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Old 19th September 2008, 03:50   #1  |  Link
olikl
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Capturing HDTV channels - 1080i?

Hi,
I have started to capture some movies on Sky Movies HD - I am wondering about the framerate and resolution.
Everyone is telling me they are broadcasting at 1080i - Why do my recordings show me 1920x1080 @ 25fps then?

I always thought 1080i should be 1920x540 @ 50fps...

So how should the workflow look like than? Initially I thought about deinterlacing the 50fps to 25fps and then going further... But don't quiet know what to do with the 1920x0180@25... Is it already deinterlaced? If so, why is that?!

Thanks!
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Old 19th September 2008, 09:47   #2  |  Link
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No, 1080i means that frame 1080 lines high is created from fields (at least two fields) so 1080i25 and 1080p25 are identical (with exception about interlaced/progressive) and means that 25 frames per second with 1080 lines is displayed and this frame can be build from at least 2 fields or is 1 frame, 1080p25 can be displayed as 1080i25 without problems (with exception for some details e.g. chroma sampling, but this is far from your question).

workflow is simple - You must detect type of source (ie interlaced vs progressive - ie did any temporal difference between two field of one frame exist - hint use AviSynth command: SeparateFields() )

Last edited by pandy; 19th September 2008 at 09:51.
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Old 24th September 2008, 03:39   #3  |  Link
olikl
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Hi,
thanks for your answer.

I separated the fields now with an .avs file and loaded it into VirtualDub. It was 50 fps now with 1920x540. I looked at each frame and I could not see any interlacing, i.e. I saw two nearly identical pictures followed by two other nearly identical pictures etc., I hope you get what I mean - Two pictures that happened at the same time followed by two pictures that happended 1/25 second later etc.

My conclusion is that this movie was broadcasted with 25 full 1920x1080 pictures each second. All internet sources say Sky is broadcasting in 1080i. Can anyone help me out here?

Thanks!
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Old 24th September 2008, 03:44   #4  |  Link
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If it is progressive content like a film, then even if it is sent at 1080i you will not see movement every field. On video content like a sports show you will.
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Old 25th September 2008, 02:41   #5  |  Link
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Ok,
thanks for your answer. Can I upload a 80MB sample .ts file so that you can have a look at it?
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Old 25th September 2008, 18:16   #6  |  Link
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Sky Movies HD is an encrypted channel.

As far as I'm aware, it's broadcast in MPEG-4 AVC at 1440x1080 pixels with (4:3 or there-abouts) aspect ratio signalling using 50 interlaced fields per second.

What equipment are you using to capture these signals?
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Old 26th September 2008, 12:22   #7  |  Link
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The "Sky HD" movie channels generally are 1920 x 1088 25fps with the "flag" set to interlaced, the film content is actually normally progressive, the adverts tend to vary, the other non movie channels within the "HD" packages vary in resolution some at 1440x1088 with others at the full resolution, they also vary with progressive and interlaced content, what the flags state isnt always the case, with regards to capture any dvb-s2 capable card & dish pointing at the right orbital slot, assuming the person is in the footprint along with a "suitable" cam and valid subscription card would work, more info on dvb specific forums.
To be honest it's not rocket science these days, the information is available on most sat forums.

Last edited by Flaarn; 26th September 2008 at 12:26.
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Old 28th September 2008, 01:20   #8  |  Link
olikl
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Thanks again for your answers.
So why does everybody state "We are very far away from FULL HD broadcasting (1920x1080p)", "1080i is the European standard" and so on? Sky shows its movies in native 1920x1080p as far as I understand it... So I don't have to deinterlace before editing the files?

Concerning capturing:
I have indeed two ways to capture it (before anyone gets bad thoughts: I have a fully legit Sky subscritption with a viewing card):
The first is a DVB-S2 capture card with a Diablo CAM to capture on PC, the second is my Dreambox DM 800 with a built-in cardreader (no CAM required) and some non-manufacturer image on it. I only need to insert the card in one of those two and I am able to use it on both, but I will not go any further in this, there are enough other forums for this.
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Old 28th September 2008, 01:35   #9  |  Link
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btw.: 1080i25 is NOT 540p50.
actually 1080i25 is much better than a imaginary 540p50 (or even 576p50).
This is due to the vertical offset every other field in the 1080i25 video.
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Old 29th September 2008, 09:22   #10  |  Link
pandy
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Quote:
Originally Posted by olikl View Post
Thanks again for your answers.
So why does everybody state "We are very far away from FULL HD broadcasting (1920x1080p)", "1080i is the European standard" and so on? Sky shows its movies in native 1920x1080p as far as I understand it...
I think that HD bring more ignorance to the whole video market than SD before ever... Worse is that plenty of "video peoples" also have problems with whole this mambos jumbos stuff...
Someone says that "ignorance is bliss"... i don't think so...

Quote:
Originally Posted by olikl View Post
So I don't have to deinterlace before editing the files?
Deinterlacing or not deinterlacing - tough problem - it is highly dependent from nature of source - if this is film (ie source is progressive for almost 100%) then probably You don't need any deinterlacing, if it is video (most of video sources are interlaced but modern video cameras can be progressive to) probably You need to deinterlace it before any processing.
I think that most important is know what type source is - interlaced or progressive.
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