iOS：用于记录网络传输的语音编码的音频格式

Question

I am looking for a good audio format to save voice recordings locally and for transport over the network. Requirements are:

• Decent quality. These clips, when received will be listened to many times
• Workflow should support trimming and fading before transport
• Decent file size

This is my current approach for recording:

// SEE IMA4 vs M4A http://stackoverflow.com/questions/3509921/recorder-works-on-iphone-3gs-but-not-on-iphone-3g
NSDictionary *recordSettings =
[[NSDictionary alloc] initWithObjectsAndKeys:
 [NSNumber numberWithFloat: 11025],               AVSampleRateKey,
 [NSNumber numberWithInt: kAudioFormatLinearPCM], AVFormatIDKey,
 [NSNumber numberWithInt: 1],                     AVNumberOfChannelsKey,
 [NSNumber numberWithBool:NO],                    AVLinearPCMIsFloatKey,
 [NSNumber numberWithInt: AVAudioQualityMax],     AVEncoderAudioQualityKey,
 nil];

NSError *error = nil;
self.audioRecorder = [[ AVAudioRecorder alloc] initWithURL:self.recordingFile settings:recordSettings error:&error];

And approach for encoding:

NSString *file = [NSString stringWithFormat:@"recordingConverted%x.caf", arc4random()];
self.filePath = [NSTemporaryDirectory() stringByAppendingPathComponent: file];
NSFileManager *fileManager = [NSFileManager defaultManager];
if ([fileManager fileExistsAtPath:self.filePath]) {
    NSError *error;
    if ([fileManager removeItemAtPath:self.filePath error:&error] == NO) {
        NSLog(@"removeItemAtPath %@ error:%@", self.filePath, error);
    }
}
NSLog(@"IN: %@", self.recordingFile);
NSLog(@"OUT: %@", self.filePath);

AVAsset *avAsset = [AVAsset assetWithURL:self.recordingFile];

// get the first audio track
NSArray *tracks = [avAsset tracksWithMediaType:AVMediaTypeAudio];
if ([tracks count] == 0) return nil;
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
                                       exportSessionWithAsset:avAsset
                                       presetName:AVAssetExportPresetAppleM4A];
if (nil == exportSession) return nil;

// create trim time range
CMTime startTime = CMTimeMake(self.speakingBeginTime*44100, 44100);
CMTime stopTime = CMTimeMake((self.speakingBeginTime+[self.duration doubleValue])*44100, 44100);
CMTimeRange exportTimeRange = CMTimeRangeFromTimeToTime(startTime, stopTime);

// create fade in time range
CMTime startFadeInTime = startTime;
CMTime endFadeInTime = CMTimeMake((self.speakingBeginTime+RECORDING_INTERVAL)*1.5*44100, 44100);
CMTimeRange fadeInTimeRange = CMTimeRangeFromTimeToTime(startFadeInTime,
                                                        endFadeInTime);

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

[exportAudioMixInputParameters setVolumeRampFromStartVolume:0.0 toEndVolume:1.0
                                                  timeRange:fadeInTimeRange]; 
exportAudioMix.inputParameters = [NSArray
                                  arrayWithObject:exportAudioMixInputParameters]; 

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

// MAKE THE EXPORT SYNCHRONOUS
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
[exportSession exportAsynchronouslyWithCompletionHandler:^{
    dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
dispatch_release(semaphore);

if (AVAssetExportSessionStatusCompleted == exportSession.status) {
    return self.filePath;
    //NSLog(@"AVAssetExportSessionStatusCompleted");
} 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
    NSLog(@"AVAssetExportSessionStatusFailed %@", exportSession.error.localizedDescription);
} else {
    NSLog(@"Export Session Status: %d", exportSession.status);
}

Currently, performance on a 3 second audio clip is: 62,228 bytes for PCM and 36,654 bytes for encoded. Seems like I could do better.

Answer 1

I have found the guide here:

http://gamua.com/blog/2010/06/sound-on-ios-best-practices/

to be helpful in picking sound formats (especially the comments)

There are also some good examples here:

How do I record audio on iPhone with AVAudioRecorder?

especially the example with various formats for export, and this answer:

https://stackoverflow.com/a/3870385/2214106

which reduced file size significantly.