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

[Robert Swain]

2008/8/18 Michael Niedermayer <michaelni at gmx.at>:
> On Mon, Aug 18, 2008 at 02:03:53PM +0100, Robert Swain wrote:
>> 2008/8/18 Michael Niedermayer <michaelni at gmx.at>:
>> > On Mon, Aug 18, 2008 at 01:47:42PM +0100, Robert Swain wrote:
>> >> 2008/8/18 Robert Swain <robert.swain at gmail.com>:
>> >> > 2008/8/18 Robert Swain <robert.swain at gmail.com>:
>> >> >> 2008/8/18 Robert Swain <robert.swain at gmail.com>:
> [...]
>> > [...]
>> >> +            for (m = 0; m < size; m++, start += inc)
>> >> +                for (i = 1; i <= FFMIN(m, order); i++)
>> >> +                    coef[start] -= coef[start - i] * lpc[i];
>> >>          }
>> >
>> > are you sure this is correct for both values of inc ?
>> > i mean one will have coef[start - i] unfiltered and one filtered as far as
>> > i can see ...
>>
>> You're right and I thought this was strange, but it's the same
>> behaviour as before from what I see and so in that sense it is as
>> correct as it was before. The spec says:
>>
>> tns_ar_filter( spectrum[], size, inc, lpc[], order )
>> {
>>  - Simple all-pole filter of order "order" defined by
>>    y(n) =  x(n) - lpc[1]*y(n-1) - ... - lpc[order]*y(n-order)
>>
>>  - The state variables of the filter are initialized to zero every time
>>
>>  - The output data is written over the input data ("in-place operation")
>>
>>  - An input vector of "size" samples is processed and the index increment
>>    to the next data sample is given by "inc"
>> }
>>
>> I think the third point suggests that it's the intended approach.
>
> but the first suggests it is not
> isnt there some reference implementation or something around?

3GPP reference code, tns.c (with cruft removed and indented sanely):


for (i=0; i<order; i++)
    state[i] = 0.0F;

if (inc == -1)
    spec += size - 1;

for (i=0; i<size; i++) {
    accu = *spec * lpc[0];

    for (j=0; j<order; j++)
        accu -= lpc[j+1] * state[j];

    for (j=order-1; j>0; j--)
        state[j] = state[j-1];

    state[0] = accu;
    *spec = accu;

    spec += inc;
}


'spec' is basically equivalent to 'coef'. The ref code stores the
states from the beginning of the state array and shifts all the states
along to add a new one to the beginning. The SoC code stores them
starting from the end of the array and works backwards, wrapping back
to the end when the index goes past the beginning of the array. That
is, the ith state[0] == b[(order-1-i)%order].

Maybe with some index trickery the above can be simplified to
something similar to what I was suggesting for the incorrect code.

Rob