Codec

In simple terms

Raw audio and video are huge. A second of 1080p60 raw video is hundreds of megabytes. A codec (“coder/decoder”) is the algorithm that squeezes that down to a few megabits per second on the way out, and reconstructs a visually-similar copy on the way back in.

More detail

Video codecs exploit:

• Spatial redundancy within a frame (like JPEG does for images).
• Temporal redundancy between frames — most of one frame looks like the previous one. Encode the difference, not the whole picture.
• Motion estimation — describe how blocks of pixels moved between frames.
• Entropy coding — pack the bits efficiently.

Frames come in types:

• I-frame (intra) — encoded from scratch. Random-access point.
• P-frame (predicted) — encoded as differences from the previous frame.
• B-frame (bidirectional) — uses both past and future references.

Major video codecs in 2026:

Codec	Family	Notes
H.264 / AVC	Mainstream	Universal compatibility, modest compression.
H.265 / HEVC	Better	Patent-encumbered; widespread on Apple devices.
AV1	Royalty-free	~30% smaller than H.265 for same quality. Hardware-decode now common.
VP9	Royalty-free	YouTube workhorse; AV1’s predecessor.
H.266 / VVC	Next-gen	Even better compression; rolling out.

Audio codecs: Opus (modern, free), AAC (Apple/streaming), MP3 (legacy), FLAC (lossless).

Codecs are wrapped in containers: MP4, WebM, MKV, MOV. The container holds video, audio, subtitles, and metadata; the codec is what’s inside.

Why it matters

Codecs are what make video streaming, video calls, and modern photography possible. They are also a battleground of patents and royalties — the move toward royalty-free codecs (AV1, Opus) is a major industry trend.

Real-world examples

• Netflix streams in AV1 (and HEVC) to save bandwidth.

• A WhatsApp voice note uses Opus.

• YouTube delivers different codecs to different devices based on what they can decode in hardware.

• A single Netflix title is encoded into dozens of variants (different codecs, bitrates, resolutions) so adaptive streaming can pick the right one for each viewer’s bandwidth.

Common misconceptions

• “MP4 is a codec.” MP4 is a container. The video inside it is usually H.264 or HEVC.
• “Lossless video is just impractical.” It’s used in production and archiving; broadcast and streaming use lossy codecs because the size difference is enormous.

Learn next

The still-image equivalent: image format. The atomic display unit: pixel.