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Definition
Transcoding is the process of converting a media file from one codec or format to another. This involves decoding the source file and re-encoding it in the target format. Transcoding is used to change video codecs (H.264 to VP9), container formats (MKV to MP4), or to adjust quality settings.
Transcoding is a decode-then-encode process. The source file is first decoded back to raw uncompressed data, then that data is encoded using the target codec and settings. This means transcoding between lossy formats always involves some quality loss — each encode cycle introduces additional compression artifacts.
Common transcoding scenarios include: converting MKV files to MP4 for broader device compatibility, re-encoding video at lower bitrate for streaming, converting H.265 to H.264 for older device support, and converting audio between formats (FLAC to MP3 for portable players).
The alternative to transcoding is transmuxing (remuxing) — changing the container format without re-encoding the media streams. For example, copying an H.264 video stream from an MKV container to an MP4 container is lossless and very fast because the video data is not re-encoded. Transmuxing is only possible when the target container supports the source codec.