Telecide guide=3 Questions

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Aleron Ives
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Telecide guide=3 Questions

Post by Aleron Ives » Sun Nov 10, 2013 1:52 am

I'm processing a video from a 1080i ATSC station that is telecined 25 -> 30 fps material. When I use Telecide(guide=3) to restore the progressive frames, I get the expected pattern of 5 "real" frames and 1 duplicate, which I can eliminate by using Decimate(cycle=6). This sets the framerate down to 24.975 fps, which leads me to my two questions:
  1. Am I correct in assuming that if I want to restore the exact 25.000 fps of the PAL stream that I would need to use AviSynth's AssumeFPS(25,1) to adjust the framerate, and then I would also need to decode the audio (Dolby Digital) and speed it up by the same amount in order to preserve the A/V sync, since the audio was already slowed down from 25.000 -> 24.975 when the video was telecined?
  2. If the above is correct, am I also correct in assuming that doing so is a lot of work for no real reason, since the 1/1000 slowdown is imperceptible and the same as the 24.000 -> 23.976 conversion done for film -> NTSC compatibility (which is equally pointless to undo)?
I haven't worked with 25 -> 30 fps material before, so I wanted to verify my thinking. I've never seen a clip with 24.975 fps before, so I was wondering if I had missed something. Thanks!

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Re: Telecide guide=3 Questions

Post by admin » Sun Nov 10, 2013 2:27 pm

Can you please post an unprocessed source sample? You can cut the TS or MPG with DGSplit. I ask for this because I have never seen broadcasted 25->29.97 telecine, so I want to verfify your analysis before advising.

Aleron Ives
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Re: Telecide guide=3 Questions

Post by Aleron Ives » Sun Nov 10, 2013 6:18 pm

Thanks for the reply. The recording device is a DVICo TViX, so I used its edit mode to cut a ~15 second sample from the original video (*.tp).
Last edited by Aleron Ives on Mon Nov 11, 2013 7:49 pm, edited 1 time in total.

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Re: Telecide guide=3 Questions

Post by admin » Mon Nov 11, 2013 7:50 am

Your analysis looks correct.

1. Yes.

2. My calculations show that if you just AssumeFPS(25) you will have appreciable AV desync by the end of a movie. Seems to me you'll have to adjust the audio if you want exactly 25 fps.

Depending on your target format you may be able to leave it at 24.975.

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Re: Telecide guide=3 Questions

Post by Aleron Ives » Mon Nov 11, 2013 8:00 pm

Thanks for the confirmation. The 0.1% slowdown of the audio would cause it to drift by 1 second every ~15 minutes if I increased the framerate to 25.0, so since I mux to Matroska instead of targetting formats such as DVD-Video or BDAV that require exactly 25.0 fps, I think I will leave it at 24.975. I've never found it necessary to increase the rate of 23.976 fps material to 24.0, so the speed difference here should be equally negligible.

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Re: Telecide guide=3 Questions

Post by admin » Mon Nov 11, 2013 10:09 pm

Assume movie length ~7200 seconds (2 hours):
-> 172,800 frames @ 24 fps
-> 172,627 frames @ 23.976 fps

AV desync by end of movie ~= (800 - 627) * 1/24 = 7.2 sec

Why you keep saying the difference is negligible?

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Re: Telecide guide=3 Questions

Post by Aleron Ives » Mon Nov 11, 2013 10:30 pm

It is negligible in the perceptual sense that you could not tell the 0.1% difference in speed if you compared 23.976 to 24.0 for the same content, unlike e.g. the 4% speed difference that occurs when you increase 24.0 to 25.0 fps for film -> PAL conversions. I'm referring to whether the animation would appear to move faster or slower than normal (and whether the audio pitch would seem different), not whether the difference can cause desync (which it can). The speed difference is therefore not enough to justify (in my opinion) converting the framerate to 25.0 and increasing the audio speed, as the 24.975 and 25.0 videos would look and sound essentially the same (and would both be in sync).

After some further research, it appears that my earlier conclusion regarding the suitability of 24.975 fps was premature, however, as this ??? thread indicates that PAL video can either be slowed by 0.1% to create a stable telecine pattern (which is what I assumed), or it can be left at its true 25.0 rate and skip a field repeat every 1001 fields, instead. Apparently the only way to know which method was used is to advance the video by 1001 frames to verify whether the irregular pattern is present, unless using guide=3 will allow Telecide to detect it automatically? I'm not sure how Decomb compares to QTGMC, since the latter is the software referenced in the ??? thread.

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Re: Telecide guide=3 Questions

Post by admin » Tue Nov 12, 2013 10:27 am

Not seeing anything further here requiring any answer from me.

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