Glossary
What is a codec?
MarkdownWhat a codec is
The word comes from coder-decoder. A codec decides how the sound or picture data is shrunk. The result is then wrapped in a container such as MP4 or MOV.
Why it matters on a Mac
A file can have a familiar extension and still not play, because the codec inside is one your app does not support. The extension tells you the container, not the codec.
How to check it on macOS
Open the file in QuickTime Player and choose Window > Show Movie Inspector. Or drop it into Media Info in Toolbox to see the codec of each video and audio track.
How Toolbox handles it
Media Info reads the codecs, resolution, frame rate and sample rate. Convert Video and Convert Audio change the codec when needed, or copy the streams as they are when the new container already fits.
Frequently asked questions
Is a codec the same as a format?
No. A format, or container, holds the streams. A codec compresses each stream inside it.
Which video codec should I use?
H.264 for compatibility, HEVC for smaller files on newer devices.
Can I change the codec without losing quality?
Re-encoding with a lossy codec loses some quality. Copying streams into a new container does not.
