[opus] Opus FEC

[Benjamin Schwartz]

The Opus encoder does not provide generic "FEC" in the conventional sense.
 You can do that yourself as a later step between the encoder and
transmission.

What Opus provides, in some modes, is an additional "Low Bitrate
Redundancy" frame within some packets to increase packet loss resilience.
 See http://tools.ietf.org/html/rfc6716#section-2.1.7 for an overview and
http://tools.ietf.org/html/rfc6716#section-4.2.5 for details.




On Mon, Jan 28, 2013 at 10:50 AM, Young, Milan <Milan.Young at nuance.com>wrote:

> Hello,
>
> I understand the encoder provides an option for FEC to provide some
> protection against packet loss, but I don't understand the details of this
> arrangement.  I'd appreciate answers to the following:
>
> * Adding FEC seems to change the encoded audio bit-stream itself, i.e., it
> doesn't just add additional protection bits, but also changes the encoded
> bits. This is easy to show by comparing two decoded versions of the same
> audio file: one encoded with no FEC, and another with FEC. Is this expected
> or is it a bug?
> * Related to the above, what does the FEC information consist of? How does
> the decoder use it? Does it simply repeat preceding packets, or interpolate
> preceding and subsequent packets, or does it do something smarter?
>  * Based on my tests, the FEC in opus seems to have a limit, i.e., it adds
> correction but only up to approx 4---10% expected packet loss, depending on
> the file. What are the parameters/limits of the FEC?
>   * Does FEC add any algorithmic delay? i.e., does it require the decoder
> to wait until post-loss packets are received in order to generate a
> replacement for the lost packets? If so, what is the maximum delay and can
> it be variably set?
>
> Thank you
>
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