Interlaced video output support?

Stu-e · 2010-09-24 12:37

Please can we have interlace video output and let the TV do the hard work of deinterlacing and pulldown detection?

I understand the biggest problem to overcome in order to achieve this is maintaining correct field order.
Taking an idea from some MythTV discussion I read a while back there is a way to maintain correct field order without relying on any field synchronisation signal from the video card.

Currently 2 fields are presented to the frame buffer at the same time something like this:

Code:

Frame A  A  B  B  C  C  D  D

Field 12 12 34 34 56 56 78 78

Video t   b t   b t   b t   b (t=top field, b=bottom field)

This is fine if the video stream (encoded with top field first) just happened to start just before the top video field is displayed.
If the stream started when the bottom field was about to be displayed, field order would be incorrect with ugly consequences:

Code:

Frame A  A  B  B  C  C  D  D

Field 12 12 34 34 56 56 78 78

Video  b t   b t   b t   b t

If instead 1 field was presented to the frame buffer per vertical sync, field order would always be correct:

Code:

Frame A  A  B  B  C  C  D  D

Field 12 32 34 54 56 76 78 98

Video t   b t   b t   b t   b

Code:

Frame A  A  B  B  C  C  D  D

Field 12 32 34 54 56 76 78 98

Video  b t   b t   b t   b t

Let us assume we always have available the current decoded frame and 1 future frame and for this discussion the video source is top field first.
To start, the current frame is written to the frame buffer. After the first vertical sync (either field 1 or 2 has just been displayed) the top field of the future frame (field 3) is written over the top field in the frame buffer (field 1) leaving the bottom field untouched (field 2). Then after the next vertical sync the bottom field of the future frame (field 4) is written over the bottom field in the frame buffer (field 2) leaving the top field untouched (field3). After the next vertical sync a new decoded frame is available. The process repeats using the new frame as the future frame.

Other issues that would need consideration are:
The video cannot be resized and must be displayed 1:1.
Video and audio might be out of sync by 1 field depending on luck.

Benefits of interlace output are leaner hardware. I envisage IGP or CPU video decode where there is not enough computing power to implement high quality deinterlace and pulldown detection. I am assuming of course the TV has a good deinterlacer and does not just do plain old bob like some cheaper flat panel TVs do.

Stu-e

**bobo1on1** · 2010-09-24 13:09

Only problem I see with this is, on interlaced resolutions, you get a vertical blank signal per field, and you have no idea which field you're on.
This could work if the tv could detect interlaced video on a progressive resolution.

Stu-e · 2010-09-24 13:32

Quote:you get a vertical blank signal per field, and you have no idea which field you're on.

The whole thrust of my post is it does not matter which field you are on. You get a vertical sync for each field. That is all that is required.

Quote:This could work if the tv could detect interlaced video on a progressive resolution.

I'm not sure I understand. Do you mean "the tv could detect progressive video on an interlaced resolution"? This is indeed how progressive content is delivered for nearly all broadcast TV and is reliant on the the TV detecting pulldown (2:2 for 50Hz and 3:2 for 60Hz). The method I have described in this post can deliver progressive content via an interlaced video stream providing the TV has adequate pulldown detection.

Stu-e

**bobo1on1** · 2010-09-24 13:44

Quote:The whole thrust of my post is it does not matter which field you are on. You get a vertical sync for each field. That is all that is required.

Ok I get it now, if you use an interlaced resolution and output interlaced video with exact alignment, it should display correctly.

Quote:I'm not sure I understand. Do you mean "the tv could detect progressive video on an interlaced resolution"? This is indeed how progressive content is delivered for nearly all broadcast TV and is reliant on the the TV detecting pulldown (2:2 for 50Hz and 3:2 for 60Hz). The method I have described in this post can deliver progressive content via an interlaced video stream providing the TV has adequate pulldown detection.

Stu-e

Some tv's can detect the combing of interlaced video on a progressive resolution, and automatically turn on the deinterlacer.

Stu-e · 2010-09-24 13:55

Quote:Some tv's can detect the combing of interlaced video on a progressive resolution, and automatically turn on the deinterlacer.

Wow! That's a really cool feature. I want one. Would you happen to know what models do that?

Stu-e

**elupus** · 2010-09-24 15:49

I like the idea alot actually.

**elupus** · 2010-09-24 15:50

Ps... Video can be resized, it just need to be weaved on output.

Stu-e · 2010-09-24 16:14

Resizing interlaced video?
That sounds difficult.

Stu-e

**bobo1on1** · 2010-09-24 20:38

It's really not that difficult actually.

Stu-e · 2010-09-25 09:35

I assume you would resize each field independently and then re-weave the 2 fields back into a frame. I was concerned about vertical detail becoming repeated. For example imagine the top half of the screen is red and the bottom half is green:

Code:

r

 r

r

 g

g

 g

x2 would become:

Code:

r

 r

r

 r

r

 g

r

 g

g

 g

g

 g

Stu-e

bobo1on1 cheapass Team-XBMC Developer Posts: 2,752 Joined: Dec 2008 Reputation: 20	2010-09-24 13:09 Post: #2 Only problem I see with this is, on interlaced resolutions, you get a vertical blank signal per field, and you have no idea which field you're on. This could work if the tv could detect interlaced video on a progressive resolution.
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Stu-e Member Posts: 83 Joined: Jan 2010 Reputation: 1	2010-09-24 13:55 Post: #5 Quote:Some tv's can detect the combing of interlaced video on a progressive resolution, and automatically turn on the deinterlacer. Wow! That's a really cool feature. I want one. Would you happen to know what models do that? Stu-e
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elupus Team-XBMC Developer Posts: 3,764 Joined: Mar 2004 Reputation: 2	2010-09-24 15:49 Post: #6 I like the idea alot actually. Always read the XBMC online-manual, FAQ and search the forum before posting. Do not e-mail XBMC-Team members directly asking for support. Read/follow the forum rules. For troubleshooting and bug reporting please make sure you read this first.
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elupus Team-XBMC Developer Posts: 3,764 Joined: Mar 2004 Reputation: 2	2010-09-24 15:50 Post: #7 Ps... Video can be resized, it just need to be weaved on output. Always read the XBMC online-manual, FAQ and search the forum before posting. Do not e-mail XBMC-Team members directly asking for support. Read/follow the forum rules. For troubleshooting and bug reporting please make sure you read this first.
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Stu-e Member Posts: 83 Joined: Jan 2010 Reputation: 1	2010-09-24 16:14 Post: #8 Resizing interlaced video? That sounds difficult. Stu-e
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bobo1on1 cheapass Team-XBMC Developer Posts: 2,752 Joined: Dec 2008 Reputation: 20	2010-09-24 20:38 Post: #9 It's really not that difficult actually.
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Stu-e Member Posts: 83 Joined: Jan 2010 Reputation: 1	2010-09-25 09:35 Post: #10 I assume you would resize each field independently and then re-weave the 2 fields back into a frame. I was concerned about vertical detail becoming repeated. For example imagine the top half of the screen is red and the bottom half is green: Code: `r r r g g g` x2 would become: Code: `r r r r r g r g g g g g` Stu-e
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