编码
void aac_rtp_create_sdp(uint8_t *file, uint8_t *ip, uint16_t port, uint16_t profile, uint16_t chn, uint16_t freq, uint16_t type) { char buff[1024] = {0}; char typeName[64] = {0}; // char demo[] = // "m=audio 9832 RTP/AVP 97 " // "a=rtpmap:97 mpeg4-generic/44100/2 " // "a=fmtp:97 sizeLength=13;mode=AAC-hbr;config=1210; " // "c=IN IP4 127.0.0.1"; char demo[] = "m=audio %d RTP/AVP %d " "a=rtpmap:%d %s/%d/%d " "a=fmtp:%d streamtype=5;profile-level-id=1;sizeLength=13;IndexLength=3;indexDeltaLength=3;mode=AAC-hbr;config=%d; " "c=IN IP4 %s"; uint16_t config = 1410, _freq = 8; int fd; strcpy(typeName, "mpeg4-generic"); // 这里先支持AAC该类型的 if(freq == 96000) _freq = 0; else if(freq == 88200) _freq = 1; else if(freq == 64000) _freq = 2; else if(freq == 48000) _freq = 3; else if(freq == 44100) _freq = 4; else if(freq == 32000) _freq = 5; else if(freq == 24000) _freq = 6; else if(freq == 22050) _freq = 7; else if(freq == 16000) _freq = 8; else if(freq == 12000) _freq = 9; else if(freq == 11025) _freq = 10; else if(freq == 8000) _freq = 11; else if(freq == 7350) _freq = 12; // 从aac adts header读取出来的profile是被减1的 config = profile + 1; config <<= 5; // 5 aac的profile, 通常情况是1, 或者2 config |= _freq; config <<= 4; //aac的采样频率的索引 config |= chn; config <<= 3; // //转成16进制 config = ((config>>12)&0xF)*1000 + ((config>>8)&0xF)*100 + ((config>>4)&0xF)*10 + ((config>>0)&0xF); snprintf(buff, sizeof(buff), demo, port, type, type, typeName, freq, chn, type, config, ip); remove(file); if((fd = fopen(file, "wt")) > 0) { fwrite(buff, strlen(buff), 1, fd); fclose(fd); } }
sdp文件
m=audio 9832 RTP/AVP 97 a=rtpmap:97 mpeg4-generic/48000/2 a=fmtp:97 streamtype=5;profile-level-id=1;sizeLength=13;IndexLength=3;indexDeltaLength=3;mode=AAC-hbr;config=2190; c=IN IP4 10.143.44.169
"m=" ：video：媒体名称 9832：端口 RTP/AVP：传输协议 97：RTP协议中的pt值
"a=rtpmap"：97：RTP协议中的pt值 mpeg4-generic：音频编码 48000：音频采样率 2：通道数
"a=fmtp"：streamtype：aac为5 profile-level-id: 1表示低复杂度类型 sizeLength：表示AU-size占用位数
IndexLength：表示AU-Index占用位数 indexDeltaLength：表示AU-Index-delta占用位数 mode：表示编码模式
config：16进制2190 转化为二进制：00100 0011 0010 000
00100：aac的profile，这里为5
0011：3，表示采样率为48000
0010: 2，表示通道数
采样率表格
| 值 | 表示的采样频率 |
|---|---|
| 0 | 96000, |
| 1 | 88200, |
| 2 | 64000, |
| 3 | 48000, |
| 4 | 44100, |
| 5 | 32000, |
| 6 | 24000, |
| 7 | 22050, |
| 8 | 16000, |
| 9 | 12000, |
| 10 | 11025, |
| 11 | 8000, |
| 12 | 7350 |
| 15 | 表示自定义的采样频率, 一般不会出现 |