Slow Motion Strobing Effect on a Time Crunch

[Andrew Swisher]

Thanks guys! Your responses helped a lot!

-Andrew

 

[Peter Borosh]

Hi Andrew,

I'm am assuming you are trying to reduce strobing from CRT (cathode ray tube) while shooting high speed.

We have to get technical:

NTSC produces a 525 line image by alternating between a odd and even line field. This gives you the 29.97 fps image. In between each image is a blanking bar (a sort of black bar).

Now this is where it gets tricky: The highest frame rate without getting a strobing image is 59.98 fps. Why? Because the odd and even alternate fields are produced at that rate. So if you film at a 120 fps, all you will see is half a field ( quarter of a frame ).

Now for the tricker part: Phasing. The timing between the camera and the CRTs have to locked at the same time in the image projection / recording cycle. If the camera is starting to record its image while the CRT is half way producing its image, there will be a black bar across the image (this is the blanking bar). The R1 has to be Genlocked to the live feed camera. If a Sync signal, usually a Black Burst, goes to the Live camera, R1 and the monitors, this will help keep the timing in sync. You may need to adjust the R1 phasing to correct the timing delay. My only concern is what kind of signal the R1 needs to be gen-locked (SDI or composite NTSC).

 

[Peter Borosh]

I've to add a correction, I've typed 59.98 fps. It should read 59.94 fps.

 

[Peter Majtan]

Providing the TV is NTSC (29.97) - shooting it at 100 FPS with 180˚ shutter will most definitely result in massive strobing.
Try using 120 FPS with 360˚ shutter - this will help a lot, but you are still dealing with issues mentioned above by Peter...
FYI - there is no 120 time base. Your time base should be the same as the final project output you are aiming for...

:sifone: Peter

 

[Peter Borosh]

I've also an another addition: The R1's shutter has to be 360 degrees.

Why, if you film at 23.98 with a 180 shutter, you will record a half image ( one field ). Not too bad. Since if you record at high speed image ( 59.94 fps ) with a 180 shutter you will record a quarter frame or half a field.

So the solution is shoot at 59.94 fps with a 360 shutter so you will record a full field.

Now.... the R1 does not do a decimal point in Varispeed so you need to shoot at a Time Base of 59.94 then convert to a 23.98 quicktime.