Glossary
What is a codec?
By the KeyMo team · Updated
A codec is the method used to compress video or audio into a smaller file and decompress it again for playback. The name joins coder and decoder. A codec is not the file type: MP4 is a container that holds streams encoded with codecs such as H.264 for video and AAC for audio.
Codec versus container
Two layers hide behind a file name. The container is the wrapper that holds the streams and keeps them in sync. The codec is how each stream inside was squeezed.
| Layer | Examples | What it does |
|---|---|---|
| Video codec | H.264 (ITU-T: Advanced video coding for generic audiovisual services), HEVC (H.265) | Compresses the picture |
| Audio codec | AAC-LC, Opus | Compresses the sound |
| Container | MP4 (ISO/IEC 14496-14), MOV (Apple's QuickTime file format) | Holds video, audio and timing together |
Apple describes the QuickTime format as the basis of the MPEG-4 standard while remaining a distinct file type. That is why MOV and MP4 look alike and are still different containers. See also H.264, MP4 and MOV.
What YouTube asks for
YouTube's upload settings page recommends an MP4 container with no edit lists and the moov atom at the front of the file. For video it recommends H.264 in High Profile, progressive scan, 4:2:0 chroma subsampling and variable bitrate. For audio it recommends AAC-LC or Opus at a 48 kHz sample rate. These are recommendations. YouTube also lists HEVC (H.265), ProRes and others among the formats it accepts.
- Recommended 1080p SDR bitrate: 8 Mbps at 24, 25 or 30 fps, and 12 Mbps at 48, 50 or 60 fps.
- YouTube says no bitrate limit is required.
- Upload at the frame rate the video was recorded at.
Example: two files with the same name
Two files are both called talk.mp4. One holds H.264 video and AAC audio and plays almost anywhere. The other holds HEVC video and plays on your phone but stutters in an older editor. The extension names the container. It does not tell you which codec is inside. Open the file's info panel, or run ffprobe, to see the real codecs.
Decision rule
- Need the widest compatibility: export H.264 video and AAC audio in an MP4.
- Source is HEVC and an editor chokes on it: export a copy as H.264 first.
- Only changed the extension from .mov to .mp4: nothing changed. Renaming does not convert a file.
- Quality dropped after a re-export: each lossy export loses some detail, so edit from the original instead of an earlier export.
Common mistakes
- Confusing codec with bitrate. The codec is the method; bitrate is how much data per second it may use.
- Raising bitrate to rescue a blurry source. It cannot add back detail.
- Assuming an audio-only file can go up as a video. YouTube says MP3, WAV and PCM files must be converted to video first.
- Padding a vertical clip with black bars. YouTube advises against adding padding or bars into the video itself.
Where KeyMo fits
KeyMo takes MP4 or MOV uploads from 5 minutes to 3 hours, up to 10 GB, and exports each clip as H.264 video with AAC audio at 1080x1920 and 30 fps, vertical 9:16, with no watermark. You do not choose a codec. Those match the video and audio codecs YouTube recommends, although YouTube's encoding guide is written around 16:9 and gives no pixel sizes for 9:16.
Sources: YouTube recommended upload encoding settings, YouTube Help (checked 11 October 2026); Supported YouTube file formats, YouTube Help (checked 11 October 2026); Video resolution & aspect ratios, YouTube Help (checked 11 October 2026); H.264: Advanced video coding for generic audiovisual services, ITU-T (checked 11 October 2026); ISO/IEC 14496-14:2020 MP4 file format, ISO (checked 11 October 2026); QuickTime File Format, Apple Developer Documentation (checked 11 October 2026).