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SDL 音频示例-打开音频设备播放和混流

发布2018-01-31浏览3335次

详情内容

SDL，即简单的DirectMedia Layer是一个跨平台开发库，旨在通过OpenGL和Direct3D提供对音频，键盘，鼠标，游戏杆和图形硬件的低级访问。 它被视频播放软件，模拟器和流行游戏使用，包括Valve的获奖产品目录和许多Humble Bundle游戏。
SDL正式支持Windows，Mac OS X，Linux，iOS和Android。 其他平台的支持可以在源代码中找到。
SDL是用C语言编写的，使用C ++进行本地工作，并且还有其他几种语言的绑定，包括C＃和Python。
SDL 2.0是根据zlib许可证分发的。 此许可证允许您在任何软件中免费使用SDL。

SDL官网地址：https://www.libsdl.org/

SDL2-CS 是C#版本的SDL工具库：https://github.com/flibitijibibo/SDL2-CS。

英文原文地址（SDL2.0.7）：http://sdl.beuc.net/sdl.wiki/Audio_Examples

英文原文地址（SDL1.2.15）：https://www.libsdl.org/release/SDL-1.2.15/docs/html/guideaudioexamples.html

打开音频设备

  1.  SDL_AudioSpec wanted;
  2.     extern void fill_audio(void *udata, Uint8 *stream, int len);
  3.  
  4.     /* Set the audio format */
  5.     wanted.freq = 22050;
  6.     wanted.format = AUDIO_S16;
  7.     wanted.channels = 2;    /* 1 = mono, 2 = stereo */
  8.     wanted.samples = 1024;  /* Good low-latency value for callback */
  9.     wanted.callback = fill_audio;
  10.     wanted.userdata = NULL;
  11.  
  12.     /* Open the audio device, forcing the desired format */
  13.     if ( SDL_OpenAudio(&wanted, NULL) < 0 ) {
  14.         fprintf(stderr, "Couldn't open audio: %s\n", SDL_GetError());
  15.         return(-1);
  16.     }
  17.     return(0);

播放音频

  1.   static Uint8 *audio_chunk;
  2.     static Uint32 audio_len;
  3.     static Uint8 *audio_pos;
  4.  
  5.     /* The audio function callback takes the following parameters:
  6.        stream:  A pointer to the audio buffer to be filled
  7.        len:     The length (in bytes) of the audio buffer
  8.     */
  9.     void fill_audio(void *udata, Uint8 *stream, int len)
  10.     {
  11.         /* Only play if we have data left */
  12.         if ( audio_len == 0 )
  13.             return;
  14.  
  15.         /* Mix as much data as possible */
  16.         len = ( len > audio_len ? audio_len : len );
  17.         SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME);
  18.         audio_pos += len;
  19.         audio_len -= len;
  20.     }
  21.  
  22.     /* Load the audio data ... */
  23.  
  24.     ;;;;;
  25.  
  26.     audio_pos = audio_chunk;
  27.  
  28.     /* Let the callback function play the audio chunk */
  29.     SDL_PauseAudio(0);
  30.  
  31.     /* Do some processing */
  32.  
  33.     ;;;;;
  34.  
  35.     /* Wait for sound to complete */
  36.     while ( audio_len > 0 ) {
  37.         SDL_Delay(100);         /* Sleep 1/10 second */
  38.     }
  39.     SDL_CloseAudio();

播放流音频，混合2个（或更多）音频流

  1. #include <iostream>
  2. #include <cmath>
  3. #include "SDL/SDL.h"
  4. #include "SDL/SDL_main.h"
  5.  
  6. /* linker options: -lmingw32 -lSDLmain -lSDL -mwindows */
  7.  
  8. using namespace std;
  9.  
  10. unsigned int sampleFrequency = 0;
  11. unsigned int audioBufferSize = 0;
  12. unsigned int outputAudioBufferSize = 0;
  13.  
  14. unsigned int freq1 = 1000;
  15. unsigned int fase1 = 0;
  16. unsigned int freq2 = 5000;
  17. unsigned int fase2 = 0;
  18.  
  19. void example_mixaudio(void *unused, Uint8 *stream, int len) {
  20.  
  21.     unsigned int bytesPerPeriod1 = sampleFrequency / freq1;
  22.     unsigned int bytesPerPeriod2 = sampleFrequency / freq2;
  23.  
  24.     for (int i=0;i<len;i++) {
  25.         int channel1 = int(150*sin(fase1*6.28/bytesPerPeriod1));
  26.         int channel2 = int(150*sin(fase2*6.28/bytesPerPeriod2));
  27.  
  28.         int outputValue = channel1 + channel2;           // just add the channels
  29.         if (outputValue > 127) outputValue = 127;        // and clip the result
  30.         if (outputValue < -128) outputValue = -128;      // this seems a crude method, but works very well
  31.  
  32.         stream[i] = outputValue;
  33.  
  34.         fase1++;
  35.         fase1 %= bytesPerPeriod1;
  36.         fase2++;
  37.         fase2 %= bytesPerPeriod2;
  38.     }
  39. }
  40.  
  41. int main(int argc, char *argv[])
  42. {
  43.  
  44.     if( SDL_Init(SDL_INIT_TIMER | SDL_INIT_AUDIO ) <0 ) {
  45.         cout << "Unable to init SDL: " << SDL_GetError() << endl;
  46.         return 1;
  47.     }
  48.  
  49.     /* setup audio */
  50.     SDL_AudioSpec *desired, *obtained;
  51.  
  52.     /* Allocate a desired SDL_AudioSpec */
  53.     desired = (SDL_AudioSpec *) malloc(sizeof(SDL_AudioSpec));
  54.  
  55.     /* Allocate space for the obtained SDL_AudioSpec */
  56.     obtained = (SDL_AudioSpec *) malloc(sizeof(SDL_AudioSpec));
  57.  
  58.     /* choose a samplerate and audio-format */
  59.     desired->freq = 44100;
  60.     desired->format = AUDIO_S8;
  61.  
  62.     /* Large audio buffers reduces risk of dropouts but increases response time.
  63.      *
  64.      * You should always check if you actually GOT the audiobuffer size you wanted,
  65.      * note that not hardware supports all buffer sizes (< 2048 bytes gives problems with some
  66.      * hardware). Older versions of SDL had a bug that caused many configuration to use a
  67.      * buffersize of 11025 bytes, if your sdl.dll is approx. 1 Mb in stead of 220 Kb, download
  68.      * v1.2.8 of SDL or better...)
  69.      */
  70.     desired->samples = 4096;
  71.  
  72.     /* Our callback function */
  73.     desired->callback=example_mixaudio;
  74.     desired->userdata=NULL;
  75.  
  76.     desired->channels = 1;
  77.  
  78.     /* Open the audio device and start playing sound! */
  79.     if ( SDL_OpenAudio(desired, obtained) < 0 ) {
  80.         fprintf(stderr, "AudioMixer, Unable to open audio: %s\n", SDL_GetError());
  81.         exit(1);
  82.     }
  83.  
  84.     audioBufferSize = obtained->samples;
  85.     sampleFrequency = obtained->freq;
  86.  
  87.     /* if the format is 16 bit, two bytes are written for every sample */
  88.     if (obtained->format==AUDIO_U16 || obtained->format==AUDIO_S16) {
  89.         outputAudioBufferSize = 2*audioBufferSize;
  90.     } else {
  91.         outputAudioBufferSize = audioBufferSize;
  92.     }
  93.  
  94.     SDL_Surface *screen = SDL_SetVideoMode(200,200, 16, SDL_SWSURFACE);
  95.     SDL_WM_SetCaption("Audio Example",0);
  96.  
  97.     SDL_PauseAudio(0);
  98.  
  99.     bool running = true;
  100.  
  101.     SDL_Event event;
  102.     while (running) {
  103.         while (SDL_PollEvent(&event)) {
  104.             /* GLOBAL KEYS / EVENTS */
  105.             switch (event.type) {
  106.             case SDL_KEYDOWN:
  107.                 switch (event.key.keysym.sym) {
  108.                 case SDLK_ESCAPE:
  109.                     running = false;
  110.                     break;
  111.                 default: break;
  112.                 }
  113.                 break;
  114.             case SDL_QUIT:
  115.                 running = false;
  116.                 break;
  117.             }
  118.             SDL_Delay(1);
  119.         }
  120.         SDL_Delay(1);
  121.     }
  122.     SDL_Quit();
  123.     return EXIT_SUCCESS;
  124. }

SDL 更多API参考

https://www.libsdl.org/release/SDL-1.2.15/docs/html/index.html


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