this process has done to their videos. Every day sees a new victim, fodder for the growing horde of good, bad and moderate 'solutions'.

Since its inception, more work has been done to undo the damage it causes than would be required to overhaul the entire system and remove it many times over.

[DrP]

There sure is. Assuming the content can make use of > 16Mbit/sec in the first place and not every single GOP from a piece of video will, then a bit of subtle edge filtering can significantly reduce the amount of bits required to encode the video with not too many people noticing the difference. There are lots and lots of tricks like that to lower the bitrate required to encode a given piece of video that most viewers won't notice, but a HD-o-phile that sits 1 metre away from their set will notice.

Unless the video is particularly complex, 8-9Mbit/sec for 720 x 576 is pretty high.

[Jan Marijniszoon]

Thanks for replying guys!

You talk about 'truly' interlaced material.

What I was wondering about if they also perform this 'duplicate-field-weave'-trick on progressive movies that are originally in 23.976 fps.

I know they make it 29.970 fps with the pulldown flag, but you can make that undone with "FORCE FILM" in DGIndex for instance.

The reason I ask this is that they broadcast these movies as 1080i but they seem to be entirely progressive; there is no combing effect. I was under the assumption that if two fields are identical and you weave them together you get a higher resolution without the combing effect.

If not, why don't they call it 1080p instead of 1080i ?

[DrP]

Because its not progressive encoded. Its interlaced encoded, even though the source is progressive and both fields come from alternate lines within a single progressive source frame. If you took the interlaced video as broadcast and simply ignored the flags and treated it as progressive, you'd be asking for colour upsampling issues.

[Jan Marijniszoon]

Sorry if I am very stupid but I still don't understand.

My source was 29.97 fps at 1920 x 1080

Then I used the option "FORCE FILM" in DGIndex and I got a nice 1920 x 1080 progressive movie at 23.976 fps. I didn't notice anything about a color problem?

[FreQi]

I always thought of Interlaced video as a sort of hack to get around a technological road block. Because bandwidth was an issue, it was hard to deliver a full framed image to a screen 60 times a second. So instead, they'd show half the image in one pass, the other half in the second pass, but only do it 30 times a second. 30 frames x 2 passes = 60fps (Why 60fps? Because the electric current to our houses runs at 60Hz, so the current used to draw the screen flips from + to - 60 times a second). Now that equipment is capable of delivering full progressive frames at 60fps, interlaced video is more of a tricky way to deliver that frame rate of 60 in half the number of frames because Native 1080i captures one image in every field.

You get two images mashed together in one frame where one image is in the even lines, the other is in the odd lines. 1920x1080i at 30fps is effectively 1920x540p at 60fps. I don't know about you but that is what convinced me that 1280x720p at 60fps is just plain better.

Things like FORCE FILM use the knowledge that some of these 30fps interlaced images are actually stepped up to that frame rate from recordings that are actually made at 24fps, which is what movies and such are filmmed at. It goes from 24 to 30 by repeating a couple fields in a known pattern called Telecine, and FORCE FILM undoes this. You should be aware that using force film on Native 1080i will produce jerky video because you loose real fields/frames of video. Native 1080i would be stuff like most live broadcasts like the Tonight Show, sports events and the like. Dramas, sitcoms and other produced video are usualy film (24fps) Telecined to 30fps.

Telecined 720p on the other hand is actually 24fps telecined to 30fps, then every frame is doubled...typically. Native 720p is just 60 frames of 1280x720 images in a line. No image mashing or weird decoding necessary.

Oh, and 60fps is really 59.96 and 30 is really 29.97, so when you force filem or IVTC you get 23.976. That frame rate mess is because they had to steal some bandwidth from somewhere to include the color information when technology got upgraded from black and white, so they just started sending slightly fewer frames.

[davidhildreth]

To have a good image on a DVD (720 x 576) an average bitrate of 8/9 Mbit/s is required.

lawl, your lucky to get 4mbps on a comercial dvd

[Kika]

1920x1080i at 30fps is effectively 1920x540p at 60fps

Only if the source is interlaced video. If the source is FILM, you get 1920x1080 at 30fps.

[Doom9]

If the source is FILM, you get 1920x1080 at 30fps.Well.. it's 1920x1080 at 23.976 really.. plus pulldown flags.. the exact same as you get with a FILM DVD, except for the resolution of course.
And 720p@60 with FILM sources make no sense.. it's just a waste of bandwidth.
The whole thing makes one wonder though why on earth they kept all those references to the analog area. With digital technology, we could transmit anything created on FILM at 24 fps at whichever resolution best fits.. then have the settop box scale it to whatever a crappy analog TV needs or keep it in its native format for any digital output.
Long gone is the time when they needed to tie screen refresh rates to AC cycles.. today, any plasma and lcd tv, yes even CRT tubes, can easily support whatever refresh rate you like. After all, we've been doing that for PCs for decades already.. imagine anyone running a CRT at 60 Hz.. you can get a headache just thinking about it.

Then I used the option "FORCE FILM" in DGIndex and I got a nice 1920 x 1080 progressive movie at 23.976 fps.That means your broadcast was from a FILM source and was telecined.. it's the same with DVDs.. they're all "480i @ 29.97" in NTSC land.. but most of them are actually from 24 fps progressive sources and just telecined.. you can restore them via force film just as you can restore those HD broadcasts.

[Kika]

Well.. it's 1920x1080 at 23.976 really.. plus pulldown flags.. the exact same as you get with a FILM DVD, except for the resolution of course.

Your right - on every thing you wrote. On HDTV, there's no need for things like pulldown flags or framerates at 23.976, 25 or 29.97 fps.
I wonder why "they" didn't take the chance to make TV-Bradcast easier for anyone.

[Jan Marijniszoon]

That means your broadcast was from a FILM source and was telecined.. it's the same with DVDs.. they're all "480i @ 29.97" in NTSC land.. but most of them are actually from 24 fps progressive sources and just telecined.. you can restore them via force film just as you can restore those HD broadcasts.

Well then I am still impressed that a progessive image of 1920 x 1080 only needs an average of around 16 Mbit/s to get a good image.

[Zag]

lawl, your lucky to get 4mbps on a comercial dvd

Exactly, even those so called Superbit versions only give you about 5mbps. Superbit my a$$.