Re: [codec] Last proposal for the update draft

Gregory Maxwell <gmaxwell@gmail.com> Mon, 20 June 2016 01:56 UTC

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From: Gregory Maxwell <gmaxwell@gmail.com>
Date: Mon, 20 Jun 2016 01:55:55 +0000
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To: Jean-Marc Valin <jmvalin@jmvalin.ca>
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Subject: Re: [codec] Last proposal for the update draft
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On Fri, Jun 17, 2016 at 12:26 PM, Jean-Marc Valin <jmvalin@jmvalin.ca> wrote:
> 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.

This appears reasonable to me, and I also do not see a way in which
the old behavior could be usefully used, or any way that this would be
likely to cause serious incompatibility.