PCM转PCMA（pcm_alaw，G711.A率）转换表 && PCM转PCMU（pcm_ulaw，G711.U率）转换表

文章目录

[PCM转PCMA（pcm_alaw，G711.A率）转换表 && PCM转PCMU（pcm_ulaw，G711.U率）转换表](#PCM转PCMA（pcm_alaw，G711.A率）转换表 && PCM转PCMU（pcm_ulaw，G711.U率）转换表)
- ffmpeg源码生成转码表
- - 使用方式
  - 生成源码

ffmpeg源码生成转码表

最近要做一个功能，int16_t的PCM数据转为uint8的PCMA数据，

网上的不太可信，找ffmpeg库，发现ffmpeg库使用的是查表发实现的，

而且还有现成的生成表数据的源代码，可信度非常高

c 复制代码

/*
 * Generate a header file for hardcoded PCM tables
 *
 * Copyright (c) 2010 Reimar Döffinger <Reimar.Doeffinger@gmx.de>
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include <stdlib.h>
#define CONFIG_HARDCODED_TABLES 0
#include "pcm_tablegen.h"
#include "tableprint.h"

int main(void)
{
    pcm_alaw_tableinit();
    pcm_ulaw_tableinit();
    pcm_vidc_tableinit();

    write_fileheader();

    WRITE_ARRAY("static const", uint8_t, linear_to_alaw);
    WRITE_ARRAY("static const", uint8_t, linear_to_ulaw);
    WRITE_ARRAY("static const", uint8_t, linear_to_vidc);

    return 0;
}

编译上有些小问题，没太关注，直接使用了注释大法，

没有用的统统干掉，编译通过，执行生成转换映射表。

不用再去研究转换公式，直接对照着表查询结果即可。

拿走不谢

使用方式

可参照ffmpeg源代码pcm.c使用

c 复制代码

    case AV_CODEC_ID_PCM_ALAW:
        for (; n > 0; n--) {
            v      = *samples++;
            *dst++ = linear_to_alaw[(v + 32768) >> 2];
        }
        break;
    case AV_CODEC_ID_PCM_MULAW:
        for (; n > 0; n--) {
            v      = *samples++;
            *dst++ = linear_to_ulaw[(v + 32768) >> 2];
        }
        break;

PCM有符号，加上32768变成正数除以4查表即可。

生成源码

表数据太大，CSDN不让保存，放到我个人服务器上了（大神手下留情不要攻击）。
http://118.126.97.61:8084/src_libs/ffmpeg_pcm_table/