SampleToChunk

SampleToChunk, in the context of the ISO base media file format (used by MP4 and QuickTime), refers to the sample-to-chunk mapping data for a track. It is typically stored in the Sample To Chunk box (stsc) within the Track’s Sample Table Box (stbl). The stsc box provides a compact description of how consecutive samples are grouped into chunks, enabling efficient indexing and access during playback, seeking, and editing.

The stsc box contains one or more entries. Each entry specifies three fields: firstChunk, samplesPerChunk, and sampleDescriptionIndex. The entries are ordered by chunk number. The firstChunk value marks the first chunk to which the rule applies; the rule remains in effect until the next entry’s firstChunk value. This structure means that a single rule can cover multiple consecutive chunks, with a new rule starting at a specified chunk.

A typical use of SampleToChunk is to determine which samples reside in a given chunk when combined with other indexing data: the Sample Size Box (stsz) provides the sizes of samples, while the Chunk Offset Box (stco or co64) provides the byte offsets of chunks. Together, these boxes allow a player or editor to locate and extract the precise samples for a given chunk or range of chunks without inspecting every sample sequentially.

SampleToChunk is part of the broader sample table framework in MP4/QuickTime, interacting with related boxes such as stsz, stco/stco64, and stsd. It supports efficient random access, streaming, and editing operations by mapping the linear sequence of samples to the physical chunked storage.