[mad-dev] MAIN_DATA buffer

Rob Leslie rob@mars.org
Fri, 12 Jul 2002 09:32:54 -0700

On Friday, July 12, 2002, at 01:09  AM, Kshitiz Malik wrote:
> I have been struggling with MAD's code for quite some time now, and have 
> managed to understand almost all of it - except I haven't been able to 
> figure out the way it manages a buffer of main_data for mp3 files. (I am 
> working solely on the MP3 part of the decoder) The problem is, the 
> function mad_frame_decode(which decodes ONE entire frame, excluding the 
> polyphase filterbank) works as follows:
> 1) Read the sideinfo(the header's already been decoded)
>    [ I don't know why, but apparently Rob had named all main_data_end 
> pointers
>      as main_data_begin. Of course, main_data_end for one frame is the 
> beginning
>      of main data for the other frame.... but they're slightly misleading 
> ]

main_data_begin is the name used by ISO/IEC 11172-3 to describe the offset 
of the beginning of a frame's main_data relative to the frame's header. It 
also happens to be the end of the previous frame's main_data, but in the 
context of decoding the current frame, "main_data_begin" should not at all 
be misleading.

Note that although it is a positive number, it denotes the number of bytes 
*before* the current frame that the main_data actually begins, not 
counting bytes used by previous frame headers or side information.

> 2) Depending on the value of main_data_end of the current frame,
>    it collects together the main_data which has been buffered till now in
>    an array, stream->(*main_data). I couldn't figure out how Rob does 
> this.
>    He actually looks at the NEXT frame, reads its main_data_end pointer, 
> calculates
>    a variable named frame_space according to:
>      frame_space = stream->next_frame - mad_bit_nextbyte(&stream->ptr);
>    This is the no. of bytes of main_data(which may belong to ANY of the 
> subsequent frames)
>    present in the current frame.
>    So far, so good.
>    Next up is the statement:
>       md_len = si.main_data_begin + frame_space - next_md_begin;
>    This has me absolutely stumped.

This calculates the effective length of the current frame's main_data: the 
number of bytes between the current frame's main_data_begin and the next 
frame's main_data_begin. frame_space is the number of bytes between the 
end of the current frame's side info and the next frame header, i.e. as 
you said, bytes of main_data which may belong to *any* of the current or 
subsequent frames. We add to this the number of bytes of main_data 
preceding the current frame (i.e. main_data_begin) and then subtract the 
number of bytes belonging to the next or subsequent frames (i.e. 

To make things clearer, imagine the case when main_data_begin is always 0.
  In this case, every frame's main_data is contained entirely between frame 
headers, and the length is exactly frame_space. Now consider one frame 
whose main_data_begin is 1, but the rest are still 0. The main_data length 
of this frame is one byte larger because it begins one byte earlier. The 
main_data length of the *previous* frame is one byte smaller.

>    From this point onward, I absolutely cannot understand the way Rob is 
> bufferring
>    up main_data. I see a couple of memcpy's(), and I can guess what's 
> happenning,
>    but I'm not really sure.
>    Also Rob, how did you manage keeping a main_data buffer without using 
> a circular
>    queue??

The general assumption is that once a frame has been decoded, its frame 
data area will no longer be accessible, so any data that might belong to 
future frames is copied to the main_data buffer. When the next frame is 
processed, the rest of the frame's main_data is appended to the buffer in 
a contiguous fashion and the audio data is decoded. Then the buffer is 
re-initialized with the remaining data that might belong to future frames,
  and the process repeats.

> 3) After all this is done , III_decode is called, which is passed a 
> pointer named ptr,
>   which happens to point to the beginning of main_data for the current 
> frame.
>   III_decode actually decodes the Huffman bits ....and does the actual 
> decoding.
> Can somebody please help me out with how the main_data buffer is built up 
> (I know the basics, i.e, main_data of the current frame may be present in 
> any of the previous frames, mani_data_end is a negative offset from the 
> end of the frame...etc...etc)

I think this might be the source of your misunderstanding: there is 
nothing called main_data_end, and main_data_begin is a negative offset 
from the *beginning* of the frame, not the end.


Rob Leslie