Re: [rtcweb] Comments on draft-ietf-rtcweb-fec-04 (Opus FEC)

This would be a pretty dramatic statement, basically indicating that the
FEC mechanism that has been built for Opus is not useful.

While I understand the rationale, I would like to have some more specific
results that we can point at before making such a claim in a RFC.

On Tue, May 16, 2017 at 2:50 PM, Bernard Aboba <bernard.aboba@gmail.com>
wrote:

> Section 3.3
>
>    For Opus, packets deemed as important are re-encoded at a lower
>    bitrate and added to the subsequent packet, allowing partial recovery
>    of a lost packet.  This scheme is fairly efficient; experiments
>    performed indicate that when Opus FEC is used, the overhead imposed
>    is about 20-30%, depending on the amount of protection needed.  Note
>    that this mechanism can only carry redundancy information for the
>    immediately preceding packet; as such the decoder cannot fully
>    recover multiple consecutive lost packets, which can be a problem on
>    wireless networks.  See [RFC6716], Section 2.1.7 <https://tools.ietf.org/html/rfc6716#section-2.1.7> for complete
>    details.
>
>
> Section 4.1
>
>
>    When using the Opus codec, use of the built-in Opus FEC mechanism is
>    RECOMMENDED.  This provides reasonable protection of the audio stream
>    against typical losses, with modest overhead.  Note that as indicated
>    above the built-in Opus FEC only provides single-frame redundancy; if
>    multi-packet protection is needed, the built-in FEC should be
>    combined with [RFC2198 <https://tools.ietf.org/html/rfc2198>] redundancy to protect the N-2th, N-3rd, etc.
>    packets.
>
>
> [BA] While loss of a single packet is by far the most likely loss
>
> mode, our tests do not indicate that there is much benefit from
>
> Opus internal FEC.  For example, when we compared Opus internal
>
> FEC against no FEC and concealment, we found little discernible
>
> difference in MOS score. On the other hand, we did see a benefit
>
> arising from RFC 2198 redundancy to protect against burst loss.
>
> So based on our test results, we cannot recommend use of built-in
>
> Opus FEC.
>
>
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