Inside Macintosh: Sound

The Format Version Chunk

One difference between the AIFF and AIFF-C file formats is that files of type AIFF-C contain a Format Version Chunk and files of type AIFF do not. The Format Version Chunk contains a timestamp field that indicates when the format version of this AIFF-C file was defined. This in turn indicates what format rules this file conforms to and allows you to ensure that your application can handle a particular AIFF-C file. Every AIFF-C file must contain one and only one Format Version Chunk.

In AIFF-C files, a Format Version Chunk has this structure:



 
TYPE FormatVersionChunk =

RECORD

    ckID:           ID;             {'FVER'}

    ckSize:         LongInt;        {4}

    timestamp:      LongInt;        {date of format version}

END;

In AIFF files, there is no Format Version Chunk.

The timestamp field indicates when the format version for this kind of file was created. The value indicates the number of seconds since January 1, 1904, following the normal time conventions used by the Macintosh Operating System. (See the chapter on date and time utilities in Inside Macintosh: Operating System Utilities for several routines that allow you to manipulate time stamps.)

You should not confuse the format version time stamp with the creation date of the file. The format version time stamp indicates the time of creation of the version of the format according to which this file is structured. Because Apple defines the formats of AIFF-C files, only Apple can change this value. The current version is defined by a constant:



 
CONST

    AIFCVersion1        = $A2805140;        {May 23, 1990, 2:40 p.m.}