Is the "jitter" in 24p pulled out of 60i noticeable?

guyburns

Active member
I have a problem with pricing for the PAL GH1, shipped into Tasmania (none stocked here by any supplier). By the time I include two 16 GB Class 6 cards, a spare battery, and shipping, the overall cost will be about AUS$2200-2400 at best and 3-4 days delivery. I can get the same items, including 3-5 day UPS shipping, for about AUS $1600 out of B&H in New York.

Except it's NTSC.

I really like the idea of the PAL model because it works at 1080p, 25 FPS. I'm almost certain I will work exclusively in 1080p, but in that mode the NTSC model only offers 60i/24 FPS. And that's what bothers me.

I want to make sure my understanding of this 60i/24 FPS thing is correct -- that is is not possible to exactly record 24p in a 60i stream no matter what you do during recording or pull down. That there will always be some small time "jitter" because the 60i fields are being recorded at 16.67 msec intervals, and that is not an integer multiple of 41.667 msec (the 24p frame period). Is that correct?

BUT... even though there is time jitter, every ten fields of 60i (1/6 sec), exactly matches 4 frames of 24p (also 1/6 sec), so there is no long-term time distortion, only a small jitter as the 24p runs ahead very slightly, then slows down very slightly, to be reset to perfect time alignment every four frames.

QUES 1
Is this jitter noticeable to a discerning eye? I'm trying to decide if the $800 saving on an NTSC model, which may come at the expense of jitter, is worth it.

QUES 2

I've seen mention in this forum of a possible flicker problem problem with PAL cameras in 60Hz countries (or was it vice versa). I don't need to go into that here, but what I would like to know -- and this info is not in the manual as far as I can tell -- when in full manual mode, what shutter speeds can be set? Where is this information made available if it is not in the manual?
 
Answer 1:
If you play it back at 60i, yes, it looks a little weird. If you do a reverse telecine (many threads on that), it pulls out all the extra frames and leaves just the true 24 fps footage. Then it looks perfectly normal.

Answer 2:
2, 2.5, 3.2, 4, 5, 6, 8, 10, 13, 15, 20, 25, 30, 40, 50, 60, 80, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1300, 1600, 2000, 2500, 3200, 4000
-found by turning on my GH1 and cycling through all the shutter speeds available in movie mode :)
 
Correction: Those are the available shutter speeds in 720, 60p

In 1080, 24P the available shutter speeds are:
30, 40, 50, 60, 80, 100, 125, 160, 200, 250,, 320, 400, 500, 640, 800, 1000, 1300, 1600, 2000, 2500, 3200, 4000
 
Thanks for the info re speeds.

My post really should have been in the workflow forum. It started off as hardware but then I introduced the jitter bit. I'll start a new post in the other forum re "jitter".
 
Trust me, if you live in a country with 50 Hz AC electricity you don't want to shoot regularly with a 24 / 60 p camera - and then deal with pulldown removal in post on top of that.
 
In case anyone visits this thread in future, I should correct the false assumption inherent in the statement in my original post:

I want to make sure my understanding of this 60i/24 FPS thing is correct -- that is is not possible to exactly record 24p in a 60i stream no matter what you do during recording or pull down.

Being new to video, when I came across the 24p/60i recording mode in the GH1, I couldn't work out why Panasonic didn't just use 24p, and I wrongly assumed it was because they already had a good algorithm for capturing 60i -- i.e. capturing 59.94 interlaced fields per second -- and had decided to use that capture mode. Thus, when a user wanted 24p data, they would do an inverse telecine on the 60i data stream, and extract an effective 24p stream -- but with some inherent jitter, jitter that would not be noticeable.

Wrong assumption! For 24p/60i the GH1 does capture data at 24 fps (actually 23.976) and if properly extracted from the 60i data stream, there is no temporal distortion. For a description of how the GH1 captures 24p/60i, see a newer thread:

http://www.dvxuser.com/V6/showthread.php?t=198579&page=3
 
Back
Top