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本篇FFMEPG实现对AAC解码，解码结果保存wav格式。对AAC编码文件来说，编码根据音频参数编码，解码根据音频参数重新构建声波，

FFMPEG构建的音频存储方式不一定支持播放, 所以需要重采样样本，例如新版ffmpeg解码AAC解码后得到AV_SAMPLE_FMT_FLTP，该格式SDKL播放不支持该格式

ffmpeg所支持的音频格式中：有的仅内部使用，有的可以外部作为音频文件格式

AVFormatContext	* pFormatCtx = avformat_alloc_context();avformat_open_input(&pFormatCtx,INPUT_FILE_NAME,NULL,NULL)‘avformat_find_stream_info(pFormatCtx,NULL);// Dump valid information onto standard errorav_dump_format(pFormatCtx, 0, INPUT_FILE_NAME, false); // Find the first audio stream👉👉int audioStreamp_codec_par = p_fmt_ctx->streams[a_idx]->codecpar;AVCodec	*pCodec=avcodec_find_decoder(pCodecCtx->codec_id);AVCodecContext	*p_codec_ctx = avcodec_alloc_context3(p_codec);avcodec_parameters_to_context(p_codec_ctx, p_codec_par);// Open codecavcodec_open2(pCodecCtx, pCodec,NULL);uint64_t iInputLayout				= av_get_default_channel_layout(pCodecCtx->channels);//使用默认声道分布int      iInputChans				= pCodecCtx->channels;AVSampleFormat eInputSampleFormat   = pCodecCtx->sample_fmt;int	     iInputSampleRate			= pCodecCtx->sample_rate;uint64_t iOutputLayout				= av_get_default_channel_layout(pCodecCtx->channels);int      iOutputChans				= pCodecCtx->channels;AVSampleFormat eOutputSampleFormat  = AV_SAMPLE_FMT_S16;int	     iOutputSampleRate			= pCodecCtx->sample_rate;SwrContext *pSwrCtx = swr_alloc_set_opts(NULL,iOutputLayout, eOutputSampleFormat, iOutputSampleRate,	iInputLayout,eInputSampleFormat , iInputSampleRate,0, NULL);swr_init(pSwrCtx);//AVPacket读取原始解码前的数据AVPacket *packet=(AVPacket *)malloc(sizeof(AVPacket));av_init_packet(packet);//1帧数据样本数int iFrameSamples = pCodecCtx->frame_size;// 存储原始数据 int iRawLineSize = 0;int iRawBuffSize  = av_samples_get_buffer_size(&iRawLineSize, iInputChans, iFrameSamples, eInputSampleFormat, 0);uint8_t *pRawBuff = (uint8_t *)av_malloc(iRawBuffSize);//原始数据保存在AVFrame结构体中AVFrame* pRawframe = av_frame_alloc();	pRawframe->nb_samples	= iFrameSamples;		//number of audio samples (per channel) described by this frame	pRawframe->format		= eInputSampleFormat;	pRawframe->channels     = iInputChans;avcodec_fill_audio_frame(pRawframe, iInputChans, eInputSampleFormat, (const uint8_t*)pRawBuff, iRawBuffSize, 0);/*	swr的接收帧pConvertframe*/// 存储转换后数据 int iConvertLineSize = 0;int iConvertBuffSize  = av_samples_get_buffer_size(&iConvertLineSize, iOutputChans, iFrameSamples, eOutputSampleFormat, 0);uint8_t *pConvertBuff = (uint8_t *)av_malloc(iConvertBuffSize);//转换后数据保存在AVFrame结构体中AVFrame* pConvertframe = av_frame_alloc();	pConvertframe->nb_samples	= iFrameSamples;	pConvertframe->format		= eOutputSampleFormat;	pConvertframe->channels     = iOutputChans;avcodec_fill_audio_frame(pConvertframe, iOutputChans, eOutputSampleFormat, (const uint8_t*)pConvertBuff, iConvertBuffSize, 0);int iGetPicture;int iDecodeRet;int iFrameNo = 0;write_wav_header(16,iOutputChans,eOutputSampleFormat,iOutputSampleRate,0,pOutFile);//自定义，按照wav格式while(av_read_frame(pFormatCtx, packet)>=0){
   	if(packet->stream_index==audioStream)	{
   		avcodec_decode_audio4( pCodecCtx, pRawframe,&iGetPicture, packet);//要解码成功&&拿到pkt		if ( iGetPicture > 0 )		{
   			printf("FrameNo:%5d\n",iFrameNo);			swr_convert(pSwrCtx, (uint8_t**)pConvertframe->data, iFrameSamples ,(const uint8_t**)pRawframe->data, iFrameSamples );			fwrite(pConvertframe->data[0],pConvertframe->linesize[0],1,pOutFile);			iFrameNo++;		}	}	av_free_packet(packet);}av_free(pRawBuff);av_free(pConvertBuff);swr_free(&pSwrCtx);avcodec_close(pCodecCtx);avformat_close_input(&pFormatCtx);fclose(pOutFile);printf("Aac encode Success!!\n");getchar();return 0;

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