FFMPEG学习----使用SDL播放PCM数据

参考雷神的代码：

/**
* 最简单的SDL2播放音频的例子（SDL2播放PCM）
* Simplest Audio Play SDL2 (SDL2 play PCM)
*
* 本程序使用SDL2播放PCM音频采样数据。SDL实际上是对底层绘图
* API（Direct3D，OpenGL）的封装，使用起来明显简单于直接调用底层
* API。
*
* 函数调用步骤如下:
*
* [初始化]
* SDL_Init(): 初始化SDL。
* SDL_OpenAudio(): 根据参数（存储于SDL_AudioSpec）打开音频设备。
* SDL_PauseAudio(): 播放音频数据。
*
* [循环播放数据]
* SDL_Delay(): 延时等待播放完成。
*
* This software plays PCM raw audio data using SDL2.
* SDL is a wrapper of low-level API (DirectSound).
* Use SDL is much easier than directly call these low-level API.
*
* The process is shown as follows:
*
* [Init]
* SDL_Init(): Init SDL.
* SDL_OpenAudio(): Opens the audio device with the desired
*                  parameters (In SDL_AudioSpec).
* SDL_PauseAudio(): Play Audio.
*
* [Loop to play data]
* SDL_Delay(): Wait for completetion of playback.
*/

#include <stdio.h>
#include <tchar.h>

extern "C"
{
#include "SDL.h"
};

Uint32  audio_len;//音频数据大小
Uint8  *audio_pos;//指向音频数据的指针


/**回调函数(由系统调用)
*  函数声明：typedef void (SDLCALL * SDL_AudioCallback) 
*            (void *userdata, Uint8 * stream, int len);
*  This function is called when the audio device needs more data.
*
*  userdata: An application-specific parameter saved in the SDL_AudioSpec structure(SDL_AudioSpec结构中的用户自定义数据，一般情况下可以不用)
*  stream:   A pointer to the audio data buffer.(该指针指向需要填充的音频缓冲区)
*  len:      The length of that buffer in bytes.(音频缓冲区的大小,以字节为单位)
*
*  Once the callback returns, the buffer will no longer be valid.
*  Stereo samples are stored in a LRLRLR ordering.
*
*  You can choose to avoid callbacks and use SDL_QueueAudio() instead, if
*  you like. Just open your audio device with a NULL callback.
*/

void  fill_audio(void *userdata, Uint8 *stream, int len)
{
	//SDL2中必须首先使用SDL_memset()将stream中的数据设置为0
	SDL_memset(stream, 0, len);
	if (audio_len == 0)		/*  Only  play  if  we  have  data  left  */
	{
		return;
	}
	len = (len > audio_len ? audio_len : len);	/*  Mix  as  much  data  as  possible  */

	/**
	*  函数声明：extern DECLSPEC void SDLCALL 
	*  SDL_MixAudio(Uint8 * dst, const Uint8 * src, Uint32 len, int volume);
	*  This takes two audio buffers of the playing audio format and mixes
	*  them, performing addition, volume adjustment, and overflow clipping.
	*  The volume ranges from 0 - 128, and should be set to ::SDL_MIX_MAXVOLUME
	*  for full audio volume.  Note this does not change hardware volume.
	*  This is provided for convenience -- you can mix your own audio data.
	*
	*  #define SDL_MIX_MAXVOLUME 128
	*/

	SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME);
	audio_pos += len;
	audio_len -= len;
}

int main(int argc, char* argv[])
{
	//初始化SDL
	if (SDL_Init(SDL_INIT_AUDIO))
	{
		printf("Could not initialize SDL - %s
", SDL_GetError());
		return -1;
	}

	//SDL_AudioSpec初始化
	SDL_AudioSpec wanted_spec;
	wanted_spec.freq = 44100;			//音频数据的采样率（常用的有48000,44100等）
	wanted_spec.format = AUDIO_S16SYS;//音频数据的格式
	wanted_spec.channels = 2;			//声道数（例如单声道取值为1，立体声取值为2）
	wanted_spec.silence = 0;			//设置静音的值
	wanted_spec.samples = 1024;		//音频缓冲区中的采样个数（要求必须是2的n次方）
	wanted_spec.callback = fill_audio;//填充音频缓冲区的回调函数

	//打开音频
	if (SDL_OpenAudio(&wanted_spec, NULL) < 0)
	{
		printf("can't open audio.
");
		return -1;
	}

	FILE *fp_pcm = fopen("..\FFmpeg_PCM\output.pcm", "rb");
	if (fp_pcm == NULL)
	{
		printf("cannot open this file
");
		return -1;
	}

	int pcm_buffer_size = 4096;//每次读取4096字节的数据，同时也是音频帧大小
	char *pcm_buffer = (char *)malloc(pcm_buffer_size);
	int data_count = 0;

	//播放音频数据
	SDL_PauseAudio(0);

	while (true)
	{
		int ret = fread(pcm_buffer, 1, pcm_buffer_size, fp_pcm);
		if (ret != pcm_buffer_size)
		{
			//这里有可能是会有剩余音频数据的，不知道这样改对不对？
			audio_pos = (Uint8 *)pcm_buffer;
			audio_len = ret;
			while (audio_len > 0)
			{
				SDL_Delay(1);
			}

			//退出
			break;
			
			//循环播放
			fseek(fp_pcm, 0, SEEK_SET);
			fread(pcm_buffer, 1, pcm_buffer_size, fp_pcm);
			data_count = 0;
		}
		printf("Now Playing %10d Bytes data.
", data_count);
		data_count += pcm_buffer_size;
		

		audio_pos = (Uint8 *)pcm_buffer;
		//Audio buffer length 
		audio_len = pcm_buffer_size;
		

		while (audio_len > 0)//Wait until finish
		{
			SDL_Delay(1);
		}
	}
	free(pcm_buffer);
	fclose(fp_pcm);
	SDL_Quit();
	return 0;
}

雷神代码中if (fread(pcm_buffer, 1, pcm_buffer_size, fp) != pcm_buffer_size){}
如果获取不到4096个字节的音频数据，就从头接着播放了，当然这意味着读到末尾了，但是剩下的音频数据没有做处理？

疑问：

wanted_spec.silence = 0; //设置静音的值
wanted_spec.samples = 1024; //音频缓冲区中的采样个数（要求必须是2的n次方）

int pcm_buffer_size = 4096;//每次读取4096字节的数据，同时也是音频帧大小