Need help understanding frame rates and "Bob" de-interlacing

[KyleMadrid]

Hi all, I just have a question about de-interlacing and frame rates.

I often encode 1080i recordings of football (soccer) matches. The source frame rate is 25 fps (interlaced video), but lately I've been encoding to 50 fps to increase the smoothness of the video, because usually when I encode to 25fps it still looks okay, but it's not as smooth as when you watch it on the TV (while it's still interlaced).

Encoding to 50fps using the 'Bob' de-interlacing option, does actually increase the smoothness of the video, it definitely looks smoother, but the thing is, I've noticed that it kind of looks like it's in slow motion... not much, but just a tiny bit. Perhaps it's just me, but I wanted to know if you guys think it's worth encoding to 50fps progressive from a 25fps interlaced source. Does this seem a little strange to you more experienced encoders?

And can someone please explain what I'm doing when i'm encoding to 50fps from 25fps? I know I'm not actually adding any new frames, but why is it that the video actually looks smoother when I do this? And could it actually make the video look like it's slightly in slow motion?

Thanks in advance

[jagabo]

Encoding to 50fps using the 'Bob' de-interlacing option, does actually increase the smoothness of the video, it definitely looks smoother, but the thing is, I've noticed that it kind of looks like it's in slow motion... not much, but just a tiny bit. Perhaps it's just me

Unless your player is having problems playing back 50 fps material, it's just you. You can easily verify this by timing the playback.

but I wanted to know if you guys think it's worth encoding to 50fps progressive from a 25fps interlaced source.

It's definitely worth it if you want smooth motion. 25p is a jerky flickery mess.

And can someone please explain what I'm doing when i'm encoding to 50fps from 25fps?

Each frame of 25i video contains two separate half-pictures, called fields, (one in all the even numbered scanlines, one in all the odd numbered scanlines) intended to be viewed separately and sequentially. Ie, 50 different fields per second. A bob filter turns each of those fields into a full frame (by filling in the missing scanlines), resulting in 50 frames per second. Simple bob filters may leave jagged and flickering edges. More sophisticated bob filters will leave fewer such artifacts.

[KyleMadrid]

Thanks Jagabo. Very helpful reply. Just one question though...

You say that "Simple bob filters may leave jagged and flickering edges. More sophisticated bob filters will leave fewer such artifacts."

I use VidCoder... any idea whether it uses more sophisticated Bob filters?

[jagabo]

I don't really know VidCoder. Does it have names for the deinterlacing filters? Like Bob, Yadif, QTGMC?

[KyleMadrid]

No, the only options it has for de-interlacing are 'Fast', 'Slow', 'Slower' and 'Bob'

[jagabo]

You'll just have to try them and see what happens. After conversion step through the video frame by frame with an editor. Look for sharp horizontal edges -- like the score board overlay. Does it appear to bounce up and down by one scan line? That's a simple bob filter (hence the name). An example cropped from a video game recording, slowed to 2 fps:

[fvisagie]

With properly interlaced video, each frame contains two fields from different points in time. A proper interlaced player or display outputs the fields at field rate, i.e. interlaced 25fps content is displayed at 50fps field rate.

Bob-deinterlacing splits every frame into its components fields, turning each into its own frame (and making the video progressive in the process, since it's no longer interlaced ).

The fact that bob-deinterlaced video displays smoother on your system probably indicates that previously interlaced content was treated as interlaced. I.e. whole frames were output at 25fps, instead of 50 fields per second.

I'm not quite sure what you mean by the slow motion effect, but my guess would be that your material wasn't true 25fps progressive before it was interlaced. Some background on interlacing and deinterlacing:

http://www.doom9.org/index.html?/capture/postprocessing_avisynth.html
http://avisynth.org/mediawiki/Interlacing_and_deinterlacing

By the way, always make sure the interlacing in your source is explicitly and correctly handled when encoding. Not doing it can lead to all kinds of preventable picture quality losses. When you encode interlaced to interlaced, make sure your encoder's settings reflect that. Likewise for interlaced to progressive, in which case you must ensure proper deinterlacing is done at some point before encoding. Some encoders can do that for you. Just make sure that it does get done at least, although external deinterlacers like Avisynth's QTGMC() usually do a better job.

To apply this to your real-life example: when encoding bob-deinterlaced 25fps interlaced source, you're encoding progressive video, so ensure the encoder is set accordingly.

I hope this helps a little .

Cheers,
Francois

[Oops, I see jagabo's beaten me to it ]

[KyleMadrid]

@ Jagabo: I've encoded a couple of matches using the Bob de-interlacer in VidCoder and I just did what you said and it looks to be all good to me!

Anyway, I always thought that encoding to 50fps would be much better, and it does look better, but the main reason I started this thread was to make sure that I wasn't doing something that was weird or unnecessary, and thanks to you I now know I have nothing to worry about. Thanks

Oh, and I suspected the slow motion thing that I mentioned was just me being paranoid. I just wanted to make sure anyway.

Thanks again, much appreciated.

@ fvisagie: Yeah, jagabo beat you to it, but I appreciate the reply anyway. Thanks very much

[KyleMadrid]

Okay, I have another question. I was thinking of making a new thread but thought this would do...

Is 25fps Interlaced, the same, or similar to 50fps Progressive?

And if 25fps progressive is not very good, then why are so many movies on DVD and Blu-Ray displayed at 24fps progressive?

Thanks

EDIT: Just to confirm, I'm asking if 25fps Interlaced would be as smooth as 50fps Progressive...

[Brad]

Yes, 25i and 50p both capture temporal information 50 times per second (50Hz). 25i just throws away every other spatial line.

There's nothing wrong with 24 or 25 fps progressive; they just aren't as smooth as 50 or 60 or 120 etc. I believe jagabo's comment about a jerky flickery mess was in regards to deinterlacing and discarding odd or even fields.

[jagabo]

I believe jagabo's comment about a jerky flickery mess was in regards to deinterlacing and discarding odd or even fields.

I meant 25 fps progressive is obviously jerky/flickery compared to 50 fps progressive.

[KyleMadrid]

Okay, I see. Thanks guys.