Film-Tech Forum ARCHIVE: Query from Home Theater Forum re: proj speed / shutter timing

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Topic: Query from Home Theater Forum re: proj speed / shutter timing

Robert Harris
Film Handler

Posts: 95
From: Bedford Hills, NY, USA
Registered: May 2003

posted 04-10-2004 03:53 PM

Hopefully this group can answer the following question posted to Home Theater Forum in regard to Phantom of the opera and all silent films in general:

"If the 1930 "International" version was shown at correct speeds, the 1925 footage would be shown at about 22fps and the 1929 sound retake footage at 24fps. Of course, audiences saw the 1925 version at whatever speed projectionists chose and 1929 audiences saw the film at a constant 24fps.

This raises a question that interests me. Perhaps someone out there can modernize my knowledge. Just how adaptable are modern cinema projectors regarding projection speed?

In my teens I frequented projection booths. In those days (the 60s) even projectors that went back to the silent era were strictly one-speed devices. In college I was the main projectionist for our public film series of art and historic films. It wasn't until several years after I graduated that they got 35mm, but the evening showings were in the hospital auditorium. It had a 15x20-ft screen with the industry-standard Altec Lansing speaker array behind. Both motorized curtains and dimmers on the stage lights and main house lights. 16mm aside, you could put on a professional show.

In the early years we had Bell and Howell auditorium projectors with arc lamps supplied by a separate generator. These machines were minimum adaptations of the B&H 300 series "classroom" projectors so you could run films at any speed you liked as long as you liked 16- or 24 fps.

Later we replaced these with a single French-made projector. It was a big machine that loaded from the port side instead of starboard and had a modern Xenon lamphouse. I had to bring the films to the booth (after cleaning them and preparing them for changeovers for the matinee performance at another location) and build them onto a single reel (5,000 feet if I remember right). Like those other professional projectors at real theaters, it ran at only sound speed. Period.

Our engineers had a special pulley machined for showing silents at 20 fps. It wasn't too difficult to switch from one to the other. Other speeds were discussed, but 20 fps was the most practical compromise. Their research into the literature together with some experimentation showed that to go lower than 20 fps, you would also have to change the shutter, which just wasn't practical.

To explain: A shutter serves two purposes in a projector. First, it eliminates travel ghost by blocking out the light when the film moves from one frame to the next. "Travel ghost" is a blur of white above or below bright objects in high contrast scenes. You see it occasionally in cinemas when the shutter is mistimed. There are even situations where you can see this effect in the natural world. (As a kid, I had a projector that didn't have a shutter, so there was a smear on all the intertitles of commercial home movies.)

The second purpose of a shutter is to reduce flicker. At the dawn of silents, many projectors had single-blade shutters which blocked the light only at the transition between frames. When a light or bright object turns off and on at or near sixteen times per second (16 Hz, the persistence-of-vision frequency), our eyes intepret that as flicker and, thus, the movies were called "flickers." If you turn the light on and off fast enough however, our eyes miss it altogether. We think the light is steady.

Professional sound projectors have two-bladed shutters so the light blinks at 48 Hz. Home projectors had three-blade shutters so a sound film blinked at 72 Hz and silents blinked at 48- or 54 Hz. 48 Hz isn't ideal. You can still see some flicker if you look for it, but 72 Hz makes the image a little dimmer, and light is at a premium when you're shooting to a cinema screen.

On the surface it would seem that lowering the projection speed and therefore the rate at which the shutter breaks the light beam wouldn't matter that much, but it does. Since our eyes are operating at a different rate (~16 Hz) there's an interaction between their behavior and what's happening on the screen. Some of the "frames" our eyes take in will see two interruptions of the shutter and some will see three, so some "frames" will appear darker than others. A pattern develops which gets interpreted as flicker.

In short, to show silent films at a wide choice of frame rates a projector needs to have both a means of altering the pulldown frequency and the shutter frequency. I would bet that the standard models have neither, but that the pulldown frequency could be altered to some degree by a knowledgeable service company. Do some cinemas have special equipment to handle silents or have modern technologies made it easier to adapt projectors for silent use?

Of course, this whole discussion begs the question of what is the "correct" speed. If I remember right, the Society of Motion Picture Engineers never quite got to the point of issuing a recommended standard (which would have then become an ASA [later ANSI] standard meant to be applied industry-wide). But there was a proposed standard on the table when sound came in and made the question moot. It called for a camera rate of 16 fps and a faster projection rate, I think 20, but I may be wrong. Most of those SMPE guys were cameramen and the proposal reflected their feeling that silents look more natural when projected a little fast. Or, maybe, just the reality that theater managers wanted to turn the house over fast."