---
title: Sound sample description extensions
framework: quicktime-file-format
role: collectionGroup
role_heading: API Collection
path: quicktime-file-format/sound_sample_description_extensions
---

# Sound sample description extensions

Extend sound sample descriptions by appending other atoms.

## Overview

Overview All extensions to the SoundDescription record are made using atoms. That means one or more atoms can be appended to the end of the SoundDescription record using the standard [size, type] mechanism used throughout the QuickTime movie architecture. Extensions were first added with sound sample description v1. To illustrate this, for sound sample description v1, the extensions are added by following the last field of the struct with QuickTime atoms. The struct implementation looks like this: struct SoundDescriptionV1 {     // original fields     SoundDescription    desc;     // fixed compression ratio information     unsigned long   samplesPerPacket;     unsigned long   bytesPerPacket;     unsigned long   bytesPerFrame;     unsigned long   bytesPerSample;     // optional, additional atom-based fields --     // ([long size, long type, some data], repeat) }; Version 2 of the sound sample description maintains the same mechanism for the addition of extensions. In the sound sample description v2 structure, the sizeOfStructOnly field value provides the offset to the extensions.

## Topics

### Extending sound sample descriptions

- [siSlopeAndIntercept atom](quicktime-file-format/sislopeandintercept_atom.md)
- [siDecompressionParam atom ('wave')](quicktime-file-format/sidecompressionparam_atom.md)
- [Format atom ('frma')](quicktime-file-format/format_atom.md)
- [Terminator atom ('0x00000000')](quicktime-file-format/terminator_atom.md)
- [MPEG-4 elementary stream descriptor atom  ('esds')](quicktime-file-format/mpeg-4_elementary_sound_stream_descriptor_atom.md)
- [Audio channel layout atom ('chan')](quicktime-file-format/audio_channel_layout_atom.md)
- [Subtitle follows track reference atom ('folw')](quicktime-file-format/subtitle_follows_track_reference_atom.md)

## See Also

### Storing audio data

- [Sound sample descriptions](quicktime-file-format/sound_sample_descriptions.md)
- [Sound sample data](quicktime-file-format/sound_sample_data.md)
- [Audio priming-handling encoder delay in AAC](quicktime-file-format/appendix_g_audio_priming_handling_encoder_delay_in_aac.md)