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Q: I am currently attempting to decompress an MP3 file into a buffer using the SoundConverter APIs and am having no luck. SoundConverterConvertBuffer seems to work fine when decoding certain files, but fails if the file is MP3 encoded. Does MP3 decompression require calling SoundConverterSetInfo? If so, what parameters do I pass in?
A: Yes, a call to SoundConverterSetInfo is required, but first you'll have to retrieve the decompression settings atom from the sound file. To do this, open the MP3 as a QuickTime movie using NewMovieFromFile. Use the GetMediaSampleDescription routine to retrieve a description of the sample data, then use GetSoundDescriptionExtension (passing in the sample description, and using the siDecompressionParams selector as parameters) to extract the decompression atom list from the sound track. Once you have the decompression atom list, call SoundConverterSetInfo using the siDecompressionParams selector as we did with the previous call. Without this setup, the decoder won't know how to interpret the sample data.
The sample routine below shows how to extract the decompression parameters.
OSErr MyGetSoundDescriptionExtension(const FSSpec *inMP3file,
AudioFormatAtomPtr *outAudioAtom) {
Movie theMovie;
Track theTrack;
Media theMedia;
short theRefNum;
short theResID = 0; // we want the first movie
Boolean wasChanged;
OSErr err = noErr;
// open the movie file
err = OpenMovieFile(inMP3file, &theRefNum, fsRdPerm);
if (err) goto bail;
// instantiate the movie
err = NewMovieFromFile(&theMovie, theRefNum, &theResID, NULL,
newMovieActive, &wasChanged);
if (err) goto bail;
CloseMovieFile(theRefNum);
theRefNum = 0;
// get the first sound track
theTrack = GetMovieIndTrackType(theMovie, 1, SoundMediaType,
movieTrackMediaType);
if (theTrack != NULL) {
// get the sound track media
theMedia = GetTrackMedia(theTrack);
if (theMedia != NULL) {
Size size;
Handle extension;
SoundDescriptionV1Handle sourceSoundDescription;
sourceSoundDescription = (SoundDescriptionV1Handle)NewHandle(0);
// get the description of the sample data
GetMediaSampleDescription(theMedia, 1,
(SampleDescriptionHandle)sourceSoundDescription);
err = GetMoviesError();
extension = NewHandle(0);
// get the "magic" decompression atom
// This extension to the SoundDescription information stores
// data specific to a given audio decompressor. Some audio
// decompression algorithms require a set of out-of-stream
// values to configure the decompressor.
err = GetSoundDescriptionExtension(
(SoundDescriptionHandle)sourceSoundDescription,
&extension, siDecompressionParams);
if (noErr == err) {
size = GetHandleSize(extension);
HLock(extension);
*outAudioAtom = (AudioFormatAtom*)NewPtr(size);
err = MemError();
// copy the atom data to our buffer...
BlockMoveData(*extension, *outAudioAtom, size);
HUnlock(extension);
} else {
// if it doesn't have an atom, that's ok
*outAudioAtom = NULL;
err = noErr;
}
DisposeHandle(extension);
DisposeHandle((Handle)sourceSoundDescription);
}
}
bail:
return err;
}
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A full sample application called MP3Player can be found on the QuickTime Sample Code page.
[Sep 22 2000]
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