On Thu, 16 Sep 1999, Andrew Coles wrote:
> I have spent some time recently working with fixed point numbers on my
> machine (StrongARM CPU - no hardware FPU .'. fixed point numbers are
> significantly faster). I'm now at a point where I would like to add some
> of these routines to LAME (not to the normal version before I get
> grilled, my aim would be to form a StrongARM specific version based
> around the main release).
>
> I don't know a great deal about how the MP3 data is stored in a file,
> is it stored as IEEE floats/doubles/long doubles or is it converted into
> some integer format before being written? One option I've thought about
> using would be to replace all the calls to the FPU emulation library
> that implements the math routines with their fixed point counterparts
> and statically linking with this library as opposed to the usual library
> thus making LAME replace all it's fixed point equivalents. Obviously I'd
> need to convert the tables used throughout the program into fixed-point
> format but apart from that I can't see any other problems occuring
> unless the floating point numbers are actually written to the file. In
> this case I could probably compile a new floating point library with
> symbol names that differ to the standard ones, compile the affected code
> segments with -S and adjust the references to point to the true float
> routines that also convert from the fixed point format.
In the quantization process floats gets converted into integers, which
then gets huffman coded and saved to the bitstream file.
I guess it should be possible to recalculate tables and convert a lot, but
it's a #� of a work to do it. The input is also integer, btw.
It's a nice idea, but lots of work.
> Just a thought, any advice greatly appreciated, even if a 100% refit is
> unfeasible then I'm sure I'll be able to add bits here and there (for
> example, replacing the FHT routine with one that uses fixed point math).
>
> Regards
>
>
> Andrew
Regards
Jan
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