Article directory

foreword
1. Introduction to SDLAPI
2. Use examples
Summarize

foreword

In the previous blog, the introduction, compilation, streaming, decoding, etc. of FFmpeg have been introduced in detail. Now following the previous blog, after the previous streaming codec, use SDL to play.

For the introduction and compilation of SDL, please check the previous blog

| Version statement: Mr. Shanhe, without the permission of the blogger, reprinting is prohibited

1. Introduction to SDLAPI

1. Initialize the subsystem

int SDL_Init(Uint32 flags);
The value of flags can be one or more of the following:

SDL_INIT_TIMER: timer subsystem
SDL_INIT_AUDIO: Audio Subsystem
SDL_INIT_VIDEO: video subsystem; automatically initializes the event subsystem
SDL_INIT_JOYSTICK: joystick subsystem; automatically initializes the event subsystem
SDL_INIT_HAPTIC: Haptic (force feedback) subsystem
SDL_INIT_GAMECONTROLLER: controller subsystem; automatically initializes the joystick subsystem
SDL_INIT_EVENTS: Events Subsystem
SDL_INIT_EVERYTHING: all subsystems above
SDL_INIT_NOPARACHUTE: Compatibility; this flag is ignored

return: 0 on success, negative error code on failure; call SDL_GetError () for more information.

Remark: After using the subsystem, you need to call SDL_Quit () to close

2. Turn on the audio device

int SDL_OpenAudio(SDL_AudioSpec * desired, SDL_AudioSpec * obtained);
参数：

desired: An SDL_AudioSpec structure representing the desired output format. See the SDL_OpenAudioDevice documentation for details on how to prepare this structure.
obtained: SDL_AudioSpec structure filled with actual parameters, or NULL.

return: Returns 0 if successful, and puts the actual hardware parameters into the pointed-to structure obtained.

Remarks: A more powerful method is to use SDL_OpenAudioDevice (), specifying the device to use

3. SDL_AudioSpec structure

Structure containing the audio output format. It also contains callbacks that are called when the audio device needs more data.

type	paraphrase
int freq	samples per second
SDL_AudioFormat format	audio data format
Uint8 channels	number of independent channels
Uint8 silence	Audio buffer mute value
Uint16 samples	audio buffer size in samples
Uint32 size	audio buffer size
SDL_AudioCallback callback	Function called when the audio device needs more data
void *userdata	the pointer to pass to the callback

The audio data format parameter values are as follows:
The number of supported channels is as follows.
The supported values after SDL2.0 are 1 (mono), 2 (stereo), 4 (quad) and 6 (5.1)
The callback function model is as follows:
void SDL_AudioCallback(void* userdata, Uint8* stream, int len)
The size of the audio buffer in the sample
Since MP3 is played in this example, the data size of one frame of MP3 is 1152 samples, so the value is 1152

4. Pause the audio device

void SDL_PauseAudio(int pause_on);
Parameters: non-zero pause, 0 to cancel the pause
Remarks: Generally, 1 is passed in to represent pause

2. Use examples

1. Rough design

The basis of playback is based on the correct streaming and encoding and decoding, so the following code is a modification and supplement to the previous example. I won’t introduce too much about the streaming and decoding APIs.

The overall design structure is as follows:
insert image description here

overall external interface

bool StartPlay(const char* pAudioFilePath);
bool StopPlay();
bool PausePlay();
bool ResumePlay();

Respectively corresponding to start playing, pause playing, resume playing, stop playing.

2. Header file

code show as below:

#pragma once

extern "C" {
    
    
#include "include/libavformat/avformat.h"
#include "include/libavcodec/avcodec.h"
#include "include/libavutil/avutil.h"
#include "include/libswresample/swresample.h"
}

#include "SDL.h"
#include <iostream>
#include <mutex>
#include <Windows.h> 
#include <vector>

namespace AudioReadFrame
{
    
    

#define MAX_AUDIO_FRAME_SIZE	192000 // 1 second of 48khz 16bit audio 2 channel
#define MAX_CACHE_BUFFER_SIZE	50*1024*1024 // 5M

	//自定义结构体存储信息
	struct FileAudioInst
	{
    
    
		long long duration;    ///< second
		long long curl_time;   ///< second
		int sample_rate;       ///< samples per second
		int channels;          ///< number of audio channels
		FileAudioInst()
		{
    
    
			duration = 0;
			curl_time = 0;
			sample_rate = 0;
			channels = 0;
		}
	};

	//拉流线程状态
	enum ThreadState
	{
    
    
		eRun = 1,
		eExit,
	};

	enum PlaySate
	{
    
    
		ePlay = 1,
		ePause,
		eStop
	};

	struct CircleBuffer
	{
    
    
		CircleBuffer() : dataBegin(0), dataEnd(0), buffLen_(0) {
    
    }

		uint32_t						buffLen_;
		uint32_t						dataBegin;
		uint32_t						dataEnd;
		std::vector<uint8_t>			decodeBuffer_;
	};

	class CAudioReadFrame
	{
    
    
	public:
		CAudioReadFrame();
		~CAudioReadFrame();

	public:
		
		bool StartPlay(const char* pAudioFilePath);

		bool StopPlay();

		bool PausePlay();
		 
		bool ResumePlay();

	private:
		//加载流文件
		bool LoadAudioFile(const char* pAudioFilePath);
		//开始读流
		bool StartReadFile();
		//停止读流
		bool StopReadFile();
		//释放资源
		bool FreeResources();
		//改变拉流线程的装填
		void ChangeThreadState(ThreadState eThreadState);
		//拉流线程
		void ReadFrameThreadProc();
		//utf转GBK
		std::string UTF8ToGBK(const std::string& strUTF8);

	private:
		uint32_t GetBufferFromCache(uint8_t* buffer, int bufferLen);
		void PushBufferCache(uint8_t* buffer, int bufferLen);
		void ClearCache();
		void AudioCallback(Uint8* stream, int len);
		static void SDL_AudioCallback(void* userdata, Uint8* stream, int len);

	private:
		typedef std::unique_ptr<std::thread> ThreadPtr;

		//目的是为了重定向输出ffmpeg日志到本地文件
#define PRINT_LOG 0
#ifdef PRINT_LOG
	private:
		static FILE* m_pLogFile;
		static void LogCallback(void* ptr, int level, const char* fmt, va_list vl);
#endif

		//目的是为了将拉流数据dump下来
#define DUMP_AUDIO 1
#ifdef DUMP_AUDIO
		FILE*					decode_file;
		FILE*					play_file;
#endif // DUMP_FILE


	private:
		bool					m_bIsReadyForRead;
		int						m_nStreamIndex;
		uint8_t*				m_pSwrBuffer;
		std::mutex				m_lockResources;
		std::mutex				m_lockThread;
		FileAudioInst*			m_pFileAudioInst;
		ThreadPtr				m_pReadFrameThread;
		ThreadState				m_eThreadState;

	private:
		SwrContext*				m_pSwrContext;		//重采样
		AVFrame*				m_pAVFrame;			//音频包
		AVCodec*				m_pAVCodec;			//编解码器
		AVPacket*				m_pAVPack;			//读包
		AVCodecParameters *		m_pAVCodecParameters; //编码参数
		AVCodecContext*			m_pAVCodecContext;	//解码上下文
		AVFormatContext*		m_pAVFormatContext;	//IO上下文

	private:
		PlaySate				m_ePlayState;
		std::mutex				cacheBufferLock;
		CircleBuffer			decodeBuffer_;
	};
}

3. Implementation file