[mad-dev] MAIN_DATA buffer
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
> as main_data_begin. Of course, main_data_end for one frame is the
> 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
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
> He actually looks at the NEXT frame, reads its main_data_end pointer,
> 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
> up main_data. I see a couple of memcpy's(), and I can guess what's
> but I'm not really sure.
> Also Rob, how did you manage keeping a main_data buffer without using
> a circular
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
> III_decode actually decodes the Huffman bits ....and does the actual
> 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.