[FFmpeg-devel] [PATCH 3/4] opus: Add Special Hybrid Folding (per RFC8251)

[Rostislav Pehlivanov]

On 2 December 2017 at 17:46, Andrew D'Addesio <modchipv12 at gmail.com> wrote:

> This decoder-side change, introduced in RFC 8251 (section 9), slightly
> improves the decoded quality of 16kbps speech in Hybrid Mode.
>
> Differences can be seen/heard in testvector05.bit, testvector06.bit,
> and testvector12.bit in the RFC 6716/8251 testvectors found here:
> https://people.xiph.org/~greg/opus_testvectors/
>
> Signed-off-by: Andrew D'Addesio <modchipv12 at gmail.com>
> ---
>  libavcodec/opus_celt.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/libavcodec/opus_celt.c b/libavcodec/opus_celt.c
> index ff56041..2bbb96b 100644
> --- a/libavcodec/opus_celt.c
> +++ b/libavcodec/opus_celt.c
> @@ -712,10 +712,22 @@ static void celt_decode_bands(CeltFrame *f,
> OpusRangeCoder *rc)
>              b = av_clip_uintp2(FFMIN(f->remaining2 + 1, f->pulses[i] +
> curr_balance), 14);
>          }
>
> -        if (ff_celt_freq_bands[i] - ff_celt_freq_range[i] >=
> ff_celt_freq_bands[f->start_band] &&
> -            (update_lowband || lowband_offset == 0))
> +        if ((ff_celt_freq_bands[i] - ff_celt_freq_range[i] >=
> ff_celt_freq_bands[f->start_band] ||
> +            i == f->start_band + 1) && (update_lowband || lowband_offset
> == 0))
>              lowband_offset = i;
>
> +        if (i == f->start_band + 1) {
> +            /* Special Hybrid Folding (RFC 8251 section 9). Copy the
> first band into
> +            the second to ensure the second band never has to use the
> LCG. */
> +            int offset = 8 * ff_celt_freq_bands[i];
> +            int count = 8 * (ff_celt_freq_range[i] -
> ff_celt_freq_range[i-1]);
> +
> +            memcpy(&norm[offset], &norm[offset - count], count *
> sizeof(float));
> +
> +            if (f->channels == 2)
> +                memcpy(&norm2[offset], &norm2[offset - count], count *
> sizeof(float));
> +        }
> +
>          /* Get a conservative estimate of the collapse_mask's for the
> bands we're
>             going to be folding from. */
>          if (lowband_offset != 0 && (f->spread != CELT_SPREAD_AGGRESSIVE ||
> @@ -728,7 +740,7 @@ static void celt_decode_bands(CeltFrame *f,
> OpusRangeCoder *rc)
>              foldstart = lowband_offset;
>              while (ff_celt_freq_bands[--foldstart] > effective_lowband);
>              foldend = lowband_offset - 1;
> -            while (ff_celt_freq_bands[++foldend] < effective_lowband +
> ff_celt_freq_range[i]);
> +            while (++foldend < i && ff_celt_freq_bands[foldend] <
> effective_lowband + ff_celt_freq_range[i]);
>
>              cm[0] = cm[1] = 0;
>              for (j = foldstart; j < foldend; j++) {
> --
> 2.15.1.windows.2
>
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>

Pushed, updated fate stddev scores, new decoded samples should be uploaded
to fate soon too.