[FFmpeg-devel] [PATCH 3/3] aaccoder: Improve IS phase rejection

[Claudio Freire]

On Fri, Jul 17, 2015 at 6:20 PM, Rostislav Pehlivanov
<atomnuker at gmail.com> wrote:
> This commit adds a slightly more robust way of determining whether the phases
> match or are too different for IS to be used.
> ---
>  libavcodec/aaccoder.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/aaccoder.c b/libavcodec/aaccoder.c
> index 17b14d6..bd232f6 100644
> --- a/libavcodec/aaccoder.c
> +++ b/libavcodec/aaccoder.c
> @@ -56,6 +56,9 @@
>  /** Frequency in Hz for lower limit of intensity stereo   **/
>  #define INT_STEREO_LOW_LIMIT 6100
>
> +/** If less than this fraction of coeff phases agree disable IS for that band **/
> +#define IS_PHASE_DIFF_LIM 0.11
> +
>  /** Total number of usable codebooks **/
>  #define CB_TOT 12
>
> @@ -1218,9 +1221,9 @@ static void search_for_is(AACEncContext *s, AVCodecContext *avctx, ChannelElemen
>                          ener01 += (coef0 + coef1)*(coef0 + coef1);
>                      }
>                  }
> -                if (!phase) { /* Too much phase difference between channels */
> +                if (fabs(phase)/(sce0->ics.group_len[w]*sce0->ics.swb_sizes[g]) < IS_PHASE_DIFF_LIM) {
>                      start += sce0->ics.swb_sizes[g];
> -                    continue;
> +                    continue; /* Too much phase difference */
>                  }
>                  phase = av_clip(phase, -1, 1);
>                  for (w2 = 0; w2 < sce0->ics.group_len[w]; w2++) {

What happened to the idea of comparing the energies of the addition
and diferrence and deciding on that?

It looked better at rejecting these cases than this one when we talked about it.