The container (file format)
MP4, MOV, MKV, WebM and AVI are containers: boxes that hold a video stream, one or more audio streams, subtitles and information such as the length and title. The container decides which streams can go in it and how they are stored together, which is why the file extension is the container's name.
The codec
A codec is the method used to compress the picture or the sound. Common video codecs are H.264 (also called AVC), H.265 (HEVC), VP9 and AV1. Common audio codecs are AAC, MP3 and Opus. A device must know the codec to play the file, whatever the container is.
Why a video will not play
- The codec is not supported: iPhones record in HEVC, which some Windows PCs and older TVs cannot decode.
- The container is not supported: many phones and TVs do not open MKV, even when the codec inside is fine.
- The audio is the problem: picture but no sound often means an audio codec the player lacks.
The combination that plays everywhere
MP4 with H.264 video and AAC audio works on practically every phone, computer, browser, smart TV and editing program. That is what our converters make: MOV to MP4 and MKV to MP4 re-encode the video to H.264 and the sound to AAC.
For websites, WebM with VP9 and Opus is a good, smaller alternative. For audio only, convert to MP3 or M4A, for example with MP4 to MP3.
Does converting lose quality?
Re-encoding with a lossy codec always discards a little detail. The settings used here keep the picture visually the same for normal viewing. Converting to a "better" codec cannot add back detail the original did not have.