stco

STCO is a term used in digital media file formats, most notably as the four-character type code for the Chunk Offset Box in the ISO base media file format (commonly MP4 and QuickTime containers). The box is identified by the code “stco” and is part of the media data indexing system that helps locate chunks of audiovisual data within a file.

The primary purpose of the stco box is to store a list of 32-bit offsets that indicate the byte positions of media chunks from the beginning of the file. Each offset corresponds to a particular chunk, enabling efficient seeking and random access during playback or processing without scanning the entire file. When very large files exceed 32-bit offsets, a 64-bit variant called co64 is used to hold larger values.

In a typical track, the stco box sits within the track’s sample table area (stbl) alongside related boxes such as stsc (Sample-To-Chunk Box), stsz (Sample Size Box), and stts (Decoding Time To Sample Box). Together, these boxes map samples to file positions, sizes, and timing information, supporting accurate decoding and playback.

Usage and significance: STCO is a standard component in MP4/QuickTime parsing and playback pipelines. Software libraries, media players, and editors rely on the chunk offset information to quickly locate and stream or edit media data.

History and standards: The concept originates from the Apple QuickTime File Format and is formalized within the ISO base media file format, particularly ISO/IEC 14496-12, which defines the box structures and their roles in media indexing.

See also: MP4, ISO base media file format, co64, stsc, stsz, stts.