音频解码流程

aac—音频解码器–pcm数据

API

avcodec_find_decoder：根据指定的AVCodecID查找注册的解码器
av_parser_init：初始化AVCodecParserContext
avcodec_alloc_context3：为AVCodecContext分配内存
avcodec_open2：打开解码器
av_parser_parse2：解析获得⼀个Packet

int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt); // 将AVPacket压缩数据给解码器

int avcodec_receive_frame ( AVCodecContext * avctx, AVFrame * frame ) // 获取到解码后的AVFrame数据

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分析：

首先读文件（原始码流），将原始码流解析为Packet, 通过解码器获得AVFrame ，最后写文件 ，循环获取Packet，写文件

伪代码

// 读文件
data_size = fread(inbuf, 1, AUDIO_INBUF_SIZE, infile); // data_size 20480 while(data_size) {av_parser_parse2()  //解析获得⼀个Packetdecode(); // (压缩AVPacket,解码AVFrame数据，之后写数据)// data_size 减少
}void decode() {ret = avcodec_send_packet； //将AVPacket压缩数据给解码器	while (ret ) {ret = avcodec_receive_frame; // 获取到解码后的AVFrame数据if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)return;  //  fwrite 结束后 ret == AVERROR(EAGAIN,需要新的输⼊, 返回av_get_bytes_per_sample //获取每个sample中的字节数fwrite();}
}

code:

#define AUDIO_INBUF_SIZE 20480
#define AUDIO_REFILL_THRESH 4096static char err_buf[128] = {0};
static char* av_get_err(int errnum)
{av_strerror(errnum, err_buf, 128);return err_buf;
}static void print_sample_format(const AVFrame *frame)
{printf("ar-samplerate: %uHz\n", frame->sample_rate);printf("ac-channel: %u\n", frame->channels);printf("f-format: %u\n", frame->format);// 格式需要注意，实际存储到本地文件时已经改成交错模式
}static void decode(AVCodecContext *dec_ctx, AVPacket *pkt, AVFrame *frame,FILE *outfile)
{int i, ch;int ret, data_size;/* send the packet with the compressed data to the decoder */ret = avcodec_send_packet(dec_ctx, pkt);if(ret == AVERROR(EAGAIN)){fprintf(stderr, "Receive_frame and send_packet both returned EAGAIN, which is an API violation.\n");}else if (ret < 0){fprintf(stderr, "Error submitting the packet to the decoder, err:%s, pkt_size:%d\n",av_get_err(ret), pkt->size)；return;}/* read all the output frames (infile general there may be any number of them */while (ret >= 0){// 对于frame, avcodec_receive_frame内部每次都先调用ret = avcodec_receive_frame(dec_ctx, frame);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)return;else if (ret < 0){fprintf(stderr, "Error during decoding\n");exit(1);}data_size = av_get_bytes_per_sample(dec_ctx->sample_fmt);if (data_size < 0){fprintf(stderr, "Failed to calculate data size\n");exit(1);}static int s_print_format = 0;if(s_print_format == 0){s_print_format = 1;print_sample_format(frame);}for (i = 0; i < frame->nb_samples; i++){for (ch = 0; ch < dec_ctx->channels; ch++)  // 交错的方式写入, 大部分float的格式输出fwrite(frame->data[ch] + data_size*i, 1, data_size, outfile);}}
}int main(int argc, char **argv)
{const char *outfilename;const char *filename;const AVCodec *codec;AVCodecContext *codec_ctx= NULL;AVCodecParserContext *parser = NULL;int len = 0;int ret = 0;FILE *infile = NULL;FILE *outfile = NULL;uint8_t inbuf[AUDIO_INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];uint8_t *data = NULL;size_t   data_size = 0;AVPacket *pkt = NULL;AVFrame *decoded_frame = NULL;if (argc <= 2){fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);exit(0);}filename    = argv[1];outfilename = argv[2];pkt = av_packet_alloc();enum AVCodecID audio_codec_id = AV_CODEC_ID_AAC;if(strstr(filename, "aac") != NULL){audio_codec_id = AV_CODEC_ID_AAC;}else if(strstr(filename, "mp3") != NULL){audio_codec_id = AV_CODEC_ID_MP3;}else{printf("default codec id:%d\n", audio_codec_id);}// 查找解码器codec = avcodec_find_decoder(audio_codec_id);  // AV_CODEC_ID_AACif (!codec) {fprintf(stderr, "Codec not found\n");exit(1);}// 获取裸流的解析器 AVCodecParserContext(数据)  +  AVCodecParser(方法)parser = av_parser_init(codec->id);if (!parser) {fprintf(stderr, "Parser not found\n");exit(1);}// 分配codec上下文codec_ctx = avcodec_alloc_context3(codec);if (!codec_ctx) {fprintf(stderr, "Could not allocate audio codec context\n");exit(1);}// 将解码器和解码器上下文进行关联if (avcodec_open2(codec_ctx, codec, NULL) < 0) {fprintf(stderr, "Could not open codec\n");exit(1);}// 打开输入文件infile = fopen(filename, "rb");if (!infile) {fprintf(stderr, "Could not open %s\n", filename);exit(1);}// 打开输出文件outfile = fopen(outfilename, "wb");if (!outfile) {av_free(codec_ctx);exit(1);}// 读取文件进行解码data      = inbuf;data_size = fread(inbuf, 1, AUDIO_INBUF_SIZE, infile);while (data_size > 0){if (!decoded_frame){if (!(decoded_frame = av_frame_alloc())){fprintf(stderr, "Could not allocate audio frame\n");exit(1);}}ret = av_parser_parse2(parser, codec_ctx, &pkt->data, &pkt->size,data, data_size,AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);if (ret < 0){fprintf(stderr, "Error while parsing\n");exit(1);}data      += ret;   // 跳过已经解析的数据data_size -= ret;   // 对应的缓存大小也做相应减小if (pkt->size)decode(codec_ctx, pkt, decoded_frame, outfile);if (data_size < AUDIO_REFILL_THRESH)    // 如果数据少了则再次读取{memmove(inbuf, data, data_size);    // 把之前剩的数据拷贝到buffer的起始位置data = inbuf;// 读取数据 长度: AUDIO_INBUF_SIZE - data_sizelen = fread(data + data_size, 1, AUDIO_INBUF_SIZE - data_size, infile);if (len > 0)data_size += len;}}/* 冲刷解码器 */pkt->data = NULL;   // 让其进入drain modepkt->size = 0;decode(codec_ctx, pkt, decoded_frame, outfile);fclose(outfile);fclose(infile);avcodec_free_context(&codec_ctx);av_parser_close(parser);av_frame_free(&decoded_frame);av_packet_free(&pkt);printf("main finish, please enter Enter and exit\n");return 0;
}