How to read ADTS header from file in C++?

Question

How can I read the header of an ADTS encoded aac file? I need it to get the buffer length for each frame to read out the whole aac file. But I can't get the right values. Here is my code to read the header and get the buffer length for each frame(Bit 30 - 43), when assuming big endian:

main(){
  ifstream file("audio_adts.m4a", ios::binary);
  char header[7],buf[1024];
  int framesize;

  while(file.read(header,7)) {
      memset(buf ,0 , 1024);

      /* Get header bit 30 - 42 */
      framesize = (header[3]&240|header[4]|header[5]&1);

      cout << "Framesize including header: "<<framesize<<endl;
      file.read(buf,framesize);

      /*Do something with buffer*/
  }

  return 0;
}

The framesize I get with this code is 65, 45 ,45, 45, -17 and then it stops because of the negative value. The actual framesizes are around 200.

Hexdump of first header:

0x000000: ff f9 50 40 01 3f fc

How do you know it is not the right value? What is the input and what is the output you expect? — wimh, Jun 14 '14 at 12:09
So you get 65 for the first header, but expect around 200. As the header is just 7 bytes, can you post the hexdump of the first header? — wimh, Jun 14 '14 at 12:13
note that `main()` is wrong. Default ints have been forbidden long time ago — phuclv, Jan 17 '19 at 13:55

score 2 · Answer 1 · answered Jun 14 '14 at 12:18

Your extraction of the framesize appears to have the shifts << missing, needed to get the extracted bit into the right locations

The bit masks does not look like they are matching the /*bit 30-42*/ comment.

Also, change the char to unsigned char as you otherwise will run into all kind of sign extension issues when you are doing this type of bit manipulation (which is the cause for your negative value error)

score 0 · Answer 2 · answered Jan 17 '19 at 10:49

0

The way I calculated it:

unsigned int AAC_frame_len = ((AAC_44100_buf[3]&0x03)<<11|(AAC_44100_buf[4]&0xFF)<<3|(AAC_44100_buf[5]&0xE0)>>5);

answered Jan 17 '19 at 10:49

YuanDa.Yu

127
7

How to read ADTS header from file in C++?

2 Answers2