AVAssetWriter如何写下采样/压缩的m4a/mp3文件

Question

我正在尝试使用本地m4a或mp3文件并压缩/下采样此文件(用于制作较小的文件).

最初,我使用AVAssetExportSession将AVAsset导出到临时目录,但我没有对压缩/下采样的任何控制(你只能使用预设,其中只有.wav文件格式支持质量下降) .

然后,按照SO上的几个例子,我尝试使用AVAssetReader/AVAssetWriter来预先形成这个'export'.

我这样创建我的读者/作者:

NSString *exportPath = [NSHomeDirectory() stringByAppendingPathComponent:@"out.m4a"];

NSURL *exportURL = [NSURL fileURLWithPath:outPath];

// reader
NSError *readerError = nil;
AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset
                                                       error:&readerError];

AVAssetTrack *track = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];    
AVAssetReaderTrackOutput *readerOutput = [[AVAssetReaderTrackOutput alloc] initWithTrack:track
                                                                          outputSettings:nil];
[reader addOutput:readerOutput];

// writer
NSError *writerError = nil;
AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:exportURL
                                                  fileType:AVFileTypeAppleM4A
                                                     error:&writerError];

AudioChannelLayout channelLayout;
memset(&channelLayout, 0, sizeof(AudioChannelLayout));
channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;

// use different values to affect the downsampling/compression
NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                [NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
                                [NSNumber numberWithFloat:44100.0], AVSampleRateKey,
                                [NSNumber numberWithInt:2], AVNumberOfChannelsKey,
                                [NSNumber numberWithInt:128000], AVEncoderBitRateKey,
                                [NSData dataWithBytes:&channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey,
                                nil];

AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio
                                                                 outputSettings:outputSettings];
[writerInput setExpectsMediaDataInRealTime:NO];
[writer addInput:writerInput];

然后我开始写......

[writer startWriting];
[writer startSessionAtSourceTime:kCMTimeZero];

[reader startReading];
dispatch_queue_t mediaInputQueue = dispatch_queue_create("mediaInputQueue", NULL);
[writerInput requestMediaDataWhenReadyOnQueue:mediaInputQueue usingBlock:^{

    NSLog(@"Asset Writer ready : %d", writerInput.readyForMoreMediaData);
    while (writerInput.readyForMoreMediaData) {
        CMSampleBufferRef nextBuffer;
        if ([reader status] == AVAssetReaderStatusReading && (nextBuffer = [readerOutput copyNextSampleBuffer])) {
            if (nextBuffer) {
                NSLog(@"Adding buffer");
                [writerInput appendSampleBuffer:nextBuffer];
            }
        } else {
            [writerInput markAsFinished];

            switch ([reader status]) {
                case AVAssetReaderStatusReading:
                    break;
                case AVAssetReaderStatusFailed:
                    [writer cancelWriting];
                    break;
                case AVAssetReaderStatusCompleted:
                    NSLog(@"Writer completed");
                    [writer endSessionAtSourceTime:asset.duration];
                    [writer finishWriting];

                    NSData *data = [NSData dataWithContentsOfFile:exportPath];
                    NSLog(@"Data: %@", data);
                    break;
            }
            break;
        }
    }
}];

当我完成写入时,我认为写入exportURL的数据为空,并且作者报告成功完成.什么想法可能会出错？

更新调用后appendSampleBuffer:的编写器状态是AVAssetWriterStatusFailed,尽管读者状态是成功的,并且似乎读取整个文件.

Answer 1

我遇到了解决方案:

使用NSHomeDirectory()NSString *exportPath = [NSHomeDirectory() stringByAppendingPathComponent:@"out.m4a"]导致编写器无法创建该文件.也不清楚为什么,或什么,我需要做的,使这项工作,但改变NSHomeDirectiory()来NSTemporaryDirectory()解决了在此期间我的问题.