[FFmpeg-devel] [PATCH 2/3] libavcodec: ac3 floating point decoder fixes

[Michael Niedermayer]

On Tue, Nov 27, 2012 at 05:27:56PM +0100, Nedeljko Babic wrote:
> Glitch caused by upmix fixed by changing the order
>  of imdct and downmix and hence eliminating upmix.
> Fate tests changed and new referent pcms generated
>  to support these changes.
> 
> Signed-off-by: Nedeljko Babic <nbabic at mips.com>
> ---
>  libavcodec/ac3dec.c |   60 +++-----------------------------------------------
>  tests/fate/ac3.mak  |    8 +++---
>  2 files changed, 8 insertions(+), 60 deletions(-)
> 
> diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
> index 7fb380c..0fe6673 100644
> --- a/libavcodec/ac3dec.c
> +++ b/libavcodec/ac3dec.c
> @@ -187,7 +187,6 @@ static av_cold int ac3_decode_init(AVCodecContext *avctx)
>              avctx->request_channels <= 2) {
>          avctx->channels = avctx->request_channels;
>      }
> -    s->downmixed = 1;
>  
>      avcodec_get_frame_defaults(&s->frame);
>      avctx->coded_frame = &s->frame;
> @@ -621,34 +620,6 @@ static inline void do_imdct(AC3DecodeContext *s, int channels)
>  }
>  
>  /**
> - * Upmix delay samples from stereo to original channel layout.
> - */
> -static void ac3_upmix_delay(AC3DecodeContext *s)
> -{
> -    int channel_data_size = sizeof(s->delay[0]);
> -    switch (s->channel_mode) {
> -    case AC3_CHMODE_DUALMONO:
> -    case AC3_CHMODE_STEREO:
> -        /* upmix mono to stereo */
> -        memcpy(s->delay[1], s->delay[0], channel_data_size);
> -        break;
> -    case AC3_CHMODE_2F2R:
> -        memset(s->delay[3], 0, channel_data_size);
> -    case AC3_CHMODE_2F1R:
> -        memset(s->delay[2], 0, channel_data_size);
> -        break;
> -    case AC3_CHMODE_3F2R:
> -        memset(s->delay[4], 0, channel_data_size);
> -    case AC3_CHMODE_3F1R:
> -        memset(s->delay[3], 0, channel_data_size);
> -    case AC3_CHMODE_3F:
> -        memcpy(s->delay[2], s->delay[1], channel_data_size);
> -        memset(s->delay[1], 0, channel_data_size);
> -        break;
> -    }
> -}
> -
> -/**
>   * Decode band structure for coupling, spectral extension, or enhanced coupling.
>   * The band structure defines how many subbands are in each band.  For each
>   * subband in the range, 1 means it is combined with the previous band, and 0
> @@ -1221,40 +1192,17 @@ static int decode_audio_block(AC3DecodeContext *s, int blk)
>          ff_eac3_apply_spectral_extension(s);
>      }
>  
> +    do_imdct(s, s->channels);
> +
>      /* downmix and MDCT. order depends on whether block switching is used for
>         any channel in this block. this is because coefficients for the long
>         and short transforms cannot be mixed. */
>      downmix_output = s->channels != s->out_channels &&
>                       !((s->output_mode & AC3_OUTPUT_LFEON) &&
>                       s->fbw_channels == s->out_channels);
> -    if (different_transforms) {
> -        /* the delay samples have already been downmixed, so we upmix the delay
> -           samples in order to reconstruct all channels before downmixing. */
> -        if (s->downmixed) {
> -            s->downmixed = 0;
> -            ac3_upmix_delay(s);
> -        }
>  
> -        do_imdct(s, s->channels);
> -
> -        if (downmix_output) {
> -            s->ac3dsp.downmix(s->output, s->downmix_coeffs,
> -                              s->out_channels, s->fbw_channels, 256);
> -        }
> -    } else {
> -        if (downmix_output) {
> -            s->ac3dsp.downmix(s->transform_coeffs + 1, s->downmix_coeffs,
> -                              s->out_channels, s->fbw_channels, 256);
> -        }
> -
> -        if (downmix_output && !s->downmixed) {
> -            s->downmixed = 1;
> -            s->ac3dsp.downmix(s->delay, s->downmix_coeffs, s->out_channels,
> -                              s->fbw_channels, 128);
> -        }
> -
> -        do_imdct(s, s->out_channels);

this change would cause the decoder to unneccessarily imdct transform
channels that are subsequently not used

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

What does censorship reveal? It reveals fear. -- Julian Assange
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