《Transparent end-to-end packet switched streaming service (PSS);3GPP file format (3GP)(Release 7)》
5.4.5 Progressive-download profile
The 3GP Progressive-download profile is branded ‘3gr7’. It is used to label 3GP files that are suitable for progressive download, i.e. a scenario where a file may be played during download (with some delay).
The following constraints shall apply to 3GP files conforming to Progressive-download profile:
- the ‘moov’ box shall be placed right after the ‘ftyp’ box in the beginning of the file;
- all media tracks (if more than one) shall be interleaved with an interleaving depth of one second or less.
NOTE 1: This profile functions as an aid and not a requirement for progressive download, which has been an inherent feature of the 3GPP file format since the first version in Release 4. By parsing a 3GP file, a client can always determine whether a file can be progressively downloaded, and then calculate the interleaving depth from the meta-data in the ‘moov’ box.
NOTE 2: The ‘interleaving depth of one second or less’ means that:
- Each chunk contains one or more samples, with the total duration of the samples being either: no greater than 1 second, or the duration of a single sample if that sample’s duration is greater than 1 second;
- Within a track, chunks must be in decoding time order within the media-data box ‘mdat’;
- It is recommended that, in ‘mdat’, regardless of media type, the chunks for all tracks are stored in ascending order by decoding time. However, this order may be perturbed so that, when two chunks from different tracks overlap in time, the chunk of one track (e.g. audio) is stored before the chunk of the other track (e.g. video), even if the first sample in the second track has a slightly earlier timestamp than the first sample in the first track.
Progressive Download Guidelines:
To enable progressive download, session information should always be put at the beginning of the file and the media tracks should be interleaved within the file. The following guidelines should be followed during the creation of ISMA 2.0 files for progressive download:
• The Movie box (‘moov’) SHOULD be placed at the start of the file, right after the File Type box (‘ftyp’).
• All media data SHOULD be present within the file (the data source of every Data Reference box should be ‘self-contained’);
• The media tracks SHOULD be interleaved inside the file. The typical interleaving depth is 1 second.
The meaning of interleaving depth is given as follows: Each chunk contains one or more samples, with the total duration of the samples being either: no greater than 1 second, or the duration of a single sample if that sample’s duration is greater than 1 second. Within a track, chunks must be in decoding time order within the Media Data box (‘mdat’).
A client that does not support progressive download can always play the file once it has been completely downloaded. A progressive download client can start playing a ISMA 2.0 file that has been created along the progressive download guidelines once it has received a first chunk of all media in the session.