使用 SDL2 播放正弦声波 - 噪音/划痕问题

Question

我的目标是创建一个 SDL 窗口，绘制不同的波形并播放该波形的不确定声音。通过按下特定按键，可以修改波的幅值、频率或波形等参数。

问题在于，即使是绘制时看起来不错的简单正弦波，听起来也很嘈杂。我不明白为什么。

代码：

#include "Graph.h"
#include <thread>
#include <iostream>
#include <sstream>
#include <string>


int main(int argc, char* argv[]){

    Graph* g = new Graph();

    int i;
    std::cin >> i;
    return 0;
}

int graphThreadFunc(void *pointer){
    Graph* grid = (Graph*)pointer;
    grid->init();

    return 0;
}



// SDL calls this function whenever it wants its buffer to be filled with samples
void SDLAudioCallback(void *data, Uint8 *buffer, int length){
    uint8_t *stream = (uint8_t*)buffer;

    Graph* graph = (Graph*)data;


    for (int i = 0; i <= length; i++){

        if (graph->voice.audioLength <= 0)
            stream[i] = graph->getSpec()->silence;      // 128 is silence in a uint8 stream
        else
        {
            stream[i] = graph->voice.getSample();
            graph->voice.audioPosition++;


            // Fill the graphBuffer with the first 1000 bytes of the wave for plotting
            if (graph->graphPointer < 999)
                graph->graphBuffer[graph->graphPointer++] = stream[i];

        }


    }
}


Graph::Graph()
{
    // spawn thread
    SDL_Thread *refresh_thread = SDL_CreateThread(graphThreadFunc, NULL, this);
}

SDL_AudioSpec* Graph::getSpec(){
    return &this->spec;
}


void Graph::init()
{
    // Init SDL & SDL_ttf
    SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER);
    SDL_zero(desiredDeviceSpec);

    desiredDeviceSpec.freq = 44100;         // Sample Rate
    desiredDeviceSpec.format = AUDIO_U8;    // Unsigned 8-Bit Samples
    desiredDeviceSpec.channels = 1;         // Mono
    desiredDeviceSpec.samples = 2048;       // The size of the Audio Buffer (in number of samples, eg: 2048 * 1 Byte (AUDIO_U8)
    desiredDeviceSpec.callback = SDLAudioCallback;
    desiredDeviceSpec.userdata = this;


    dev = SDL_OpenAudioDevice(NULL, 0, &desiredDeviceSpec, &spec, SDL_AUDIO_ALLOW_FREQUENCY_CHANGE);
    if (dev == 0) {
        printf("\nFailed to open audio: %s\n", SDL_GetError());
    }
    else {
        SDL_PauseAudioDevice(dev, 1); /* pause! */
        SDL_PauseAudio(1);
    }

    // Create an application window with the following settings:
    window = SDL_CreateWindow(
        WINDOW_TITLE.c_str(),              // window title
        SDL_WINDOWPOS_UNDEFINED,           // initial x position
        SDL_WINDOWPOS_UNDEFINED,           // initial y position
        WINDOW_WIDTH,                      // width, in pixels
        WINDOW_HEIGHT,                     // height, in pixels
        SDL_WINDOW_SHOWN                  // flags - see below
        );

    // Check if the window was successfully created
    if (window == NULL) {
        // In case the window could not be created...
        printf("Could not create window: %s\n", SDL_GetError());
        return;
    }
    else{
        voice.waveForm = Graph::Voice::WaveForm::SINE;
        voice.amp = 120;
        voice.frequency = 440;
        SDL_PauseAudioDevice(dev, 1);        // play
        graphPointer = 0;

        voice.audioLength = 44100;
        voice.audioPosition = 0;

        SDL_PauseAudioDevice(dev, 0);        // play
        SDL_Delay(200);

        drawGraph();


        mainLoop();
        return;
    }
}

void Graph::mainLoop()
{
    while (thread_exit == 0){
        SDL_Event event;
        bool hasChanged = false;

        while (SDL_PollEvent(&event)) {
            switch (event.type)
            {
            case SDL_KEYDOWN:
            {
                hasChanged = true;

                if (event.key.keysym.scancode == SDL_SCANCODE_SPACE){
                    //pause_thread = !pause_thread;
                    switch (voice.waveForm){
                    case Voice::SINE:
                    {
                        voice.waveForm = Graph::Voice::WaveForm::TRIANGLE;
                        break;
                    }
                    case Voice::TRIANGLE:
                    {
                        voice.waveForm = Graph::Voice::WaveForm::RECT;
                        break;
                    }
                    case Voice::RECT:
                    {
                        voice.waveForm = Graph::Voice::WaveForm::SAWTOOTH;
                        break;
                    }
                    case Voice::SAWTOOTH:
                    {
                        voice.waveForm = Graph::Voice::WaveForm::SINE;
                        break;
                    }
                    default:
                        break;
                    }
                }
                else if (event.key.keysym.scancode == SDL_SCANCODE_ESCAPE){
                    exit();
                }
                else if (event.key.keysym.scancode == SDL_SCANCODE_RETURN){

                }
                else if (event.key.keysym.scancode == SDL_SCANCODE_LEFT){
                    voice.frequency -= 2;
                }
                else if (event.key.keysym.scancode == SDL_SCANCODE_RIGHT){
                    voice.frequency += 2;
                }
                else if (event.key.keysym.scancode == SDL_SCANCODE_UP){
                    voice.amp += 2;
                }
                else if (event.key.keysym.scancode == SDL_SCANCODE_DOWN){
                    voice.amp -= 2;
                }
                else{

                }

                break;
            }

            case SDL_QUIT:
            {
                exit();
                return;
                break;
            }
            default: /* unhandled event */
                break;
            }
        }

        if (!pause_thread && hasChanged)
        {

            //SDL_PauseAudioDevice(dev, 1);      // play
            graphPointer = 0;

            voice.audioLength = 44100;
            voice.audioPosition = 0;

            SDL_PauseAudioDevice(dev, 0);        // play
            SDL_Delay(200);

            drawGraph();
        }


        //voice.waveForm = Voice::WaveForm::TRIANGLE;       

        //SDL_Delay(n);                      // delay the program to prevent the voice to be overridden before it has been played to the end
        //SDL_PauseAudioDevice(dev, 1);      // pause   

        SDL_Delay(REFRESH_INTERVAL);
        //SDL_PauseAudioDevice(dev, 1);      // pause   
    }


    return;
}

void Graph::drawGraph()
{

    SDL_Renderer *renderer = SDL_GetRenderer(window);
    if (renderer == nullptr)
        renderer = SDL_CreateRenderer(window, 0, SDL_RENDERER_ACCELERATED);

    // Set background color
    SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);

    // Clear winow
    SDL_RenderClear(renderer);


    SDL_SetRenderDrawColor(renderer, 22, 22, 22, 255);
    for (int x = 0; x < WINDOW_WIDTH; x++){
        uint8_t y = graphBuffer[x];
        SDL_RenderDrawPoint(renderer, x, WINDOW_HEIGHT - y);
    }


    SDL_RenderPresent(renderer);


    return;
}

void Graph::exit(){
    thread_exit = 1;
    // Close and destroy the window
    SDL_DestroyWindow(window);
    // Clean up
    SDL_Quit();
}


uint8_t Graph::Voice::getSample(){
    switch (waveForm){
    case SINE:
    {
        float sineStep = 2 * M_PI * audioPosition * frequency / 44100;
        return (amp * sin(sineStep)) + 128;
        break;
    }
    case RECT:
        break;

    case SAWTOOTH:
        break;

    case TRIANGLE:
        break;

    default:
        return 0;
    }
}

还有头文件：

#ifndef GRAPH_H
#define GRAPH_H
#include "SDL.h"
#include "SDL_audio.h"
#include <stdio.h>
#include <cmath>
#include <string>
#include <stack>

/* Constants */
const int REFRESH_INTERVAL = 50;                // mseconds
const int WINDOW_WIDTH = 1000;
const int WINDOW_HEIGHT = 255;
const std::string WINDOW_TITLE = "Wave Graph";

class Graph
{
private:
    SDL_Window *window;          // Declare a pointer

    // SDL audio stuff
    SDL_AudioSpec desiredDeviceSpec;
    SDL_AudioSpec spec;
    SDL_AudioDeviceID dev;

    int thread_exit = 0;
    bool pause_thread = false;

public:
    Graph();
    void init();
    void mainLoop();
    void drawGraph();

    void exit();
    SDL_AudioSpec* getSpec();

    struct Voice{
        int frequency;              // the frequency of the voice
        int amp;                    // the amplitude of the voice

        int audioLength;            // number of samples to be played, eg: 1.2 seconds * 44100 samples per second
        int audioPosition = 0;      // counter

        enum WaveForm{
            SINE = 0, RECT = 1, SAWTOOTH = 2, TRIANGLE = 3
        } waveForm;


        uint8_t getSample();
    } voice;
    int graphPointer = 0;
    uint8_t graphBuffer[1000];


};


#endif

Answer 1

您的SDLAudioCallback() 正在从buffer 的末尾写入一个额外的字节：

void SDLAudioCallback(void *data, Uint8 *buffer, int length)
{
    ...
    for (int i = 0; i <= length; i++)
    //                ^^ huh?
    {
        ...
    }
}

将<= 更改为仅< 修复了我系统上的裂纹。

通常 C 风格的“字节指针 + 长度”API 需要一个左闭右开区间：[0, length)。即，您可以访问buffer[length - 1]，但不能访问buffer[length]。