[opus] FEC and Stereo

[Jean-Marc Valin]

Hi Jon,

On 27/01/17 08:31 AM, Jon Lederman wrote:
> We are trying to use Opus in a VoIP environment for sending stereo
> audio.  We have noticed a phenomenon where when FEC is enabled and
> packet_loss_percentage>0, that there is a mixing of audio from the
> left channel into the right channel and vice versa.  That is, rather
> than hearing each channel in its pristine form as it was in the file,
> there is a combination of right and left channels that is heard on
> the left and right playback.
> 
> First, is this the correct behavior?  If so, why does this occur?

It is the correct behaviour, at least to some extent. Opus may
deliberately introduces cross-talk between the channels to make the
signal easier to code when it doesn't have enough bits to do a good
enough job with the original stereo audio. If you increase the bitrate,
then that cross-talk with be reduce and eventually disappear. Now the
only difference that FEC does is that it increases the threshold for the
cross-talk.

> Finally, is there a way to prevent this other than disabling FEC
> (which we don’t want).

Well, obviously if you're not going to use the FEC, then you shouldn't
waste bits to encode it.

Cheers,

	Jean-Marc