[FFmpeg-devel] [PATCH] AAC decoder round 8

[Robert Swain]

2008/8/18 Robert Swain <robert.swain at gmail.com>:
> 2008/8/17 Michael Niedermayer <michaelni at gmx.at>:
>> On Sun, Aug 17, 2008 at 11:12:55PM +0100, Robert Swain wrote:
>>> 2008/8/15 Robert Swain <robert.swain at gmail.com>:
>>> > 2008/8/15 Michael Niedermayer <michaelni at gmx.at>:
>>> >> On Fri, Aug 15, 2008 at 09:04:24AM +0100, Robert Swain wrote:

>>> [...]
>>>
>>> >>> Index: libavcodec/aacdectab.h
>>> >>> ===================================================================
>>> >>> --- libavcodec/aacdectab.h    (revision 14767)
>>> >>> +++ libavcodec/aacdectab.h    (working copy)
>>> >
>>> >>> +
>>> >>> +/* @name tns_tmp2_map
>>> >>> + * Tables of the tmp2[] arrays of LPC coefficients used for TNS.
>>> >>> + * The suffix _M_N[] indicate the values of coef_compress and coef_res
>>> >>> + * respectively.
>>> >>> + * @{
>>> >>> + */
>>> >>> +static const float tns_tmp2_map_1_3[TNS_MAX_ORDER] = {
>>> >>> +     0.00000000,  0.43388373, -0.64278758, -0.34202015,
>>> >>> +     0.97492790,  0.78183150, -0.64278758, -0.34202015,
>>> >>> +    -0.43388373, -0.78183150, -0.64278758, -0.34202015,
>>> >>> +    -0.78183150, -0.43388373, -0.64278758, -0.34202015,
>>> >>> +     0.78183150,  0.97492790, -0.64278758, -0.34202015
>>> >>> +};
>>> >>> +
>>> >>> +static const float tns_tmp2_map_0_3[TNS_MAX_ORDER] = {
>>> >>> +     0.00000000,  0.43388373,  0.78183150,  0.97492790,
>>> >>> +    -0.98480773, -0.86602539, -0.64278758, -0.34202015,
>>> >>> +    -0.43388373, -0.78183150, -0.97492790, -0.97492790,
>>> >>> +    -0.98480773, -0.86602539, -0.64278758, -0.34202015,
>>> >>> +     0.78183150,  0.97492790,  0.97492790,  0.78183150
>>> >>> +};
>>> >>> +
>>> >>> +static const float tns_tmp2_map_1_4[TNS_MAX_ORDER] = {
>>> >>> +     0.00000000,  0.20791170,  0.40673664,  0.58778524,
>>> >>> +    -0.67369562, -0.52643216, -0.36124167, -0.18374951,
>>> >>> +     0.99452192,  0.95105648,  0.86602539,  0.74314481,
>>> >>> +    -0.67369562, -0.52643216, -0.36124167, -0.18374951,
>>> >>> +    -0.20791176, -0.40673670, -0.58778530, -0.74314487
>>> >>> +};
>>> >>> +
>>> >>> +static const float tns_tmp2_map_0_4[TNS_MAX_ORDER] = {
>>> >>> +     0.00000000,  0.20791170,  0.40673664,  0.58778524,
>>> >>> +     0.74314481,  0.86602539,  0.95105654,  0.99452192,
>>> >>> +    -0.99573416, -0.96182561, -0.89516330, -0.79801720,
>>> >>> +    -0.67369562, -0.52643216, -0.36124167, -0.18374951,
>>> >>> +    -0.20791176, -0.40673670, -0.58778530, -0.74314487
>>> >>> +};
>>> >>
>>> >> iam not sure if the code is correct but i think several of these
>>> >> elements can never be accessed
>>> >
>>> > I'll check the code.
>>>
>>> The current code only accesses elements 0-15 by my reckoning.
>>
>> actually i think it accesses fewer
>>
>> lets just look at the table 3rd table, to access this implicates that
>> tmp2_idx=2
>> which implicates
>> coef_res=0
>> coef_compress=1
>>
>> and
>> coef_len = coef_res + 3 - coef_compress;
>> which makes it 2
>> thus only 4 elements of the 3rd table could be accssed
>> unless i missed something
>
> I don't think you did. I reached the same conclusion shortly after
> sending the e-mail. The code in the spec for this section is
> absolutely horrible. Working through their code, I don't obtain the
> same values as are in the tns_tmp2_map_M_N tables though I expect I'm
> making a mistake as I don't quite understand the intention of some of
> the code.

See attached. I'll work on optimising the code in apply_tns() a bit tomorrow.

Regards,
Rob
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