How to Fix Common Audio/Video Sync Problems

In the early days of motion picture, audio sync was easy: There wasn’t any. When you’re dealing with silent films, you have plenty of room to play fast and loose with frame rates.

The first hand-cranked cameras used in the industry could shoot footage at rates anywhere from 16 to 18 frames per second; there was no standardization. When the finished silent movies were screened for audiences, they were often played back considerably faster than that, at rates over 20 frames per second.

This system allowed the studios to save money on film stock, and let the movie theaters earn more money by turning audiences over at a healthy clip.

But with the birth of the “talkies”, we quickly started to standardize our frame rates to make accommodations for audio. Throw sound into the picture, and all of a sudden people start to notice when Charlie Chaplin starts sounding like Mickey Mouse.

Video Frame Rates for Audio People

Photo courtesy of University of Houston Libraries.

Even when sound was first added to picture, workflow remained fairly straightforward for a little while.

In the U.S., we began to standardize the speed of film at 24 frames per second in the mid 1920s. This allowed for smooth motion capture and reliable audio sync, and it worked nicely with the 60Hz AC frequency coming out of our power outlets.

On the consumer end, movie houses figured out that they could cut down on “flicker”, by simply flashing each of these 24 frames two times a piece for a total of 48 distinct illuminations per second.

Complications began with the advent of television. 24 frames per second may have looked pretty good in the low-light of the movie theaters, but with the greater brightness of TV sets, it caused noticeable flicker.

To combat this, 30 frames per second quickly became the U.S. standard for black-and-white video broadcast. On top of that, “interlacing”, a method of drawing each frame twice, was used to achieve a full 60 “fields” of illumination per second and cut down on flicker even further.

Converting to this new format wasn’t terribly difficult. A process called “Telecine Transfer” was invented for the U.S. market. The 24 frames of a film could be converted to 30 frames of video through “2:3 pulldown”; The frames would alternately be drawn either 2 or 3 times each – effectively stretching 4 frames of film across 5 frames of video. But…

Click here to read more about Frame Rates and Audio Sync Issues on SonicScoop

This entry was posted in All Stories, Beyond The Basics, January 2013, SonicScoop, Sub-Feature 2. Bookmark the permalink. Both comments and trackbacks are currently closed.
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