[FFmpeg-devel] [PATCH] atrac decoder

[Michael Niedermayer]

On Tue, Aug 11, 2009 at 07:32:30PM +0200, Benjamin Larsson wrote:
> Michael Niedermayer wrote:
> > On Mon, Aug 10, 2009 at 11:10:40PM +0200, Benjamin Larsson wrote:
> >> Michael Niedermayer wrote:
> >>> On Mon, Jun 08, 2009 at 09:40:18PM +0200, Benjamin Larsson wrote:
> > [...]
> >>> [...]
> >>>> +static void at1_imdct_transform(ATRAC1Context *q, float *spec, float *out, int nbits, int reverse_spectrum)
> >>>> +{
> >>>> +    float* window;
> >>>> +    MDCTContext* mdct_context;
> >>>> +    int transf_size = 1 << nbits;
> >>>> +
> >>>> +    switch(nbits) {
> >>>> +        case 5:
> >>>> +            window = short_window;
> >>>> +            mdct_context = &q->mdct_ctx[0];
> >>>> +            break;
> >>>> +        case 7:
> >>>> +            window = mid_window;
> >>>> +            mdct_context = &q->mdct_ctx[1];
> >>>> +            break;
> >>>> +        case 8:
> >>>> +        default:
> >>>> +            window = long_window;
> >>>> +            mdct_context = &q->mdct_ctx[2];
> >>>> +    }
> >>>> +
> >>>> +    if (reverse_spectrum) {
> >>>> +        int i;
> >>>> +        for (i=0; i<transf_size/2; i++)
> >>>> +            FFSWAP(float, spec[i], spec[transf_size-1-i]);
> >>>> +    }
> >>> i think that can be avoided by decong in reverse order ...
> >>>
> >>
> >> Would be possible if there where no short blocks. The logic to untangle
> >> those are not worth it.
> > 
> > elabrate please
> 
> A subband that is reversed can use short or long mdct sizes. If it is
> long then decoding in reverse order would work.
> 
> 12,11,10,9,8,7,6,5,4,3,2,1
> 
> But when the short order is used (to code transients), the blocks are
> reversed in groups.
> 
> 3,2,1,6,5,4,9,8,7,12,11,10

these groups dont coincide with some for loop ?
and they arent powers of 2 ? 4321 8765 ...



> 
> > 
> > 
> >>>> +
> >>>> +    ff_imdct_calc(mdct_context,out,spec);
> >>> cant imdct_half be used?
> >>>
> >> Changed to
> >>
> >> memset
> >> fmul
> >> fmul_rev
> >> memset
> >>
> >> It was slightly faster and uses less memory.
> > 
> > you still use the full imdct
> 
> Yes, that has to do with windowing. As I shouldn't multiply with 0 and 1
> I have to do it this way. To use imdct_half I need to build the complete
> windows again (Use more memory etc).

iam not convinced but i didnt verify it can be done ...


> 
> > 
> > 
> > [...]
> >>> [...]
> >>>> +
> >>>> +static int at1_unpack_dequant(GetBitContext* gb, AT1SUContext* su, float spec[AT1_SU_SAMPLES])
> >>>> +{
> >>>> +    int bits_used, band_num, bfu_num, i;
> >>>> +
> >>>> +    /* parse the info byte (2nd byte) telling how much BFUs were coded */
> >>>> +    su->num_bfus = bfu_amount_tab1[get_bits(gb, 3)];
> >>>> +
> >>>> +    /* calc number of consumed bits:
> >>>> +        num_BFUs * (idwl(4bits) + idsf(6bits)) + bsm(8bits) + info_byte(8bits)
> >>>> +        + info_byte_copy(8bits) + bsm_copy(8bits) */
> >>>> +    bits_used = su->num_bfus * 10 + 32 +
> >>>> +                bfu_amount_tab2[get_bits(gb, 2)] * 4 +
> >>>> +                bfu_amount_tab3[get_bits(gb, 3)] * 6;
> >>>> +
> >>>> +    /* get word length index (idwl) for each BFU */
> >>>> +    for (i=0 ; i<su->num_bfus ; i++)
> >>>> +        su->idwls[i] = get_bits(gb, 4);
> >>>> +
> >>>> +    /* get scalefactor index (idsf) for each BFU */
> >>>> +    for (i=0 ; i<su->num_bfus ; i++)
> >>>> +        su->idsfs[i] = get_bits(gb, 6);
> >>>> +
> >>>> +    /* zero idwl/idsf for empty BFUs */
> >>>> +    for (i = su->num_bfus; i < AT1_MAX_BFU; i++)
> >>>> +        su->idwls[i] = su->idsfs[i] = 0;
> >>>> +
> >>>> +    /* read in the spectral data and reconstruct MDCT spectrum of this channel */
> >>>> +    for (band_num=0 ; band_num<AT1_QMF_BANDS ; band_num++) {
> >>>> +        for (bfu_num=bfu_bands_t[band_num] ; bfu_num<bfu_bands_t[band_num+1] ; bfu_num++) {
> >>>> +            int         pos;
> >>>> +
> >>>> +            int num_specs = specs_per_bfu[bfu_num];
> >>>> +            int word_len = idwl2wordlen[su->idwls[bfu_num]];
> >>> that remaping could be done outside the loops
> >>
> >> I think I understood this one time, now I just don't see how that could
> >> be done.
> > 
> > for all bfu_num
> >     table[bfu_num] = idwl2wordlen[get_bits()];
> > 
> > ...
> > for (band_num=0 ; band_num<AT1_QMF_BANDS ; band_num++) {
> >     for (bfu_num=bfu_bands_t[band_num] ; bfu_num<bfu_bands_t[band_num+1] ; bfu_num++) {
> >         int         pos;
> > 
> >         int num_specs = specs_per_bfu[bfu_num];
> >         int word_len = table[bfu_num];
> > 
> > 
> > [...]
> > 
> >> +/* idwl to wordlen translation table */
> >> +static const uint8_t idwl2wordlen[16]= {0, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16};
> > 
> > doxy
> > 
> 
> Ok. How should the split be done, svn copy or not (complete atrac3.c in
> history or not) ?

is there some reason not to do svn copy? if not id say do copy


[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

In fact, the RIAA has been known to suggest that students drop out
of college or go to community college in order to be able to afford
settlements. -- The RIAA
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