Question

我正在尝试在SDL2中播放纯正弦波，但是我发现似乎无法获得完全纯净的音调。它听起来很接近真实的正弦波，但是后面有一个轻微的次级频率，听起来更接近方波。我已经记录了声音，并验证了在两个不同系统上通过两组扬声器播放时的确不正确（与纯正弦波相比）

在这一点上，我已经尝试了很多事情，包括从堆栈溢出中实现多个正弦波，并尝试改编Handmade Hero中的代码。但是每次都会出现同样的问题。我的怀疑是位精度有问题，某个地方的转换不正确，或者与SDL音频工作的特定方式有关，我无法正确导航

这是我目前正在使用的主要音频回调函数，以及最近一次尝试将正弦波写入缓冲区的操作：

#define Pi32 3.14159265359f
#define Tau32 (2.0f * Pi32)

void
AudioCallback(void* UserData, u8* Stream, int Length)
{
    audio_user_data* AudioUserData = (audio_user_data*)UserData;
    static u32 Count = 0;

    u16* SampleBuffer = (u16*)Stream;
    int SamplesToWrite = Length / AudioUserData->BytesPerSample;
    for(int SampleIndex = 0; SampleIndex < SamplesToWrite; SampleIndex++)
    {
        u16 ToneValue = round((AudioUserData->ToneVolume * sin(Tau32 * (f32)Count / (f32)AudioUserData->WavePeriod)));
        *SampleBuffer++ = ToneValue;
        *SampleBuffer++ = ToneValue;
        ++Count;
    }
}

如果有帮助，我很乐意提供更多背景信息

编辑-其他上下文：

#include "x:\SDL2-2.0.10\include\SDL.h"

#define Pi32 3.14159265359f
#define Tau32 (2.0f * Pi32)

#define INITIAL_SCREEN_WIDTH (8*80)
#define INITIAL_SCREEN_HEIGHT (8*60)

typedef struct audio_user_data audio_user_data;
struct audio_user_data
{
    int SamplesPerSecond;
    int BytesPerSample;
    int SampleIndex;
    int ToneHz;
    int ToneVolume;
    int WavePeriod;
    u32 FileLength;
    u16* BufferLocation;
};

void
AudioCallback(void* UserData, u8* Stream, int Length)
{
    audio_user_data* AudioUserData = (audio_user_data*)UserData;
    static u32 Count = 0;

    u16* SampleBuffer = (u16*)Stream;
    int SamplesToWrite = Length / AudioUserData->BytesPerSample;
    for(int SampleIndex = 0; SampleIndex < SamplesToWrite; SampleIndex++)
    {
        u16 ToneValue = (0.5f + (AudioUserData->ToneVolume * sin(Tau32 * Count / AudioUserData->WavePeriod)));
        *SampleBuffer++ = ToneValue;
        *SampleBuffer++ = ToneValue;
        ++Count;
    }
}

int
main(int argc, char* argv[])
{
    SDL_Init(SDL_INIT_VIDEO|SDL_INIT_AUDIO);

    SDL_Window* Window = SDL_CreateWindow("Spell Checker", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, INITIAL_SCREEN_WIDTH*2, INITIAL_SCREEN_HEIGHT*2, 0);
    SDL_Renderer* Renderer = SDL_CreateRenderer(Window, 0, SDL_RENDERER_SOFTWARE);
    SDL_PixelFormat* Format = SDL_AllocFormat(SDL_PIXELFORMAT_RGB888);
    SDL_Texture* Screen = SDL_CreateTexture(Renderer, Format->format, SDL_TEXTUREACCESS_STREAMING, INITIAL_SCREEN_WIDTH, INITIAL_SCREEN_HEIGHT);

    audio_user_data AudioUserData = {0};
    AudioUserData.SamplesPerSecond = 44100;
    AudioUserData.BytesPerSample = 2 * sizeof(int16);
    AudioUserData.SampleIndex = 0;
    AudioUserData.ToneVolume = 3000;
    AudioUserData.ToneHz = 440;
    AudioUserData.WavePeriod = AudioUserData.SamplesPerSecond / AudioUserData.ToneHz;

    SDL_AudioSpec Want, Have;
    SDL_AudioDeviceID AudioDeviceID;
    Want.freq = AudioUserData.SamplesPerSecond;
    Want.format = AUDIO_S16;
    Want.channels = 2;
    Want.samples = 4096;
    Want.callback = &AudioCallback;
    Want.userdata = &AudioUserData;
    AudioDeviceID = SDL_OpenAudioDevice(0, 0, &Want, &Have, 0);

    SDL_PauseAudioDevice(AudioDeviceID, 0); // Start playing

    u32* PixelMap = calloc(INITIAL_SCREEN_WIDTH * INITIAL_SCREEN_HEIGHT, sizeof(PixelMap));

    int PixelMapLocation = 0;
    int Running = 1;
    while(Running)
    {

        SDL_Event Event;
        while(SDL_PollEvent(&Event))
        {
            if(Event.type == SDL_QUIT)
            {
                Running = 0;
                break;
            }
        }

        // Test colors
        PixelMapLocation = 0;
        for(int Row = 0; Row < INITIAL_SCREEN_WIDTH; ++Row)
        {
            for(int Col = 0; Col < INITIAL_SCREEN_HEIGHT; ++Col)
            {
                PixelMap[PixelMapLocation++] = 0xFF00FF;
            }
        }
        for(int Row = 0; Row < INITIAL_SCREEN_WIDTH; ++Row)
        {
            for(int Col = 0; Col < INITIAL_SCREEN_HEIGHT; ++Col)
            {
                PixelMap[PixelMapLocation++] = 0x00FFFF;
            }
        }

        SDL_UpdateTexture(Screen, 0, PixelMap, INITIAL_SCREEN_WIDTH * sizeof(PixelMap));
        SDL_RenderClear(Renderer);
        SDL_RenderCopy(Renderer, Screen, 0, 0);
        SDL_RenderPresent(Renderer);
    }

    return(0);
}

EDIT2：我录制了我在这里听到的音频（可能需要调高音量才能听到问题）： https://www.youtube.com/watch?v=-V2IMhK2Zis&feature=youtu.be

我还运行了一个时序测试，并在每次AudioCallback函数运行时将其取回： https://imgur.com/a/9pqCte0

EDIT3：示波器读数-

我的正弦波：

纯正弦波：

（我看不到明显的差异，但是也许其他人可以看到吗？）编辑：哦，等等，在示波器的左侧，两个波形之间有明显的差异（尽管它们没有出现在主要阅读）。试图找出问题所在–由于我尝试了几种不同的算法，目前我仍不确定

EDIT4：这是一张想要/必须显示在调用SDL_OpenAudioDevice之后的所有内容（大小除外）都相同的图片：

EDIT5：问题（某种）已经解决！将AudioUserData.SamplesPerSecond设置为48000会产生纯正弦的正弦波。但是问题仍然存在：为什么它只能在48000下工作？

难以获得纯正弦的正弦波（目前很难获得正弦波）

0 个答案: