AVAssetWriter如何写下采样/压缩的m4a / mp3文件

问题描述

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）。

I'm trying to take a local m4a or mp3 file and compress/down-sample this file (for the purposes of making a smaller file).

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）。

Originally, I was using the AVAssetExportSession to export an AVAsset to a temp directory, but I didn't have any control over compression/down-sampling (you can only use presets, which of them, only .wav file formats support quality degradation).

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'出口'。

Then, following several examples here on SO, I tried using AVAssetReader/AVAssetWriter to preform this 'export'.

我这样创建我的读者/作者：

I create my reader/writer as such:

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;
        }
    }
}];

当我写完文件时，我认为写入exportURL的数据为空，并且作者报告成功完成。可能出现问题的任何想法？

When I'm done writing, the data i've supposedly written to the exportURL is null, and the writer reports a successful completion. Any ideas what might be going wrong?

更新调用后的作者状态appendSampleBuffer：是AVAssetWriterStatusFailed，虽然读者状态是成功的，并且似乎读取整个文件。

Update The writer status after calling appendSampleBuffer: is AVAssetWriterStatusFailed, though the readers status is successful, and seems to read through the entire file.

推荐答案

我遇到了解决方案：

使用NSHomeDirectory（）在 NSString * exportPath = [NSHomeDirectory（）stringByAppendingPathComponent：@out.m4a] 导致编写器无法创建该文件。不完全确定原因，或者我需要做些什么才能使其工作，但将 NSHomeDirectiory（）更改为 NSTemporaryDirectory（）在此期间解决了我的问题。

Using NSHomeDirectory() in NSString *exportPath = [NSHomeDirectory() stringByAppendingPathComponent:@"out.m4a"] was causing the writer to not be able to create the file. Not exactly sure why, or what I would need to do to allow this to work, but changing NSHomeDirectiory() to NSTemporaryDirectory() has solved my problems in the meantime.

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