Quick answer: A codec is the method used to compress audio or video for storage and decompress it for playback, such as H.264, HEVC or AAC.
Codec (Coder-decoder) explained
A codec is the method a device uses to compress video or audio when saving it and to decompress it again for playback. Raw video is enormous, so it is almost always compressed with a codec. Common video codecs are H.264, HEVC (H.265), AV1, VP9 and ProRes. Common audio codecs are AAC, MP3, Opus and FLAC.
The codec is separate from the file extension. An .mp4 or .mov file can contain different codecs, which is why one MP4 plays on a TV and another does not.
When a file will not play for someone, the codec is the first thing to check. Re-encoding to a widely supported codec like H.264 with AAC audio usually solves it.
Common codecs at a glance
| Codec | Type | Where you meet it | Plays on |
|---|---|---|---|
| H.264 | Video | MP4 files, older phones, the web | Almost everything |
| HEVC (H.265) | Video | iPhone and Android recordings, 4K | Most recent devices |
| AV1, VP9 | Video | Web video and streaming | Modern browsers, newer devices |
| ProRes | Video | Professional editing | Editing software |
| AAC | Audio | The sound in most MP4 and MOV files | Almost everything |
| MP3 | Audio | Music files | Everything |
| FLAC | Audio | Lossless music | Most modern devices and players |
How to fix a video that will not play
- Find out the codecs: open the file in VLC and look at the codec information, or use MediaInfo.
- Try VLC on the recipient's computer. It has its own decoders and plays most files.
- If they need to use a particular player, TV or app, convert a copy to MP4 with H.264 video and AAC audio.
- Send both the original and the converted copy if quality matters, and label them clearly.
Why the codec matters when sending files
A download link does not change the codec: the recipient gets exactly the file you uploaded. Chat apps and social networks are different. Many of them re-encode videos with their own settings, which can lower quality.
So if a video looks perfect on your phone but bad after sending through an app, the app's re-encoding is the likely reason, not the original codec.
Frequently asked questions
How do I find out which codec a video uses?
Media players such as VLC show it under media or codec information, and file info tools like MediaInfo list it in detail.
Is MP4 a codec?
No. MP4 is a container format. The codec inside it is often H.264 or HEVC for video and AAC for audio.
Why does a video play but have no sound?
Usually the player does not support the audio codec, for example AC-3 or DTS inside some MKV files. Converting the audio to AAC normally fixes it.
Do I need to install codecs?
Rarely these days. A player like VLC includes its own decoders, or you can convert the file to MP4 with H.264 and AAC.
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