What is Digital Video?

Digital Video: Simple Definition

Digital video is a sequence of still images shown fast enough to read as motion, almost always with synchronised audio alongside. Each image is a frame, and the rate at which they are displayed is measured in frames per second.

Stored naively, video is impossibly large. A single second of uncompressed 1080p runs to hundreds of megabytes, which is why every practical video format is built around compression rather than treating it as an optimisation applied afterwards.

Frame rate is a creative decision, not just a technical one. Film has used 24 frames per second for a century and the slight motion blur reads as cinematic; 60 frames per second looks hyper-real, which suits sport and games and unsettles people watching drama.

Why Video Compresses So Well

Successive frames usually differ only slightly, so video codecs encode most frames as changes from neighboring ones rather than as full pictures.^[3] Self-contained keyframes are interspersed with predicted frames that store only motion and differences, which is how an hour of footage shrinks to a fraction of its raw size.^[3] This temporal compression is why a mostly static shot compresses far smaller than fast action with constant scene changes: the more each frame resembles the last, the less the codec has to store.

Frame Rate and Scanning

Beyond resolution, video is defined by its frame rate and whether it is progressive (each frame drawn whole) or interlaced (alternating half-frames of odd and even lines), a legacy of broadcast television.^[3] Higher frame rates smooth fast motion but multiply the data the codec must handle.^[3] Common rates are 24 fps for a cinematic look, 30 fps for general video, and 60 fps for smooth motion in gaming and sports footage.

What Decides a Video's Size

Four factors together set a video file's size and quality: resolution (how many pixels per frame), frame rate (how many frames per second), bitrate (how many bits per second of data), and the codec (how efficiently those bits are used). Raising resolution or frame rate increases the data the codec must compress; raising bitrate preserves more detail but grows the file; a more efficient codec gets the same quality from fewer bits. Understanding these four explains why one 1080p video is tiny and another is huge.

Streams Inside a Container

A finished video file is rarely a single thing: a container interleaves the compressed video with one or more audio tracks, subtitles and metadata, keeping them in sync during playback.^[1] Utilities like FFmpeg can extract, transcode or repackage these individual streams.^[2]

How Digital Video Works

The saving comes from time. Consecutive frames are nearly identical, so a codec stores occasional complete keyframes and, in between, only what changed: which regions moved and where to. A static shot compresses enormously; fast motion or a scene cut forces the encoder to spend far more data.

A video file therefore has two layers, and confusing them causes most playback problems. The container, such as MP4 or MKV, packages and synchronises the tracks. The codecs, such as H.264 or AAC, compress the content inside. A player that rejects a file usually lacks the codec, not the container.

Resolution, frame rate, and bitrate compound. Doubling resolution quadruples the pixels; doubling frame rate doubles the frames. 4K at 60 frames per second is therefore many times the data of 1080p at 30, and giving it too little bitrate produces something visibly worse than the lower-resolution original.

Examples of Digital Video