Frame rate & refresh
How motion is built from still frames, and why 60 Hz vs. 30 Hz changes your whole signal path.
Video doesn't really move. It's a rapid sequence of still images — frames — shown fast enough that your brain fuses them into motion. How many frames are shown each second is the frame rate, measured in frames per second (fps) or hertz (Hz).
41.7 ms
1,440
0.8×
The common rates
| Rate | Where it comes from | Typical use |
|---|---|---|
| 24 fps | Film tradition | Cinema, many streaming movies |
| 25 / 50 fps | 50 Hz power grids (Europe, much of Asia) | Broadcast in those regions |
| 30 / 60 fps | 60 Hz power grids (North America, Japan) | Broadcast, computers, conferencing |
| 120 fps and above | Gaming and high-end displays | Smooth motion; rare in corporate AV |
- 24 fps
- Where it comes fromFilm tradition
- Typical useCinema, many streaming movies
- 25 / 50 fps
- Where it comes from50 Hz power grids (Europe, much of Asia)
- Typical useBroadcast in those regions
- 30 / 60 fps
- Where it comes from60 Hz power grids (North America, Japan)
- Typical useBroadcast, computers, conferencing
- 120 fps and above
- Where it comes fromGaming and high-end displays
- Typical useSmooth motion; rare in corporate AV
You'll often see odd-looking figures like 29.97 and 59.94. These are a legacy of colour television in 60 Hz regions, where the rate was nudged down by 0.1% to fit colour into the existing signal. Modern equipment handles both the round and fractional rates, but they are technically different formats.
Progressive and interlaced
In progressive video (the p in 1080p), every frame contains every line of the picture. Interlaced video (the i in 1080i) is an older bandwidth-saving trick from broadcast: each pass draws only the odd lines, then only the even lines, so each field carries half the picture. Modern displays are progressive, so interlaced signals have to be de-interlaced before display. You'll still meet 1080i from some broadcast and cable TV sources.
Why frame rate matters in AV
Double the frame rate and you double the data. A 4K signal at 60 fps needs twice the bandwidth of the same signal at 30 fps. That's the difference between a connection that works and one that doesn't — the very thing the bandwidth lesson at the end of this chapter puts numbers to.
- Computers almost always output 60 Hz. Mouse movement and scrolling look sluggish at 30.
- Conferencing cameras commonly run at 30 or 60 fps, depending on the camera and the platform's settings.
- Shared content in a call is often sent at a lower frame rate than camera video, because slides change rarely and the bandwidth is better spent on sharpness.
Key takeaways
- 01Video is a sequence of still frames; frame rate is how many are shown each second.
- 0224, 25/50 and 30/60 fps are the common families; 29.97 and 59.94 are legacy variants.
- 03Frame rate belongs to the signal; refresh rate belongs to the display.
- 04Progressive (p) draws every line each frame; interlaced (i) alternates odd and even lines.
- 05Doubling frame rate doubles bandwidth.
Check your understanding
Q1What does the 'p' in 1080p stand for?
Q2A 4K signal runs at 30 fps. Roughly how much more bandwidth does it need at 60 fps?
Q3A user says the mouse feels sluggish on the room display but the image looks fine. What's a likely cause?