[Stu-e]

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