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Topic: Extra byte in frame header (Read 5203 times) previous topic - next topic
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Extra byte in frame header

Alright, well I'm almost done with my decoder but there's some twilight zone stuff going on.  I'm working on decoding a .flac file and when it gets to frame 0x80 I read:


[Sync][Block Size ][Sample Rate][Ch. Assign][Sample Size]          [Frame #][CRC8]
FF F8    3           9                  0                   8            C2        80              AF

and the subsequent frame headers have the extra c2 in front of the frame number.

Once I get to frame 0xC0 i get:

FF F8    3           9             0                     8          C3        80            BA         

What am I supposed to do with the extra byte, and when should I expect it?  I encoded with the front end encoder and I also noticed these flac files won't work with my windows media player codec, while all the other flac files I've had would.  I encoded with max lpc order = 0 if that makes any difference.

Extra byte in frame header

Reply #1 –
The frame number is stored as a UTF-8 encoded value, which takes up a variable amount of bytes as it grows.  You can use something like the following in order to decode it:
Code: [Select]
int read_utf8(bitstream) {
  int total_bytes = read_unary(bitstream, 0);   /*count the number of 1 bits before the next 0 bit*/
  int value = read(bitstream, 7 - total_bytes); /*grab the remainder of the first byte as our initial value*/
  for (; total_bytes > 1; total_bytes--) {
     assert(read(bitstream, 2) == 2);           /*the top 2 bits of each subsequent byte should be 10b*/
     value = (value << 6) | read(bitstream, 6); /*prepend the remaining 6 bits to our value*/
  }

  return value;
}

Extra byte in frame header

Reply #2 –
Thanks for the help again.  I hope I can return the favor.

Quote from: tuffy on
The frame number is stored as a UTF-8 encoded value, which takes up a variable amount of bytes as it grows.  You can use something like the following in order to decode it:
Code: [Select]
int read_utf8(bitstream) {
  int total_bytes = read_unary(bitstream, 0);   /*count the number of 1 bits before the next 0 bit*/
  int value = read(bitstream, 7 - total_bytes); /*grab the remainder of the first byte as our initial value*/
  for (; total_bytes > 1; total_bytes--) {
     assert(read(bitstream, 2) == 2);           /*the top 2 bits of each subsequent byte should be 10b*/
     value = (value << 6) | read(bitstream, 6); /*prepend the remaining 6 bits to our value*/
  }

  return value;
}