QTGMC: When to use "bob"?

[Sunk]

I've been slowly learning how to use QTGMC recently and one thing I still don't really understand is when to use the "bob" setting to double the frame rate. Most of my tapes contain recordings from TV channels in 25 fps, but I also have some licensed cartoons (in 25 fps as well). Is there a general "rule" when and when not to use "bob" or does it depend entirely on the recording itself?

[jagabo]

After bobbing with QTGMC examine the video frame by frame during a motion shot -- a horizontal panning shot is best. If you see motion at each frame you need to bob. Otherwise you don't. In fact you may not even want to use QTGMC but rather a field matcher like TFM.

[Sunk]

After bobbing with QTGMC examine the video frame by frame during a motion shot -- a horizontal panning shot is best. If you see motion at each frame you need to bob. Otherwise you don't. In fact you may not even want to use QTGMC but rather a field matcher like TFM.

I remember in my previous topic you've mentioned that some of my frames were progressive. Unless I'm misunderstanding something, but shouldn't VCR footage be fully interlaced instead of progressive? Did I capture that last footage of mine wrong? I'm still learning this, so please excuse me potentially mixing things up.

[manono]

...shouldn't VCR footage be fully interlaced instead of progressive?

No, not necessarily. If you follow his advice from his earlier reply, you'll know when and when not to bob with QTGMC. And in cases when it appears interlaced but shows those tell-tale duplicate frames after bobbing, then TFM will restore it to its original progressive form.

[lollo]

Unless I'm misunderstanding something, but shouldn't VCR footage be fully interlaced instead of progressive?

No. A useful guide here:

[Bwaak]

shouldn't VCR footage be fully interlaced instead of progressive?

The stream from an old-school VCR is always interlaced. But the content can be progressive, and by using different pulldown patterns you can encode different image rates up to the native field rate.

https://hometheaterhifi.com/volume_7_4/dvd-benchmark-part-5-progressive-10-2000.html

If you always bob, you won't do worse than an average TV set does.

[Alwyn]

Lollo, I didn't find that helpful at all. Lots of telecine talk but it doesn't seem to cover the issue here, which is analogue capture of progressive VHS.

Well, got a Headache? Aspirin helps!

Yep!

[jagabo]

Analog standard definition video is always transmitted one field at a time. In that sense it is always interlaced. Live video, like from a camcorder, live sports, news, etc. is fully interlaced -- the contents of each field comes from a different point in time, at 1/50 second intervals for PAL. The fields are intended to be seen on at a time -- so you see 50 different pictures per second. When film sources are transmitted on PAL systems the 24 fps film is often sped up to 25 fps. Then each film frame is sent for two successive fields. If you join the two fields together (as is usually the case when digitizing) you can restore the 25 fps film frames. In that sense the content of the interlaced video is progressive frames.

[lollo]

Lollo, I didn't find that helpful at all. Lots of telecine talk but it doesn't seem to cover the issue here, which is analogue capture of progressive VHS.

I 'll try to summarize, hoping to not be repetitive after jagabo's excellent explaination.

In the analog video there are the field architecture and the relation between the fields.

Analog video is always broadcasted, or recorded on tape, or "transferred" to tape with an architecture of 50 fields per seconds.

The fields building a frame can be from two different moment in time (pure interlaced video), or from the same moment in time (what we call "progressive video" here).

In addition, when "trasferring" a movie to VHS there can be many options on the relation of the fields, that are analyzed in the document I linked, and that I found several times on my captures, for both commercial tapes and VHS recorded from TVs.

When capturing an anime, there are additional relations that sometimes are difficult to detect, but that's another story.

Going back to your question of anlogue capture of "progressive" VHS, simply capture the analog video at 50 fields per second and then do not deinterlace. As a side note sometime we use QTGMC to repair damages done on a "progressive" video because bad processing, not to deinterlace it.

[oln]

Which "bob" setting? QTGMC outputs full framerate by default (i.e 50fps for PAL content). There are some options to switch between "bob" and more sofisticated algorithms for some of the internal bits, for changing the output to half framerate in cases that works better you can use fpsdivisor instead. Though you generally don't want that for material that is actually interlaced and for film/animation that is not a full 50 or 59.94 fields video you want ideally to use the various detelecine plugins instead (unless you have content that is a mix like animation or advertisements or content without a consistent pull-down pattern like a home transfer of a 8mm film to video, in those cases it might work better to just throw qtgmc at it.)

[Alwyn]

Lollo, as I said, largely irrelevant to the current discussion.

[lollo]

Lollo, as I said, largely irrelevant to the current discussion.

Because after all this time of you beeing here, you don't understand too much (it is enough to read all your posts about interlaced video).

What is really irrelevant, are your useless comment!

[Sunk]

Which "bob" setting?

I was referring to the one you can tick in Hybrid. Not sure how it'd look like in a "raw" script.

[oln]

Ah I see. When you use it in a script bob is what's done unless you explicitly specify "fpsdivisor=2" to output at half frame rate, so hybrid's author changed around a bit how it's specified in the GUI.

[manono]

Which "bob" setting? QTGMC outputs full framerate by default (i.e 50fps for PAL content).

PAL is 25fps (frames per second). QTGMC is a bobber outputting double framerate, or 50fps. That FPSDivisor=2 returns it to the original PAL framerate.

[Skiller]

That FPSDivisor=2 returns it to the original PAL framerate.

...But literally throws away half the video while doing so. In my opinion, if QTGMC is the right tool for the job, output should be 50 fps. It's not 20 years ago where 50 fps would have been a burden to encode and playback. There's really no good reason to halve the frame rate.

[lollo]

+1. If a video need to be deinterlaced, output should be at 50 frames per second.