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# Sync sample atom ('stss')

An atom that identifies the key frames in the media.

## Overview

In a media that contains compressed data, key frames define starting points for portions of a temporally compressed sequence. The key frame is self-contained — that is, it is independent of preceding frames. Subsequent frames may depend on the key frame.

The sync sample atom provides a compact marking of the random access points within a stream. The table is arranged in strictly increasing order of sample number. If this table is not present, every sample is implicitly a random access point.

Sync sample atoms have an atom type of `'stss'`. The sync sample atom contains a table of sample numbers. Each entry in the table identifies a sample that is a key frame for the media. If no sync sample atom exists, then all the samples are key frames.

The layout of a sync sample atom is as follows.

|Sync sample atom data field                                                                    |Bytes   |
|-----------------------------------------------------------------------------------------------|--------|
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sync_sample_atom/Size``             |4       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sync_sample_atom/Type`` = `'stss'`  |4       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sync_sample_atom/Version``          |1       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sync_sample_atom/Flags``            |3       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sync_sample_atom/Number_of_entries``|4       |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Sync_sample_atom/Sync_sample_table``|Variable|

## Topics

### Data fields

[`Size`](/documentation/quicktime-file-format/Sync_sample_atom/Size)

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

[`Type`](/documentation/quicktime-file-format/Sync_sample_atom/Type)

A 32-bit integer that identifies the atom type.

[`Version`](/documentation/quicktime-file-format/Sync_sample_atom/Version)

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

[`Flags`](/documentation/quicktime-file-format/Sync_sample_atom/Flags)

A 3-byte space for sync sample flags.

[`Number of entries`](/documentation/quicktime-file-format/Sync_sample_atom/Number_of_entries)

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

[`Sync sample table`](/documentation/quicktime-file-format/Sync_sample_atom/Sync_sample_table)

A table of sample numbers.


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