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

Profile Atom GuidelinesUniversal FeaturesWriter ResponsibilitiesA writer of the average video bit rate feature should record a value that is equal to or greater than the averagebit rate for the video 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 video encoder during initialization so long as thevalue equals or exceeds the average calculated from the resulting file.Feature Value AlgorithmIdeally, 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 video encoder. If the video track is edited and the average video bit rate recalculated, it may becalculated as an overall average based on the sample table.Reader ResponsibilitiesA reader of the average video bit rate feature value should compare the recorded value with its own limits todetermine if the content can be played. The reader should not perform an equality comparison (=) but insteada relative comparison (=).Because a writer may record a larger value than the actual video bit rate, a reader should not interpret this asthe actual video bit rate. To determine the current or actual bit rate, the reader may need to perform aninspection of the video track's sample table.CommentsThe value of this feature should be deducible from information found in the sample table. Track edits must beconsidered in its calculation. Note that for highly variable bit rate video, the average rate may not be a typicalrate.Maximum Audio Bit RateContaining profile atomTrack (sound), moviepart-ID0x20202020 (universal feature)feature-code'mabr'2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.401