Transcoding

Codec, container and the terms around transcoding

Most confusion about video transcoding comes from mixing up the codec and the container.

Term	Meaning	Example
Codec	The method that compresses the picture or sound	H.264, HEVC, AV1, ProRes, AAC
Container	The file format that holds the encoded tracks and their timing	MP4, MOV, MKV, MXF
Encoding	Compressing raw or uncompressed footage for the first time	A camera writing its recording
Transcoding	Decoding an encoded file and encoding it again differently	ProRes to H.264
Transmuxing	Moving the same encoded tracks into another container, without re-encoding	H.264 in MOV repackaged as MP4
Transrating	Re-encoding at a lower bitrate, same codec and resolution	A 20 Mbit/s file reduced to 5 Mbit/s

Transmuxing is fast and lossless because the picture is not recompressed. Transcoding takes far more processing time and loses some quality each time, since every lossy codec discards information. That is why transcodes should always be made from the highest-quality source available, never from a previous transcode. See MOV files for how one container can hold very different codecs.

Why video is transcoded

• Playback compatibility. Editing codecs such as ProRes, and many camera formats, do not play in a browser. H.264 in an MP4 or HLS stream plays almost everywhere.
• File size. An intra-frame editing master can run to hundreds of megabits per second. A web stream of the same scene needs a few megabits.
• Different screens and connections. A sales rep on hotel Wi-Fi and a designer on a fibre line need different versions of the same product video.
• Platform specifications. Social networks, broadcasters and stock agencies publish their own resolution, codec and bitrate requirements.

Adaptive bitrate streaming and the bitrate ladder

Instead of one playable file, a video is usually transcoded into a bitrate ladder: several versions (rungs) of the same video at different resolutions and bitrates. Each rung is cut into short segments of a few seconds. The player measures the connection and switches rung between segments, so playback continues at lower quality instead of stopping to buffer.

The most widely used streaming format is HTTP Live Streaming (HLS), published by the IETF as RFC 8216 in 2017. An HLS stream consists of a master playlist (.m3u8) listing the rungs, one playlist per rung, and the media segments. Segments are increasingly stored as fragmented MP4 following CMAF, the Common Media Application Format (ISO/IEC 23000-19).

A typical ladder for marketing video:

Rung	Resolution	Typical use
1080p	1920 × 1080	Desktop, presentation screens
720p	1280 × 720	Laptops, good mobile connections
480p	854 × 480	Mobile data, embedded players
360p	640 × 360	Weak connections, small previews

A ladder should never go above the source: upscaling a 720p source to a 1080p rung makes a larger file with no extra detail.

Transcoding in day-to-day DAM work

A product marketing team uploads a 3 GB launch video. Without transcoding, every reviewer has to download the master before watching it, and the dealer portal cannot show it at all. With transcoding at upload, the video plays immediately in the library, on share pages and in embeds on partner websites, while the master stays in the library for the agency that cuts the next version.

Common mistakes:

• Sending the master file by link to people who only need to watch it.
• Transcoding a transcode, which adds compression artefacts with each generation.
• Deleting the master after making web versions, which rules out any future re-edit or higher-quality export.
• Assuming a .mp4 extension guarantees playback; the codec inside decides.

Transcoding in Freedam

• Every uploaded video is transcoded automatically into an adaptive HLS stream, encoded as H.264 video with AAC audio in fragmented MP4 segments of six seconds.
• The default ladder has 1080p, 720p, 480p and 360p rungs. Rungs taller than the source are skipped, and a source smaller than every rung gets one rung at its own height.
• Sources that browsers cannot play, such as ProRes masters with 4:2:2 colour, are converted to 4:2:0 H.264 so they play in the browser.
• The player chooses the quality level by itself, and viewers can pick one from its quality menu.
• The same stream plays on the asset page, in share links and in video embeds on other websites, with its subtitle tracks.
• Four candidate poster frames are extracted from every video, and editors can pick one, capture any frame or upload a cover. See video poster and subtitles.
• Audio files play from the original and get a waveform preview.

Frequently asked questions

What is the difference between transcoding and encoding?

Encoding compresses footage for the first time. Transcoding takes a file that is already encoded, decodes it and encodes it again with different settings or a different codec.

Does video transcoding reduce quality?

Yes, to some degree, whenever a lossy codec is used: each encode discards information. Transcoding from the original master once per target keeps the loss small. Transmuxing, which only changes the container, does not reduce quality.

Is transcoding the same as converting MOV to MP4?

Not always. If the MOV holds H.264 video, it can be transmuxed into MP4 without re-encoding. If it holds ProRes or another codec MP4 players do not support, it has to be transcoded.