用核心音频实时产生正弦音

Question

我想创建一个实时正弦发生器使用苹果核心音频框架。我想做低水平，这样我就可以学习和理解基本原理。

    while (OSXIsGameRunning())
    {
       OSXProcessPendingMessages(&GameData);            

       [GlobalGLContext makeCurrentContext];

       CGRect WindowFrame = [window frame];
       CGRect ContentViewFrame = [[window contentView] frame];

       CGPoint MouseLocationInScreen = [NSEvent mouseLocation];
       BOOL MouseInWindowFlag = NSPointInRect(MouseLocationInScreen, WindowFrame);
       CGPoint MouseLocationInView = {};

       if (MouseInWindowFlag)
       {
          NSRect RectInWindow = [window convertRectFromScreen:NSMakeRect(MouseLocationInScreen.x,                                                                        MouseLocationInScreen.y,                                                                 1,                                                                         1)];
          NSPoint PointInWindow = RectInWindow.origin;
          MouseLocationInView= [[window contentView] convertPoint:PointInWindow fromView:nil];
       }
       u32 MouseButtonMask = [NSEvent pressedMouseButtons];

       OSXProcessFrameAndRunGameLogic(&GameData, ContentViewFrame,
                                           MouseInWindowFlag, MouseLocationInView,
                                           MouseButtonMask);

#if ENGINE_USE_VSYNC
       [GlobalGLContext flushBuffer];
#else        
       glFlush();
#endif

     }

通过使用VSYNC，我可以将循环降低到60 fps。时间不是很紧，但相当稳定。我也有一些代码来手动使用马赫计时，这甚至更不精确。为了可读性我把它省略了。不使用VSYNC或使用马赫定时来获得每秒60次迭代也会造成音频故障。

定时日志：

CyclesElapsed: 8154360866, TimeElapsed: 0.016624, FPS: 60.155666
CyclesElapsed: 8174382119, TimeElapsed: 0.020021, FPS: 49.946926
CyclesElapsed: 8189041370, TimeElapsed: 0.014659, FPS: 68.216309
CyclesElapsed: 8204363633, TimeElapsed: 0.015322, FPS: 65.264511
CyclesElapsed: 8221230959, TimeElapsed: 0.016867, FPS: 59.286217
CyclesElapsed: 8237971921, TimeElapsed: 0.016741, FPS: 59.733719
CyclesElapsed: 8254861722, TimeElapsed: 0.016890, FPS: 59.207333
CyclesElapsed: 8271667520, TimeElapsed: 0.016806, FPS: 59.503273
CyclesElapsed: 8292434135, TimeElapsed: 0.020767, FPS: 48.154209

这里重要的是函数OSXProcessFrameAndRungameLogic。它每秒被调用60次，并传递给它一个包含基本信息的结构，如用于呈现的缓冲区、键盘状态和声音缓冲区，如下所示：

    typedef struct osx_sound_output
    {
       game_sound_output_buffer SoundBuffer;
       u32 SoundBufferSize;
       s16* CoreAudioBuffer;
       s16* ReadCursor;
       s16* WriteCursor;

       AudioStreamBasicDescription AudioDescriptor;
       AudioUnit AudioUnit;  
    } osx_sound_output;

    typedef struct game_sound_output_buffer
    {
       real32 tSine;
       int SamplesPerSecond;
       int SampleCount;
       int16 *Samples;
    } game_sound_output_buffer;

void OSXProcessFrameAndRunGameLogic(osx_game_data *GameData, CGRect WindowFrame,
                                    b32 MouseInWindowFlag, CGPoint MouseLocation,
                                    int MouseButtonMask)
{
    GameData->SoundOutput.SoundBuffer.SampleCount = GameData->SoundOutput.SoundBuffer.SamplesPerSecond / GameData->TargetFramesPerSecond;

    // Oszi 1

    OutputTestSineWave(GameData, &GameData->SoundOutput.SoundBuffer, GameData->SynthesizerState.ToneHz);

    int16* CurrentSample = GameData->SoundOutput.SoundBuffer.Samples;
    for (int i = 0; i < GameData->SoundOutput.SoundBuffer.SampleCount; ++i)
    {
        *GameData->SoundOutput.WriteCursor++ = *CurrentSample++;
        *GameData->SoundOutput.WriteCursor++ = *CurrentSample++;

        if ((char*)GameData->SoundOutput.WriteCursor >= ((char*)GameData->SoundOutput.CoreAudioBuffer + GameData->SoundOutput.SoundBufferSize))
        {
            //printf("Write cursor wrapped!\n");
            GameData->SoundOutput.WriteCursor  = GameData->SoundOutput.CoreAudioBuffer;
        }
    }
}

void OutputTestSineWave(osx_game_data *GameData, game_sound_output_buffer *SoundBuffer, int ToneHz)
{
    int16 ToneVolume = 3000;
    int WavePeriod = SoundBuffer->SamplesPerSecond/ToneHz;

    int16 *SampleOut = SoundBuffer->Samples;
    for(int SampleIndex = 0;
        SampleIndex < SoundBuffer->SampleCount;
        ++SampleIndex)
    {
        real32 SineValue = sinf(SoundBuffer->tSine);
        int16 SampleValue = (int16)(SineValue * ToneVolume);

        *SampleOut++ = SampleValue;
        *SampleOut++ = SampleValue;

        SoundBuffer->tSine += Tau32*1.0f/(real32)WavePeriod;
        if(SoundBuffer->tSine > Tau32)
        {
            SoundBuffer->tSine -= Tau32;
        }
    }
}

void OSXInitCoreAudio(osx_sound_output* SoundOutput)
{
    AudioComponentDescription acd;
    acd.componentType         = kAudioUnitType_Output;
    acd.componentSubType      = kAudioUnitSubType_DefaultOutput;
    acd.componentManufacturer = kAudioUnitManufacturer_Apple;

    AudioComponent outputComponent = AudioComponentFindNext(NULL, &acd);

    AudioComponentInstanceNew(outputComponent, &SoundOutput->AudioUnit);
    AudioUnitInitialize(SoundOutput->AudioUnit);

    // uint16
    //AudioStreamBasicDescription asbd;
    SoundOutput->AudioDescriptor.mSampleRate       = SoundOutput->SoundBuffer.SamplesPerSecond;
    SoundOutput->AudioDescriptor.mFormatID         = kAudioFormatLinearPCM;
    SoundOutput->AudioDescriptor.mFormatFlags      = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsNonInterleaved | kAudioFormatFlagIsPacked;
    SoundOutput->AudioDescriptor.mFramesPerPacket  = 1;
    SoundOutput->AudioDescriptor.mChannelsPerFrame = 2; // Stereo
    SoundOutput->AudioDescriptor.mBitsPerChannel   = sizeof(int16) * 8;
    SoundOutput->AudioDescriptor.mBytesPerFrame    = sizeof(int16); // don't multiply by channel count with non-interleaved!
    SoundOutput->AudioDescriptor.mBytesPerPacket   = SoundOutput->AudioDescriptor.mFramesPerPacket * SoundOutput->AudioDescriptor.mBytesPerFrame;



    AudioUnitSetProperty(SoundOutput->AudioUnit,
                         kAudioUnitProperty_StreamFormat,
                         kAudioUnitScope_Input,
                         0,
                         &SoundOutput->AudioDescriptor,
                         sizeof(SoundOutput->AudioDescriptor));

    AURenderCallbackStruct cb;
    cb.inputProc = OSXAudioUnitCallback;
    cb.inputProcRefCon = SoundOutput;

    AudioUnitSetProperty(SoundOutput->AudioUnit,
                         kAudioUnitProperty_SetRenderCallback,
                         kAudioUnitScope_Global,
                         0,
                         &cb,
                         sizeof(cb));

    AudioOutputUnitStart(SoundOutput->AudioUnit);
}

OSStatus OSXAudioUnitCallback(void * inRefCon,
                              AudioUnitRenderActionFlags * ioActionFlags,
                              const AudioTimeStamp * inTimeStamp,
                              UInt32 inBusNumber,
                              UInt32 inNumberFrames,
                              AudioBufferList * ioData)
{
#pragma unused(ioActionFlags)
#pragma unused(inTimeStamp)
#pragma unused(inBusNumber)

    //double currentPhase = *((double*)inRefCon);

    osx_sound_output* SoundOutput = ((osx_sound_output*)inRefCon);


    if (SoundOutput->ReadCursor == SoundOutput->WriteCursor)
    {
        SoundOutput->SoundBuffer.SampleCount = 0;
        //printf("AudioCallback: No Samples Yet!\n");
    }

    //printf("AudioCallback: SampleCount = %d\n", SoundOutput->SoundBuffer.SampleCount);

    int SampleCount = inNumberFrames;
    if (SoundOutput->SoundBuffer.SampleCount < inNumberFrames)
    {
        SampleCount = SoundOutput->SoundBuffer.SampleCount;
    }

    int16* outputBufferL = (int16 *)ioData->mBuffers[0].mData;
    int16* outputBufferR = (int16 *)ioData->mBuffers[1].mData;

    for (UInt32 i = 0; i < SampleCount; ++i)
    {
        outputBufferL[i] = *SoundOutput->ReadCursor++;
        outputBufferR[i] = *SoundOutput->ReadCursor++;

        if ((char*)SoundOutput->ReadCursor >= (char*)((char*)SoundOutput->CoreAudioBuffer + SoundOutput->SoundBufferSize))
        {
            //printf("Callback: Read cursor wrapped!\n");
            SoundOutput->ReadCursor = SoundOutput->CoreAudioBuffer;
        }
    }

    for (UInt32 i = SampleCount; i < inNumberFrames; ++i)
    {
        outputBufferL[i] = 0.0;
        outputBufferR[i] = 0.0;
    }

    return noErr;
}

这就是它的主要内容。这是相当长的，但我没有看到一个方法，以一个更紧凑的方式呈现所有需要的信息。我想展示所有，因为我绝不是一个专业的程序员。如果你觉得缺少了什么，请告诉我。

我的感觉告诉我时机有问题。我觉得函数OSXProcessFrameAndRungameLogic有时需要更多的时间，这样核心音频回调在OutputTestSinewave完全写入之前就已经从缓冲区中提取样本了。

实际上，在OSXProcessFrameAndRungameLogic中还有更多的内容，我在这里没有展示这些内容。我是“软件渲染”到一个framebuffer中的非常基本的东西，然后由OpenGL显示，我也在那里做按键检查，因为是的，这是功能的主要功能。在未来，这是一个地方，我想处理多个振荡器，滤波器和东西的控制。无论如何，即使每次迭代都停止调用渲染和输入处理，我仍然会遇到音频故障。

pthread_t soundThread;
pthread_create(&soundThread, NULL, RunSound, GameData);
pthread_join(soundThread, NULL);

null

谢谢：）

Answer 1

你的基本问题是，你试图推音频从你的游戏循环，而不是让音频系统拉它；例如，而不是总是具有（或能够快速创建*)足够的音频样本，以准备由音频回调请求的量由音频回调拉动。“总是”必须考虑到足够的斜率来覆盖游戏循环中的定时抖动（被调用的时间太晚或太早或太少）。

(*没有锁、信号量、内存分配或目标C消息）