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Re: [codec] Last proposal for the update draft

Felicia Lim <flim@google.com> Tue, 21 June 2016 09:22 UTC

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From: Felicia Lim <flim@google.com>
Date: Tue, 21 Jun 2016 09:22:22 +0000
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Subject: Re: [codec] Last proposal for the update draft
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+1

On Fri, Jun 17, 2016 at 2:26 PM Jean-Marc Valin <jmvalin@jmvalin.ca> wrote:

> Hi,
>
> I've been working on completing the update draft and have come across
> one last thing that I think should be updated in the spec. This was
> discovered through running hybrid mode at low rates, around 16 kb/s.
>
> At those rates, we sometimes only have enough bits to code a single
> CELT band (8 - 9.6 kHz). When that happens in hybrid mode (unlike
> CELT-only), the second band (CELT band 18, from 9.6 to 12 kHz) cannot
> use folding because it is wider than the amount already coded, and
> falls back to LCG noise. Because it can also happen on transients
> (e.g. stops), it can cause audible pre-echo.
>
> My proposed solution is to fix the folding behaviour so that it is
> never forced to fall back to LCG due to not enough folding data. This
> is achieved by simply repeating part of the first band in the folding
> of the second band. The proposed patch is available at:
> https://git.xiph.org/?p=opus.git;a=commitdiff;h=949c7f33181
> It's a decoder-only change and it fixes the low-bitrate pre-echo
> problem. I uploaded a short example that demonstrates the problem and
> the fix. This is before the change:
> http://jmvalin.ca/misc_stuff/fix_before.wav
> and this is after the change:
> http://jmvalin.ca/misc_stuff/fix_after.wav
> Note especially the "t" in the word "tour", which has noticeable HF
> pre-echo in the "before" file. Both files are decoded from the same 16
> kb/s CBR bit-stream.
>
> The fix does not impact compatibility, because the improvement does
> not depend on the encoder doing anything special. I also do not see
> any case where an encoder could have effectively used the current
> broken behaviour to improve quality over the proposed change.
>
> For these reasons, I think it should be included in the update draft.
>
> Cheers,
>
>         Jean-Marc
>
> _______________________________________________
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> codec@ietf.org
> https://www.ietf.org/mailman/listinfo/codec
>

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