What Is a Container Format? MP4, MKV and M4A

What is a container format? It is a file type that holds one or more streams, such as audio, video and subtitles, together in a single file. MP4, MKV and Ogg are containers. The container stores the streams and their timing, while codecs compress each stream inside.

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Key facts

One container can carry several audio tracks, for instance two languages or a commentary, plus subtitle tracks.
The extension names the container, not the codec; two files that end in .mp4 can hold different audio codecs.
A player that opens the container but lacks the codec will show the picture and stay silent, or the reverse.
For transcription only the audio stream matters, so extracting it can turn a huge video file into a small one.

What it looks like

// list the streams inside a container
ffprobe -hide_banner lecture.mp4
//   Stream #0:0: Video: h264, 1920x1080
//   Stream #0:1: Audio: aac, 48000 Hz, stereo
//   Stream #0:2: Subtitle: mov_text

// keep only the audio, without re-encoding
ffmpeg -i lecture.mp4 -vn -c:a copy lecture.m4a

See the full list of languages, the tools and the glossary.

Frequently asked questions

Is MP4 a codec?

No. MP4 is a container. The video inside is often H.264 and the audio AAC, but the container is only the box that keeps the streams together.

Why does my video have several audio tracks?

Containers can store multiple audio streams, such as different languages or a separate microphone track. Pick the one with the voice you need before transcribing.

Do I need to extract audio from a video before transcribing?

No. The Potok bot accepts video files directly. Extraction only helps when a file is too large or you want to choose a specific audio track.

How large a video file can I send?

Files up to 2 GB are accepted, and larger ones go through the Upload button in the bot's mini app. A copy of the audio alone is far smaller.

What is the difference between a container and a codec?

A container organises streams in a file. A codec compresses and decompresses one stream. A single container can hold streams made with several different codecs.

Sources

Each source was opened by hand on the date shown. Pages change, so open the source if a detail matters to you.

Container format (Wikipedia), Wikipedia · checked 2026-10-18
MP4 file format, MPEG-4 Part 14 (Wikipedia), Wikipedia · checked 2026-10-18

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