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
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.
Europe however, came to settle on a standard of 25fps, which made good sense for their AC frequency of 50Hz. They ended up sticking with this frame rate for both film and video, even if their TVs exhibited a bit more flicker early on.
Since the Europeans adopted a single frame rate for both formats, no conversion was necessary for their own domestic productions. But when American films were shown on TV in Europe, the stations would simply speed them up by about 4%.
This leads us to Common Sync Problem #1: Audible Pitch Shift.
Today, digital technology now allows us to speed up sound without increasing its pitch. Although this is supposed to be part of the contemporary U.S.-Europe conversion process, it’s not always done as it should be.
If you’re dealing with audio that has been converted from one of these film frame rates to the other, keep an ear open for audible pitch shift. It happens less often now than in the past, but it’s still worth listening for.
At 4% a change in pitch can be significantly noticeable in a way that a change in motion is not. This becomes even more apparent in cases where the internal camera sound is properly pitch-shifted but externally recorded audio is not.
Keep in mind that this 4% pitch drift can occur in situations where sound and picture appear to be in proper sync. Test tones, like the conventional 1kHz “2 pop” can help you evaluate sync and pitch on completed projects. When dealing with raw footage, particularly on smaller-budget foreign-market projects, you may want to refer back to the original unconverted rate, and adjust pitch if needed.
The Coming of Color
Things really started to get complicated when color entered the picture. In the U.S., the 30fps video standard just didn’t leave enough extra bandwidth to include color information along with the picture.
The solution was almost brilliantly simple: slow down the video frame rate by 0.1%. No one would notice the difference in picture or in pitch, but this new frame rate of 29.97 would free up enough bandwidth to include color in the broadcast.
Ironically, it’s the brilliant simplicity and transparent fidelity of this solution that leads to so many of our synchronization headaches today.
And that’s what leads us to Common Sync Problem #2: Sound Drifts Against Picture.