IVTC with frame rate at 23.976 fps or 29.970 fps?

[venky_venkateshwar]

I have a doubt which has arisen after reading and re-reading the guide. I was ripping a DVD and DVD2AVI was unable to activate ForceFilm for that DVD (Film ratio < 95%). Hence the FPS on loading the d2v file in Gordian Knot was 29.970

Now the guide for Gordian Knot for Xvid encoding mentions

If FPS equals 29.970 you did not choose Force Film in DVD2AVI and will have to perform IVTC (explained later on in the guide). For now set FPS to 23.976. In all other cases (FPS = 25.00 or FPS = 23.976) you don't have to do anything.

That implied to me that change the FPS from 29.970 to 23.976 and in the save and encode menu change the field operation to inverse telecining. Call this the first case.

In the "Force Film, IVTC, and Deinterlacing" guide it mentions that

TELECINE is a process where the FILM first is slowed down to 23.976 frames/second. Then for every 4 frames, an extra frame is created from fields of adjacent frames. If a frame consists of two fields, top (t) and bottom (b) and the original sequence is 1t1b 2t2b 3t3b 4t4b, then the telecined sequence would be 1t1b 2t2b 2t3b 3t4b 4t4b (commonly called 2:3 pull down because of the alternating 2 field, 3 field progression). This also means that the frame rate increases to 29.97 frames/second.

and

FORCE FILM is DVD2AVI's way of trying to reverse the telecining process. This is a quite simple function and will therefore fail when there is too much garbage (i.e. NTSC) in the stream. This will present itself in the preview window where the type value will switch between NTSC xx% and FILM yy%. If the final value is FILM at 95% or lower or NTSC at 5% or higher then you will have to use some other method for doing an Inverse Telecine, or IVTC.

This implied to me that I should keep the FPS at 29.970 and check ivtc as the field operation in the save and encode menu thinking that ivtc would automatically junk the ~6 extra frames per second (since telecining is insertions of those frames in the first place). Call this case II.

So my confusion is that with sources which DVD2AVI fails to force into film should I set the FPS to 23.976 and use inverse telecining as a field operation or keep the FPS at 29.970 and use inverse telecining with this setting as the field operation.

Also for case I, I get the Bits/(pixels*frame) value as ~52 and for case II I get the value as ~42. On visual inspection of the two avi files I did not see any noticeable difference.

So which is the correct (or more correct) way of proceeding?

[Slogra]

I would say: enable Inverse Telecine in gknot and save the .avs file. Then play the avs file in your media player and check if it looks good and smooth.

OR

Maybe you can give us a sample of your movie? cause it's hard to tell what you're dealing with. You can use Chopper XP to cut a small part from the movie, preferably a scene with panning. Just a few seconds will do.


Merry Xmas!

[killingspree]

the framerate selector on the gknot bitrate tab is only used for calculating the bit(frame*prixels) value (compressability value). it does not have any effect on the outcome of your final movie (as you visually already noticed). since you selected IVTC in the save & encode tab of gknot, you'll always end up with a 23.76 fps avi.

the only difference this selection makes appears in the comp check value - which you also noticed...

so the right way of proceeding depends on wether you perform a comp test or not. if you do so, set the fps to 23.76 since that is what you are going to end up with due to using IVTC and if you do not, just ignore it, it won't have any use for you...

oh and btw, those comp check values are rather low, i'd recommend you go for a higher bitrate (e.g. more cds, or smaller frame size)

hth
steVe

[venky_venkateshwar]

Originally posted by killingspree
[B]the framerate selector on the gknot bitrate tab is only used for calculating the bit(frame*prixels) value (compressability value). it does not have any effect on the outcome of your final movie (as you visually already noticed). since you selected IVTC in the save & encode tab of gknot, you'll always end up with a 23.76 fps avi.

the only difference this selection makes appears in the comp check value - which you also noticed...

so the right way of proceeding depends on wether you perform a comp test or not. if you do so, set the fps to 23.76 since that is what you are going to end up with due to using IVTC and if you do not, just ignore it, it won't have any use for you...

Thanks a lot. It finally makes sense now.

oh and btw, those comp check values are rather low, i'd recommend you go for a higher bitrate (e.g. more cds, or smaller frame size)

The video looks good compared to the original DVD. I usually go for 50-60% as the xvid guide recommends that 40% should be the threshold and in the example given in the guide the conversion proceeds at a comp check value of ~25

Thanks again for the advice. It cleared up the confusion.