---
title: "Sync sample atom ('stss')"
framework: quicktime-file-format
role: symbol
role_heading: Atom
path: quicktime-file-format/sync_sample_atom
---

# Sync sample atom ('stss')

An atom that identifies the key frames in the media.

## Overview

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.  |   |   |   |   |   |   |

## Topics

### Data fields

- [Size](quicktime-file-format/sync_sample_atom/size.md)
- [Type](quicktime-file-format/sync_sample_atom/type.md)
- [Version](quicktime-file-format/sync_sample_atom/version.md)
- [Flags](quicktime-file-format/sync_sample_atom/flags.md)
- [Number of entries](quicktime-file-format/sync_sample_atom/number_of_entries.md)
- [Sync sample table](quicktime-file-format/sync_sample_atom/sync_sample_table.md)

## See Also

### Describing samples

- [Sample table atom ('stbl')](quicktime-file-format/sample_table_atom.md)
- [Seeking with a QuickTime file](quicktime-file-format/seeking_with_a_quicktime_file.md)
- [Sample description atom ('stsd')](quicktime-file-format/sample_description_atom.md)
- [Time-to-sample atom ('stts')](quicktime-file-format/time-to-sample_atom.md)
- [Creating video tracks at 30 frames per second](quicktime-file-format/creating_video_tracks_at_30_frames_per_second.md)
- [Creating video tracks at 29.97 frames per second](quicktime-file-format/creating_video_tracks_at_2997_frames_per_second.md)
- [Creating sound tracks at 44.1 kHz](quicktime-file-format/creating_sound_tracks_at_441_khz.md)
- [Composition offset atom ('ctts')](quicktime-file-format/composition_offset_atom.md)
- [Composition shift least greatest atom ('cslg')](quicktime-file-format/composition_shift_least_greatest_atom.md)
- [Using composition offset and composition shift least greatest atoms](quicktime-file-format/using_composition_offset_and_composition_shift_least_greatest_atoms.md)
- [Partial sync sample atom ('stps')](quicktime-file-format/partial_sync_sample_atom.md)
- [Sample-to-chunk atom ('stsc')](quicktime-file-format/sample-to-chunk_atom.md)
- [Referencing two data files with a single track](quicktime-file-format/referencing_two_data_files_with_a_single_track.md)
- [Sample size atom ('stsz')](quicktime-file-format/sample_size_atom.md)
- [Chunk offset atom ('stco')](quicktime-file-format/chunk_offset_atom.md)