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

Some Useful Examples and ScenariosPreparing Sound and Subtitle Alternate Groups for Use with Apple DevicesRelationships Across Alternate GroupsAn audio alternate group and subtitle alternate group may need to use different languages because the firstis the spoken language and the second is the written language and the BCP 47 language tags may differ. Ifthere is not a need to differentiate, tracks in both alternate groups should use the same language code andoptionally extended language tag.Sound tracks can have a track reference of type 'folw' (for “follows”) to a single subtitle track; this track isthe default to select if the sound track is selected. Use of a 'folw' reference is a fallback in cases where thelanguage tagging cannot be used for some reason (for example, the written language and spoken languageuse different extended language tags, as with Norwegian).All subtitle tracks found in a subtitle track alternate group are candidate tracks for any chosen sound track. Nofacility to restrict the set of available subtitle tracks for a particular selected sound track is currentlydefined. However, to guide a player's behavior in making a selection among subtitle alternates in the absenceof a user selection or preference, a single subtitle track may be associated with a sound track as follows:●●●●●A sound track may include a 'folw' (“follows”) track reference to a corresponding subtitle track.If a sound track has a 'folw' track reference to a subtitle track, that referenced subtitle track is the defaultsubtitle track for that sound track.Each sound track can have either zero or one 'folw' track references to a subtitle track.If a subtitle track pair is to be made the default, the sound track should have a 'folw' track reference tothe forced subtitle track of the track pair.If there is no 'folw' track reference to a subtitle track, a player most commonly determines the defaultsubtitle track by matching aspects of the audio and subtitle tracks, typically by matching the extendedlanguage tag (or language code if there is no language tag) in the set of candidate tracks.If no match is found by language, a player might choose another candidate track based upon a userpreference (for example, from a list of preferred languages). Another player might choose the first trackin 'trak' box order in the 'moov' among the candidate tracks. Another player might have another mechanismto choose the default. Possible fall-back approaches are neither enumerated nor restricted here. Note thatselecting no subtitle track may be appropriate.●If the default matching between audio language tagging and subtitle track language tagging cannot beused, a 'folw' track reference must be authored in the media file. (For example, Norwegian as spoken usesa different language tag than the two language tags for Norwegian as written.)2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.336