ReplayKit2 采集音视频回调格式分析

一、iOS中的音视频都是采用一个叫做CMSampleBuffer的格式封装的

　　比如回调的App音频

Printing description of sampleBuffer:
CMSampleBuffer 0x143d06560 retainCount: 1 allocator: 0x25bae95e0
    invalid = NO
    dataReady = YES
    makeDataReadyCallback = 0x0
    makeDataReadyRefcon = 0x0
    formatDescription = <CMAudioFormatDescription 0x281940990 [0x25bae95e0]> {
    mediaType:'soun'
    mediaSubType:'lpcm'
    mediaSpecific: {
        ASBD: {
            mSampleRate: 44100.000000 
            mFormatID: 'lpcm'
            mFormatFlags: 0xe 
            mBytesPerPacket: 2 
            mFramesPerPacket: 1 
            mBytesPerFrame: 2 
            mChannelsPerFrame: 1 
            mBitsPerChannel: 16     } 
        cookie: {(null)} 
        ACL: {(null)}
        FormatList Array: {(null)} 
    } 
    extensions: {(null)}
}
    sbufToTrackReadiness = 0x0
    numSamples = 22528
    sampleTimingArray[1] = {
        {PTS = {619877934457375/1000000000 = 619877.934}, DTS = {INVALID}, duration = {1/44100 = 0.000}},
    }
    dataBuffer = 0x281940360

　　比如回调的micphone音频

Printing description of sampleBuffer:
CMSampleBuffer 0x149e053b0 retainCount: 1 allocator: 0x25bae95e0
    invalid = NO
    dataReady = YES
    makeDataReadyCallback = 0x0
    makeDataReadyRefcon = 0x0
    formatDescription = <CMAudioFormatDescription 0x281b47b10 [0x25bae95e0]> {
    mediaType:'soun'
    mediaSubType:'lpcm'
    mediaSpecific: {
        ASBD: {
            mSampleRate: 44100.000000 
            mFormatID: 'lpcm'
            mFormatFlags: 0xc 
            mBytesPerPacket: 2 
            mFramesPerPacket: 1 
            mBytesPerFrame: 2 
            mChannelsPerFrame: 1 
            mBitsPerChannel: 16     } 
        cookie: {(null)} 
        ACL: {(null)}
        FormatList Array: {(null)} 
    } 
    extensions: {(null)}
}
    sbufToTrackReadiness = 0x0
    numSamples = 1024
    sampleTimingArray[1] = {
        {PTS = {27273930501/44100 = 618456.474, rounded}, DTS = {INVALID}, duration = {1/44100 = 0.000}},
    }
    dataBuffer = 0x281b47c30

　从这个结构中，可以得到这个音频数据的描述信息

// Get samples.
      CMBlockBufferRef audioBuffer = CMSampleBufferGetDataBuffer(sampleBuffer);
      size_t lengthAtOffset;
      size_t totalLength;
      char *samples;
      CMBlockBufferGetDataPointer(audioBuffer, 0, &lengthAtOffset, &totalLength, &samples);
 
      // Get format.
      CMAudioFormatDescriptionRef format = CMSampleBufferGetFormatDescription(sampleBuffer);
      const AudioStreamBasicDescription *description = CMAudioFormatDescriptionGetStreamBasicDescription(format);

　　输出为格式的描述信息

Printing description of format:
<CMAudioFormatDescription 0x28282f7b0 [0x25bae95e0]> {
    mediaType:'soun'
    mediaSubType:'lpcm'
    mediaSpecific: {
        ASBD: {
            mSampleRate: 44100.000000 
            mFormatID: 'lpcm'
            mFormatFlags: 0xc 
            mBytesPerPacket: 2 
            mFramesPerPacket: 1 
            mBytesPerFrame: 2 
            mChannelsPerFrame: 1 
            mBitsPerChannel: 16     } 
        cookie: {(null)} 
        ACL: {(null)}
        FormatList Array: {(null)} 
    } 
    extensions: {(null)}
}

　　关于音频格式有几个关键信息：格式、采样率、采样精度、声道数

　　关于音频数据除了上面的格式信息之外，还有 PTS 、 samples（帧数）描述信息

　　比如上面的PCM数据中，采样率是44100hz、采样深度是16位、声道数是1，samples是1024帧，最后计算这段声音的长度信息是 (1/44100) * 1024 = 23ms , 这个声音的数据大小是 1024 * 2byte = 2014 Byte

　　如果是录制声音的话，需要先设置需要录制的音频格式信息　　

AudioStreamBasicDescription format;
memset(&format, 0, sizeof(format));
format.mSampleRate = 44100;
format.mFormatID = kAudioFormatLinearPCM;
format.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked;
format.mChannelsPerFrame = 1;
format.mBitsPerChannel = 16;
format.mBytesPerFrame = (format.mBitsPerChannel/8) * format.mChannelsPerFrame;
format.mFramesPerPacket = 1;
format.mBytesPerPacket = format.mBytesPerFrame * format.mFramesPerPacket;