Re: random seeking with MAD

On Nov 28, 2003, at 4:47 PM, Jedediah Smith wrote:
> Super.. now is there I way I can get the mad api to tell me how many 
> frames back the bit pool goes from a given frame? I could store this 
> along with the frame offsets to optimize.

Not simply. You can poke around in the frame yourself, if you like:

   unsigned int main_data_begin;

   {
     struct mad_bitptr peek;
     unsigned long header;

     mad_bit_init(&peek, stream->this_frame);

     header = mad_bit_read(&peek, 32);
     if (!(header & 0x00010000L))  /* protection_bit */
       mad_bit_skip(&peek, 16);  /* crc_check */

     main_data_begin =
       mad_bit_read(&peek, (header & 0x00080000L) /* ID */ ? 9 : 8);

     mad_bit_finish(&peek);
   }

This reads the current frame's main_data_begin which tells you how many 
bytes before the frame its main_data begins. Note however that this 
count does not include header or side info bytes of previous frames. 
Headers are 4 or 6 bytes depending whether protection_bit is clear (see 
above). Layer III side information has the following number of bytes:

   lsf = frame->header.flags & MAD_FLAG_LSF_EXT
   nch = MAD_NCHANNELS(&frame->header)

                 lsf  !lsf
               +----------
     nch == 1  |   9    17
     nch != 1  |  17    32

The total byte size of the frame is stream->next_frame - 
stream->this_frame. This should be enough information to calculate the 
number of frames prior the current frame's main_data begins and thus 
where to start decoding in order to fully decode the current frame.

Note that with the exception of total frame byte size, the above 
discussion is only applicable to Layer III. Layers I and II have 
different frame structures but do not use a bit reservoir so they can 
always begin decoding at any frame.

-- 
Rob Leslie
[email protected]