Technical Q&A QA1730

AVAssetExportSession - Exporting a Trimmed Audio Asset

Q:  When creating .m4a output files, how do I configure an AVAssetExportSession object to trim audio assets and also perform a fade in?

A: When creating .m4a output files, how do I configure an AVAssetExportSession object to trim audio assets and also perform a fade in?

An AVAssetExportSession object is used to transcode the contents of an AVAsset source and has a number of configurable properties allowing the export operation to be customized. Output is described by a specified export preset, for example AVAssetExportPresetAppleM4A which will produce audio-only .m4a files.

Specifying the duration of the asset being exported may be done with the timeRange property. This property is a CMTimeRange describing start time and duration.

Another useful property is audioMix. By specifying a configured AVAudioMix object for the audioMix export property, you can perform custom audio processing on audio tracks during export.

Listing 1 demonstrates the basic configuration of an AVAssetExportSession object required to export an audio asset trimmed to 20 seconds with a 10 second fade in. The trim being set up in the code snippet takes place at the 30 second mark of the asset and therefore the track duration should be at least 50 seconds.

For more information regarding the AV Foundation APIs see the AV Foundation Programming Guide.

Listing 1  Exporting a trimmed audio asset with a fade in.

- (BOOL)exportAsset:(AVAsset *)avAsset toFilePath:(NSString *)filePath {

    // we need the audio asset to be at least 50 seconds long for this snippet
    CMTime assetTime = [avAsset duration];
    Float64 duration = CMTimeGetSeconds(assetTime);
    if (duration < 50.0) return NO;

    // get the first audio track
    NSArray *tracks = [avAsset tracksWithMediaType:AVMediaTypeAudio];
    if ([tracks count] == 0) return NO;

    AVAssetTrack *track = [tracks objectAtIndex:0];

    // create the export session
    // no need for a retain here, the session will be retained by the
    // completion handler since it is referenced there
    AVAssetExportSession *exportSession = [AVAssetExportSession
    if (nil == exportSession) return NO;

    // create trim time range - 20 seconds starting from 30 seconds into the asset
    CMTime startTime = CMTimeMake(30, 1);
    CMTime stopTime = CMTimeMake(50, 1);
    CMTimeRange exportTimeRange = CMTimeRangeFromTimeToTime(startTime, stopTime);

    // create fade in time range - 10 seconds starting at the beginning of trimmed asset
    CMTime startFadeInTime = startTime;
    CMTime endFadeInTime = CMTimeMake(40, 1);
    CMTimeRange fadeInTimeRange = CMTimeRangeFromTimeToTime(startFadeInTime,

    // setup audio mix
    AVMutableAudioMix *exportAudioMix = [AVMutableAudioMix audioMix];
    AVMutableAudioMixInputParameters *exportAudioMixInputParameters =
            [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:track];

    [exportAudioMixInputParameters setVolumeRampFromStartVolume:0.0 toEndVolume:1.0
    exportAudioMix.inputParameters = [NSArray

    // configure export session  output with all our parameters
    exportSession.outputURL = [NSURL fileURLWithPath:filePath]; // output path
    exportSession.outputFileType = AVFileTypeAppleM4A; // output file type
    exportSession.timeRange = exportTimeRange; // trim time range
    exportSession.audioMix = exportAudioMix; // fade in audio mix

    // perform the export
    [exportSession exportAsynchronouslyWithCompletionHandler:^{

        if (AVAssetExportSessionStatusCompleted == exportSession.status) {
        } else if (AVAssetExportSessionStatusFailed == exportSession.status) {
            // a failure may happen because of an event out of your control
            // for example, an interruption like a phone call comming in
            // make sure and handle this case appropriately
        } else {
            NSLog(@"Export Session Status: %d", exportSession.status);

    return YES;

Document Revision History


New document that demonstrates how to configure an export session to trim an audio asset and add a fade in.