Lam Yick Yan wrote:

I have downloaded a Perl Script to record mp3 stream from ShoutCast 

server. Now my task is need to determine the property of the mp3 stream:

1. whether it is stereo, joint-stereo or mono

2. whether it is a 11kHz, 22kHz, or 44kHz sampling rate stream

3. whether the bit rate is 16KHz, 32kHz, 64kHZ or 128kHz

   The problem is there are a lot of junk before the stream begin,

and there is so many consecutive 12 or 11 1's in the program ( for those who dont understand what I mean, please see document of mpeg header). I have trouble determining the begin of of MP3 stream!! Some decoder does not have this ability and I must tell it the begin of MP3 stream.

Can any one point out where I can find the C/C++ source code

to do that?

This algorithm works pretty well to find correct sync words:

1. Search forward for the next sync word (0xfff or 0xffe for MPEG 2.5). 2. Try to decode the header. If any field is invalid ("forbidden"), the sync word was false; go back to 1. 3. Use the layer, bitrate, sampling frequency, and padding bit to calculate the frame size (and hence, beginning of the next frame.) 4. Verify that a sync word follows at the calculated location. If not, the original sync word was false; go back to 1.

This is what libmad does, generally. For additional verification of a correct sync, you can also:

5. Decode the next frame header. If any field is invalid ("forbidden"), the original sync word was false; go back to 1. 6. Verify that the layer and sampling frequency of the second header match the first. If not, the original sync word was likely false; go back to 1.

These additional checks are performed by libmad only when trying to determine a free format bitrate.

If the stream has CRC protection, you could also use that for additional verification, but the above is pretty effective in most cases.