AVCaptureDevice，采集特定采样频率的WAV音频

Question

所以问题很简单：如何使用AVCaptureDevice、AVCaptureSession、AVCaptureDeviceInput和AVCaptureAudioFileOutput以特定的采样频率、比特率等来捕获WAV文件？我有代码：

captureSession = [[AVCaptureSession alloc] init];
AVCaptureDevice *audioCaptureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeAudio];
NSError *error = nil;
AVCaptureDeviceInput *audioInput = [AVCaptureDeviceInput deviceInputWithDevice:audioCaptureDevice error:&error];
AVCaptureAudioFileOutput *audioOutput = [[AVCaptureAudioFileOutput alloc] init];
if (audioInput && [captureSession canAddInput:audioInput] && [captureSession canAddOutput:audioOutput]) {
        [captureSession addInput:audioInput];
        [captureSession addOutput:audioOutput];
        [captureSession startRunning];
    }

之后，我在其他函数中处理启动：

- (void)captureSessionStartedNotification:(NSNotification *)notification
{
    AVCaptureSession *session = notification.object;
    id audioOutput  = session.outputs[0];
    .......
    [audioOutput startRecordingToOutputFileURL:[NSURL fileURLWithPath:filePath] outputFileType:AVFileTypeWAVE recordingDelegate:self];
}

在哪里可以设置捕捉属性之类的？ P.S。 setSessionPreset 根本不起作用。

Answer 1

好吧，这并不完全符合我的预期，但无论如何我已经想出了解决方案。在我的例子中，我需要 AVSampleRateKey eq 到 16000，AVLinearPCMBitDepthKey eq 到 16 和 1 个通道。但不幸的是，设置这些属性并尝试记录 wav 格式有点复杂。要设置我使用的属性：

NSDictionary *recordSettings = [NSDictionary
                                    dictionaryWithObjectsAndKeys:
                                    [NSNumber numberWithInt:kAudioFormatLinearPCM],
                                    AVFormatIDKey,
                                    [NSNumber numberWithInt:16],
                                    AVLinearPCMBitDepthKey,
                                    [NSNumber numberWithBool:NO],
                                    AVLinearPCMIsFloatKey,
                                    [NSNumber numberWithInt: 1],
                                    AVNumberOfChannelsKey,
                                    [NSNumber numberWithFloat:16000.0],
                                    AVSampleRateKey,
                                    nil];

    [audioOutput setAudioSettings:recordSettings];

并且有两个选项：

[audioOutput startRecordingToOutputFileURL:[NSURL fileURLWithPath:filePath] outputFileType:AVFileTypeCoreAudioFormat recordingDelegate:self];
[audioOutput startRecordingToOutputFileURL:[NSURL fileURLWithPath:filePath] outputFileType:AVFileTypeWAVE recordingDelegate:self];

在第一种情况下，我什至可以编写 .wav 文件，但由于某些原因，我在上面设置的一些属性根本不起作用（例如，频率）。在第二种情况下，我什至无法写任何东西。所有行都出现 w/o 错误，但在某些时候，文件没有创建。所以做出了决定：用我的参数编写 .caf，然后将其转换为所需的 .wav 文件。所以上面的设置 + 第一种情况可以编写简单的 .caf 文件。之后我使用了转换函数：

-(BOOL)convertFromCafToWav:(NSString*)filePath
{
    NSError *error = nil ;

    NSDictionary *audioSetting = [NSDictionary dictionaryWithObjectsAndKeys:
                                  [ NSNumber numberWithFloat:16000.0], AVSampleRateKey,
                                  [ NSNumber numberWithInt:1], AVNumberOfChannelsKey,
                                  [ NSNumber numberWithInt:16], AVLinearPCMBitDepthKey,
                                  [ NSNumber numberWithInt:kAudioFormatLinearPCM], AVFormatIDKey,
                                  [ NSNumber numberWithBool:NO], AVLinearPCMIsFloatKey,
                                  [ NSNumber numberWithBool:0], AVLinearPCMIsBigEndianKey,
                                  [ NSNumber numberWithBool:NO], AVLinearPCMIsNonInterleaved,
                                  [ NSData data], AVChannelLayoutKey, nil ];

    NSString *audioFilePath = filePath;
    AVURLAsset * URLAsset = [[AVURLAsset alloc]  initWithURL:[NSURL fileURLWithPath:audioFilePath] options:nil];

    if (!URLAsset) return NO ;

    AVAssetReader *assetReader = [AVAssetReader assetReaderWithAsset:URLAsset error:&error];
    if (error) return NO;

    NSArray *tracks = [URLAsset tracksWithMediaType:AVMediaTypeAudio];
    if (![tracks count]) return NO;

    AVAssetReaderAudioMixOutput *audioMixOutput = [AVAssetReaderAudioMixOutput
                                                   assetReaderAudioMixOutputWithAudioTracks:tracks
                                                   audioSettings :audioSetting];

    if (![assetReader canAddOutput:audioMixOutput]) return NO ;

    [assetReader addOutput :audioMixOutput];

    if (![assetReader startReading]) return NO;

    NSString *outPath = [NSString stringWithFormat:@"%@/%@.wav", NSHomeDirectory(), @"myWav"]; //for example

    NSURL *outURL = [NSURL fileURLWithPath:outPath];
    AVAssetWriter *assetWriter = [AVAssetWriter assetWriterWithURL:outURL
                                                          fileType:AVFileTypeWAVE
                                                             error:&error];
    if (error) return NO;
    AVAssetWriterInput *assetWriterInput = [ AVAssetWriterInput assetWriterInputWithMediaType :AVMediaTypeAudio
                                                                                outputSettings:audioSetting];
    assetWriterInput. expectsMediaDataInRealTime = NO;
    if (![assetWriter canAddInput:assetWriterInput]) return NO ;
    [assetWriter addInput :assetWriterInput];
    if (![assetWriter startWriting]) return NO;
    [assetWriter startSessionAtSourceTime:kCMTimeZero ];
    dispatch_queue_t queue = dispatch_queue_create( "assetWriterQueue", NULL );
    [assetWriterInput requestMediaDataWhenReadyOnQueue:queue usingBlock:^{
        while (TRUE)
        {
            if ([assetWriterInput isReadyForMoreMediaData] && (assetReader.status == AVAssetReaderStatusReading)) {

                CMSampleBufferRef sampleBuffer = [audioMixOutput copyNextSampleBuffer];

                if (sampleBuffer) {
                    [assetWriterInput appendSampleBuffer :sampleBuffer];
                    CFRelease(sampleBuffer);
                } else {
                    [assetWriterInput markAsFinished];
                    break;
                }
            }
        }
        [assetWriter finishWritingWithCompletionHandler:^{}];

        NSFileManager *fileManager = [NSFileManager defaultManager];
        BOOL isDir;
        NSString* resultDir = NSHomeDirectory(); //for example
        if([fileManager fileExistsAtPath:resultDir isDirectory:&isDir]){
            if ([fileManager fileExistsAtPath:cafPath]){
                [fileManager removeItemAtPath:cafPath error:nil];
            }
        }

    }];
    return YES;
}

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