Video Frame Rate
Video frame rate describes how many complete pictures are presented per second.
At 24 frames per second, a video supplies a new picture every 1/24 second, or about 41.67 ms. At 60 fps, the interval is 16.67 ms. Frame rate describes this temporal sampling: how frequently the scene is represented by a new picture.
Imagine an object moving steadily across the image at 600 picture positions per second. At 24 fps, it moves 25 positions between captures. At 60 fps, it moves 10. The second recording supplies more intermediate positions, so motion can be followed in smaller steps.
| Constant picture rate | Interval between pictures | Object movement at 600 positions/s |
|---|---|---|
| 24 fps | 41.67 ms | 25 positions |
| 25 fps | 40 ms | 24 positions |
| 30 fps | 33.33 ms | 20 positions |
| 50 fps | 20 ms | 12 positions |
| 60 fps | 16.67 ms | 10 positions |
An ideal steady-motion example, not a measurement of a camera.
Exposure happens inside that timing
The camera also integrates light for an exposure. If this moving object is exposed for 1/48 second, it travels 12.5 positions during the exposure. Its image can therefore cover more than one sharply defined position in a single frame.
Frame interval and exposure duration do different jobs. More frames provide more recorded motion steps. A shorter exposure can make each step crisper; a longer one can blur the movement within it. A recording can have frequent but blurred pictures, or infrequent but sharply edged ones.
The player's task is then to present these existing pictures at their assigned times. Simply repeating them more often does not change the recorded exposure.
Put a 24 fps film on a 120 Hz screen
A screen's refresh rate is how often it updates. At exactly 120 Hz, the screen has five refresh intervals for each 24 fps picture:
120 ÷ 24 = 5.
It can hold A for five refreshes, B for five, then C for five. Each picture remains visible for 5/120 second = 1/24 second. The display refreshes frequently, but the video still supplies 24 different pictures per second.
At 60 Hz, the ratio is 2.5. A common cadence alternates two and three refreshes. Some pictures remain for 33.33 ms and others for 50 ms, rather than a uniform 41.67 ms. During a steady pan, this alternating hold can produce judder.
Frame-rate conversion can allocate pictures to another schedule; interpolation can estimate additional pictures. Choosing a display mode with an evenly divisible refresh rate is a separate route that preserves the existing picture sequence.
Source pictures, converted output, and display refresh
In an Emby session, the source rate describes the library track. A video transcode has an output picture rate, and the display has an active refresh rate. Observing each stage explains whether a motion change came from the source, picture conversion, or screen cadence.
Interlaced sources need their unit named as well. A stream reported as 25 frame pairs per second can contain 50 field capture times; its fields are not 25 complete progressive motion samples.
Short labels may also round 24000/1001 to “24.” Exact timing calculations use the rational rate and the actual presentation timestamps. The interval between recorded pictures and the interval between screen refreshes remain separate quantities.