如何使用AVAssetWriter在ios中寫出AAC音頻？

Question

我正在使用AVCaptureSession從設備麥克風和攝像頭捕獲音頻和視頻樣本。

然后我嘗試編寫通過AVCaptureSessions委托方法返回的CMSampleBuffers（使用AVAssetWriter和AVAssetWriterInputs）

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:    (CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection

當我的音頻AVAssetWriterInput配置為以Apple Lossless格式（kAudioFormatAppleLossless）寫出數據時，這可以正常工作但是如果我嘗試配置音頻AVAssetWriterInput以使用AAC（kAudioFormatMPEG4AAC）它成功寫入視頻和音頻樣本大約500ms然后失敗跟隨錯誤

writer has failed with Error Domain=AVFoundationErrorDomain Code=-11821 "Cannot Decode" UserInfo=0x4b2630 {NSLocalizedFailureReason=The media data could not be decoded. It may be damaged., NSUnderlyingError=0x4ad0f0 "The operation couldn’t be completed. (OSStatus error 560226676.)", NSLocalizedDescription=Cannot Decode}

這是我用來創建AVAssetWriter和AVAssetWriterInputs的代碼

NSError *error = nil;
m_VideoCaputurePath = [[NSString stringWithFormat:@"%@/%@.mp4",[UserData getSavePath],[UserData getUniqueFilename]] retain];

if( USE_AAC_AUDIO )
{
    m_audioAndVideoWriter = [[AVAssetWriter alloc] initWithURL:[NSURL fileURLWithPath:m_VideoCaputurePath] fileType:AVFileTypeMPEG4 error:&error];
}
else
{
    m_audioAndVideoWriter = [[AVAssetWriter alloc] initWithURL:[NSURL fileURLWithPath:m_VideoCaputurePath] fileType:AVFileTypeQuickTimeMovie error:&error];
}

//\Configure Video Writer Input
NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                               AVVideoCodecH264, AVVideoCodecKey,
                               [NSNumber numberWithInt:640], AVVideoWidthKey,
                               [NSNumber numberWithInt:480], AVVideoHeightKey,
                               nil];

m_videoWriterInput = [[AVAssetWriterInput
                       assetWriterInputWithMediaType:AVMediaTypeVideo
                       outputSettings:videoSettings] retain];

m_videoWriterInput.expectsMediaDataInRealTime = YES;

//\Configure Audio Writer Input

AudioChannelLayout acl;
bzero(&acl, sizeof(acl));
acl.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;

NSDictionary*  audioOutputSettings;
if( USE_AAC_AUDIO )
{
    audioOutputSettings = [ NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithInt:kAudioFormatMPEG4AAC], AVFormatIDKey,
                                          [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,
                                          [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                          [ NSData dataWithBytes: &acl length: sizeof( acl ) ], AVChannelLayoutKey,
                                          [ NSNumber numberWithInt: 96 ], AVEncoderBitRateKey,
                                          nil];
}
else
{
    audioOutputSettings = [ NSDictionary dictionaryWithObjectsAndKeys:                       
                                          [ NSNumber numberWithInt: kAudioFormatAppleLossless ], AVFormatIDKey,
                                          [ NSNumber numberWithInt: 16 ], AVEncoderBitDepthHintKey,
                                          [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                          [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,                                      
                                          [ NSData dataWithBytes: &acl length: sizeof( acl ) ], AVChannelLayoutKey, nil ];
}

m_audioWriterInput = [[AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:audioOutputSettings] retain];

m_audioWriterInput.expectsMediaDataInRealTime = YES;

//\Add inputs to Write
NSAssert([m_audioAndVideoWriter canAddInput:m_audioWriterInput], @"Cannot write to this type of audio input" );
NSAssert([m_audioAndVideoWriter canAddInput:m_videoWriterInput], @"Cannot write to this type of video input" );

[m_audioAndVideoWriter addInput:m_videoWriterInput];
[m_audioAndVideoWriter addInput:m_audioWriterInput];

有沒有人知道如何使用配置為寫入AAC的AVAssetWriterInput正確編寫從AVCaptureSession返回的音頻樣本？

Answer 1

我設法通過將AVEncoderBitRateKey參數更改為96到64000的所需輸出設置來使其工作。

我現在用於初始化能夠編寫AAC音頻的AVAssetWriterInput的音頻設置如下所示

    NSDictionary*  audioOutputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                    [ NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
                                    [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,
                                    [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                    [ NSData dataWithBytes: &acl length: sizeof( AudioChannelLayout ) ], AVChannelLayoutKey,
                                    [ NSNumber numberWithInt: 64000 ], AVEncoderBitRateKey,
                                    nil]