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# Audio variable bit rate indication

## Overview

- Containing profile atom: Track (sound), movie
- `part-ID`: `0x20202020` (universal feature)
- `feature-code`: `'avbr'`
- `feature-value`: Unsigned `int(32)` holding the value `0` if the audio is constant bit rate or `1` if the audio is variable bit rate

## Feature Values

The feature value holds one of the following two values: `0` if the audio is constant bit rate, or `1` if the audio is variable bit rate.

## Writer Responsibilities

A writer of the Audio Variable Bit Rate Indication feature should determine if the audio frames are constant or variable bit rate in nature and record either `0` or `1`, respectively.

## Feature Value Algorithm

Consult the audio sample descriptions. If the `compressionID` field in the sample descriptions is `0` or `-1`, then the audio is constant bit rate. If the field is `-2`, then the same algorithm as for video applies: if all the samples have both constant duration and constant size, then the audio is constant bit rate; otherwise it is variable.

## Reader Responsibilities

A reader of this feature code should only expect the values `0` or `1`.

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