iphone - 在 iPhone 上以 IMA4 格式录制单声道

Question

我正在使用 Apple 开发者网站上的 SpeakHear 示例应用来创建录音应用。我正在尝试使用 kAudioFormatAppleIMA4 系统常量直接录制为 IMA4 格式。这被列为可用格式之一，但每次我设置音频格式变量并传递和设置它时，我都会收到“fmt？”错误。这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;

}

这里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;
} 

感谢您的帮助! -马特

Answer 1

我不确定您传递的所有格式标志是否正确； IMA4(IIRC，代表 IMA ADPCM 4:1)是 4 位(从 16 位压缩为 4:1)，带有一些 header 。

根据 AudioStreamBasicDescription 的文档:

• mBytesPerFrame 应为 0，因为格式已压缩。
• mBitsPerChannel 应为 0，因为格式已压缩。
• mFormatFlags 可能应该为 0，因为没有什么可供选择。

根据 afconvert -f caff -t ima4 -c 1 blah.aiff blah.caf 后跟 afinfo blah.caf:

• mBytesPerPacket 应为 34，并且
• mFramesPerPacket 应为 64。您可以将它们设置为 0。

引用算法在original IMA spec没那么有用(它是扫描的 OCR，该站点也有扫描)。