[FFmpeg-devel] [PATCH] AAC decoder

[Robert Swain]

2008/5/28 Michael Niedermayer <michaelni at gmx.at>:
> On Wed, May 28, 2008 at 12:37:04PM +0100, Robert Swain wrote:
>> 2008/5/26 Michael Niedermayer <michaelni at gmx.at>:
>> > On Mon, May 26, 2008 at 12:42:37PM +0100, Robert Swain wrote:
>> >> The tables are still different as intensity uses pow(0.5, (i-100)/4.)
>> >> and the other cases use pow(2.0, (i-100)/4.).
>> >
>> > pow(0.5, (i-100)/4.) == pow(2.0, (100-i)/4.)
>> >
>> > and
>> >
>> > pow(2.0, (100-i)/4.) / 1024 == pow(2.0, (100-i)/4.-10) ==pow(2.0, (100-i-40)/4.)
>> >
>> > possibly these allow the 2 tables to be merged, i mean
>> >
>> > pow2sf_tab[i] and intensity_tab[i]
>> > to
>> > pow2sf_tab[i+C] and pow2sf_tab[-i]
>>
>> OK, I've thought about this a bit more. I think either sf_scale should
>> be 'applied' just before downmixing/float_to_int16 conversion as in
>> ac3dec.c or sf_scale can effectively be merged into this table almost
>> entirely thanks to being representable as a power of 2 in either the C
>> or SIMD float_to_int16 case.
>>
>> >From what I see there are 3 cases.
>>
>> - intensity table:
>>   pow(0.5, (i-100)/4) = pow(2, (100-i)/4)
>>   which would have indices [100-255, 100-0] = [-155, 100]
>>
>> - sf table when sf_scale is -1/1024:
>>   pow(2, (i-100)/4) * -pow(2, -10) = -pow(2, (i-140)/4)
>>   ignoring the sign issue, it would have indices [0-140, 255-140] = [-140, 115]
>>
>> - sf table when sf_scale is -1/(1024*32768):
>>   pow(2, (i-100)/4) * -pow(2, -25) = -pow(2, (i-200)/4)
>>   [0-200, 255-200] = [-200, 55]
>>
>> So, the range of indices into the table should be [-200, 115],
>> sf_scale can be replaced by a constant integer offset into the table
>> and we handle the signs with a little branching or something. Does
>> that sound like a good idea? Any suggestions for alterations before I
>> implement it?
>
> yes that was my idea (note ive not checked if the ranges are correct)

Patch attached.

Rob
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