使用SDL2播放正弦聲波 - 噪音/劃痕問題

Question

我的目標是創建一個SDL窗口，繪製不同波形並播放此波的無限聲音。通過按特定鍵，可以修改波形的參數，如幅度，頻率或波形。

問題是，即使是一個簡單的正弦波，看起來不錯，當繪製時，聽起來很嘈雜。我不明白爲什麼。

代碼：

#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; 
    } 
} 

而頭文件：

#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

你SDLAudioCallback()被關閉的buffer末寫一個額外的字節：

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

改變<=只<修復了我的crack啪聲ystem。

通常C風格的「字節指針+長度」API需要左封閉的右開區間：[0, length)。即，您可以訪問buffer[length - 1]而不是buffer[length]。