[opus] FEC and Stereo
jmvalin at jmvalin.ca
Fri Jan 27 16:38:03 UTC 2017
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
> 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.
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