Modify Motor to NTSC

It is possible to modify  CP Multi-Speed Motors to shoot NTSC TV screens without roll-bar flickering. While there are several ways to photograph an NTSC TV-screen onto FILM to eliminate the roll-bar effect (such as playing back the TV image with 24-frame video, or using an LCD display where apparently camera speed/shutter angle don’t matter), one of the most tried and true ways is to use a camera speed of 23.976 frames per second and a 144-degree shutter. Even many of the professional camera guys seem to have forgotten about this — but it’s the way kinescopes (film prints of TV shows that went out live on video) were done for decades. In fact, Auricon made a 16mm camera that was dedicated to this.

An easy and relatively low-cost way to get your CP multi-speed ACL motor to roll at 23.976 fps is to replace one of the existing crystals in your motor (in the CP motor only two of the speeds use crystals —  24fps and 25fps; in the ACL II multi-speed motor, all speeds are crystal) with a 3.928232 crystal*.  This is a reasonably  straight-forward soldering job for someone who knows what they are doing.

Pictured Below: (1) Crystal; (2) CP Motor circuit; (3) 25 fps Crystal is in front, 24 fps in rear

If, for example, you never use 25 fps (the 25 fps crystal is 4.096000; the 24 fps crystal is 3.932160), then you could turn that setting on your dial into the speed of your choice.  This can save money in an external sync controller, (and the ACL 1 and 1.5 motors have no such external sync input to begin with).

Shooting at 23.976, will “freeze” the rolling effect, but a wide white bar will still be present. The bar can be positioned anywhere in the frame by “phasing” the speed — in this case just flick the speed switch between 24 and 23.976. When at 24fps, the bar will move. At 23.976 it will freeze.

In order to shrink the bar down to a very small size that can be rolled out of the main area of the TV picture, it is necessary to use a 144-degree shutter. A 144-degree shutter for the ACL is available (manufactured for the Super-16 HD-144 conversion, these shutters will also work in a Regular-16 ACL for shooting an NTSC TV screen). Although in theory the bar becomes infinitely small at 144.0000000000 degrees — in practice the shutter may be very close to 144 but not to every decimal place — yielding 2 thin lines. These can be rolled to a less noticeable area of the picture; one line can usually be buried in the frameline (“vertical interval” in video-speak). If the shutter is a fraction of a degree over 144, the bar will be white. If it is a fraction of a degree under, the bar will be black. The current batch of HD-144 shutters from March 2001 to the present yields a very thin black line after the second line is hidden. But because the bar isn’t rolling, it is hard to detect — short of a tight insert.

At 144-degrees, the shutter speed for setting exposure is 1/60th of a second  (a 1/3rd stop exposure increase over the old 175-degree shutter is necessary.

Crystal  replacement should only be done by a soldering pro. If you don’t know what you’re doing, you could fry your circuit board. In general, use a soldering pencil of no more than 15 watts, fine electronic solder that is 97/3, and place heat sinks onto the ends of the lead you are soldering to draw heat away from the crystal (clamping a pair of needlenose pliers onto the end of the lead will accomplish this). I also suggents dampening a small piece of tissue and wrapping it around the crystal’s housing to draw off heat. A possible source of crystals (at about $10 each) is Tobin Cinema Systems.

Finally, remember that as your camera rolls at 23.976 fps and you look though the ACL’s viewfinder at the NTSC TV-screen, you are not actually seeing the image that will be going onto the film — you are, in fact, seeing the exact inverse that occurs while the claw is advancing the film. So you will view, instead, a large ugly bar effect; it can even look like a sideways v-shape at times! But this is not what the film will see! Run some tests to get a feel for what you need to see through the finder to know that you have sucessfully phased the bar to where you want it.

*If you are shooting a project at 30 fps for transfer straight to video, a camera speed of 29.970 fps would be used for shooting an NTSC screen — the crystal is 4.910290.   If you want to shoot at 30 fps without TVs in the shot —  also for transfer directly to video — a 4.9152 crystal would be required.

Note that shooting at 29.970 fps is best done with a shutter as close to 180 degrees as possible for the narrowest bar, which can then be moved out of the picture by “phasing” your speed until the bar goes away. (The ACL Regular-16 shutter is 175 degrees — close.)

ALSO — make certain that you inform your transfer facility that you shot at a “non-standard” camera speed (anything not 24 fps), so that they can correct for it in the transfer. Your transfer house may suggest using a 59.94 Htz crystal on a 1/4″ Nagra instead of the usual 60 Htz crystal as a way to accomplish this audio correction “in the field” (usually these 59.94 crystals can be rented from your audio rental place). With DAT, it is a lttle more complex to correct in production. Check before shooting!