"Strobed" effect on AVCHD files after rendering with Premiere CS5

R

RobTrujillo

Active member
Hi, i've been looking as i have time for a few weeks to a solution to this, so far i haven't found one.

I've been using the gh1 with the hack for a few months. We shoot multicam video for live music mostly, and most of the other cameras don't have nearly as good IQ as the gh1. Cameras have to look somewhat like they're shooting the same show, so one way to help with that, i usually shot with 720p. Lower resolution, and the 60p is a better match for 60i far as motion is concerned than 24p.

Anyway, more recently i've been shooting static wides for some bands. For the ones that aren't multicam, i've been using 1920x1080 (24p). I have the gh17 hack installed, and the 24p patch is enabled.

After i export the files, there is a strobbed effect in the video that comes and goes. For a few seconds, it will play smoothly, and then suddenly it starts to have a subtle strobe effect.

I went back to the source file and played it directly, it plays with horizontal lines running through it, so it looks like playing an interlaced file as progressive, when i enable the realtime deinterlacer in vlc player, it looks ok, but my computer can't keep up with it, so i can't use that to see if it has the same problem.

Anyway, i thought that having the 24p patch enabled would make it so it's progressive, and vlc player should play it fine, in other words, i thought that with the hack, the video would be recorded as native 24p, instead of schenagglefied within a 60i wrapper. My workflow was to take the source into a 24p timeline (the 1920x1080 24p preset if i remember right), apply some filters and export using the same settings as source. The results with that had the same problem. The latest version of it i used a 29.97 timeline, thinking that maybe the video is still wrapped in a 60i file, but same results. Here is a link to the video. It's a static wide shot, and it's easiest to tell when the problem starts by watching the drummer (faster motion).

I'd just keep working with it until i find a solution, but the thing that's got me stuck, the problem comes and goes, seemingly randomly. As far as i can tell, any specific setting that's wrong should affect the entire video, not just random segments..

Thanks,

Rob Trujillo:furious3:
 
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Not sure what to look for in your file ...

The strobe effect, so called because it resembles a live scene lit with a flashing strobe light, is typically created when the shutter speed is too high -- each frame is frozen in a moment of time shorter than the frame duration, so objects make a small 'jump' between frames. The slower the frame rate (i.e. 24Fps) the easier this effect is to create. As a simple example, if the frame rate is 24Fps and the shutter speed is 1/24 of a second, there can be no strobing as each frame immediately follows the previous ... shoot the same scene with a frame rate of 1/100 of a second and suddenly there is three times as much time between frames as that included in each frame.

All that said, I don't think you are talking about strobing at all -- watching the drummers sticks for instance (a very good way to spot strobing, even modest strobing) and I don't see anything noteworthy.

Cheers,
GB
 
It's subtle but it's there. On shows where we have different lighting, closeups, camera motion, lots of action, it's obvious almost to the point of being unusable except for fx type shots (the feel of the video needs to match what the other cameras are shooting). I just looked again, the video opens with that effect, and at 11 seconds it vanishes instantly and then keeps coming and going, seemingly randomly.
 
RobTrujillo said:
It's subtle but it's there. On shows where we have different lighting, closeups, camera motion, lots of action, it's obvious almost to the point of being unusable except for fx type shots (the feel of the video needs to match what the other cameras are shooting). I just looked again, the video opens with that effect, and at 11 seconds it vanishes instantly and then keeps coming and going, seemingly randomly.
Click to expand...

In the first few seconds, when the drummer swings his sticks they blur -- this is by definition NOT the strobe effect. I don't like the look of the frame rate in those early frames, and maybe that's what you mean -- but by specifying strobe I think you're describing something that is not the problem.

Cheers,
GB
 
If there's a better word, feel free.. You're exactly right, i see the sticks blur at those points where the motion is like that. I guess rather than spending time deciding on which word to give it, i was hoping to figure out a way to eliminate it.. It seems odd that those first frames are like that, then everything plays smooth for awhile, then back again.. I wish i had some video that i could make public that would be a better example.. As far as i can tell, my workflow should be right:

Since the gh17 patch is installed and the 24p option is enabled, and i'm shooting at 1920x1080 (24p), the video is recorded 24p native so no pulldown required, the video should be placed on a 24p 1920x1080 timeline and exporting with these same settings should be the correct way. Very simple stuff now that the 24p patch enabled native 24p and no pulldown steps are needed. I'll try reinstalling the g17 patch before the next show, maybe something got messed in the settings, though i left everything default (i used one of the high reliability patches).
 
No, good thought though, it's easy to scroll that wheel when handheld, but this was mounted on a tripod shooting static, and was set full manual.
 
Just a followup, i did find the problem. It was twofold.

1. For whatever reason, the fields were being reversed in premiere. I might have rendered the wrong field order when i made proxies, or maybe premiere was having a problem interpreting field order from the files, i made them with tmpgenc, standard mpg2 files, 720x480. Project was shot on ten cameras using both HD and SD (both fff and lff) so i just used "interpret footage" to reverse the way premiere interprets it.

2. One of the effects was causing some image tearing, was easy to find once i disabled all of the added fx, went away, reneabled one by one and found the problem that way. Now it seems like an obvious t/s step, never though about it tho..
 
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