ExtAudioFile - ExtAudioFileTell Incorrect Position Work Around

Q:  I'm calling ExtAudioFileTell for a .m4a file containing AAC audio (immediately after I set the client format) and the API returns a value of -2112 instead of 0. Is this a know issue?

A: I'm calling ExtAudioFileTell for a .m4a file containing AAC audio (immediately after I set the client format) and the API returns a value of -2112 instead of 0. Is this a know issue?

ExtAudioFileTell currently always returns a value which is the number of priming-frames too small.

The ExtAudioFile APIs should take care of this priming offset on behalf of the client however, as the AAC bitstream contains 2112 priming frames, ExtAudioFileTell returns a value which is priming-frames too small, in this case -2112 instead of 0 (zero).

The work around requires the client to compensate:

• Make an initial call to ExtAudioFileTell immediately after opening the file and setting the client format.

• If the returned position is negative, all future calls to ExtAudioFileTell will be off by this value and the client should compensate.

Listing 1  Figure out the offset correction value.

...

    // call ExtAudioFileTell immediately after opening the file and setting the client format
    // and get the correction value to add to all future calls
    SInt64 frameOffset = 0;
    SInt64 frameOffsetCorrection = 0;

    ExtAudioFileTell(sourceFile, &frameOffset);

    if (frameOffset < 0) frameOffsetCorrection = -frameOffset;

...

Listing 2  As a general work around always add this correction when calling ExtAudioFileTell.

...      ExtAudioFileTell(sourceFile, &frameOffset);     frameOffset += frameOffsetCorrection;  ...

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