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# Sample-to-chunk atom ('stsc')

An atom that stores chunk information for the samples in a media.

## Overview

As samples are added to a media, they are collected into chunks that allow optimized data access. A chunk contains one or more samples. Chunks in a media may have different sizes, and the samples within a chunk may have different sizes. The sample-to-chunk atom stores chunk information for the samples in a media.

Sample-to-chunk atoms have an atom type of `'stsc'`. The sample-to-chunk atom contains a table that maps samples to chunks in the media data stream. By examining the sample-to-chunk atom, you can determine the chunk that contains a specific sample.

The layout of the sample-to-chunk atom is as follows.

|Sample-to-chunk atom data field                                                                        |Bytes   |
|-------------------------------------------------------------------------------------------------------|--------|
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sample-to-chunk_atom/Size``                 |4       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sample-to-chunk_atom/Type`` = `'stsc'`      |4       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sample-to-chunk_atom/Version``              |1       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sample-to-chunk_atom/Flags``                |3       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sample-to-chunk_atom/Number_of_entries``    |4       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sample-to-chunk_atom/Sample-to-chunk_table``|Variable|

## Topics

### Data fields

[`Size`](/documentation/quicktime-file-format/Sample-to-chunk_atom/Size)

A 32-bit integer that specifies the number of bytes in this sample-to-chunk atom.

[`Type`](/documentation/quicktime-file-format/Sample-to-chunk_atom/Type)

A 32-bit integer that identifies the atom type.

[`Version`](/documentation/quicktime-file-format/Sample-to-chunk_atom/Version)

A 1-byte specification of the version of this sample-to-chunk atom.

[`Flags`](/documentation/quicktime-file-format/Sample-to-chunk_atom/Flags)

A 3-byte space for sample-to-chunk flags.

[`Number of entries`](/documentation/quicktime-file-format/Sample-to-chunk_atom/Number_of_entries)

A 32-bit integer containing the count of entries in the sample-to-chunk table.

[`Sample-to-chunk table`](/documentation/quicktime-file-format/Sample-to-chunk_atom/Sample-to-chunk_table)

A table that maps samples to chunks.


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