Reference · Video timing

Frame rate, CFR, VFR and time base

Frame rate describes how frequently pictures are presented. Media files also carry timestamps, and the clock used to express those timestamps is often called a time base. Those concepts are related, but they are not the same thing.

Diagram separating frame rate, time base and presentation or decoding timestamps. — Frame rate, clock units and timestamps solve different timing problems.

Key distinction: 30 fps describes picture cadence. A time base such as 1/90000 describes the units used to represent timestamps. Variable-frame-rate video may not have one fixed interval between every frame.

What frame rate means

Common rates include 24, 25, 30, 50 and 60 frames per second, plus fractional broadcast-derived rates such as 23.976, 29.97 and 59.94. Higher frame rates can represent motion more frequently, but do not by themselves guarantee better picture quality.

Nominal name	Exact/common rate	Typical context
24p	24 fps	Cinema and true-24 digital production
23.976p	24000/1001 ≈ 23.976	NTSC-derived film/video workflows
25p	25 fps	50 Hz television regions
29.97p/i	30000/1001 ≈ 29.970	NTSC-derived television/video
50p	50 fps	High-motion 50 Hz workflows
59.94p	60000/1001 ≈ 59.940	NTSC-derived high-frame-rate video

Why 23.976 and 29.97 exist

The familiar 1000/1001 rates come from colour NTSC-era timing compromises. Modern digital systems inherited those rates for compatibility, which is why software often distinguishes 24 from 23.976 and 30 from 29.97.

Treating them as identical can cause long-duration sync or timing errors even though the numerical difference looks tiny.

Constant versus variable frame rate

Mode	Meaning	Typical use
CFR	Frames follow a regular presentation cadence.	Broadcast, editing, many camera and delivery workflows.
VFR	Intervals between presented frames may vary.	Phones, screen capture, conferencing and content that benefits from avoiding redundant frames.

VFR does not mean “bad” or “broken.” It means timestamps, rather than one fixed cadence, are essential to correct playback.

Time base, PTS and DTS

Compressed video often uses timestamps to say when a frame should be decoded and presented. PTS is commonly the presentation timestamp; DTS is commonly the decoding timestamp. They can differ when frames are decoded in a different order from display order, as happens with B-frames.

A time base is the unit used for those timestamp values. It is not necessarily “1 divided by the frame rate.” Containers and streams may use convenient clock scales independent of presentation cadence.

Common confusion

23.976 is not exactly 24.

The exact 24000/1001 rate matters over long durations.

60 fps does not mean every file stores sixty complete independent pictures every second.

Inter-frame codecs may describe many pictures partly from neighbouring pictures.

Time base is not bitrate.

It is a timing scale, not a measure of compressed data per second.