by Larry Jordan (Republished from LarryJordan.com)
My goal in this article is to discuss the challenges in converting frame rates. If everything you shoot, edit and output is a single frame rate, then don’t change anything. This is the ideal way to work. However, as you start to integrate elements that originate at different frame rates, frame rate conversion rears its very ugly head.
Think of a video clip as a series of wooden children’s blocks connected by a piece of string. Each block represents a frame of video. As we pull the string, tugging the blocks along in a line, the frame “rate” represents the number of blocks (or images or frames) that pass an observer each second. Frame rate is measured in frames per second; “fps.”
Changing the speed of a clip is NOT the same as changing the frame rate.
- Changing the speed of a clip always speeds up or slows down the action displayed in the clip by repeating or removing frames.
- Changing the frame rate of a clip changes the number of frames passing an observer without changing the perceived speed of the action displayed by the clip.
This difference is significant. The first is easy, the second is hard.
We change the speed of a clip to create a visual effect. We change the frame rate of a clip to match the settings of our clip to the project. If you don’t need to match settings, don’t mess with changing frame rates.
There are two sides to a frame rate discussion:
There is a lot of debate as to which is the “best” frame rate. Some feel that 24 fps is more “cinematic,” while 60 fps is more “real.” As you should know by now, there is no “best.” Just as there is no “best” car, camera, or restaurant; there are simply choices.
Converting to a 24 fps frame rate will NOT make your movie look “filmic.” It will, generally, just make it look worse. The “cinematic look” is a combination of: lenses, lighting, depth of field, shutter speed, shutter angle, motion blur and frame rate. Changing the frame rate only affects the frame rate, not the look.
There are no right answers, just louder voices.
Also, to keep this article to a manageable length, I will ignore:
- Frame blending
- Optical Flow
- Field-based editing
These special cases don’t alter the basic rules of frame rates, though they can complicate understanding.
THE BASIC RULES
- Where possible, always shoot, edit and output the same frame rate.
- Camera-native frame rates always look better than converted frame rates.
- Frame rates are irrelevant on the web. Streaming and downloadable files can play at any frame rate.
- Frames are indivisible. (Think about those children’s building blocks. They can’t be split, stretched, squeezed or discombobulated.)
- The frame rate of your project/sequence takes precedence over the frame rate of your source media.
- The duration of a frame is one frame. Not more, not less.
- Partial-frame durations do not exist for video, though they do for audio. (You can’t display images for half-a-frame, or a quarter-frame. If your project/sequence is 25 fps, then the shortest duration you can display an image is 1/25th of a second.)
- Video editing software automatically “conforms” (converts) clips with different frame rates to match the frame rate of the project/sequence.
- Projects/sequences can only have one frame rate, though they can contain clips that use different frame rates.
- It is “easier” and “better” to convert from a faster frame rate (i.e. 50 fps) to a slower one (i.e. 25 fps), than from a slower frame rate to a faster one.
HOW WE GOT TO TODAY
In the early days of film, say 1890 – 1915, all cameras were hand-cranked. During this time, frame rates wandered from 8 fps to 30 fps, often in the same scene. In those days, the value of a camera operator was not based on their composition, but on the consistency of their cranking.
NOTE: This is one of the reasons comedies were so prevalent in the early days of film. Speed changes are inherently comedic and physical comedy does not require dialog.
As films grew in popularity and profitability, standards developed allowing cameras to be cranked by a motor, rather than by hand. Also, at this time, the industry settled on a frame rate of 16 fps.
Why? Because film was expensive and producers were, um, cheap. 18 fps provided the illusion of smooth movement without wasting a lot of film, and money.
This standard continued up until the advent of talkies which exploded on the scene in 1927 with The Jazz Singer. The problem was that 18 fps was not fast enough to support high quality audio. This frame rate yielded audio roughly equivalent to a telephone call.
So, a new frame rate standard needed to be developed – and the industry chose 24 fps.
Why? Because film was expensive and producers were still, um, cheap. 24 fps provided the illusion of smooth movement and relatively high-quality sound without wasting a lot of film, and money.
NOTE: Sound quality continued to improve over time, not by increasing the frame rate, but by shifting audio from an optical track to a magnetic track.
When video arrived, in the 1930’s, we had a major timing problem. How to get the TV receiver to “pulse” in sync with the transmitter? The solution was AC power. All across the US, power “pulsed” at 60 cycles per second.
Television engineers adopted this “universal” pulse as the basic timing circuit for video. Since video in those days was interlaced, where a single frame (complete image) was composed of two fields (a portion of the image consisting of all the odd or even scan lines), each field pulsed at 1/60th of a second.
Ta-dah! 30 fps video.
Except, over time it was discovered that high-voltage electricity “evaporated” from transmission lines when the cycle rate was too high. 50 cycles preserved more power over distance than 60 cycles (now called Hz). So, when much of the world was rebuilt after World War II, the utility companies, to save money and power, dropped the cycle rate to 50 Hz.
From there, the video industry derived 25 fps video, because interlacing was still in vogue.
So, at the dawn of the HD era in the early 1990’s, we had three principle frame rates: 24, 25, and 30 (which, with the advent of color was slightly modified to 29.97 fps, because why should this story be particularly simple?)
And, as we all know, with the rise of HD, our industry came together as a group and standardized on a single frame size and single frame rate.
– – –
Sigh… No such luck.
At last count, we now have nine different frame rates: 23.98, 24, 25, 29.97, 30, 48, 50, 59.94 and 60. (And, yes, 100 and 120 fps are knocking on the door. Please keep that door shut…!)
No WONDER we’re all confused. We’ve been handed a veritable Gordian Knot of frame rates!
Larry looks at what are your conversion options and how video can be compressed to deal with various frame rates over at LarryJordan.com.