Dear Gene,
this looks ok to me. The only potential problem
I do see is the merging of the two main data sections
because of the ancillary bit. After the second granule
in both channels there might be ancillary bit. If you concatenate the data
there must be no ancillary data in between the
data blocks. So you must determine and remove these bits.
It is simple to determine the exact location and size of
the main data based on the side information. But then
you need a bitwise concatenation. While the main data of the second
channel will start on a byte boundary it may or may not do so
when inserted in the combined stream. You may not
change the size information (main_data_bits) in the side
information, because this is used to determine the end
of the small values section in the huffman decoder.
An other problem (you dont say much about it) is the determination
of the new header, which must take into account the new bitrate
(which is about halve of the original bitrate but not quite.
The amount of data per frame almoust doubles but the number of
samples per frame stays constant. With the new bitrate
again you have a new frame size and may need more or less
ancillary bit.
If that isnt the problem, I guess its a case for intense
debugging.
Yours
Martin Ruckert
Gene Williams wrote:
Hi!
I'm trying to programmatically take 2 mono mp3 file inputs, and output a
stereo mp3 file with 1 input on ch0 and the other input on ch1 - without
decoding and re-encoding. I'm using ISO 11172-3 for reference, thought
I understood the bit organization, and I've put together some code that
creates the output I wanted it to, but the audio's integrity is not
maintained. It seems like I've very close, because the peaks and
valleys of each channel of the output seem to match the peaks and
valleys of their corresponding input, but the audio is garbled. I think
bits are slightly skewed
Anybody have any ideas? Below is what I'm doing.
I'm arranging the output's side-info like this:
- the proper header
- main_data_begin
- private_bits (3)
- scfsi from input 1
- scfsi from input 2
- the 59 bits of the remaining side-info for input 1's gr0
- the 59 bits of the remaining side-info for input 2's gr0
- the 59 bits of the remaining side-info for input 1's gr1
- the 59 bits of the remaining side-info for input 2's gr1
and the output's main data like this:
- input 1's gr0 main data according to its corresponding part2_3_len
- input 2's gr0 main data according to its corresponding part2_3_len
- input 1's gr1 main data according to its corresponding part2_3_len
- input 2's gr1 main data according to its corresponding part2_3_len
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--
-------------------------------------
Prof.Dr.Martin Ruckert
Munich University of Applied Sciences
FB07 Mathematics and Computer Science
Lothstrasse 34
D-80335 Munich
GERMANY
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