FFmepg--音频编码流程--pcm编码为aac-编程知识

文章目录

- - 基本概念
  - 流程
  - api
  - code(核心部分)

基本概念

从本地⽂件读取PCM数据进⾏AAC格式编码，然后将编码后的AAC数据存储到本地⽂件。

PCM样本格式：未经压缩的⾳频采样数据裸流
参数：

Sample Rate : 采样频率
Sample Size : 量化位数
Number of Channels : 通道个数
Sign : 表示样本数据是否是有符号位
Byte Ordering : 字节序
Integer Or Floating Point : 整形或浮点型

流程

请添加图片描述

api

avcodec_find_encoder：根据指定的AVCodecID查找注册的编码器
avcodec_alloc_context3：为AVCodecContext分配内存
avcodec_open2：打开编码器
avcodec_send_frame：将AVFrame⾮压缩数据给编码器
avcodec_receive_packet：获取到编码后的AVPacket数据
设置AVFrame参数: format 、nb_samples、channel_layout、width/height
av_frame_get_buffer: 为⾳频或视频帧分配新的buffer
av_frame_make_writable：确保AVFrame是可写的
av_samples_fill_arrays 填充⾳频帧

code(核心部分)

static int encode(AVCodecContext *ctx, AVFrame *frame, AVPacket *pkt, FILE *output)
{int ret;/* send the frame for encoding */ret = avcodec_send_frame(ctx, frame);if (ret < 0) {fprintf(stderr, "Error sending the frame to the encoder\n");return -1;}while (ret >= 0) {ret = avcodec_receive_packet(ctx, pkt);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {return 0;} else if (ret < 0) {fprintf(stderr, "Error encoding audio frame\n");return -1;}size_t len = 0;if((ctx->flags & AV_CODEC_FLAG_GLOBAL_HEADER)) {// 需要额外的adts header写入uint8_t aac_header[7];get_adts_header(ctx, aac_header, pkt->size);len = fwrite(aac_header, 1, 7, output);if(len != 7) {fprintf(stderr, "fwrite aac_header failed\n");return -1;}}len = fwrite(pkt->data, 1, pkt->size, output);if(len != pkt->size) {fprintf(stderr, "fwrite aac data failed\n");return -1;}}return -1;
}int main(int argc, char **argv)
{char *in_pcm_file = NULL;char *out_aac_file = NULL;FILE *infile = NULL;FILE *outfile = NULL;const AVCodec *codec = NULL;AVCodecContext *codec_ctx= NULL;AVFrame *frame = NULL;AVPacket *pkt = NULL;int ret = 0;int force_codec = 0;     // 强制使用指定的编码char *codec_name = NULL;in_pcm_file = argv[1];      // 输入PCM文件out_aac_file = argv[2];     // 输出的AAC文件enum AVCodecID codec_id = AV_CODEC_ID_AAC;codec_ctx = avcodec_alloc_context3(codec);if (!codec_ctx) {fprintf(stderr, "Could not allocate audio codec context\n");exit(1);}codec_ctx->codec_id = codec_id;codec_ctx->codec_type = AVMEDIA_TYPE_AUDIO;codec_ctx->bit_rate = 128*1024;codec_ctx->channel_layout = AV_CH_LAYOUT_STEREO;codec_ctx->sample_rate    = 48000; //48000;codec_ctx->channels       = av_get_channel_layout_nb_channels(codec_ctx->channel_layout);codec_ctx->profile = FF_PROFILE_AAC_LOW;    //if(strcmp(codec->name, "aac") == 0) {codec_ctx->sample_fmt = AV_SAMPLE_FMT_FLTP;} else if(strcmp(codec->name, "libfdk_aac") == 0) {codec_ctx->sample_fmt = AV_SAMPLE_FMT_S16;} else {codec_ctx->sample_fmt = AV_SAMPLE_FMT_FLTP;}/* 检测支持采样格式支持情况 */check_sample_fmt(codec, codec_ctx->sample_fmt);check_sample_rate(codec, codec_ctx->sample_rate);check_channel_layout(codec, codec_ctx->channel_layout);codec_ctx->flags = AV_CODEC_FLAG_GLOBAL_HEADER;  //ffmpeg默认的aac是不带adts，而fdk_aac默认带adts，这里我们强制不带/* 将编码器上下文和编码器进行关联 */avcodec_open2(codec_ctx, codec, NULL);// 打开输入和输出文件infile = fopen(in_pcm_file, "rb");outfile = fopen(out_aac_file, "wb");/* packet for holding encoded output */pkt = av_packet_alloc();/* frame containing input raw audio */frame = av_frame_alloc();// 设置frame参数frame->nb_samples     = codec_ctx->frame_size;frame->format         = codec_ctx->sample_fmt;frame->channel_layout = codec_ctx->channel_layout;frame->channels = av_get_channel_layout_nb_channels(frame->channel_layout);/* 为frame分配buffer */av_frame_get_buffer(frame, 0);// 计算出每一帧的数据 单个采样点的字节 * 通道数目 * 每帧采样点数量int frame_bytes = av_get_bytes_per_sample(frame->format) \* frame->channels \* frame->nb_samples;uint8_t *pcm_buf = (uint8_t *)malloc(frame_bytes);uint8_t *pcm_temp_buf = (uint8_t *)malloc(frame_bytes);int64_t pts = 0;printf("start enode\n");for (;;) {memset(pcm_buf, 0, frame_bytes);size_t read_bytes = fread(pcm_buf, 1, frame_bytes, infile);if(read_bytes <= 0) {printf("read file finish\n");break;}/* 确保该frame可写, 如果编码器内部保持了内存参考计数，则需要重新拷贝一个备份目的是新写入的数据和编码器保存的数据不能产生冲突*/av_frame_make_writable(frame);if(AV_SAMPLE_FMT_S16 == frame->format) {// 将读取到的PCM数据填充到frame去ret = av_samples_fill_arrays(frame->data, frame->linesize,pcm_buf, frame->channels,frame->nb_samples, frame->format, 0);} else {// 将读取到的PCM数据填充到frame去// 将本地的f32le packed模式的数据转为float palanarmemset(pcm_temp_buf, 0, frame_bytes);f32le_convert_to_fltp((float *)pcm_buf, (float *)pcm_temp_buf, frame->nb_samples);ret = av_samples_fill_arrays(frame->data, frame->linesize,pcm_temp_buf, frame->channels,frame->nb_samples, frame->format, 0);}// 设置ptspts += frame->nb_samples;frame->pts = pts;       // 使用采样率作为pts的单位，具体换算成秒 pts*1/采样率ret = encode(codec_ctx, frame, pkt, outfile);if(ret < 0) {printf("encode failed\n");break;}}/* 冲刷编码器 */encode(codec_ctx, NULL, pkt, outfile);// 关闭文件fclose(infile);fclose(outfile);// 释放内存if(pcm_buf) {free(pcm_buf);}if (pcm_temp_buf) {free(pcm_temp_buf);}av_frame_free(&frame);av_packet_free(&pkt);avcodec_free_context(&codec_ctx);printf("main finish, please enter Enter and exit\n");getchar();return 0;
}