What is a video codec?

Compression, separate from the file

Uncompressed video is enormous. A single 1080p frame with 8 bits for each of three colour channels is about 6 megabytes, so one second at 30 frames would be close to 190. A codec, short for coder and decoder, shrinks that by describing what changes instead of storing every pixel of every frame. The common codecs are published standards. The ITU publishes H.264, known as Advanced Video Coding, and H.265, known as High Efficiency Video Coding, while the Alliance for Open Media publishes AV1. The container is a separate layer on top. An MP4 file can hold H.264, HEVC, VP9 or AV1 video, and the file extension alone never tells you which codec is inside it.

How a codec compresses

Most video codecs share one design. Some frames are stored whole as key frames, and most of the rest are stored as predictions: this block of pixels matches a block in an earlier frame, shifted slightly, plus a small correction. MDN's reference describes AVC as a motion compensation based codec for exactly this reason. Newer codecs add larger and more flexible blocks, better predictions and cleverer ways to discard detail the eye will not miss. MDN notes that HEVC can in theory produce files half the size of AVC at comparable quality, and that AV1 achieves compression as much as 50 percent better than AVC. The trade is effort: newer codecs take more computing power to encode, and smooth playback often needs hardware decoding.

When the choice matters

It matters for compatibility first. H.264 plays in all major browsers and on almost every device, which is why it remains the safe choice for a file that will be uploaded, emailed or embedded by someone else. It matters for licensing. MDN describes AVC and HEVC as proprietary formats covered by patents, with licensing managed through patent pools, while VP9 and AV1 are royalty free. It matters for file size at high resolutions, where HEVC and AV1 save the most. And it matters for how long the encode takes, since the most efficient codecs are also the slowest to produce. For uploads to platforms that re-encode everything anyway, compatibility usually beats efficiency, because the platform decides what viewers finally receive.

Common mistakes

Treating the file extension as the codec is the first, and it explains most files that play on a laptop but not on a phone. Check the codec with a media information tool before blaming the file. The second is sending HEVC to a client or a platform without checking their playback, since support still varies by device, operating system and browser. The third is converting again and again between codecs, losing a little detail at every generation. The fourth is using an editing codec, a large intermediate format built for smooth scrubbing on a timeline, as the delivery file. Such files are excellent in the edit and far too heavy to send, stream or upload over an ordinary connection. The fifth is choosing a codec for the smallest possible file without testing playback on a modest, older phone, which is often where a new codec first fails in front of a real viewer.

Where it shows up in the product

The video builder here exports H.264 video in an MP4 container at 30 frames per second, with no codec choice to make. That is the most widely playable combination, so the file can go straight to a social platform, a website or an email. Uploaded clips are accepted as MP4, WebM or MOV files up to 200 MB, and can be trimmed and set to cover or fit their box. Because the builder plays clips in the browser, a clip using a codec the browser cannot decode is the likely cause of one that refuses to play, and re-exporting it as an H.264 MP4 is the dependable fix. The presentation recorder in the slides module is the one place that writes WebM, using VP9 or VP8 video with Opus audio.

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