最近遇到在 iOS 平台上实时播放 AAC 音频数据流, 一开始尝试用 AudioQueue 直接解 AAC 未果, 转而将 AAC 解码为 PCM, 最终实现了 AAC 实时流在 iOS 平台下的播放问题.
AAC 转 PCM 需要借助解码库来实现, 目前了解到有两个库能干这个事 : faad 和 ffmpeg.
- faad 算是轻量级的解码库, 编译出来全平台静态库文件大小 2M 左右, API 也比较简单, 缺点是功能单一只处理 AAC , 它还有一个对应的编码库叫 faac.
- ffmpeg 体积庞大, 功能丰富, API 略显复杂.
下面分别梳理使用这两个库完成解码的过程.
faad
- 下载源码
#下载
wget http://downloads.sourceforge.net/faac/faad2-2.7.tar.gz
#解压缩
tar xvzf faad2-2.7.tar.gz
#重命名
mv faad2-2.7 faad
- 写编译脚本, vi build-faad.sh
#!/bin/sh
CONFIGURE_FLAGS="--enable-static --with-pic"
ARCHS="arm64 armv7s armv7 x86_64 i386"
# directories
SOURCE="faad"
FAT="fat-faad"
SCRATCH="scratch-faad"
# must be an absolute path
THIN=`pwd`/"thin-faad"
COMPILE="y"
LIPO="y"
if [ "$*" ]
then
if [ "$*" = "lipo" ]
then
# skip compile
COMPILE=
else
ARCHS="$*"
if [ $# -eq 1 ]
then
# skip lipo
LIPO=
fi
fi
fi
if [ "$COMPILE" ]
then
CWD=`pwd`
for ARCH in $ARCHS
do
echo "building $ARCH..."
mkdir -p "$SCRATCH/$ARCH"
cd "$SCRATCH/$ARCH"
if [ "$ARCH" = "i386" -o "$ARCH" = "x86_64" ]
then
PLATFORM="iPhoneSimulator"
CPU=
if [ "$ARCH" = "x86_64" ]
then
SIMULATOR="-mios-simulator-version-min=7.0"
HOST=
else
SIMULATOR="-mios-simulator-version-min=5.0"
HOST="--host=i386-apple-darwin"
fi
else
PLATFORM="iPhoneOS"
if [ $ARCH = "armv7s" ]
then
CPU="--cpu=swift"
else
CPU=
fi
SIMULATOR=
HOST="--host=arm-apple-darwin"
fi
XCRUN_SDK=`echo $PLATFORM | tr '[:upper:]' '[:lower:]'`
CC="xcrun -sdk $XCRUN_SDK clang -Wno-error=unused-command-line-argument-hard-error-in-future"
AS="$CWD/$SOURCE/extras/gas-preprocessor.pl $CC"
CFLAGS="-arch $ARCH $SIMULATOR"
CXXFLAGS="$CFLAGS"
LDFLAGS="$CFLAGS"
CC=$CC CFLAGS=$CXXFLAGS LDFLAGS=$LDFLAGS CPPFLAGS=$CXXFLAGS CXX=$CC CXXFLAGS=$CXXFLAGS $CWD/$SOURCE/configure \
$CONFIGURE_FLAGS \
$HOST \
--prefix="$THIN/$ARCH" \
--disable-shared \
--without-mp4v2
make clean && make && make install-strip
cd $CWD
done
fi
if [ "$LIPO" ]
then
echo "building fat binaries..."
mkdir -p $FAT/lib
set - $ARCHS
CWD=`pwd`
cd $THIN/$1/lib
for LIB in *.a
do
cd $CWD
lipo -create `find $THIN -name $LIB` -output $FAT/lib/$LIB
done
cd $CWD
cp -rf $THIN/$1/include $FAT
fi
保存编译脚本到解压出的 faad 目录同一级目录下, 并添加可执行权限
chmod a+x build-faad.sh
编译
./build-faad.sh
当前目录下 fat-faad 即为编译结果所在位置, 里面有头文件和支持全平台(armv7,armv7s,i386,x86_64,arm64)的静态库添加静态库到工程依赖 (鼠标拖 fat-faad 目录到 xcode 工程目录下), 创建解码文件
FAACDecoder.h,FAACDecoder.mFAACDecoder.h
//
// FAACDecoder.h
// EasyClient
//
// Created by 吴鹏 on 16/9/3.
// Copyright © 2016年 EasyDarwin. All rights reserved.
//
#ifndef FAACDecoder_h
#define FAACDecoder_h
typedef struct {
NeAACDecHandle handle;
int sample_rate;
int channels;
int bit_rate;
}FAADContext;
FAADContext* faad_decoder_create(int sample_rate, int channels, int bit_rate);
int faad_decode_frame(FAADContext *pParam, unsigned char *pData, int nLen, unsigned char *pPCM, unsigned int *outLen);
void faad_decode_close(FAADContext *pParam);
#endif /* FAACDecoder_h */
- FAACDecoder.m
//
// FAACDecoder.m
// EasyClient
//
// Created by 吴鹏 on 16/9/3.
// Copyright © 2016年 EasyDarwin. All rights reserved.
//
#import <Foundation/Foundation.h>
#import "FAACDecoder.h"
#import "faad.h"
uint32_t _get_frame_length(const unsigned char *aac_header)
{
uint32_t len = *(uint32_t *)(aac_header + 3);
len = ntohl(len); //Little Endian
len = len << 6;
len = len >> 19;
return len;
}
FAADContext* faad_decoder_create(int sample_rate, int channels, int bit_rate)
{
NeAACDecHandle handle = NeAACDecOpen();
if(!handle){
printf("NeAACDecOpen failed\n");
goto error;
}
NeAACDecConfigurationPtr conf = NeAACDecGetCurrentConfiguration(handle);
if(!conf){
printf("NeAACDecGetCurrentConfiguration failed\n");
goto error;
}
conf->defSampleRate = sample_rate;
conf->outputFormat = FAAD_FMT_16BIT;
conf->dontUpSampleImplicitSBR = 1;
NeAACDecSetConfiguration(handle, conf);
FAADContext* ctx = malloc(sizeof(FAADContext));
ctx->handle = handle;
ctx->sample_rate = sample_rate;
ctx->channels = channels;
ctx->bit_rate = bit_rate;
return ctx;
error:
if(handle){
NeAACDecClose(handle);
}
return NULL;
}
int faad_decode_frame(FAADContext *p, unsigned char *pData, int nLen, unsigned char *pPCM, unsigned int *outLen)
{
FAADContext* pCtx = (FAADContext*)pParam;
NeAACDecHandle handle = pCtx->handle;
long res = NeAACDecInit(handle, pData, nLen, (unsigned long*)&pCtx->sample_rate, (unsigned char*)&pCtx->channels);
if (res < 0) {
printf("NeAACDecInit failed\n");
return -1;
}
NeAACDecFrameInfo info;
uint32_t framelen = _get_frame_length(pData);
unsigned char *buf = (unsigned char *)NeAACDecDecode(handle, &info, pData, framelen);
if (buf && info.error == 0) {
if (info.samplerate == 44100) {
//src: 2048 samples, 4096 bytes
//dst: 2048 samples, 4096 bytes
int tmplen = (int)info.samples * 16 / 8;
memcpy(pPCM,buf,tmplen);
*outLen = tmplen;
} else if (info.samplerate == 22050) {
//src: 1024 samples, 2048 bytes
//dst: 2048 samples, 4096 bytes
short *ori = (short*)buf;
short tmpbuf[info.samples * 2];
int tmplen = (int)info.samples * 16 / 8 * 2;
for (int32_t i = 0, j = 0; i < info.samples; i += 2) {
tmpbuf[j++] = ori[i];
tmpbuf[j++] = ori[i + 1];
tmpbuf[j++] = ori[i];
tmpbuf[j++] = ori[i + 1];
}
memcpy(pPCM,tmpbuf,tmplen);
*outLen = tmplen;
}else if(info.samplerate == 8000){
//从双声道的数据中提取单通道
for(int i=0,j=0; i<4096 && j<2048; i+=4, j+=2)
{
pPCM[j]= buf[i];
pPCM[j+1]=buf[i+1];
}
*outLen = (unsigned int)info.samples;
}
} else {
printf("NeAACDecDecode failed\n");
return -1;
}
return 0;
}
void faad_decode_close(void *pParam)
{
if(!pParam){
return;
}
FAADContext* pCtx = (FAADContext*)pParam;
if(pCtx->handle){
NeAACDecClose(pCtx->handle);
}
free(pCtx);
}
几个主要 API :
- NeAACDecOpen
- NeAACDecGetCurrentConfiguration
- NeAACDecSetConfiguration
- NeAACDecInit
- NeAACDecDecode
- NeAACDecClose
ffmpeg
添加 ffmpeg 静态库到工程依赖, 创建解码文件
AACDecoder.h,AACDecoder.mAACDecoder.h
#ifndef _AACDecoder_h
#define _AACDecoder_h
void *aac_decoder_create(int sample_rate, int channels, int bit_rate);
int aac_decode_frame(void *pParam, unsigned char *pData, int nLen, unsigned char *pPCM, unsigned int *outLen);
void aac_decode_close(void *pParam);
#endif
- AACDecoder.m
#include "AACDecoder.h"
#include "libavformat/avformat.h"
#include "libswresample/swresample.h"
#include "libavcodec/avcodec.h"
typedef struct AACDFFmpeg {
AVCodecContext *pCodecCtx;
AVFrame *pFrame;
struct SwrContext *au_convert_ctx;
int out_buffer_size;
} AACDFFmpeg;
void *aac_decoder_create(int sample_rate, int channels, int bit_rate)
{
AACDFFmpeg *pComponent = (AACDFFmpeg *)malloc(sizeof(AACDFFmpeg));
AVCodec *pCodec = avcodec_find_decoder(AV_CODEC_ID_AAC);
if (pCodec == NULL)
{
printf("find aac decoder error\r\n");
return 0;
}
// 创建显示contedxt
pComponent->pCodecCtx = avcodec_alloc_context3(pCodec);
pComponent->pCodecCtx->channels = channels;
pComponent->pCodecCtx->sample_rate = sample_rate;
pComponent->pCodecCtx->bit_rate = bit_rate;
if(avcodec_open2(pComponent->pCodecCtx, pCodec, NULL) < 0)
{
printf("open codec error\r\n");
return 0;
}
pComponent->pFrame = av_frame_alloc();
uint64_t out_channel_layout = channels < 2 ? AV_CH_LAYOUT_MONO:AV_CH_LAYOUT_STEREO;
int out_nb_samples = 1024;
enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;
pComponent->au_convert_ctx = swr_alloc();
pComponent->au_convert_ctx = swr_alloc_set_opts(pComponent->au_convert_ctx, out_channel_layout, out_sample_fmt, sample_rate,
out_channel_layout, AV_SAMPLE_FMT_FLTP, sample_rate, 0, NULL);
swr_init(pComponent->au_convert_ctx);
int out_channels = av_get_channel_layout_nb_channels(out_channel_layout);
pComponent->out_buffer_size = av_samples_get_buffer_size(NULL, out_channels, out_nb_samples, out_sample_fmt, 1);
return (void *)pComponent;
}
int aac_decode_frame(void *pParam, unsigned char *pData, int nLen, unsigned char *pPCM, unsigned int *outLen)
{
AACDFFmpeg *pAACD = (AACDFFmpeg *)pParam;
AVPacket packet;
av_init_packet(&packet);
packet.size = nLen;
packet.data = pData;
int got_frame = 0;
int nRet = 0;
if (packet.size > 0)
{
nRet = avcodec_decode_audio4(pAACD->pCodecCtx, pAACD->pFrame, &got_frame, &packet);
if (nRet < 0)
{
printf("avcodec_decode_audio4:%d\r\n",nRet);
printf("avcodec_decode_audio4 %d sameles = %d outSize = %d\r\n", nRet, pAACD->pFrame->nb_samples, pAACD->out_buffer_size);
return nRet;
}
if(got_frame)
{
swr_convert(pAACD->au_convert_ctx, &pPCM, pAACD->out_buffer_size, (const uint8_t **)pAACD->pFrame->data, pAACD->pFrame->nb_samples);
*outLen = pAACD->out_buffer_size;
}
}
av_free_packet(&packet);
if (nRet > 0)
{
return 0;
}
return -1;
}
void aac_decode_close(void *pParam)
{
AACDFFmpeg *pComponent = (AACDFFmpeg *)pParam;
if (pComponent == NULL)
{
return;
}
swr_free(&pComponent->au_convert_ctx);
if (pComponent->pFrame != NULL)
{
av_frame_free(&pComponent->pFrame);
pComponent->pFrame = NULL;
}
if (pComponent->pCodecCtx != NULL)
{
avcodec_close(pComponent->pCodecCtx);
avcodec_free_context(&pComponent->pCodecCtx);
pComponent->pCodecCtx = NULL;
}
free(pComponent);
}
