【音视频学习】三、FFmpeg音频编码过程详解

前言

linux版本：22.04.1-Ubuntu x86_64 GNU/Linux
ffmpeg版本： 7.1.1
gcc版本：11.4.0

本文以FFmpeg\doc\examples\encode_audio.c为例来说明。

1、FFmpeg编解码器的编码流程概述

encode_audio.c文件的源码如下：

/**
 * @file libavcodec encoding audio API usage examples
 * @example encode_audio.c
 *
 * Generate a synthetic audio signal and encode it to an output MP2 file.
 */

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

#include <libavcodec/avcodec.h>

#include <libavutil/channel_layout.h>
#include <libavutil/common.h>
#include <libavutil/frame.h>
#include <libavutil/samplefmt.h>

/* check that a given sample format is supported by the encoder */
static int check_sample_fmt(const AVCodec *codec, enum AVSampleFormat sample_fmt)
{
   
   
    const enum AVSampleFormat *p = codec->sample_fmts;

    while (*p != AV_SAMPLE_FMT_NONE) {
   
   
        if (*p == sample_fmt)
            return 1;
        p++;
    }
    return 0;
}

/* just pick the highest supported samplerate */
static int select_sample_rate(const AVCodec *codec)
{
   
   
    const int *p;
    int best_samplerate = 0;

    if (!codec->supported_samplerates)
        return 44100;

    p = codec->supported_samplerates;
    while (*p) {
   
   
        if (!best_samplerate || abs(44100 - *p) < abs(44100 - best_samplerate))
            best_samplerate = *p;
        p++;
    }
    return best_samplerate;
}

/* select layout with the highest channel count */
static int select_channel_layout(const AVCodec *codec, AVChannelLayout *dst)
{
   
   
    const AVChannelLayout *p, *best_ch_layout;
    int best_nb_channels   = 0;

    if (!codec->ch_layouts)
        return av_channel_layout_copy(dst, &(AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO);

    p = codec->ch_layouts;
    while (p->nb_channels) {
   
   
        int nb_channels = p->nb_channels;

        if (nb_channels > best_nb_channels) {
   
   
            best_ch_layout   = p;
            best_nb_channels = nb_channels;
        }
        p++;
    }
    return av_channel_layout_copy(dst, best_ch_layout);
}