transmuxing

Transmuxing, also known as remuxing, is the process of changing a multimedia file’s container format without re-encoding the audio or video streams. The original codecs remain intact; only the packaging is changed to fit the target container, such as moving from MPEG-TS to MP4 or from MKV to WebM.

Transmuxing is distinct from transcoding. In transmuxing, the elementary streams (the encoded video and audio data) are copied directly into a different container. This preserves quality and avoids the computational cost of re-encoding. It is often used to improve compatibility with devices, players, or streaming workflows, rather than to alter the media’s content.

Common workflows involve repackaging video and audio into a target container suitable for a platform or protocol. Examples include remuxing MPEG-2 Transport Stream files to MP4 for offline playback, packaging to MP4 or WebM for web players, or preparing segments for streaming formats such as DASH or HLS. In streaming, containers are often fragmented (for example, fMP4 in DASH or HLS) to enable efficient delivery and seeking.

The transmuxing process generally involves demultiplexing the input container to obtain separate streams, copying the streams into the target container, and adjusting container-specific metadata. Key considerations include ensuring codec compatibility with the target container, handling timestamp and timebase conversions to preserve synchronization, and updating indexes or metadata (such as moov boxes in MP4) for proper playback and seeking.

Common tools include FFmpeg (with the -c copy option to copy streams), MP4Box from the GPAC project, and various media players or transcoding suites that offer remuxing capabilities. Transmuxing enables flexible packaging with minimal processing, making it a staple in media preparation and delivery workflows.