AAC音频转WAV

将aac编码格式的音频转为lpcm编码的音频,转出来的音频会比原来大个10倍左右.

/**
 把.m4a转为.caf格式
 @param originalUrlStr .m4a文件路径
 @param destUrlStr .wav文件路径
 @param completed 转化完成的block
 */
- (void)convetM4aToWav:(NSURL *)originalUrlStr
               destUrl:(NSString *)destUrlStr
             completed:(void (^)(NSError *error)) completed {
    if ([[NSFileManager defaultManager] fileExistsAtPath:destUrlStr]) {
        [[NSFileManager defaultManager] removeItemAtPath:destUrlStr error:nil];
    }

    NSURL *originalUrl = originalUrlStr;
    NSURL *destUrl     = [NSURL fileURLWithPath:destUrlStr];
    AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:originalUrl options:nil];
    
    //读取原始文件信息
    NSError *error = nil;
    AVAssetReader *assetReader = [AVAssetReader assetReaderWithAsset:songAsset error:&error];
    if (error) {
        NSLog (@"error: %@", error);
        completed(error);
        return;
    }
    AVAssetReaderOutput *assetReaderOutput = [AVAssetReaderAudioMixOutput
                                              assetReaderAudioMixOutputWithAudioTracks:songAsset.tracks
                                              audioSettings: nil];
    if (![assetReader canAddOutput:assetReaderOutput]) {
        NSLog (@"can't add reader output... die!");
        completed(error);
        return;
    }
    [assetReader addOutput:assetReaderOutput];
    
    AVAssetWriter *assetWriter = [AVAssetWriter assetWriterWithURL:destUrl
                                                          fileType:AVFileTypeCoreAudioFormat
                                                             error:&error];
    if (error) {
        NSLog (@"error: %@", error);
        completed(error);
        return;
    }
    AudioChannelLayout channelLayout;
    memset(&channelLayout, 0, sizeof(AudioChannelLayout));
    channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
    NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                    [NSNumber numberWithInt:kAudioFormatLinearPCM], AVFormatIDKey,
                                    [NSNumber numberWithFloat:44100], AVSampleRateKey,
                                    [NSNumber numberWithInt:2], AVNumberOfChannelsKey,
                                    [NSNumber numberWithInt:16], AVLinearPCMBitDepthKey,
                                    [NSNumber numberWithBool:NO], AVLinearPCMIsNonInterleaved,
                                    [NSNumber numberWithBool:NO],AVLinearPCMIsFloatKey,
                                    [NSNumber numberWithBool:NO], AVLinearPCMIsBigEndianKey,
                                    nil];
    AVAssetWriterInput *assetWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio
                                                                              outputSettings:outputSettings];
    if ([assetWriter canAddInput:assetWriterInput]) {
        [assetWriter addInput:assetWriterInput];
    } else {
        NSLog (@"can't add asset writer input... die!");
        completed(error);
        return;
    }
    
    assetWriterInput.expectsMediaDataInRealTime = NO;
    
    [assetWriter startWriting];
    [assetReader startReading];
    
    AVAssetTrack *soundTrack = [songAsset.tracks objectAtIndex:0];
    CMTime startTime = CMTimeMake (0, soundTrack.naturalTimeScale);
    [assetWriter startSessionAtSourceTime:startTime];
    
    __block UInt64 convertedByteCount = 0;
    
    dispatch_queue_t mediaInputQueue = dispatch_queue_create("mediaInputQueue", NULL);
    [assetWriterInput requestMediaDataWhenReadyOnQueue:mediaInputQueue
                                            usingBlock: ^
     {
         while (assetWriterInput.readyForMoreMediaData) {
             CMSampleBufferRef nextBuffer = [assetReaderOutput copyNextSampleBuffer];
             if (nextBuffer) {
                 // append buffer
                 [assetWriterInput appendSampleBuffer: nextBuffer];
                 NSLog (@"appended a buffer (%zu bytes)",
                        CMSampleBufferGetTotalSampleSize (nextBuffer));
                 convertedByteCount += CMSampleBufferGetTotalSampleSize (nextBuffer);
                 
             } else {
                 [assetWriterInput markAsFinished];
                 [assetWriter finishWritingWithCompletionHandler:^{
                     
                 }];
                 [assetReader cancelReading];
                 completed(nil);
                 break;
             }
             
             //释放内存
             CMSampleBufferInvalidate(nextBuffer);
             CFRelease(nextBuffer);
             nextBuffer = NULL;
         }
     }];
}