[mad-dev] MAIN_DATA buffer
Fri, 12 Jul 2002 01:09:26 -0700 (PDT)
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
[ 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 ]
2) Depending on the value of main_data_end of the
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 -
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 -
This has me absolutely stumped.
||| header +sideinfo | main_data |||
head + si | main_data |||
If frame_space was the length of main data in the
second frame above,
md_len would be "the no. of main_data bytes in the
next frame". But frame_space
is the length of the first frame (above), so I
don't really know what md_len stands for.
Can someone please help me out here???
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
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)
Thanks a lot!!
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