以IMA4格式在iPhone上录制单声道

Question

我正在使用苹果开发人员网站上的SpeakHear示例应用程序来创建音频记录应用程序。我尝试使用kAudioFormatAppleIMA4系统常量直接记录到IMA4格式。这被列为可用的格式之一，但每次我设置音频格式变量并传递和设置它时，我都会得到一个'fmt?‘错误。下面是我用来设置音频格式变量的代码：

#define kAudioRecordingFormat kAudioFormatAppleIMA4
#define kAudioRecordingType kAudioFileCAFType
#define kAudioRecordingSampleRate 16000.00
#define kAudioRecordingChannelsPerFrame 1
#define kAudioRecordingFramesPerPacket 1
#define kAudioRecordingBitsPerChannel 16
#define kAudioRecordingBytesPerPacket 2
#define kAudioRecordingBytesPerFrame 2

- (void) setupAudioFormat: (UInt32) formatID {

    // Obtains the hardware sample rate for use in the recording
    // audio format. Each time the audio route changes, the sample rate
    // needs to get updated.
    UInt32 propertySize = sizeof (self.hardwareSampleRate);

    OSStatus err = AudioSessionGetProperty (
        kAudioSessionProperty_CurrentHardwareSampleRate,
        &propertySize,
        &hardwareSampleRate
    );

    if(err != 0){
        NSLog(@"AudioRecorder::setupAudioFormat - error getting audio session property");
    }

    audioFormat.mSampleRate = kAudioRecordingSampleRate;

    NSLog (@"Hardware sample rate = %f", self.audioFormat.mSampleRate);

    audioFormat.mFormatID           = formatID;
    audioFormat.mChannelsPerFrame   = kAudioRecordingChannelsPerFrame;
    audioFormat.mFormatFlags        = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
    audioFormat.mFramesPerPacket    = kAudioRecordingFramesPerPacket;
    audioFormat.mBitsPerChannel     = kAudioRecordingBitsPerChannel;
    audioFormat.mBytesPerPacket     = kAudioRecordingBytesPerPacket;
    audioFormat.mBytesPerFrame      = kAudioRecordingBytesPerFrame;

}

下面是我使用该函数的地方：

- (id) initWithURL: fileURL {
    NSLog (@"initializing a recorder object.");
    self = [super init];

    if (self != nil) {

        // Specify the recording format. Options are:
        //
        //      kAudioFormatLinearPCM
        //      kAudioFormatAppleLossless
        //      kAudioFormatAppleIMA4
        //      kAudioFormatiLBC
        //      kAudioFormatULaw
        //      kAudioFormatALaw
        //
        // When targeting the Simulator, SpeakHere uses linear PCM regardless of the format
        //  specified here. See the setupAudioFormat: method in this file.
        [self setupAudioFormat: kAudioRecordingFormat];

        OSStatus result =   AudioQueueNewInput (
                                &audioFormat,
                                recordingCallback,
                                self,                   // userData
                                NULL,                   // run loop
                                NULL,                   // run loop mode
                                0,                      // flags
                                &queueObject
                            );

        NSLog (@"Attempted to create new recording audio queue object. Result: %f", result);

        // get the recording format back from the audio queue's audio converter --
        //  the file may require a more specific stream description than was 
        //  necessary to create the encoder.
        UInt32 sizeOfRecordingFormatASBDStruct = sizeof (audioFormat);

        AudioQueueGetProperty (
            queueObject,
            kAudioQueueProperty_StreamDescription,  // this constant is only available in iPhone OS
            &audioFormat,
            &sizeOfRecordingFormatASBDStruct
        );

        AudioQueueAddPropertyListener (
            [self queueObject],
            kAudioQueueProperty_IsRunning,
            audioQueuePropertyListenerCallback,
            self
        );

        [self setAudioFileURL: (CFURLRef) fileURL];

        [self enableLevelMetering];
    }
    return self;
} 

谢谢你的帮助！-Matt

Answer 1

我不确定您传递的所有格式标志是否都是正确的；IMA4 ( IIRC，代表IMA ADPCM 4:1)是4位的( 16位的4:1压缩)，其中包含一些头部。

根据AudioStreamBasicDescription的文档

• mBytesPerFrame应该是0，因为格式是compressed.
• mBitsPerChannel应该是0，因为格式是compressed.
• mFormatFlags应该是0，因为没有什么可选择的。

A根据afconvert -f caff -t ima4 -c 1 blah.aiff blah.caf，然后是afinfo blah.caf

• mBytesPerPacket应为34，and
• mFramesPerPacket应为64。您也可以将它们设置为0。

original IMA spec中的参考算法没有那么有用(它是扫描的光学字符识别，网站也有扫描)。

Answer 2

在@tc的顶部。已经说过，使用以下代码根据ID自动填充您的描述会更容易：

AudioStreamBasicDescription streamDescription;
UInt32 streamDesSize = sizeof(streamDescription);
memset(&streamDescription, 0, streamDesSize);
streamDescription.mFormatID         = kAudioFormatiLBC;

OSStatus status;
status = AudioFormatGetProperty(kAudioFormatProperty_FormatInfo, 0, NULL, &streamDesSize, &streamDescription);
assert(status==noErr);

这样，您就不需要费心去猜测某些格式的特性了。请注意，尽管在本例中kAudioFormatiLBC不需要任何其他附加信息，但其他格式需要(通常是通道数和采样率)。