[FFmpeg-devel] [PATCH 4/4] opus: Don't invert phase when downmixing to mono (per RFC8251)

[Rostislav Pehlivanov]

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

> When decoding to downmixed mono, don't put the channels out of phase,
> as they will cancel out and create audible artifacts. (See
> RFC 8251 sec. 10.)
>
> Signed-off-by: Andrew D'Addesio <modchipv12 at gmail.com>
> ---
>  libavcodec/opus_pvq.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/libavcodec/opus_pvq.c b/libavcodec/opus_pvq.c
> index 2f7aa74..f18c010 100644
> --- a/libavcodec/opus_pvq.c
> +++ b/libavcodec/opus_pvq.c
> @@ -643,7 +643,13 @@ static av_always_inline uint32_t
> quant_band_template(CeltPVQ *pvq, CeltFrame *f,
>                  }
>              } else {
>                  inv = (b > 2 << 3 && f->remaining2 > 2 << 3) ?
> ff_opus_rc_dec_log(rc, 2) : 0;
> +
> +                /* Don't put the channels out of phase if we are decoding
> to downmixed
> +                 * mono as this subjectively sounds bad (RFC 8251 section
> 10). */
> +                if (f->channels == 1)
> +                    inv = 0;
>              }
> +
>              itheta = 0;
>          }
>          qalloc = opus_rc_tell_frac(rc) - tell;
> --
> 2.15.1.windows.2
>
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>

RFC8251 made this optional, pushed a new version which added an option to
toggle this.