在网上找了两个关于aac音频帧头adts的格式介绍,这两篇文章介绍的挺全面的,这里就不多阐述:
为什么在将视频中的音频提取出来的时候要加入adts帧头?因为adts帧头中包含了播放器要如何播放该帧的音频。
下面直接就引用前面文章使用到的生成adts帧头代码:
#define ADTS_HEADER__LEN 7
const int sampling_frequencies[] = {
96000, // 0x0
88200, // 0x1
64000, // 0x2
48000, // 0x3
44100, // 0x4
32000, // 0x5
24000, // 0x6
22050, // 0x7
16000, // 0x8
12000, // 0x9
11025, // 0xa
8000 // 0xb
// 0xc d e f是保留的
};
int adts_header(char * const p_adts_header, const int data_length,
const int profile, const int samplerate,
const int channels)
{
int sampling_frequency_index = 3; // 默认使用48000hz
int adtsLen = data_length + 7;
int frequencies_size = sizeof(sampling_frequencies) / sizeof(sampling_frequencies[0]);
int i = 0;
for(i = 0; i < frequencies_size; i++)
{
if(sampling_frequencies[i] == samplerate)
{
sampling_frequency_index = i;
break;
}
}
if(i >= frequencies_size)
{
printf("unsupport samplerate:%d\n", samplerate);
return -1;
}
p_adts_header[0] = 0xff; //syncword:0xfff 高8bits
p_adts_header[1] = 0xf0; //syncword:0xfff 低4bits
p_adts_header[1] |= (0 << 3); //MPEG Version:0 for MPEG-4,1 for MPEG-2 1bit
p_adts_header[1] |= (0 << 1); //Layer:0 2bits
p_adts_header[1] |= 1; //protection absent:1 1bit
p_adts_header[2] = (profile) << 6; //profile:profile 2bits
p_adts_header[2] |= (sampling_frequency_index & 0x0f) << 2; //sampling frequency index:sampling_frequency_index 4bits
p_adts_header[2] |= (0 << 1); //private bit:0 1bit
p_adts_header[2] |= (channels & 0x04) >> 2; //channel configuration:channels 高1bit
p_adts_header[3] = (channels & 0x03) << 6; //channel configuration:channels 低2bits
p_adts_header[3] |= (0 << 5); //original:0 1bit
p_adts_header[3] |= (0 << 4); //home:0 1bit
p_adts_header[3] |= (0 << 3); //copyright id bit:0 1bit
p_adts_header[3] |= (0 << 2); //copyright id start:0 1bit
p_adts_header[3] |= ((adtsLen & 0x1800) >> 11); //frame length:value 高2bits
p_adts_header[4] = (uint8_t)((adtsLen & 0x7f8) >> 3); //frame length:value 中间8bits
p_adts_header[5] = (uint8_t)((adtsLen & 0x7) << 5); //frame length:value 低3bits
p_adts_header[5] |= 0x1f; //buffer fullness:0x7ff 高5bits
p_adts_header[6] = 0xfc; //11111100 //buffer fullness:0x7ff 低6bits
// number_of_raw_data_blocks_in_frame:
// 表示ADTS帧中有number_of_raw_data_blocks_in_frame + 1个AAC原始帧。
return 0;
}
下面是adts_header()函数的使用:
pkt = av_packet_alloc();
// 读取数据包
while (av_read_frame(fmt_ctx, pkt) >= 0)
{
// 只处理自己想要的流的信息
if(audio_index == pkt->stream_index)
{
char adts_header_buf[7] = {0};
adts_header(adts_header_buf, pkt->size,
fmt_ctx->streams[audio_index]->codecpar->profile,
fmt_ctx->streams[audio_index]->codecpar->sample_rate,
fmt_ctx->streams[audio_index]->codecpar->channels);
fwrite(adts_header_buf, 1, 7, fp); // 写adts header , ts流不适用,ts流分离出来的packet带了adts header
int len = fwrite(pkt->data, 1, pkt->size, fp);
if(len != pkt->size)
{
av_log(NULL, AV_LOG_WARNING, "fwrite len not equal.\n");
}
av_packet_unref(pkt);
}
}