Quicktime File Format (2012-08-14).pdf

Profile Atom GuidelinesUniversal FeaturesReader ResponsibilitiesA reader of this feature code should compare the recorded value with its own limits to determine if the contentcan be played. The reader should not perform an equality comparison (=) but instead a relative comparison(=).Because this value may be an over-estimate of the true maximum bit rate, the reader should not refuse to playa file on the basis of this value, although a warning may be appropriate. To determine the current or actualbit rate, the reader may need to perform an inspection of the video track's sample table.Average Audio Bit RateContaining profile atomTrack (sound), moviepart-ID0x20202020 (universal feature)feature-code'avab'feature-valueUnsigned int(32) indicating average audio bit rate in bits per secondFeature ValuesThe value is an unsigned 32-bit integer indicating the average audio bit rate in bits per second.Example: 128 kbps = 128000.Writer ResponsibilitiesA writer of the average audio bit rate feature should record a value that is equal to or greater than the averagebit rate for the sound track, measured across all media samples. A writer (such as a CE device) may choose torecord a constant value so long as that value is greater than or equal to the average bit rate that may beencoded. It is also permitted to record a value set by the audio encoder during initialization so long as thevalue is never exceeded on average.Feature Value AlgorithmNormally, the long-term average: total sample sizes divided by total sample durations.If the feature is written for a newly encoded track (as by a CE device), it is permitted to record a value used toinitialize the audio encoder. If the sound track is edited and the average video bit rate recalculated, it may becalculated as an overall average based on the sample table.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.403