iOS音频单元 - 创建立体声正弦波

Question

在周末,我遇到了一个学习如何在iOS上编程音频合成的绊脚石.我已经在iOS上开发了好几年了,但我刚刚进入音频合成方面.现在,我只是编写演示应用程序来帮助我学习这些概念.我目前能够在音频单元的回放渲染器中构建和堆叠正弦波而没有任何问题.但是,我想了解渲染器中发生了什么,因此我可以在每个左右声道中渲染2个单独的正弦波.目前,我假设在我的init音频部分,我需要进行以下更改:

从:

AudioStreamBasicDescription audioFormat;
    audioFormat.mSampleRate = kSampleRate;
    audioFormat.mFormatID = kAudioFormatLinearPCM;
    audioFormat.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
    audioFormat.mFramesPerPacket = 1;
    audioFormat.mChannelsPerFrame = 1;
    audioFormat.mBitsPerChannel = 16;
    audioFormat.mBytesPerPacket = 2;
    audioFormat.mBytesPerFrame = 2;

至:

AudioStreamBasicDescription audioFormat;
    audioFormat.mSampleRate = kSampleRate;
    audioFormat.mFormatID = kAudioFormatLinearPCM;
    audioFormat.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
    audioFormat.mFramesPerPacket = 1;
    audioFormat.mChannelsPerFrame = 2;
    audioFormat.mBitsPerChannel = 16;
    audioFormat.mBytesPerPacket = 4;
    audioFormat.mBytesPerFrame = 4;

但是,渲染器对我来说有些希望.我一直在处理我能找到的任何教程或示例代码.我可以使单声道信号的给定上下文工作,但我无法使渲染器生成立体声信号.我想要的只是左声道中的一个不同频率和右声道中的不同频率 - 但老实说,我不理解渲染器足以让它工作.我已经尝试将memcpy函数导入mBuffers [0]和mbuffers [1],但这会导致应用程序崩溃.我的渲染在下面(它目前包含堆叠的正弦波,但对于立体声示例,我可以在每个通道中使用一个设定频率的波).

#define kOutputBus 0
#define kSampleRate 44100
//44100.0f
#define kWaveform (M_PI * 2.0f / kSampleRate)

OSStatus playbackCallback(void *inRefCon,
                          AudioUnitRenderActionFlags *ioActionFlags,
                          const AudioTimeStamp *inTimeStamp,
                          UInt32 inBusNumber,
                          UInt32 inNumberFrames,
                          AudioBufferList *ioData) {

        HomeViewController *me = (HomeViewController *)inRefCon;

    static int phase = 1;
    static int phase1 = 1;

    for(UInt32 i = 0; i < ioData->mNumberBuffers; i++) {

        int samples = ioData->mBuffers[i].mDataByteSize / sizeof(SInt16);

        SInt16 values[samples];

        float waves;
        float volume=.5;
        float wave1;

        for(int j = 0; j < samples; j++) {


            waves = 0;
            wave1 = 0;

            MyManager *sharedManager = [MyManager sharedManager];


            wave1 = sin(kWaveform * sharedManager.globalFr1 * phase1)*sharedManager.globalVol1;
            if (0.000001f > wave1) {
                [me setFr1:sharedManager.globalFr1];
                phase1 = 0;
                //NSLog(@"switch");
            }

            waves += wave1;
            waves += sin(kWaveform * sharedManager.globalFr2 * phase)*sharedManager.globalVol2;
            waves += sin(kWaveform * sharedManager.globalFr3 * phase)*sharedManager.globalVol3;
            waves += sin(kWaveform * sharedManager.globalFr4 * phase)*sharedManager.globalVol4;
            waves += sin(kWaveform * sharedManager.globalFr5 * phase)*sharedManager.globalVol5;
            waves += sin(kWaveform * sharedManager.globalFr6 * phase)*sharedManager.globalVol6;
            waves += sin(kWaveform * sharedManager.globalFr7 * phase)*sharedManager.globalVol7;
            waves += sin(kWaveform * sharedManager.globalFr8 * phase)*sharedManager.globalVol8;
            waves += sin(kWaveform * sharedManager.globalFr9 * phase)*sharedManager.globalVol9;
            waves *= 32767 / 9; // <--------- make sure to divide by how many waves you're stacking

            values[j] = (SInt16)waves;
            values[j] += values[j]<<16;

            phase++;
            phase1++;

        }

        memcpy(ioData->mBuffers[i].mData, values, samples * sizeof(SInt16));

    }


    return noErr;

}

在此先感谢您的帮助!

Answer 1

OP 似乎已经解决了他的问题，但我认为发布明确的答案会对我们其他人有所帮助。

我有同样的问题，想要将音调独立引导到左声道和右声道。最容易用 Matt Gallagher 的现在标准的An iOS 音调生成器（AudioUnits 简介）来描述。

要进行的第一个更改是在方法中设置（遵循 @jwkerr）streamFormat.mChannelsPerFrame = 2;（而不是streamFormat.mChannelsPerFrame = 1;）createToneUnit。完成后，每帧中有两个通道/缓冲区，您需要独立填充左右缓冲区RenderTone()：

// Set the left and right buffers independently
Float32 tmp;
Float32 *buffer0 = (Float32 *)ioData->mBuffers[0].mData;
Float32 *buffer1 = (Float32 *)ioData->mBuffers[1].mData;

// Generate the samples
for (UInt32 frame = 0; frame < inNumberFrames; frame++) {
    tmp = sin(theta) * amplitude;

    if (channelLR[0]) buffer0[frame] = tmp; else buffer0[frame] = 0;
    if (channelLR[1]) buffer1[frame] = tmp; else buffer1[frame] = 0;

    theta += theta_increment;
    if (theta > 2.0 * M_PI) theta -= 2.0 * M_PI;
}

当然channelLR[2]是一个bool数组，您可以设置其元素来指示相应通道是否可听。请注意，程序需要明确地将无声通道的帧设置为零，否则您会得到一些有趣的音调。